(S.A. Patney: Interyc volume 2, 2000. JKAI Publications)

ARTICLE ON STRABISMUS-SHORT REVIEW

Incidence

Various figures from the literature vary from 3% to 73%! The older figures are higher and more recent ones lower^{27, 28}. Moreover, the wide variation in the figures of incidence reported may be due to the fact that they were taken at various lengths of time after surgery. Immediately postoperatively the incidence seems to be very high but as many cases recover spontaneously as the time goes by, the incidence goes down.

Multiple factors are responsible for the strabismus occurring after scleral buckling and other procedures to repair a retinal detachment. Some of them predispose to the heterotropia while others play a more causative role. They are discussed in short in the following text.

1. Pre-existing conditions
2. Causative factors

1. Pre-existing heterophoria or even undetected heterotropia
2. Superior oblique palsy
3. Myasthenia gravis
4. Graves’ ophthalmopathy
5. Congenital ocular motility disorders
6. Amblyopia

2. Causative factors

1. Sensory
2. Motor

1. Sensory factors:

1. Any condition leading to diminution of vision in one eye (due to occlusion, opacity in the media, retino-neural diseases etc.) can cause a disruption of fusion leading to a sensory strabismus. Mostly they are temporary²⁸ and recover as the visual acuity improves but may become permanent particularly in cases with prolonged or permanent loss of vision. A pre-existing heterophoria makes it worse.

• Scleral buckle can give rise to myopia with or without astigmatism. If the encircling or segmental buckle causes the axial length of the eye to increase by about 1 mm (0.99 mm to be exact), -2.75 D of myopia is created^{29, 30}. Astigmatism often changes in degree and may occasionally be very high (6-7 D) but usually it is lower and temporary³¹. It can be irregular astigmatism³².
• There is increased risk of refractive errors if the height of the buckle is increased^{32, 33}.
• Use of silicone oil as temponade for retinal detachment repair has been found to lead to diplopia and other disturbances including astigmatism³⁴.

2. Distortion of the image: If there is an image distortion after the retina has been reattached (due to a fold in the central retina or a membrane in the central area) diplopia may result. Later suppression and secondary (sensory strabismus) may develop.

2. Motor factors: They can be divided in to the following types:

1. Mechanical
2. Structural
3. Traumatic
4. Miscellaneous

1. Mechanical factors: (a) Adhesions^{35, 36, 37} resulting from the retinal reattachment surgery are a common cause of strabismus and diplopia. These adhesions can be formed between muscles, sclera, scar tissue, orbital fat and exoplant. They can cause significant limitation of the actions of various muscles leading to incomitant strabismus and diplopia.

• Adhesions and restrictions
• a change in the direction of the effective part of the muscle and its pull
• the area and arc of contact with the globe
• length-tension relationship and
• geometry of action of the muscle concerned

2. Structural factors include the following^{35, 38, 40}:

• Direct muscle trauma, which is mainly due to excessive and prolonged muscle pulling and stretching
• Slipped muscle
• Ischemia of the muscle due to compression by the buckle for prolonged periods, sometimes leading to muscle erosion³⁷.
• Disinsertion of the muscle with or without changed and faulty reinsertion has been blamed by many surgeons^{38, 41, 42 43}. The frequency of occurrence of strabismus is directly proportional to the number of detached muscles. A definite muscle stump should always be left when the muscle is disinserted from the globe so that the insertion can be recognized and the muscle can be

3. Traumatic factors:

• As mentioned elsewhere also, prolonged severe stretching of extraocular muscles.
• Another cause is the retrobulbar injection (direct trauma with needle and the anesthetic toxicity).

4. Miscellaneous factors:

1. Involvement of superior or inferior oblique muscle can lead to vertical, torsional or cyclovertival strabismus.
2. Nerve ischemia or trauma may lead to muscle paresis, which in turn can cause contracture of its antagonist.
3. Macular dystopia:
             (a) Macular displacement after reattachment of retina

(b) Formation of an epiretinal membrane can give rise to macular diplopia, which may be intractable as described under the heading of macular dystopia. The text appears immediately following the table 44-1.

Symptomatology and clinical picture

• The main and most bothersome symptom is diplopia
• There is usually a strabismus, mostly incomitant due to restriction of ocular motility in certain directions.

1. Manifestation of a pre-existing heterophoria due to dissociation caused by decreased vision in one eye
2. Secondary (sensory) horizontal strabismus consequent on a loss of vision in one eye.
3. Iatrogenic squint as a result of the retinal reattachment surgery can occur in various ways detailed later. It may be restrictive due to involvement

• The direction of deviation can be any of the three types or a combination of two or all three types as follows:

1. Horizontal strabismus after retinal reattachment surgery may be sensory secondary to loss of vision in the affected eye. At other times however, it is due to adhesions and restrictions or palsy of horizontal muscles. Its incidence is reported as 12%-30% by various surgeons^{35 and 52}.
2. Vertical strabismus after retinal detachment repair surgery is the most common of the three types^{35, 40} and is more likely to be restrictive or paretic. The usual type of deviation in this category is a hypotropia with a restriction of elevation in adduction, sometimes simulating Brown’s syndrome with a positive forced duction test during elevation in adduction. The diplopia is vertical and often torsional. The images are therefore difficult to fuse. The site of exoplant placement and that of muscle scarring may not be co-related³⁵.
3. Cyclodeviation (torsional deviation) is more common than it is supposed to be. A 46% incidence has been reported¹⁶. However generally it is supposed to be less common than this figure. It is possible that cyclodeviation remains undetected in many cases and the patients hardly ever describe a tilted image.

1. Anterior displacement of the reflected tendon of superior oblique (SO) tendon by its involvement with the encircling band/exoplant.
2. Adhesions between nasal border of superior rectus (SR) and the SO tendon. The patients show a noncomitant hypotropia, excyclotorsion / incyclotorsion and restriction of downgaze.
3. Adhesions between SO, SR, band/exoplant and sclera.
4. Accidental or planned tenotomy of SO resulting in SO palsy.
5. Presence of a large exoplant under the inferior rectus muscle or formation of adhesions between the exoplant and the inferior rectus causes hypotropia and restricted elevation and excyclodeviation
6. Rarely SR may be disinserted
7. IO may be involved while isolating the inferior rectus and may become tucked again causing restricted elevation and positive forced duction test.
8. Oblique astigmatism may result after scleral buckling procedures, particularly when the exoplant is large or placed radially. The cyclodeviation is produced in an effort to cyclofuse the images to get rid of diplopia.
   

Diagnosis of this type of strabismus is not difficult provided comprehensive examination is carried out as follows:

1. History of previous ocular surgery and its relationship with the strabismus as regards the timing.
2. The refractive errors should be assessed again as the previous refraction changes after retinal reattachment surgery, particularly scleral buckle, which can lead to an increase in axial length of the globe resulting in myopia as already mentioned in the previous text.
3. A thorough orthoptic examination / ocular motility workout is necessary in order to plan future course of action. If there are adhesions or a pre-existing or recent palsy the deviation is incomitant and the angle of deviation should therefore be measured in various cardinal directions of gaze. Presence of fusion with a good range on major amblyoscope indicates that it is possible to overcome diplopia. In case of unavailability of major amblyoscope presence of fusion can be determined by Wirt / Titmus stereogram test, Worth Four Dots test, Bagolini test (for peripheral fusion), using other stereograms and trying to fuse the two images with appropriate prisms.
4. Slit lamp (biomicroscopic) examination should also be carried out to localize various adhesions and areas of anterior segment ischemia if present.
5. Laboratory tests:

I Prevention of iatrogenic strabismus after retinal detachment repair surgery
II Treatment of the post- retinal reattachment surgery strabismus and diplopia

I Prevention

Before taking up the actual management of a case of iatrogenic strabismus resulting from retinal detachment repair surgery it is in order to outline the precautions that can be taken (during retinal reattachment surgery) to prevent its occurrence. They are:

1. Paying attention to surgical technique that should be meticulous.
2. Causing least trauma to the muscles by not stretching them too much.
3. If retrobulbar injection is used for local anesthesia, an expert who is thoroughly conversant with the anatomy and the technique should only give it.
4. One should handle the tissues carefully causing the least degree of trauma. This will prevent severe postoperative inflammation that causes scarring and adhesions^{53, 64, 94, 99,105, 120.}
5. The relationship between various layers of membranes, sheaths, fascia and sclera should be respected to prevent adhesions between them causing restrictions of ocular motility.
6. In my opinion the most important point is that Tenon’s membrane should be respected and handled carefully to prevent unwanted adhesions after the surgery.

1. Nonsurgical
2. Surgical

1. Nonsurgical treatment must be tried before deciding on surgery. Depending on the type of diplopia and the results of various examinations, including history taking, the following types of management modalities are used.

1. Conservative therapy: Observation: Wait and watch for as long as the picture is changing. This period may be a few weeks to a few months. Many of these cases show spontaneous improvement. Mostly the power of fusion is used to overcome the postoperative ocular motility problems. Sometimes however, this is possible by developing suppression to get rid of diplopia. Often the condition improves objectively also due to the resolution of scars.
2. Prismotherapy can help⁴⁴ in many cases provided a proper assessment is made and appropriate prisms are prescribed. The guidelines for the prescription of prisms will be given in short in the chapter on prismotherapy. It can be combined with surgery. Smaller prisms will be required after the angle of deviation and incomitance has been reduced. Prismotherapy is more likely to succeed in smaller comitant deviations and can be given in the form of Fresnel prisms or directly ground in the spectacle lenses^{38, 46, and 47.} However, diplopia due to macular distortion and displacement (dystopia) resulting from retinal reattachment, can not be relieved by prisms or any other means except retinal stripping and laser treatment⁴⁵.

The main aim of prismotherapy is to increase the field of binocular fixation (the same thing as the field of binocular single vision) and to move it as near to primary position as possible so that the patient does not have to adopt a compensatory head posture.

According to some studies reported in literature about 1/3^rd of the patients can be helped with prisms alone^{46, 47}.

3. Some surgeons use denervation with Botulinum toxin. It can be used as a temporary measure or may result in permanent relief⁴⁸. The angle of deviation may be reduced making it possible to fuse the two images. It also makes the deviation less incomitant.

1. Where surgery is contraindicated for some reason or the other.
2. When there is an increased risk of anterior segment ischemia.
3. When there is increased risk of perforation.
4. When there is a possibility of recurrence of retinal detachment.

1. The effect is temporary
2. The injections have to be repeated
3. As the anatomy is altered as a result of scleral buckle, there is an enhanced risk of perforating the globe with the injection needle.
4. If the surgery is planned one must wait for 2-3 months for the effect of the toxin to wear off.
5. Vertical deviation has been reported⁴⁹ after an injection of Botulinum toxin in to the medial rectus muscle.
6. The results can not be predicted with any degree of accuracy.

4. Orthoptic treatment may help in some cases with pre-existing fusion, by increasing the amplitudes of fusional vergence.
5. Occlusion of one eye, partial or total is employed if everything else fails to relieve an intolerable diplopia. This can be done by using frosted lens, high power plus lens, giving total occlusion or by leaving the high refractive error in one eye uncorrected.

2. Surgical treatment

1. All the investigations and tests described should be performed and thoroughly assessed.
2. Planning the surgery should be done carefully; keeping in mind that every subsequent operation will lead to more scarring that might make the condition worse.
3. Slit lamp examination should be thorough and all the sites of scarring, exoplant and encircling etc should be located as far as possible.
4. The patients should be warned in advance about the complicated nature of the problem and advised not to expect too much.
5. Before every retinal reattachment surgery the patient should be advised about the possibility of diplopia, strabismus and some restriction of motility.
   

Certain basic points that apply to strabismus surgery in general are kept in mind and then details are worked out. The main points are given below.

1. If the forced duction test is positive for a certain muscle, it is obvious that the muscle has developed significant adhesions and/or contracture and therefore

has to be tackled to achieve any improvement in the condition.

2. If the force generation test and saccadic velocity analysis reveal presence of some function in the muscle showing restriction, the choice depends on the result of forced duction test (FDT):

• If the FDT is positive, a large recession of the tight muscle may be quite sufficient. However, if the FDT is strongly positive and the muscle seems to be severely damaged contracted and tight, even free or hang-back tenotomy may have to be performed.

For very large angled deviations, resection of the antagonist in addition to the free tenotomy on the tight muscle will result in a correction of larger degree. However, a resection of the antagonist of a tight muscle should be avoided if a free tenotomy has not been performed. This is so because it might cause an enophthalmos. If it has to be done it should be a small resection.

• If the FDT is negative in the muscle showing restriction of action, it is apparently a case of paresis. If the force generation test shows presence of function, resection should be effective. For large angle deviations resection has to be supplemented by a recession of the direct antagonist or the contralateral synergist (whichever is showing overaction).
  
• Surgery can be performed on the normal eye and is in fact preferable as it will avoid the difficulties encountered while operating on an operated eye with lots of adhesions. Also the results are more predictable and postoperative reaction is much less.
  
• If a tight muscle is weakened usually it is necessary to recess the conjunctiva also (4mm-7mm or even more in bad cases) to enhance the effect and/or to prevent the eye from going back to the deviating position when the conjunctiva is sutured.
  
• Adjustable suture technique^{35, 39 and 46} may be used in severe cases because a fair prediction of the result is impossible in these cases.
  
• Pre, intra and post-operative forced duction test must be performed to get a good result.
  
• When bad adhesions are suspected a limbal incision is particularly preferable.

Choice of procedures:

Certain conditions have been specified in the preceding text about some choices. One can use any technique one is comfortable with, provided it is based on those general principles. The following procedures are used more often:

1. Weakening procedures: Recession, free tenotomy, hang back recession or tenotomy, marginal myotomy (particularly of an operated tight muscle)
2. Strengthening procedures: Resection, muscle transplant
3. Recession and resection (R&R) of the overacting and underacting rectus muscles respectively.
4. Faden operation (posterior fixation suture) in the other eye to limit the action of an overacting muscle. This operation is done in order to limit or reduce the incomitance and increase the field of binocular fixation and single vision.
5. For hyperdeviation: Weakening of the overacting vertical muscle (usually it is superior rectus as a consequence of inferior rectus paresis. Inferior oblique weakening for IO overaction due to SO palsy (e.g., due to anterior displacement of the reflected tendon of SO.

Vertical transposition of the horizontal muscles for hyperdeviation: For a smaller hypertropia both horizontal recti may be transposed (insertions shifted) downwards. For hypotropia both horizontal recti are transposed upwards. These procedures have already been mentioned under management of vertical deviations in chapter thirty-seven.

6. For cyclodeviations: Surgery for cyclodeviations is discussed in chapter thirty-seven. Depending on the type of defect the following procedures are done as indicated:

• For excyclotropia with hypertropia, more in upgaze: Recession, myectomy or anteriorization of the overacting inferior oblique IO)
• For incyclotropia with hypotropia increasing in downgaze: Recession, tenotomy or tenectomy of superior oblique (SO). A tenectomy of anterior 7/8^th of SO is done for incyclodeviation without hypotropia, correction achieved being 7-10 degrees.
• For large excyclodeviation (more than 10 degrees) associated with hypertropia worse in depression: SO tucking. However, personally I do not favor this procedure as it might cause a tightening of SO tendon giving rise to a picture simulating Brown’s syndrome. This condition is known as acquired Brown’s syndrome. There are many more reports of acquired Brown’s syndrome in American literature than they are seen here, as SO tucking is a fairly popular operation.
• Anterior lateralization of the anterior fibers of the SO tendon (Harada Ito procedure)⁵⁶ is the most favored procedure for excyclodeviation.
• Horizontal transposition of vertical rectus muscles^{57, 58 and 59}.

1. Failure to resolve the diplopia despite the best attempts to align the eyes³⁵: This happens due to various reasons already mentioned (cyclodeviation, macular dystopia causing disruption of central fusion etc).
2. Recurrence of retinal detachment may occur on removing the scleral buckle or the encircling band. However, if a period of at least 6 months is allowed after retinal reattachment surgery, the chances of recurrence are significantly reduced. Fortunately it is not a common occurrence⁶⁰.
3. Perforation of the globe⁶¹ is a real risk because of the presence of adhesions, changed anatomy due to buckling etc. and thinning of the sclera. I have noticed that sclera becomes friable after repeated surgeries. One therefore has to be extremely careful in dissecting the adhesions, detaching a muscle and passing the needle through the sclera.
4. Rupture of a muscle may take place⁶¹ when it is handled. This is because of erosion or segmental necrosis of the muscle due to pressure of a buckle or an encircling band.
5. Perforation of the globe⁶¹ is a real risk because of the presence of adhesions, changed anatomy due to buckling etc. and thinning of the sclera. I have noticed that sclera becomes friable after repeated surgeries. One therefore has to be extremely careful in dissecting the adhesions, detaching a muscle and passing the needle through the sclera.
6. Migration of the implant and extrusion may occur after the removal of the band or the buckle. To prevent it only a part of the buckle or the band is removed³⁵.

(NOTE: The III and final part of this article on strabismus after ocular surgery will appear in InteRyc volume 3, 2000. However, the "short review article on strabismus" series will be continued.)