(S. A. Patney: Strabismology Desk Reference, chapter 51, JKA Publications)

NYSTAGMUS

Nystagmus is among the oculomotor disorders that are relatively more difficult to manage. In most cases it is congenital. Its etiology, mechanism and other factors are not yet well understood.

In this chapter we shall include the more important points relating to nystagmus, mainly congenital, as strabismologists have to deal with the congenital form. The acquired nystagmus is mainly due to neurological causes and concerns the neurologist and neuro-ophthalmologist.

The common association of nystagmus and infantile esotropia is well known and there may be reason to suspect that primary brain dysfunction may be responsible for both, the strabismus and the congenital nystagmus (latent and manifest). The primary brain dysfunction may have been due to brain injuries¹ caused during delivery of the infant. However, other authors have indicated that latent nystagmus may be secondary to strabismus², which in turn may have been a consequence of optokinetic asymmetry.

Nystagmus is involuntary, rhythmic and pendular/jerky to and fro (oscillatory) movement of the eyes.

Prevalence: An incidence³ of 1 in 6550 was reported by Hemmes in 1927.

A preponderance of males over females has been reported repeatedly in literature.^{4, 5}

Congenital nystagmus, particularly the sensory type, is not uncommonly hereditary. We have a family on record that has about 9 members that have oculo-cutaneous albinism and nystagmus among first and second cousins. Two of them have esotropia also.

The mode of transmission in some of the cases of sensory nystagmus is given in table 51-1 on the following page.

Terminology related to nystagmus

1. Type: A. Pendular: Phases (to-and-fro movements) are of equal velocity

B. Jerky: Phases are of unequal velocity.

2. Direction: The side (direction) of the fast component. However, the pathological movement is the slow one.
3. Frequency: Rapid / Slow
4. Amplitude: Coarse (large) / Medium / Fine
5. Trajectory: Horizontal / Vertical / Diagonal / Rotary / Circular / Elliptical
6. Dissociated: The amplitude in the two eyes is different.
7. Null Zone: The field of gaze in which the intensity is minimal.
8. Conjugacy: (a) Conjugate: The nystagmus in both eyes is similar.

(b) Disjugate: The movements in the two eye are different, e.g., horizontal in one eye and circular in the other eyes.

9. Neutral Zone: The field of gaze in which the bilateral jerky nystagmus reverses its direction.

There are various ways of classifying nystagmus. Different books give different classifications. The following are a few examples:

Classification example 1:

Noorden⁶ has classified nystagmus as: Congenital and Acquired. The latter not being of concern to ophthalmologist / strabismologist, he has divided the congenital variety as summarized below.

Classification example 3:

A neurological classification is given in table 51-2. Although strabismologists are not concerned with other than congenital type of nystagmus, but having an idea of the huge variety of nystagmus and related movements may not be out of place here.

The traditional classification of congenital nystagmus into motor and sensory types is not recognized now by neurologists.⁴⁰ The reason for this is as follows:

The recording of the eye movements in congenital nystagmus is the same in the case of congenital nystagmus associated with visual dysfunction and that without it. A causal relationship between the visual defect and the nystagmus can only be proved if the nystagmus has been proved to follow the visual loss. Otherwise the two of them can coexist without the loss of vision being responsible for nystagmus.

Comments: In my opinion none of the existing classifications that have come to my notice are comprehensive. But I would like to suggest a new classification of nystagmus as given below.

Physiological nystagmus

1. Optokinetic nystagmus (OKN)

When a patient looks at alternate black stripes as in the case of an optokinetic drum, the eyes show nystagmus, known as Optokinetic nystagmus. The main points are:

• OKN is a combination of pursuit movements (SEM) and Saccade (compensatory FEM) to take up fixation of the next target on the drum.
• The function of the OKN is to facilitate image stabilization when looking at a constantly moving field.
• Normal OKN is symmetric.
• Asymmetric OKN is found in some cases of deep parietal lesions with hemianopia and in infantile esotropia. In the former OKN is decreased when the stripes are moving towards the side of the lesion. In the later the OKN has a nasal bias.
• OKN is used to test the visual acuity in an infant.
• It can also be useful in cases of functional blindness where it can show the presence of vision if present

2. Caloric nystagmus⁸²

• It is a combination of SEM (vestibular / pursuit) and compensatory FEM (saccade).
• SEM is produced by stimulation or inhibition of one or more of the semicircular canals.
• Depending on the position of head, unilateral irrigation leads to horizontal, circular or oblique nystagmus.
• Bilateral simultaneous irrigation leads to vertical nystagmus. With cold water the fast phase is upwards and with warm water it is downwards.

3. Rotational nystagmus

• It is a jerky nystagmus, which is produced as a result of rotation or fast movements of head.
• It is useful in estimating the oculomotor status in infants.
• The eyes deviate tonically in the direction of the movement during the nystagmus.
• The quick phase of nystagmus is towards the opposite side.

4. End position nystagmus

• Fatigue nystagmus: It occurs after rather prolonged deviation of eyes in side gaze (about 10-15 seconds).
• Sustained end position nystagmus: It comes along when eyes are in extreme side gaze. It is symmetrical and fine. It stops when eyes come back to PP.
• Unsustained end position nystagmus: Occurs initially in extreme side gaze. It stops spontaneously in 5-10 seconds.

The etiology varies according to the type of nystagmus. The main types are given in the following text.

Among the commonest causes of manifest congenital nystagmus are:

1. Oculocutaneous albinism^{6, 7}
2. Congenital cataracts
3. Congenital glaucoma
4. Down’s syndrome⁸
5. Aniridia
6. Achromatopsia
7. High myopia
8. Optic nerve hypoplasia⁷
9. Leber’s amurosis⁷
10. Coloboma of choriod (in our experience)
11. Other conditions causing congenital defective vision (in our experience)

1) The basic cause in these cases is insufficient image formation on the fovea. This in turn is caused by some disorder of the anterior visual pathway that interferes with the formation of a clear image on the fovea. The feedback from the foveal region is thus inadequate /abnormal, resulting in disruption f normal oculomotor control of the fixation mechanism, leading to nystagmus.

According to Cogan the normal oculomotor stabilization requires that normal sensory input be there. This is only possible if the afferents are normal. His view was supported by the work done by Doesschate¹⁰, which demonstrated the occurrence of pendular nystagmus after the need and thus the stimulation for fixation was removed by making the foveal image stable.

2) But the mechanism of production of nystagmus seems to depend on more than just one factor. No doubt that establishment of normal oculomotor stabilization depends on the feedback from normal sensory afferent, but there must be other factors also because the congenital sensory nystagmus does not go away in dark nor when Fresnel lenses are used (when all stimulation to fixate is removed) ^{11, 12}.

The cause of motor defect (congenital) nystagmus is a defective efferent system. The seat of lesion could be either in the centers or the pathways of the binocular (conjugate) ocular movements. No ocular anomalies are found. The visual acuity may be better in one of the directions of gaze because of the nystagmus being less or absent in that direction.

Various factors have been held responsible for this type of nystagmus. The better known theories / hypotheses are summarized below.

• A difference in the quality of the two retinal images³²: It is considered to be the main etiological factor, in the presence or absence (as in strabismus) of binocular vision.
• A lack of stability of oculomotor coordination³³: It may have been caused by a defective development of monocular and binocular fixation reflexes.³⁴
• A nasal drift bias originating centrally: This factor is supposed to be caused to a defect of spatial localization and directionalization.^{28, 35}

Age of onset: Usually manifest congenital nystagmus starts between 3-4 months of age. However, it can occur at any age, later in infancy, childhood or teenage. It has even been reported to start in adult age⁹.

Clinical course: Manifest congenital nystagmus is found to disappear after some time in quite a number of cases. The percentage of cases in which nystagmus disappears varies in various clinical reports in the literature. In one report¹⁰ it has been found to be from 30% (in cases with strabismus and neurological disorders) to 70% where no other anomaly is present. These workers found that nystagmus went away in 50% of cases by the age of 5 years. Other authors find these figures to be on the higher side⁶.

General features of nystagmus are given in short below followed by special features each of the main types.

1. Visual acuity and Amblyopia

Nystagmus amblyopia is one of the main types in von Noorden's classification of amblyopia (see chapter 23, page327-328. It can be easily realized how constant movements of the eyes will adversely affect the visual acuity. However, it is not clear if the nystagmus is the cause or the result of reduced visual acuity. In every case of bilateral amblyopia one should always look out for the presence of nystagmus that is not visible to naked eye. To the naked eye, in some cases, the nystagmus is not visible. However, when the patient is asked to look at the fixation object in the visuscope or the ophthalmoscope, the fine fast nystagmus can be easily seen. Micronystagmus particularly is detected in this way. The underlying causes of amblyopia in these cases can be multiple as elaborated a little later on.

The main points are given below.

• It is extremely important to examine both the binocular and uniocular visual acuity (VA) for near and for distance, with and without CHP, in cases of nystagmus. In latent and manifest-latent nystagmus the visual acuity is greatly reduced when one eye is occluded. In these cases binocular VA is significantly better than uniocular VA.

• However, in some cases uniocular VA is found to be better when one eye is occluded to examine the VA the nystagmus is seen to decrease leading to improvement in VA.²⁵

• In cases of sensory defect nystagmus the VA is reduced according to the type and severity of the organic disease. In the motor defect type of nystagmus the VA is reduced due the nystagmus and is often found to be severely affected, e.g., 6/60 or even sometimes lower.^{26, 2}.

• But in most cases there is a combination of factors that is responsible for the reduction of VA. Apart from the constant movement due to nystagmus, refractive error, sensory defects (organic disease like congenital cataract), underdevelopment of fovea (particularly in cases of oculocutaneous albinism, strabismus amblyopia, ametropic amblyopia and visual (stimulus) deprivation amblyopia may all co-exist.

• Patients’ whose null point is situated in the periphery improve their VA by adopting a compensatory (anomalous) head posture (CHP).

• If a compensatory (anomalous) head posture (CHP) is present the visual acuity should be tested with as well as without the CHP. If the CHP is due to nystagmus, the VA will be found to be better with CHP. Without CHP and with the test types placed in primary position the VA is found to be severely reduced.

• Visual acuity is found to be reduced in all kinds of nystagmus Even in latent nystagmus as soon as one of the eyes is occluded to test the VA the nystagmus manifests and VA is reduced. However, it varies according to the particular type of nystagmus and in the directions of gaze in which nystagmus becomes worse. The visual acuity (VA) improves when the nystagmus is reduced either by placing the eyes in the "null point" or, in the case of latent nystagmus when both eyes are open. In latent- manifest nystagmus the reduced visual acuity due to the manifest component decreases further when one eye is occluded.

• If the nystagmus is fast and jerky the decrease in VA is more marked because the foveation time (the time during which the image of the object of attention stays on the fovea) in these patients is quite short.

• However, the motor features of nystagmus, its frequency, its amplitude, and velocity do not always co-relate with the visual acuity. Often it is difficult to explain the degree of visual defect. This may be due to the presence of a nystagmus amblyopia that is obviously due to visual deprivation as is the case in ametropic amblyopia. The nystagmus amblyopia further complicates an already complicated clinical picture.

• In my practice I have found that some of these cases have an underdeveloped fovea. This is particularly true of patients of oculocutaneous albinism with nystagmus. Also, refractive errors are fairly common in cases of congenital idiopathic nystagmus.

• In every case of nystagmus it is important to measure the near as well as distance VA. The former is often much better than the latter. These patients often hold the print very close but may be able to read N5! Like distance VA, near VA should also be measured with each eye in turn and also binocularly.

• Correction of refractive error helps therefore it should always be done before starting any other therapy.

• In the patients of nystagmus with strabismus, presence of a strabismic amblyopia complicates things further. If the nystagmus does not become exacerbated with one eye occluded, occlusion therapy for it can still be carried out.
• The visual acuity can be improved by prisms (Fresnel’s). This is done in one of the two ways given below: Firstly, by moving the null point to primary position and secondly, base out prisms in front of both eyes stimulate convergence as the patient has to converge to fuse the images (that have been displaced nasally by prisms) to avoid diplopia.²⁸ The fact that convergence reflex suppresses nystagmus is well known.

• It can also be bettered by surgery that places the eyes in the neutral zone (at null point). Details are given under the heading of surgery for nystagmus.

2. Oscillopsia

• This symptom of seeing even stationary objects moving is rarely present in cases of congenital nystagmus. Usually it accompanies acquired nystagmus.
• The reason could of absence of oscillopsia in congenital nystagmus cases may be one of the following two:

• Compensatory or anomalous head posture (CHP)
  • Quite often the nystagmus is not there in all the directions. For instance, it may be present in horizontal, vertical or oblique direction only, in more than one of these. For instance, nystagmus may occur in dextroversion only and absent in levoversion and so on (see case report-1).

Case report -1 A girl aged 6 years came to us for defective vision in both eyes. The number of corrective glasses was: OD: -3 sph.-3.5 cylinder axis 20 degrees and OS: -3 sph.-1.5 cyl. Axis 150 degrees. No significant deviation is there. She has a rather noticeable face turn to right, slight head tilt to right and some chin depression. The cause is a nystagmus which is only present in Dextroversion, dextroelevation and dextrodepression. The nystagmus is of maximum intensity in dextrodepression. There is a latent mild nystagmus in primary position, which is not always demonstrable but as the eyes turn towards right, it goes on increasing. The null point is, naturally, in levoelevation (opposite of dextrodepression) and therefore the eyes are maintained in that position by adopting the CHP.

 

• CHP is adopted to place the eyes at the null point where the nystagmus is either absent or markedly reduced. It is only in cases where null point is to one side or in elevation or depression that CHP can be of use. It is obvious that if the null point is in primary position (PP) the best visual acuity is obtained in PP. It is only when the null point is situated in the periphery that the head has to be placed so that the eyes are looking towards the direction in which the nystagmus is absent/reduced. This is done to get the best possible visual acuity.

• If the null point is only slightly displaced to one side, the CHP is also slight and the patient needs no therapy to correct the anomalous head posture. However, if the CHP is marked because the null point is right at the periphery a marked CHP can be present that is uncomfortable and is cosmetically intolerable. If the head is straightened the visual acuity is severely reduced as the foveation (the time the image of the object of attention stays at the fovea by a steady fixation) time is drastically reduced. In such cases if the null point is moved to the center by prisms or surgery, the CHP disappears/improves leading to marked improvement of VA in primary position.

• If surgery is decided upon, it is better to try appropriate prisms prior to surgery to see if the CHP can be corrected by placing the eyes at the null point and whether the visual acuity improves.

• The type of CHP depends on the situation of the null point (the point at which the nystagmus is either absent or significantly reduced resulting in marked improvement of visual acuity (VA). For instance, if the null point is situated in levoversion, the head has to be turned to right to place the eyes in levoversion. This leads to a face turn to right. Other CHPs are as follows:

Null point in dextroversion: Face turn to left

Null point in deorsumversion (depression): Chin elevation

Null point in sursumversion (elevation): Chin depression

Null point in an oblique direction: Face-turn associated with head tilt to have the eyes looking in that direction.

• Face turn is by far the most common type of CHP in cases of nystagmus

4. Strabismus

• Strabismus is quite often found to be present in cases of nystagmus, especially in motor defect type. Even sensory defect nystagmus can be associated with strabismus as is seen in cases of congenital cataract and other conditions causing defective vision.
• Latent / manifest latent nystagmus is commonly associated with infantile esotropia while manifest nystagmus is not.
• A thorough orthoptic examination (ocular motility workout) is important in every case of nystagmus as a small strabismus can be easily missed.
• It is rather difficult to conduct an orthoptic examination in a case of nystagmus because of the constant movements but keen observation on repeated cover test is usually successful in detecting even a small strabismus.
• An association of nystagmus, strabismus and oculocutaneous albinism is not an uncommon occurrence.
• In my practice there have been many cases of nystagmus associated with strabismus and in some of them at least, correction of refractive error and/or surgery for strabismus reduced the nystagmus.
• Unilateral amblyopia can be explained easily if a small strabismus can be detected. The latter is rather difficult as small movements on cover test with the eyes constantly moving due to nystagmus are usually always missed or can be confused with the refixation movement of the eye.
• In some patients esotropia is brought on to block the nystagmus: Also called as the "Nystagmus blockage syndrome", it is discussed under a separate heading in this chapter.

A short description of the various types of nystagmus follows.

Sensory defect nystagmus of Cogan

• Presence of anterior ocular (visual pathway) disease, leading to indistinct/defective image formation on the fovea.
• Bilateral
• Horizontal
• Pendular (velocity of oscillations is equal in both directions)
• Becomes jerky in extreme gazes.

 

Motor defect nystagmus of Cogan

• No ocular disease present
• May be asymmetrical/unequal in the two directions getting better/absent in one direction leading to
• Improvement in visual acuity in that direction and adoption of:
• Compensatory (anomalous) head posture to place the eyes in the direction of the "Null point¹³/neutral zone¹⁴/priviledged area¹⁵ (direction in which the nystagmus is least/absent)

 

Manifest congenital nystagmus

• Usually starts during early infancy(first 3-4 months of life) but onset can be at any age, even adult age.⁹
• Often associated with one of the following conditions: congenital cataract, oculocutaneous albinism, congenital glaucoma, Leber’s amaurosis, aniridia, achromatopsia, high myopia Down’s syndrome and optic nerve hypoplasia, the most common of these being oculocutaneous albinism.
• Heredity: Congenital nystagmus may be sporadic or it may follow X-linked, autosomal dominant or autosomal recessive inheritance patterns. Chromosome 6p 12 is the first reported genetic locus in cases of autosomal dominant congenital nystagmus.
• Natural history: In some patients the nystagmus disappears by the age of 5 years, especially if strabismus or a neurological disease does not accompany it. The incidence of disappearance of nystagmus in this group varies.^{6, 19}
• Uniocular and binocular visual acuity are the same, as the nystagmus remains the same with uniocular and binocular fixation.
• Main characteristic of the congenital manifest nystagmus is a slow drift of the visual target off the fovea followed by a correctional movement in the form of a rapid saccadic movement.^{20, 21}
• Congenital nystagmus is generally horizontal but it may be⁶⁰ vertical, rotary (circular), elliptical / oblique.
• Congenital nystagmus may be pendular or jerky.
• Amplitude and frequency of the congenital manifest nystagmus remain the same when one eye is closed or both eyes are open.²²
• Sometimes the manifest nystagmus is associated with latent nystagmus and in such cases occlusion of one eye (as for instance, for examining VA) makes the nystagmus worse.²³
• Electronystagmography: In cases of manifest congenital nystagmus nystagmograph (recording of the eye movements in nystagmus) usually shows equal frequency regardless of whether the fixation is maintained with one or both eyes. It is biphasic, usually pendular and has an increasing velocity slow phase. This last is the most important feature distinguishing it from latent nystagmus. No change is observed when one eye is covered. Difference between a decreasing and increasing velocity slow phase cannot be made out without electronystagmography.^{22, 24}
• Direction of the nystagmus remains the same despite a change of fixation from one eye to the other.
• Association with infantile esotropia is uncommon.
• According to Dell'Osso and Daroff'
• Needs treatment if CHP is significant.

 

Latent and Manifest-latent congenital nystagmus²⁸

• True latent nystagmus is only present when one eye is occluded or becomes amblyopic or blind due to some cause. With both eyes open there is no nystagmus.
• The manifest-latent nystagmus is there when both eyes are open but becomes worse / more intense when one eye is covered. This means that occluding one eye leads to manifestation of the latent part of the nystagmus. The manifest-latent nystagmus is also known as "manifested latent nystagmus".²⁹
• The amplitude of manifest-latent type of nystagmus increases in abduction and decreases in adduction. Its fast phase is invariably towards the side of the fixating eye.
• There has been, in literature, some confusion between true latent nystagmus and manifest-latent nystagmus. This is so because sometimes the manifest element is very fine and is not visible to the eye. In such cases the correct diagnosis can only be made by electronystagmography (recording the nystagmus electrographically).

The main difference between the latent nystagmus and the manifest-latent nystagmus is only quantitative (that is, in degree).³¹ Otherwise findings as visible to the naked eye and in electronystagmography are similar. As already mentioned earlier, the true latent nystagmus is only there when one eye is occluded. However, in some cases the nystagmus is so fine with both eyes open that it is invisible clinically and is only found on nystagmography.

• It has been generally agreed upon that the differentiation between manifest nystagmus and latent nystagmus (or manifest-latent nystagmus) is sometimes difficult or even impossible without electronystagmography.
• The main difference between the manifest nystagmus and latent (or manifest-latent) nystagmus is the slow phase velocity. Manifest nystagmus has an increasing velocity slow phase while latent (or manifest-latent) nystagmus has a decreasing velocity slow phase. This is the most important differentiating feature between the two types of nystagmus.
• Latent or manifest-latent nystagmus is usually accompanied by infantile esotropia.³⁰ It is not so with manifest nystagmus, which only coexists with infantile esotropia in occasional cases.³²

Ciancia's syndrome: Association of esotropia with latent or latent -manifest nystagmus is known as Ciancia's syndrome. Also there is a head turn towards the adducting eye and bilateral limitation of abduction.⁴⁶

 

Nystagmus blockage syndrome

• Definition: Nystagmus blockage syndrome (NBS) was so named by Adelstein and Cuppers.³³ It was defined by them as a condition with the following components:

1. Infantile esotropia (with onset in infancy),

2. Frequent history of nystagmus starting before esotropia comes on,

3. Pseudopalsy of lateral recti,

4. Straight eyes under deep anesthesia and a large manifest nystagmus during the induction,

4. Nystagmus is present when eyes are straight and infant is not attentive but absent / significantly reduced with the infant attentive and esotropic.
5. Manifest nystagmus when fixing eye is moved out of its adducted position and goes into abduction,

6. Anomalous head-posture on occlusion of either eye so that adduction is maintained.

• According to present thinking, it includes all the ocular movements (vergences and versions) that are used to reduce nystagmus⁴¹. For instance, when the nystagmus is worse in dextroversion the eyes are turned to reduce nystagmus and consequently improve visual acuity. Adopting a face turn to right does this. This is an example of compensation by versions. When nystagmus is reduced by exerting convergence leading to esotropia it s called compensation by vergence.
• The only aim of Nystagmus blockage syndrome is to improve the visual acuity.
• This is done by, either turning the eyes in a direction (compensation by version), in which nystagmus is reduced / absent or by inducing vergence, e.g., convergence. The latter is typically identified as Nystagmus blockage syndrome.
• As it is difficult to keep the eyes turned in version and walk, a compensatory head posture is adopted with the head placed in a direction opposite to that of the eyes. Thus nystagmus is a common cause of anomalous head posture.
• It is important to remember that though nystagmus is often blocked by excessive convergence causing esotropia consequent upon nystagmus, the two may co-exist coincidentally. Thus every case of esotropia with nystagmus should not be diagnosed as "Nystagmus blockage syndrome".
• Terminology: It is also known as " Nystagmus compensation syndrome", "Nystagmus blocked syndrome" and "Nystagmus dampening".
• Clinical features: Nystagmus blockage syndrome is a clinical entity with certain special features, which are as follows (case reports 50-2 and 50-3) :
1. There is a pre-existing congenital nystagmus.
2. An esotropia develops to block the nystagmus. Bringing in the convergence mechanism tends to dampen the nystagmus by augmenting the adduction of the fixing eye.
3. Usually the esotropia is unilateral but it could be alternating, with the infant changing the compensatory head posture (CHP) from one side to the other.
4. There is an eccentric "Null Zone" in the converged position.
5. As soon as the eye moves out of the adducted position, one can see a congenital horizontal nystagmus, with the fast phase towards the abducted eye. That means the nystagmus is jerky in waveform and the jerky phase is in the direction of the gaze.
6. The esotropia is non-accommodational and therefore the corrective glasses do not make any difference in the angle of deviation.
7. Constriction of pupil during the esotropic phase has been reported^{42, 43} by some while others deny its presence.⁴⁴
8. Base out prisms fail to produce abduction of the fixing eye.
9. Surgery may not be effective.
• Summary: Doubts have been raised about the existence of NBS by some authors.⁴⁴ Present reviewed definition of NBS is as follows:⁴⁵

The acute phase of NBS shows an esotropia of varying angle, in which whenever the patient is concentrating and is exerting visual attention, there is a manifest convergent deviation with no nystagmus. However, when the child is not concentrating (visual inattention) there is orthotropia with manifest nystagmus. Thus the nystagmus intensity is inversely proportional to the degree of esotropia.

However, Dell'Osso⁴⁴ wonders if NBS is congenital (manifest) nystagmus (CN), manifest-latent nystagmus (MLN) or both? After a careful study involving quantitative oculography on one patient of NBS and two of suspected NBS, he is of the opinion that NBS has been diagnosed indiscriminately in the past. His patient with NBS demonstrated the presence of two different kinds of nystagmus. For distance there were waveforms typical of CN while for near the convergence innervation did not dampen it. However, when one of the eyes became esotropic for near, the nystagmus suddenly changed to MLN.

The differentiation between the two types can only be diagnosed by oculography. This indicated that there are two different mechanisms causing the nystagmus this case. This conclusion has been reached on the basis of the findings in the only case with ongoing CN where oculography has been done. There was no change from CN to MLN.

Some of the cases of Nystagmus blockage syndrome (Nystagmus blockage syndrome) reported in the past may not have had NBS at all, as the diagnosis was only made clinically and congenital nystagmus (CN) and manifest-latent nystagmus (MLN) can only be differentiated on the basis of oculography and not clinically.⁴⁵

 

Other compensatory / dampening / blocking mechanisms

1. The Nystagmus blockage syndrome described above is just one of several mechanisms being used in different patients of nystagmus. In nystagmus blockage syndrome convergence is used to block /dampen / compensate the nystagmus.
2. There is another condition in which dampening of nystagmus by convergence innervation takes place. Cases of intermittent divergent squint have been described in which there was nystagmus with exotropia. When the exotropia was overcome by exerting fusional convergence nystagmus disappeared. On occlusion of one eye, however, as on a cover test, exotropia was precipitated along with a return of nystagmus. These cases should not be confused with those of latent nystagmus. This implies that if the exodeviation is under-corrected in these patients, fusional convergence will be able to control the exotropia and consequently the nystagmus most of the time.
3. Dampening by versions is quite commonly seen. The nystagmus has its null point in one direction where is disappears or becomes significantly less. In the opposite direction the nystagmus becomes exacerbated. The lessening of nystagmus may happen in more than one direction0..

Acquired nystagmus

It has already been mentioned that all cases of acquired nystagmus should be referred to neurologist / neuro-ophthalmologist for management. However, before this can be done the strabismologist has to be able to diagnose these cases. A single well-defined null-zone is almost never seen in cases of acquired nystagmus. The main features of some of the types of acquired nystagmus are given below.

The various types of Acquired nystagmus

The strabismologist mostly has to deal, almost exclusively, with congenital nystagmus. Cases of acquired nystagmus are usually referred to a neurologist for investigations and treatment. However, a few selected types of acquired nystagmus are summarized below.

The symptoms in a case of acquired nystagmus are quite marked and can be disabling. For this reason the strabismologist / ophthalmologist has to have a broad idea of how to manage the case until it is referred or in association with a neurologist.

1. Periodic alternating nystagmus
2. See-Saw nystagmus
3. Vestibular nystagmus
4. Upbeat and downbeat nystagmus
5. Oculopalatal myoclonus
6. Opsoclonus
7. Others

Each of them is described in short in the following text.

Periodic alternating nystagmus

This type of nystagmus is usually caused by acquired lesions of posterior fossa. Main features are:

• Etiology: It can be congenital or acquired. The various causes mentioned are as follows: Congenital Ideopathic, congenital hydrocephalus, posterior fossa lesions, Arnold Chiari syndrome and myelomeningocele.^{56 and 58} Causes mentioned⁵⁹ by other authors are: (1) Congenital, (2) Vestibulocerebellar diseases like strokes, multiple sclerosis, Arnold Chiari malformation and spinocerebellar degeneration, (3) Bilateral severe loss of vision (advanced cataracts, vitreous hemorrhage, optic atrophy etc.), (4) Cruetzfeldt-Jacob disease and (5) Anticonvulsant therapy.
• Clinical features: Jerk nystagmus while eyes in primary position. Fast phase to one side for a certain period, usually for about 2 minutes, followed by the fast phase⁵⁸ to the other side for about 2 minutes. This cycle is repeated every 4 minutes. The nystagmus is horizontal in horizontal as well as vertical planes, that is to say, it is unipolar. There may be a periodic CHP accompanying the nystagmus, to decrease it. The CHP changes from one to the other side according to the side of the fast phase of nystagmus.
• Differential diagnosis: It should be differentiated from cases of congenital nystagmus with a null zone in both the horizontal directions, dextroversion and levoversion^.57
• Treatment: Improvement of vision by surgery if indicated may get rid of nystagmus.

 

See-Saw nystagmus:

The main features are:

• Etiology: 1) Frequent association with suprasellar space occupying lesion, bitemporal hemianopia and loss of vision, 2) Head injury, 3) Midbrain stroke, 4) Arnold Chiari malformation, 5) Multiple sclerosis and 6) Congenital.
• Clinical characteristics: 1. Pendular nystagmus, 2. A conjugate rotary (torsional nystagmus co-exists with disjugate vertical component. One eye elevates and intorts, the other depresses and extorts. Occasionally reverse is true in congenital See-Saw nystagmus.
• Has been reported to occur along with retinitis pigmentosa, oculo-cutaneous albinism and optic nerve hypoplasia⁵⁸.

Vestibular nystagmus⁵⁹

Etiology of vestibular nystagmus: Defects of (1) Vestibular end organs, (2) The nerve nuclear complex or the (3) Brain stem connections.

Types: 1) Peripheral vestibular nystagmus, 2) Central vestibular nystagmus

1) Peripheral vestibular nystagmus

Etiology: Labyrinthitis, neuritis, trauma, vascular ischemia, Meniere's disease, toxicity and benign paroxysmal positional vertigo. In rare cases noises induce peripheral vestibular nystagmus (Tullio phenomenon).

Clinical characteristics of vestibular nystagmus: The main features are given below:

• A mixed type of nystagmus, usually horizontal and rotary (torsional)
• Primary position jerk nystagmus
• Maximum amplitude when the gaze is in the direction of fast phase (Alexander's law)
• Nystagmus is suppressed by attention (fixation of an object of attention), increased by inattention (when fixation is removed)
• Fast phase away from the direction of the diseased end organ
• Frequently associated with tinnitus, deafness and vertigo
• Labyrinthine disease usually causes suppression of labyrithine input. Putting water in the ear leads to the same effect (as in the cold water caloric test). Water in the left ear produces the same effects as left labyrinthine disease.

Natural history of peripheral vestibular nystagmus: Usually it resolves within a few days to a few weeks by central compensation of the asymmetric vestibular input and / or visual suppression.

2) Central vestibular nystagmus

Etiology: Brainstem lesions like trauma, space-occupying lesions (SOL), Demyelinating diseases and stroke (refer to downbeat, upbeat and periodic alternating nystagmus) also.

Clinical characteristics:

• Jerky nystagmus that may change its direction on change in gaze or on exertion of convergence
• Vertical / horizontal /rotary (only one of them, the most common being vertical). It is a unidirectional nystagmus.
• No significant effect of attentiveness (fixation). Removal of effect of fixation as with high plus lenses, Fresnel prisms or goggles does not make much difference.
• Usually long standing
• Linear, increasing or decreasing velocity slow phase on nystagmography
• Romberg direction of fall does not vary with head position

 

Downbeat nystagmus

Etiology:

1. Lesions at the craniocervical junction, e.g., Arnold Chiari syndrome, Paget's disease
2. Brain stem lesions, such as hydrocephalus with increased intracranial pressure, multiple sclerosis, cerebellar degeneration, brainstem stroke, cerebellar space occupying lesions, head injuries
3. Toxic and metabolic causes: Lithium toxicity, magnesium deficiency, vitamin B12 deficiency, Wernicke's encephalopathy and anticonvulsants.
4. Can be congenital in some cases.

Clinical characteristics:

• In primary position the fast phase is downwards
• Downbeat nystagmus is not uncommonly more marked in downward lateral gaze.

 

Upbeat nystagmus

Etiology:

The main causes of upbeat nystagmus are: brainstem lesions, such as multiple sclerosis, cerebellar degeneration, brainstem stroke, posterior fossa lesions, Wernicke's encephalopathy. Could be congenital in some cases.

Clinical characteristics of upbeat nystagmus

• In primary position and lateral gaze the fast phase is upwards.
• Amplitude may be small or large.

Convergence-Retraction nystagmus

Etiology:⁵⁸ It is one of the various components of Dorsal midbrain syndrome, as briefed below:

1. Convergence-Retraction nystagmus
2. Defective vertical gaze, particularly elevation
3. Lid retraction (Collier's sign)
4. Spasm or paresis of convergence
5. Spasm or paresis of accommodation
6. Dissociation between the pupillary reflex to light and near vision
7. Skew deviation

Relationship between age and etiology: J. Lawson Smith has suggested the following etiology for the various age groups (decades):

1. 0 (Infant): Congenital stenosis of aqueduct
2. 10 years: Pinealoma
3. 20 years: Head injury
4. 30 years: Vascular malformation in the brainstem
5. 40 years: Multiple sclerosis (M.S.)
6. 50 years: Basilar artery stroke

Clinical characteristics of Convergence-Retraction nystagmus

• It is a jerky type of movements that are combinations of convergence and retraction
• It is more marked on attempted convergence or elevation
• It is due to co-contraction of extraocular muscles
• It is best seen when the optokinetic targets are moved downwards so that upward saccades are produced.

 

Spasmus Nutans

Etiology: (1) Congenital / infantile (2) Acquired monocular has been reported as an initial sign of anterior visual pathway glioma.

Clinical characteristics of Spasmus Nutans:

• There is triad of head turn, head nodding and nystagmus.
• Starts in the first 18 months of life and goes away within the first decade.
• Horizontal or vertical
• Low amplitude high frequency nystagmus of pendular type
• May be unilateral or bilateral
• When bilateral, can be of different amplitude
• Amplitude and phase may be different in each eye.
• Patients with acquired monocular nystagmus must be investigated properly. MRI should be done to rule out a lesion of anterior visual pathway, particularly a glioma.

 

Voluntary nystagmus

Etiology: It is found in hysterical people or malingerers.

Clinical characteristics

• Fast (10-20 beats per seconds) and short-lived nystagmus, which can not be sustained for more than 30 seconds.
• Often accompanied by facial grimacing, lid fluttering and convergence.
• If the patient is asked to change the direction of gaze the nystagmus can be momentarily stopped.

 

Rebound nystagmus

Etiology: Brain stem and cerebellar disease.

Clinical characteristics

Usually seen in one of the two forms described in short below:

1. Gaze-evoked nystagmus that is followed by a jerk nystagmus with the fast phase towards the PP.
2. Jerky nystagmus that occurs when eyes come back to PP after sustained lateral (eccentric) gaze.

Saccadic intrusions and oscillations

The function of saccades is to facilitate foveal fixation of the object of attention / fixation. When they do not fulfill this function they are abnormal and unwanted and therefore termed " saccadic intrusions". If they occur repeatedly they may cause ocular oscillations.

Some of them are listed below with their main features.

 

Opsoclonus

• Etiology: There are various causes, some of them being: (a) In infants: Neuroblastoma and autoimmune encephalitis (treatable with ACTH). (b) In infants and adults: It may occur as a benign self-limited ocular problem due to parainfectious encephalopathy. (c) Optoclonus may be due to a remote effect of cancer of lung, viscera, breast or ovaries. Rarely, it may be due to toxicity or hyperosmolar coma.
• Clinical characteristics: Optoclonus is also known as "Saccadomania". It is multivectorial or multidirectional (horizontal, vertical and diagonal). It is involuntary, rapid, conjugate, fast and unpredictable (chaotic or irregular). It stops during sleep. There is no intersaccadic interval. Before total recovery ocular flutter often replaces opsoclonus. In cases of infants with neuroblastoma opsoclonus is accompanied by myoclonus and ataxia. It is therefore also known as "dancing eyes and dancing feet".

 

Oculopalatal myoclonus

• Note: Ocular myoclonus is called oculopalatal myoclonus, if accompanied by contractions of palate (sometimes those of face, pharynx, diaphragm or extremities)
• Etiology: A lesion in the triangle of Guillain and Mollaret (see * below) causes it. A hypertrophy of the inferior olive is responsible for it. It is not an acute lesion. There is a latent period of 2-49 months.
• Clinical characteristics: It is usually a slow vertically pendular nystagmus (about two cycles per second). There may be synchronous contractions of other organs like palate and / or face, pharynx, diaphragm and extremities as enumerated above. These movements may persist during sleep.

*Triangle of Guillain and Mollaret is formed by the red nucleus, ipsilateral olive and contralateral dentate nucleus.

 

Ocular dysmetria

• Etiology: Often the cause is cerebellar disease.
• It is one of the various types of gaze-evoked nystagmus.
• The eyes overshoot when they comes back to primary position from a side gaze.
• When the eyes change fixation they overshoot the mark and it is only after a few oscillations that they come to the fixation object. This phenomenon is for the eyes as past-pointing is for extremities.

Clinical characteristics: There is nystagmus in side gaze (eccentric gaze). When the gaze is returned to primary position (PP) the eyes overshoot the mark (primary position). This overshoot (hypermetria) occurs every time the eyes come back to PP or any other new fixation point. It is followed by several oscillations before the eyes come to rest.

 

Ocular bobbing

Etiology: (1) Massive pontine lesion like hemorrhage, infarct or malignant tumour. The patient is usually comatose. (2) Metabolic encephalopathy (3) Obstructive hydrocephalus

Clinical characteristics

• Fast, conjugate downward eye movements followed by a slow upward drift back to the PP.
• Generally in these patients reflex / spontaneous / doll's eye horizontal movements can not be demonstrated.

 

Other types of ocular bobbing are:

• 1) Reverse: Etiology: Metabolic encephalopathy. Can also accompany ocular bobbing. Characteristics: Upward movements followed by slow downward drift back to PP.
• 2) Inverse: Etiology: Ischemic brain stem lesion or metabolic disorder, also known as "Ocular dipping". Clinical characteristics: Slow conjugate downward eye movements followed by quick saccades back to PP after some delay.
• 3) Converse: It is also known as reverse ocular dipping. Etiology: Aids, pontine infections. Clinical characteristics: Slow upward eye movements followed by rapid return to PP.

 

Ocular flutter

• They are unidirectional (purely horizontal)
• It occurs when the patient tries to fix in the PP, in the shape of spontaneous and intermittent bursts of a few (3-4) back to back saccades.
• There is no intersaccadic interval as is seen in cases of square wave jerk nystagmus.
• It may be accompanied by ocular dysmetria.

 

Investigations in a case of nystagmus

1. Routine ophthalmologic examination, particularly fundus and refraction: Usually the fovea is illdeveloped in cases of congenital nystagmus. Refractive error is found in many cases. Occurrence of esotropia is usual in cases of latent or latent-manifest nystagmus. However, it is unusual in cases of manifest nystagmus. If nystagmus is associated with oculocutaneous albinism, a very common occurrence, the retina shows a lack of pigment giving it a typical look of an albino fundus.
2. Visual acuity: As elaborated earlier, uniocular and binocular visual acuity for near and distance, with and without compensatory head posture (CHP) should be taken in every case of nystagmus. Visual acuity is always reduced to various extents. The reasons may be more than one, underdevelopment of fovea, nystagmus amblyopia, insufficiency of foveation periods, presence of ametropic amblyopia, strabismic amblyopia and visual deprivation amblyopia etc. Binocular visual acuity may be better than uniocular, particularly if manifest nystagmus is associated with a latent one. The latter manifests when one eye is covered and exacerbates the manifest nystagmus already present.
3. Orthoptic check-up to determine the state of binocularity. If the nystagmus has a null point in the periphery a trial with prisms is indicated to find out if they can reduce the nystagmus in PP by moving the null point to the center or by stimulating the convergence reflex by interposition of base out prisms.
4. Electronystagmography is the recording of nystagmus. As already indicated it helps in differentiating the types of nystagmus when there is a doubt.
5. Neurological investigations should be carried out in cases of acquired nystagmus (See case report 51-1). The patient therefore is referred to a neurologist or neuro-ophthalmologist.

 

Case report -2 A young man came to us a few years ago who had suddenly developed fast mixed nystagmus (vertical with a rotary element). He was also unsteady of gait. He was referred to a local neurologist who in turn referred him to a large hospital in another town. Although I had requested his relatives to let me know about his progress, they never did that. Later I came to know that the patient had died a few days later!

 

Treatment of nystagmus

1. Conservative
2. Surgical

Aims of the treatment of nystagmus are as follows:

1. To stabilize the eyes so that the patient can have a better visual acuity
2. To shift the null point (the direction in which the nystagmus is least) to the primary position if it is situated on one side, say, the right and an anomalous head posture has to be adopted to place the eyes in that zone
3. To decrease the oscillopsia
1. Conservative treatment
1. Spectacles and contact lenses
2. Concave (minus) lenses
3. Prisms
4. Drugs
5. Acupuncture and biofeedback

Spectacles and contact lenses:

Spectacles: I have found the incidence of refractive errors in cases of nystagmus quite high. Any significant refractive error must be corrected to give the patient maximum improvement of visual acuity. I have often observed a decrease in nystagmus after the correction of significant refractive errors. Other ophthalmologists have also reported dramatic improvement of nystagmus after correction of refractive error¹³.

Note: Retinoscopy should be done in the null point position.

Contact lenses: Lessening of nystagmus has been reported^{14, 15} by the use of contact lenses. They have the advantage of moving with eye so that the visual axis coincides with the optical center of the lenses. Apart from the improvement of visual acuity obtained with the corrective contact lenses, Dell’ Osso and associates think that apart from improvement of visual acuity there is some kind of tactile feedback from the contact lenses.

Concave (minus) lenses

Overcorrection of myopia with minus lenses stimulates convergence, which in turn has a nystagmus dampening effect. The latter leads to improvement in visual acuity.¹³

Prisms

Prisms can fulfill all the three aims mentioned above. This to say that in selected cases they can improve visual acuity, reduce oscillopsia and improve head posture. The null point can be shifted to the primary position by appropriate prisms. The various ways in which they can help are:

1. If binocular vision is present: base out prisms stimulate convergence that has a dampening effect on nystagmus and thus it brings about improvement of visual^{16, 17} acuity by decreasing the nystagmus. While this therapy is favored by some workers¹others have found encouraging results in a few cases only.¹⁸
2. To improve the head posture the null point has to be shifted to the primary position. This is done by placing the prisms base out towards the direction away from the direction of null point. Thus if the nystagmus is minimum in levoversion (null point) the head turn will be to right. In such a case the prisms are prescribed with base towards the right side. In the right eye the prisms are given base out and in left eye base in. The power of the prisms depends on the amount of the shift we desire.

d) Drugs

There are two types of uses of drugs in cases of nystagmus as described in short in the following text.

Systemic medication

Systemic use of drugs for nystagmus is not popular. Alcohol, barbiturates, tranquilizers and Baclofen have been tried by various workers from time to time but all of them have a common drawback, that of side effects making it difficult to continue treatment for long periods.⁴⁹

The most often used drug is Baclofen, which is used for the following types of nystagmus:

1. Congenital nystagmus⁵¹
2. Sea-Saw nystagmus⁵²
3. Periodic alternating nystagmus⁵³

Pharmacological denervation:

Injections of Botulinum A given every 1-3 months have been used.⁵⁰ The drawback here is that the effect does not last long and injections have to be repeated at 1-3 month interval.

Dose: 20-25 units of Botulinum toxin A are injected as follows⁵⁴:

Two options are available:

(1) Injection into individual muscle under guidance with electromyographic machine. All the four rectus muscles are injected.

(2) Retrobulbar injection.

Side effects / complications of pharmacological denervation:

1. Diplopia resulting from unequal palsy of the various extraocular muscles.
2. Ptosis is a common problem resulting from this therapy. It is particularly severe and longer lasting after a direct injection into the superior rectus muscle.

Advantages of the retrobulbar injection (over those of direct injection into the muscles)⁵⁴ are given below:

1. No special equipment (like electromyographic machine and special electrodes) is required.
2. A specialist is not required. Any ophthalmologist can give the R/B injection.
3. The technique of retrobulbar injection is known to every ophthalmologist.
4. Chances of side effects like ptosis and diplopia are more common in the case of direct injection into the muscle.
5. The side effects are stronger and longer lasting because of a more severe palsy that results by the injection of the toxin directly into the muscle.
5. Acupuncture and biofeedback

Acupuncture and biofeedback have been mentioned in one of the recently published books on strabismus⁵⁴ but no details have been given. The only comment made is:

"Acupuncture and biofeedback have been claimed to improve visual acuity and lessen nystagmus but no peer reviewed reports have substantiated these anecdotal reports".


1. Surgical treatment

There are three main aims of surgery:

1. To correct / reduce disabling / noticeable CHP (compensatory or anomalous head posture): This is achieved by bringing the null zone to or nearest to the PP.
2. To improve the visual acuity by reducing the nystagmus: This is achieved by dampening the ocular motility to increase the foveation fraction / period, leading to improvement of VA.
3. To reduce the oscillopsia: This is also done by reducing the nystagmus by dampening ocular motility. Using the null zone also helps.

The experts have not decided upon the best time for surgery. However, Rosenbaum et al⁶² have suggested the following guidelines:

1. Congenital nystagmus with no strabismus: Surgery can be postponed until school age (about 4 years) so that repeated orthoptic assessments have been made for the various findings to be reliable.
2. Congenital nystagmus with strabismus: Early surgery so that bifoveal fixation can be achieved.
3. Acquired nystagmus with CHP: At least 1 year is given so that information about stability and direction of nystagmus and CHP is fairly reliable.

Special precaution:

Every patient / parent should be explained in detail about the prognosis and a need to understand, anticipate and accept a realistic expectation of the result. As regards the results, undercorrection is quite common. A residual CHP which is not too noticeable should be accepted as a good result.

Pre-requisites of surgery:

1. Prism test should be performed to see if turning the eyes in a certain required direction (towards the null zone) is likely to help reduce the CHP and /or improve the visual acuity. Put the required power of prisms with apex towards the direction in which the eyes are to be turned by surgery. If the head becomes straight and /or visual acuity improves and nystagmus lessens one can go ahead with the surgery.
2. If null zone exists in one direction only (right or left) surgery will bring it to primary position and the CHP will be reduced. However, if the null zone exists in both the directions (right and left) and the patient keeps changing the face turn from right to left surgery is not indicated.
3. Visual acuity should improve with eyes in the null zone by at least 2 or more lines on Snellen's chart.

Planning the surgery

The following important information must be obtained before planning surgery for nystagmus:

1. If nystagmus is accompanied by strabismus surgery has to be modified according to the type and degree of strabismus.
2. If binocular functions are present, particularly a good fusion potential: When the usual recession-resection surgery is performed on all the four horizontal rectus muscles (Kestenbaum's procedure or one of its modifications) the chances of a consecutive heterotropia are minimal if fusion is present.⁵⁵

Various surgical techniques used in cases of nystagmus

1. Weakening procedures: Usually recession of rectus muscles
2. Strengthening procedures: Usually resection of rectus muscles
3. Recession-Resection may be combined with posterior fixation (Faden) suture
4. Some people use adjustable sutures
5. On cyclovertical muscles: (a) for chin elevation / depression and (b) for head tilt. For head tilt surgery can be performed on cyclovertical muscles (recession-advancement /resection) or horizontal rectus muscles (supra or infraplacement as required (see under the heading of (1)c surgery for head tilt..

The exact nature and amount surgery is decided according to the following factors:

1. The purpose of surgery
2. Type of compensatory head posture (CHP)
3. Degree of head turn
4. Degree of reduction in VA
5. Severity of nystagmus
1. Surgery to correct / reduce disabling / noticeable CHP

As already mentioned this is achieved by bringing the null zone to or nearest to the PP.

(1)a: surgery for horizontal CHP:

Many options are available, most of them being modifications of Kestenbaum-Anderson procedure. They have been in use for many years.

Note: Most surgeons now do all the four horizontal rectus muscles at one sitting. If there is a strabismus accompanying the nystagmus, appropriate adjustments are made in the recession- resection procedures. Mostly, amount of the adjustments for the presence of strabismus has to be by guess.

The various techniques used are given below.

1. The Kestenbaum-Anderson procedure
2. The Parks' modification of Kestenbaum-Anderson⁶³ procedure
3. The classic + 50%
4. The classic + 60%
5. Pratt-Johnson's maximum surgery
6. Von Noorden's modification of Anderson's two-muscle surgery

In all Kestenbaum-Anderson procedures the lateral rectus of the abducted eye is recessed and medial rectus resected while the medial rectus of the adducted eye is recessed and the lateral rectus resected. The net effect of these is a turning of the eyes towards the direction of the face turn, thus straightening the face by a variable degree. The amount of recession-resection is different in various modifications. Each of the above procedures will be described in short below.

1. The Parks' modification of Kestenbaum-Anderson procedure: Also known as "The classic maximum", this has been the most often used procedure. The main points are as follows:
   
• Recommended for a head turn of 20 degree or less.
• The basic principle is the same, that is to say, the operation is done to turn the eyes towards the direction of the face/head turn.
• All the four horizontal recti are tackled at one sitting.
• Amount of recession-resection in each eye is given in table 51-3. It uses a dosage of 5-6-7-8 mm for the various four horizontal muscles. The amount of surgery is distributed in such a way that each eye gets a total dose of 13 mm of surgery. In the adducted eye the MR is recessed by 5 mm and the LR is resected by 8 mm. In the abducted eye the LR is recessed by 6 mm and MR is resected by 7 mm.

 

Table -3 (Surgery for face turn):

Kestenbaum-Anderson procedure & modifications	Degree of face-turn	Amount of recession of LR of abducted eye	Amount of resection of MR of abducted eye	Amount of recession of MR of adducted eye	Amount of resection of LR of adducted eye
Kestenbaum's procedure	Moderate	5 mm	5 mm	5 mm	5 mm
Anderson procedure	Moderate	4-5 mm		4-5 mm
Parks' modification (Classic maximum)	20 or less72	7 mm	6 mm	5 mm	8 mm
Classic+40%	30 or more	40% more	40% more	40% more	40% more
Classic 60%	>45	60% more	60% more	60% more	60% more
Calhoun & Harley64	Severe	9 mm	8 mm	7 mm	10 mm
Pratt-Johnson36	35	10 mm	10 mm	10 mm	10 mm
Spielmann's modification	Severe	Recession + Faden suture	Resection	Recession + Faden suture	Resection
Cooper & Sandall71	FT in degrees X 2 = FT in PD	Appropriate amount	Appropriate amount	Appropriate amount	Appropriate amount

 

• The name classic maximum was given due to the fact that in nineteen fifties the MR was not recessed by more than 5 mm.
• Preoperative and postoperative oculomyographic recordings have confirmed that the intensity of the nystagmus decreases in all directions and the null zone or the zone of minimal nystagmus is not only shifted nearer to the PP but also broadened⁶³.
• Undercorrections are common (Note: Therefore augmented modifications of Kestenbaum-Anderson procedure were suggested).

2. The classic + 40% (Augmented modification of the Classic maximum)⁶⁴:

• This procedure is recommended for more pronounced CHP: 45 degree or more⁶⁵ / 30 degree or more⁶⁶). The surgical dosage is increased by 40%, hence the name.
• The disadvantage of this procedure is the common occurrence of limitation of ductions^{64 and 65} and rarely a new deviation (strabismus).

3. The classic + 60% (Augmented modification of the classic maximum):

• This procedure has been recommended⁶⁶ for a head turn of 45 degree or more.
• 60% more surgery is done than in the classic procedure.
• An undercorrection, resulting in a face (head) turn of 10-15 degrees should be acceptable to all (the surgeon and the patient).
• The patients / parents should always be warned in advance about the possibility of undercorrection.
• Limitation of duction is to be expected postoperatively because of the large recessions.
• The effect may gradually wear off partially and re-operation may be required after about 6 years⁶⁷.

4. Bietti and Bagolini's maximum surgery

• First reported by Bietti and Bagolini ⁶⁹ in 1960, all horizontal rectus muscles are operated upon. Each muscle gets a dose of 10-12 mm, whether it is recession or resection, whether it is a medial or lateral rectus muscle.
• It did not become popular for some time, then it was revived by Limon⁶⁹ in 1986.
• Pratt-Johnson recommends this procedure. He claims an absence of complications^{55, 67}. In his patients fusion was never lost if it was present preoperatively. No overcorrection and a head turn to opposite side resulted postoperatively if there were no two null zones, one to the right and the other to the left. This technique is contraindicated if there are two null zones.
• The most suitable patients are those with presence of fusion, a single null zone congenital nystagmus with a head turn of 35 degrees or more.
• The best results were seen after 1 month.
• Undercorrection may follow after about a year.
• Von Noorden has also reported good results⁶⁹ with this technique.
5. von Noorden's modification of Anderson's procedure or enhanced Anderson's operation
• Anderson was the first surgeon to report recession of the pair of yoke muscles, one in each eye. For instance, in a case of face turn to the left the right lateral rectus and the left medial rectus are to be recessed 4-5 mm so that both the eyes are turned to left.
• Noorden recommends an enhanced version of Anderson's two-muscle surgery. He recesses each yoke muscle by 10-12 mm (instead of 4-5 mm as suggested by Anderson). He explains the advantage of this procedure as follows: After a maximum dose-four-muscle surgery (one of the modifications of Kestenbaum's procedure) if re-operation is needed to correct an undercorrection / recurrence of a face turn, it is difficult to decide the next course of action. All the four horizontal muscles have already been operated upon. By doing a 2-muscle surgery instead, the other pair of yoke muscles is still available for further surgery.⁷⁰
• Because a recession of medial rectus results in more correction than a recession of lateral rectus, medial rectus (of the adducted eye) is recessed by 10 mm⁷⁰ and the lateral rectus (of the abducted eye) is recessed by 12 mm.
• Noorden has reported the effectiveness of enhanced Anderson's operation.⁷⁰
• Good results⁷⁰ have been reported for the medium term and two unoperated horizontal muscles are still there.
• If there is a strabismus accompanying the nystagmus appropriate change in the amount of recessions is made.
6. Spielmann's modifications of Kestenbaum-Anderson procedure:

Spielmann suggested⁷² this procedure for CHP of more than 20 degrees. Posterior fixation suture is added to the recessed muscles.

7. Surgery for nystagmus associated with strabismus
   
• There are three options:
1. One stage surgery: One has to guess about the dosage of the amount of surgery for nystagmus and strabismus. This plan is quite unpredictable⁷⁵ but in our experience it does work more often than not.
2. Two (or more) stage surgery: Surgery for CHP is carried out on the fixing eye in the first stage. In the second stage the strabismus is corrected.
3. One or two of the muscles to be operated are given adjustable sutures so that the result can be modified after operation.

 

(1)b Surgery for vertical CHP⁷⁹

• Vertical CHP (to reduce vertical nystagmus) is much less common than the horizontal. Few reports are available, none of them after long-term studies.
• For chin elevation the surgery is carried out to move the eyes upwards. This is done by recession of inferior rectus muscles and resection of superior rectus muscles.
• For chin depression the eyes are moved down by large recession of superior rectus and recession and anteriorization of inferior oblique.⁷⁷

 

(1)c Surgery for head tilt (torsional nystagmus)

• To correct head tilt / torsional nystagmus one has to operate on cyclovertical muscles.
• One has to be more careful and plan the surgery thoroughly because surgery on cyclovertical muscles is more likely to cause consecutive heterotropia and therefore diplopia, particularly if it is asymmetrical and unbalanced.
• The eyes are to be tilted in the direction of the head tilt. The eye on the side of he head tilt is to be extorted (excyclorotated) and that on the opposite side intorted (incyclorotated).

 



• The literature on management of head tilt adopted to compensate for the torsional nystagmus is quite small.
• The following techniques have been reported:
1. Conrad and Decker⁸⁰ advance or recess the appropriate oblique muscles, strengthening the extortors and weakening the intortors on the side of the head tilt. On the opposite side the intortors are strengthened and extortors weakened. This surgery was more effective if advancement was combined with resection.
2. de Decker⁸¹ recently advised a vertical transposition of horizontal rectus muscles. To cyclo-rotate the eyes to right (extort the right eye and intort the left eye) for a head tilt to the right the right lateral rectus and the left medial rectus are supraplaced and the right medial rectus and the left lateral rectus are infraplaced. This technique is simpler and has less chances of a consecutive heterotropia (more common with surgery on oblique muscles).

(2)A Surgery to improve visual acuity by reducing the mobility to reduce nystagmus

• This is achieved by dampening the ocular motility to increase the foveation fraction / period, leading to improvement of VA.
• There are no long-term reports available.
• All the four horizontal recti are recessed and reattached behind the equator. The amount of recession varies in different reports (table 51-4).
• Complications are uncommon. Surprisingly, even limitation of ocular motility is not significant. Incidence of consecutive heterotropia (e.g., exotropia) is rare.
• Precaution: Patients should be warned in advance to be realistic about the results. At the best, improvement in CHP, reduction of nystagmus in PP and slight (1-2 line) improvement can be expected.
• This surgery should only be carried out for marked and unsightly nystagmus.

Table 51-4, amount of surgery to reduce nystagmus:

Name of surgeon	Recession of BMR	Recession of BLR
Bietti, G.B.	To behind limbus	To behind limbus
Noorden and Sprunger76	10-12 mm	10-12 mm
Helveston et al78	11.5 mm	13 mm
Rosenbaum79	7.5 mm	10 mm

 

(2)B Surgery to improve visual acuity by creating artificial divergence

• Pre-requisites for this plan are:
1. Presence of fusion
2. Base out prisms dampen the nystagmus
• The amount of surgery is determined by finding out the power of the prisms (base out) that improve the visual acuity without producing diplopia. Medial rectus recession of 5-12 mm has been advised.⁸¹ however; the results were not very encouraging in this series. Moreover, some patients developed intractable diplopia.
• The procedure involves bilateral medial rectus recession or unilateral medial rectus recession + LR resection sufficient to produce exodeviation with intact fusion. In these cases attempt to converge the eyes results in dampening the nystagmus. When divergence is produced by surgery the effort to maintain fusion by exerting convergence leads to reduction in nystagmus and consequently improves the visual acuity.
• Some cases may get exotropia leading to non-fusible diplopia.

 

(2) Surgery to reduce oscillopsia

• Oscillopsia usually is a problem of acquired nystagmus, which is fortunately much less common than congenital nystagmus as far as practice of strabismology is concerned. The acquired cases are mostly sent to the neurologist or the neuro-ophthalmologist to be dealt with. However, if surgery is indicated, the procedures are the same as those to reduce nystagmus.

 

(3) Surgery for Nystagmus Blockage syndrome

(See case report 51-3)

• There is no clear favorite among the two most often used procedures, given below.
• Unilateral MR Recession-LR Resection
• Bilateral large MR recessions (7 mm or more)
• Both have been reported to cause undercorrection / overcorrection.
• Preoperative prism-adaptation test helps in deciding the amount of surgery. First the maximum angle is measured with which there is binocular fusion for distance fixation and then it is reduced until minimum measurement is obtained with which fusion is maintained. The amount of surgery depends on this angle.

 

Case report 51-3: A female infant aged 4 months came with vertical and horizontal nystagmus. We did not see her again until 3 months later when the nystagmus was found to have disappeared. It had been replaced by a large esotropia (infantile ET). She had mixed astigmatism of +3.5 D with small minus spheres. She was advised to use glasses. Occlusion was carried out for the severe amblyopia she had developed in left eye. The vision was equalized within a few weeks and LET was converted into alternating ET. Surgery was advised and bilateral medial rectus recession was carried out at the age of 11 months. She now has orthotropia, i.e., the alignment of the eyes is within 8 PD (less than 4 degrees of esotropia). There is a small left to alternating esotropia.

Case report 51-3 Baby aged 1 year, a case of Nystagmus blockage syndrome.See case report 51-3.

 

Complications of surgery for nystagmus

1. Main complication is a consecutive heterotropia. It is less likely after balanced operations like Kestenbaum-Anderson type procedures and more common after four horizontal rectus muscles surgery and operations inducing artificial divergence for nystagmus blockage syndrome.
2. Undercorrections: They are quite common. A residual head turn of 15 degrees or so should be left alone. It is likely to become with time and repeat surgery may have to be performed after a few years.
3. Overcorrections: They are less common and the effect tends to wear off rapidly. One should wait for at least for 3 months before deciding that this is a case of overcorrection and needs surgery.
4. Opposite head posture: It is likely to occur but most surgeons insist that it only happens if the existence of an periodic alternating nystagmus is missed.³⁶

 

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