(S. A. Patney: Strabismology Desk Reference, chapter 47, JKA Publications)

DOUBLE ELEVATOR PALSY (Monocular Elevation Deficiency)

As there are varied causes of absence of elevation of an eye, including nonparalytic ones, the term "Monocular elevation deficiency" (MED) is more apt. Apparently in such cases it appears as if both the elevators (the superior rectus and the inferior oblique) are not functioning but recently it has been shown that a paralysis of superior rectus muscle alone can cause a deficiency of elevation in abduction as well as in adduction. This is due to the fact that superior rectus is the main elevator of the eye and elevates the eye not only in abduction and primary position but also in adduction.^{1, 2} The inferior oblique is the main extortor. However, there are some cases in which there is true palsy of both the elevators resulting from a supranuclear involvement affecting upgaze.

Double elevator palsy or monocular elevation deficiency refers to a condition in which the eye can not be elevated in abduction, adduction or from primary position. It is associated with ptosis.

1864: Bilateral upgaze palsy described first by Henoch³

1883: Perinaud described upgaze, downgaze and total vertical gaze palsies³.

1942: White⁴ reported congenital paralysis of elevation associated with hypotropia and ptosis of the affected eye. The deficiency of elevation was thought to be due to paralysis of both, superior rectus (SR) and inferior oblique (IO).

1954 Dunlap⁵ named the condition as "double elevator palsy".

Prevalence

Double elevator palsy is not a common problem but it is certainly seen much more frequently than double depressor palsy or DDP (also termed as monocular depression deficiency or MDD). Exact figures are not available.

Double elevator palsy is also known as "Monocular elevation paresis" or "Monocular elevation deficiency".

The following is a modified classification of Double elevator palsy (DEP) or monocular elevation deficiency (MED):

NOTE: Congenital cases occur sporadically. Acquired cases have a varied etiology. Thorough systemic and neurological examination is necessary to find the cause.

Etiology

In these cases it is presumed that there is an interruption of supranuclear input from riMLF¹⁰ (rostral interstitial nucleus of Medial Longitudinal Fasciculus that mediates upgaze) into III CN nucleus. Various causes³ of congenital and acquired DEP have been mentioned in the table 47-1 on page 907.

It has been shown relatively recently¹ that superior rectus is the main muscle for elevation of the eye, be it in abduction, adduction or primary position from where the eyes are elevated. In some of the cases the defective elevation can therefore be explained by the presence of a marked superior rectus palsy alone.

To understand the etiology a basic knowledge of anatomy of supranuclear pathways is necessary. The main points are given in the following text:

• The supranuclear pathways for upgaze are situated in the pretectum in midbrain.
• The important structures are rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) and the posterior commissure.
• The efferent fibres for upgaze leave the riMLF and decussate in the midline of the posterior commissure, pass through the pretectum and enter the subnucleus of the superior rectus (SR) in the oculomotor nucleus.
• After leaving the SR subnucleus the upgaze fibres to the SR cross the midline again.
• After the double decussation of the upgaze fibres the SR is innervated from the ipsilateral riMLF and from the contralateral pretectum and the SR subnucleus.
• According to a hypothesis the supranuclear fibres for up and down gaze may be compartmentalized⁹. This borne out by the fact that one may come across cases in which saccadic recordings may be found to be abnormal in upgaze and normal in down gaze.

Table 47-1:

Symptomatology / Clinical Picture

The typical clinical picture of MED has the following components:

1. Associated systemic symptoms may be present in acquired MED according to the type of involvement.
2. Ocular symptoms: Typically, no symptoms in congenital MED. In acquired MED however, vertical diplopia is present in elevation (upgaze) only, none in PP. Diplopia in PP is there only if the affected eye is hypotropic in PP also. In acquired MED the onset of diplopia is acute and it is present in PP and elevation.
3. Compensatory Head Posture (CHP): Various types of CHPs have been seen with MED (DEP). If binocular vision is present a chin elevation by tilting the head back is usually seen. Head posture may be normal if there is no hypotropia in PP or if the hypotropic or the hypertropic eye is amblyopic.

Ptosis: A hypotropic eye shows ptosis because of the fascial attachments between the levator palpebrae superioris and the superior rectus muscle. Usually it is a pseudo-ptosis that disappears when the hypotropic eye is made to take up fixation in primary position. Ptosis is most noticeable when the non-affected eye fixes. As the hypotropic eye moves up to primary position to fixate, the ptosis disappears if there is no true ptosis.

Presence of true ptosis must be ruled out by covering the normal eye. It may be present in about 50% cases¹². In these cases when the patient is made to fix with the affected eye the ptosis recovers only partly.

Marcus Gunn phenomena may be present¹² with the ptosis. DEP may also be associated with other complications in some patients with intracranial involvement.

4. Deviation: Hypotropia of the affected eye when normal eye is fixing; hypertropia of the normal eye when the affected eye is fixing. Usually it is the hypotropia of the affected eye and the normal eye fixates. But if the normal eye is amblyopic (and sometimes if the hypotropic eye is the dominant eye and is preferred for fixation) it may be the constantly deviating eye with hypertropia.
5. Monocular limitation of upgaze above midline (horizontal plane) is present in all the three horizontal positions, i.e., abduction, midline position and adduction.
6. Bell's phenomenon is usually present unless inferior rectus has developed contracture.
7. Amblyopia: Amblyopia may be present if there is a constant deviation or if there is anisometropia. A hypotropic eye is usually not found to be amblyopic because depression is the most often used position. Presence or absence of amblyopia depends on fixation preference. If the non-paretic eye is the constantly deviating (hypertropic eye it may develop amblyopia. If there is no fusion present the hypotropic paretic eye is likely to become amblyopic.

Characteristic signs of supranuclear MED:

1. It is generally congenital.
2. Vertical movements above the horizontal plane are absent in the affected eye in abduction, midline and adduction. It is a monocular deficiency of elevation.
3. Forced duction test is negative, meaning thereby that there is no resistance to forced (passive) elevation¹³ of the affected eye.
4. Bell's phenomenon is present indicating a normal III cranial nerve (oculomotor) nucleus , fasciculus.^{6, 13}
5. Saccadic velocity in vertical direction is normal or only slightly reduced below the horizontal plane but absent above it (in upgaze).

Examination

The following points are particularly important in the examination of cases of MED:

1. History: It is important to take a thorough history in order to find out whether the MED is congenital or acquired. If acquired one has to decide the cause by taking history of systemic diseases, malignancy etc.
2. Pupillary examination should be carried out to rule out / identify anomalies of pupil (important in neurological disorders).
3. Cover test: It should be done in all the three vertical planes, fixing each eye. In elevation there is marked hypotropia of the affected eye. In PP no deviation to some hypotropia and in depression no vertical deviation. There may be a co-existent horizontal deviation.
4. Ocular motility examination: Both versions and ductions should be tested in the 9 cardinal directions of gaze. A special feature of MED is an equal degree of defect of elevation in adduction as well as in abduction.
5. The Park's three-step test and Bielschowsky's head tilting test is carried out to distinguish MED from superior oblique palsy.
6. Bell's phenomenon, if present means the palsy is supranuclear.
7. Examination on a major amblyoscope is useful in estimating the grade of binocular vision (SMP, Fusion and its range and stereopsis) present, measuring the deviation (vertical, horizontal and torsional) in various directions of gaze and determining the presence, type, direction and degree of suppression. It is usual to find suppression in upgaze and sometimes in PP also. Good binocular functions are generally present in downgaze with no suppression. Angle of deviation fixing each eye in turn, should be measured in the 9 cardinal directions of gaze.
8. Stereo-tests: Results on Titmus / Wirt fly test, TNO test etc give a fair idea of the state of binocular vision even if major amblyoscope is not available in a clinic.
9. Worth Four Dots test also helps in estimating the state of binocularity, presence / absence of suppression, presence, directions and type of diplopia.
10. Prism Bar cover test will give a measurement of the deviation in various cardinal directions of gaze, particularly in upgaze, PP and downgaze.
11. Forced duction test (FDT): In case of primary palsy the FDT is negative while in cases of MED due to contracture of inferior rectus, it is positive.
12. Active force generation test is done to decide the severity of palsy. One should be aware of the possibility of contracture of inferior rectus (IR).
13. If recording of saccades is not possible, they can be tested clinically without any equipment. The patient is asked to look to and fro between the PP and the upgaze (to test saccades in upgaze) and to and fro between PP and downgaze to test saccades in downgaze. If there is a primary paresis there are floating saccades. In the cases of restrictive strabismus and in supranuclear MED the saccades are rapid and they stop suddenly.
14. Objective recording of saccades is done by electro-oculography or scleral search coil technique, the latter being more accurate. Vertical saccadic velocity is of help in differentiating between MED, inferior rectus (IR) restriction and SR paresis.
15. Hess chart gives an idea of the amount of deviation in primary position and the degree of underaction in various fields, e.g., elevation in adduction and elevation in abduction.
16. Field of BSV: It helps in deciding the extent BSV.
17. Radiological diagnosis: CT scan and MRI of orbits help in diagnosing the missing and defective muscles (e.g., contracture). In cases of acquired palsy with systemic, particularly neurological complications, the patient must be referred for a thorough neurological examination.

Diagnosis

NOTE: The diagnosis of congenital MED is made clinically. No laboratory and radiological tests are necessary. However, they can be performed for academic and research purposes.

Acquired MED needs further tests to determine the cause.

The following points are particularly to be kept in mind while making a diagnosis of MED:

1. If the patient is a child the parents will usually be able to tell the age of onset.
2. Presence of amblyopia indicates a long duration and therefore point to the presence of a congenital MED.
3. In most cases the supranuclear MED is congenital. Bell's phenomenon is present indicating a normal III CN, fasciculus and nucleus¹⁴.
4. Presence of systemic disorders like cardiovascular disease, malignancy etc will be in favor of acquired palsy.
5. Presence of neurological signs like loss of consciousness, vertigo, ataxia, tinnitus, deafness and others will go in favor of acquired palsy of SR. A thorough neurological examination is a must in these cases.
6. Positive forced duction test on the direct antagonist of superior rectus (SR), that is, the ipsilateral inferior rectus (IR), will indicate the presence of contractures / tightness of IR leading to secondary SR restriction. If the inferior rectus is tight and fibrotic SR will not be able to move the eye up satisfactorily.
7. If there is no primary palsy of SR the saccades will be normal otherwise the saccadic velocity will show an upward slowing in upgaze, may be in PP and will be normal / or show mild slowing in down gaze.
8. Radiological tests like plane (x-rays) radiogram, CT scan and MRI of skull / orbits may be of great help, even in cases of congenital MED if the cause is a missing SR or the IR is fibrotic.
9. Neurological (Neuro-ophthalmological) examination is a must in all cases of acquired MED.

Legend: CV = Cardiovascular; IR = Inferior rectus; XT = Exotropia; w = With; w/o = Without; CN = Cranial nerve

Treatment

1. Correction of refractive error
2. Treatment of Congenital MED
3. Treatment of acquired MED

Treatment of deviation (hypotropia)

Treatment of ptosis

1. Treatment of Amblyopia: The amblyopia can be of various types, namely strabismic, anisometropic and ametropic (binocular). The treatment is carried out in conventional way. The main points are as follows:

1. Upto the age of 7-8 years every case of strabismic / anisometropic amblyopia

should be treated by conventional occlusion provided the fixation is central and refractive error has been corrected.

2. Above the age of 8 years (visual maturity):

1. Strabismic amblyopia with central fixation is treated only if binocular functions (fusion with a definite range) are present.
2. Anisometropic amblyopia with central fixation and fusion is treated by occlusion as usual. I have found CAM useful in quite a few of these cases.

3. Amblyopia with eccentric fixation is treated conventionally only if the patient is a young child (upto the age of 2-3 years). These cases are also divided into 2 types:

1. If the eccentric fixation is fixed (well established) at one point inverse occlusion is given for 6 week and then surgery is performed. If however, the fixation changes after the 6 weeks, conventional occlusion can be tried to see if the fixation improves (becomes unsteady or moves nearer to macula).
2. If the eccentric fixation is unsteady, conventional occlusion is given a chance to see if it improves. If it does not, inverse occlusion for 6 weeks is followed by surgery.

1. Adequate correction of refractive error and constant use of glasses can gradually lead to improvement of visual acuity in young children.
2. Use of yellow glasses has been credited with improvement of visual acuity in cases of ametropic amblyopia without any strabismus of anisometropia. We are carrying out a study on use of yellow glasses in ametropic amblyopia and await the results.

2. Treatment of significant CHP: As the CHP is adopted because of a hypotropia in PP, surgery is indicated to correct it. It should be carried out while the patient is still young so that any deformities in the spine that have been brought about by a long-standing CHP can be corrected. Significant degree of elevation of chin can lead to antero-posterior changes in spinal curvature (while a head tilt can cause a sideways change).
3. Treatment of deviation (hypotropia): Surgery has to be resorted to. It is only indicated if it is present in PP and can be a cause of amblyopia, loss of fusion, pseudoptosis and CHP (and possibly spinal changes). If there is no hypotropia in horizontal plane and in downgaze, no active treatment is required. The patient is called for checks from time to time to see if the deviation is becoming manifest.
4. Treatment of ptosis: It is only needed if there is true ptosis present that poses a cosmetic problem. A pseudoptosis is automatically corrected when hypotropia is taken care of. There may be both factors present and if ptosis is not fully corrected and cosmetically acceptable after the hypotropia is gone or significantly reduced (to become a hypophoria), ptosis correction is carried out by surgery.

Treatment of acquired MED

1. Conservative

• Medical and neurological
• Orthoptic for suppression and amblyopia
• Observation for any signs of change for better or worse

2. Surgical

As there is a wide variety of causes of acquired MED, thorough investigations (general, ophthalmologic, neurologic (neuro-ophthalmologic and orthoptic (ocular motility work out) have to be undertaken. In a case of recent palsy one has to wait for at least 6 months to allow for spontaneous recovery.

The main points are given below:

• The underlying systemic neurologic conditions must be treated first.
• Observation is the best policy. Surgery should not be carried out as long as there is spontaneous recovery. A period of six months after the onset of palsy should elapse before surgery is resorted to.
• The indications of surgery in acquired MED are as follows:

1. Vertical deviation in PP
2. Persistent vertical diplopia in PP
3. Suppression
4. Amblyopia
5. Small field of BSV

• Ptosis correction by surgery should only be done if it persists after the vertical deviation (and hence the pseudo-ptosis) is eliminated.

The aims of surgery

1. To increase the field of BSV by achieving orthotropia with fusion in as large a field as possible. The field of BSV should preferably be centered in primary gaze (PP).
2. To eliminate any significant degree of CHP (chin elevation) by achieving BSV in PP.
3. To eliminate hypotropia in PP (and depression where diplopia is hardly ever present)
4. To treat ptosis, which is usually pseudoptosis due the presence of hypotropia in PP. Eliminating hypotropia gets rid of it.
5. To eliminate manifest deviation (hypotropia with or without horizontal deviation)

Pre-requisites of surgery for acquired MED

1. There should be no change in the condition of MED for a few weeks on two consecutive checks.
2. At least 6 months must have elapsed after the onset even if the condition is stable.
3. Systemic conditions like diabetes and hypertension must be under control.
4. Neurologic conditions like encephalitis, meningitis, and intracranial space occupying lesion (SOL) etc. should have been taken care of.
5. Condition of stroke patients should have been stabilized.
6. Suppression and amblyopia should have been attended to. They occur only after a significant length of time, specially the latter.

Precautions before surgery

1. The patient / parents must be explained, particularly in cases of SR palsy and supranuclear palsy that full elevation may not be restored. In some cases no improvement in elevation (upgaze) is possible.
2. The patient / parents must be informed that more than one operations may be required for the strabismus.
3. The patient / parents must be cautioned that ptosis correction by surgery may be required later if eliminating hypotropia fails to get rid of it.

Techniques of surgery

1. If the forced duction test is positive for restriction of the ipsilateral inferior rectus (IR) due to fibrosis / contracture: These cases fall into two groups, namely 1. Those with primary IR restriction / tightness / contracture / fibrosis and 2. Those with IR restriction secondary to SR palsy.

Group 1 cases: recession of IR (5-8 mm) with recession of conjunctiva (4 mm) is indicated. Good results have been reported¹⁴ after this procedure.

The surgery on a tight rectus muscle is difficult and risky. Three serious complications that should be guarded against are perforation of the globe, avulsion of inferior rectus if it is pulled with force and slippage of the muscle (particularly if pre-placed sutures are not applied).

Another precaution during surgery on IR is that connections between the IR and the lower lid should be dissected thoroughly to avoid a postoperative lower lid retraction.

Forced duction test is repeated after the IR is disinserted (detached) from the globe. The eye should now elevate freely if all the adhesions and tight IR fibres have been severed.

Finally the forced duction test is repeated after the IR is reattached. Elevation of the eye by at least 20-25 degrees¹⁴ should be possible.

Group 2 cases: Fibrosis of the IR in these cases is secondary to SR palsy. Therefore IR recession alone may often be inadequate to eliminate hypotropia in PP and Knapp's procedure^{15, 16} has to be performed in addition to the IR recession. It is safer to perform Knapp's procedure in the second stage after 4-5 months to reduce the risk of anterior segment ischaemia. An alternative is to do them in one sitting with the sparing of the ciliary vessels to the recti. It is easier in case of vertical rectus muscles than in the case of horizontal rectus muscles.

.

NOTE: In Knapp's procedure^{15, 16} insertions of ipsilateral MR and LR are vertically transposed to that of SR.

2. If the forced duction test is negative for restriction of the ipsilateral inferior rectus (IR): In these patients there is either a SR palsy or supranuclear MED. In both these conditions Knapp's procedure^{15, 16} is indicated. This technique causes insignificant improvement in upgaze but corrects about 20 to 35 PD of hypotropia^{6, 15, 16, 17, 18, 19, 20} in PP. For various degrees of hypotropia the effect of the Knapp's procedure can be graded by using it after IR recession and / or using the various modifications of the Knapp's procedure as suggested below:

1. Hypotropia of 10 PD or less in PP: There are two options depending upon the degree of elevation above midline: A) If there is no elevation possible above midline: partial tendon transposition of horizontal rectus muscles (modified Knapp's procedure) without any previous surgery (e.g., IR recession).

B) If there is some elevation present above midline: resection of SR²¹ (as an alternative to horizontal rectus muscles transposition) with or without one of the other secondary procedures may deliver the desired result.

2. Hypotropia of 25 PD or less: IR recession followed by partial tendon Knapp's procedure can take care of the hypotropia.
3. Hypotropia of 25-35 PD: IR recession followed by full tendon vertical transposition of horizontal rectus muscles near the SR tendon is recommended. As already mentioned, the two procedures should be performed in two sittings to avoid anterior segment ischaemia. However, if the two procedures are to be done at one session sparing of ciliary vessels to the recti is done.
4. Hypotropia of more than 35 PD in cases of MED without IR restriction: Classic Knapp's operation with vertical transposition of the full horizontal rectus muscles to the SR insertion + posterior fixation suture on the transposed muscles²¹ has been advised.

If there is true ptosis present it will persist after the correction of hypotropia. These are the cases that need surgery for ptosis correction. A resection of the levator muscle by external route is indicated if there is some function in that muscle. One must also make sure that there is some function in the SR and the eye can be raised at least partially, to avoid the risk of exposure keratitis.

Complications and problems after surgery for MED

Names of the main problems that can occur in the postoperative period are given below:

1. Undercorrection
2. Overcorrection
3. Worsening of diplopia
4. Anterior segment ischaemia
5. Horizontal tropia
6. Retraction of lower lid

Undercorrection: If there is residual hypotropia after the Knapp's procedure one should wait and watch, as improvement has been reported¹⁵ after some time. The management strategy may be devised²¹ as follows:

1. After Knapp's procedure: Recession of ipsilateral IR can be done, particularly if there is even the slightest indication of tightness in the muscle.
2. An alternative to recession of the IR, is a recession of the SR of the other eye. A plus point of this procedure is that it lessens the incomitance in the upgaze that s due to underaction of SR / MED.
3. If Knapp's procedure has not been performed and only a recession of IR has been done in the first stage, a partial horizontal rectus muscles vertical transposition is indicated.

Overcorrection: In case of overcorrection leading to hypertropia and advent / worsening of diplopia, the problem is not likely to improve. According to reports the hypertropia tends to get worse^{15, 17} with time. These cases are managed as follows:

1. If the overcorrection is a result of Knapp's procedure: The insertions of the lateral and medial rectus muscles that were supraposed (transposed superiorly) are lowered.
2. If the IR recession had been performed: the IR id advanced by 1mm for a hypertropia of 3 PD, 2mm for 6 PD of hypertropia and so on.

Worsening of diplopia: This situation develops if the IR on the affected side has been recessed excessively leading to a weakness of the eye in downgaze as compared to the other eye. Thus a hypertropia of the operated eye is created in downgaze, the IR on the sound side lowering the eye more than the operated IR on the operated side. The usual procedure advised for these patients is recession of the IR of the sound unoperated eye to match the position of the two eyes in downgaze. If however this procedure does not produce the desired amount of weakening and the diplopia persists in downgaze, prismatic correction is prescribed.

Anterior segment ischaemia: It may result if more that two rectus muscles are operated upon at one sitting. To prevent this complication a ciliary vessels sparing procedure may be carried out.

Horizontal tropia: It may be produced if the two horizontal rectus muscles were not transposed equally.

Retraction of lower lid may result if the attachments between the eyelid and the IR that was recessed were not severed sufficiently.

References

1. Boeder, P.: The co-operation of extraocular muscles, Am. J. Ophthalmol. 51:469, 1961.
2. Jampel, R.S.: Extraocular muscle action from brainstem stimulation of Macaque, Invest. Ophthalmol. 1:565, 1962.
3. Ziffer, A.: Clinical strabismus management: Principles and surgical techniques. Editors: Rosenbaum, Arthur L. and Santiago, A.P., 1999, Philadelphia, PA, W.B. Saunders Company, p. 272.
4. White, J.W.: Paralysis of the superior rectus and the inferior oblique muscles of the same eye, Arch. Ophthalmol. 27:366, 1942.
5. Dunlap, E.A.: Diagnosis and surgery of double elevator palsy, Trans. Am. Ophthalmol. Soc. 3:1554, 1952.
6. Barsoum Homsy, M.: Congenital double elevator palsy, J. Pediatr. Ophthalmol. Strabismus 20:185, 1983.
7. Bell, J.A. et al: Congenital double elevator palsy in identical twins, J. Clin. Neuroophthalmol. 10:32, 1990.
8. Hitz, J.B.: Discussion: Paralysis of the superior rectus and the inferior oblique muscles of the same eye, 27:366, 1942.
9. Kirkham, T.H. and Kline L.B.: Monocular elevation paresis, Argyll Robertson Pupils and Sarcoidosis, Can. J. Ophthalmol., 1976, 11:330.
10. Kline, L.B. and Bajandas, F.J.: Neuro-Ophthalmology: Review Manual, Fifth edition, 2000, Slack Incorporated, Thorofare, NJ 08086, p.73.
11. Barsoum-Homsy, M.: Congenital Double Elevator Palsy, J. Pediatr. Ophthalmol. Strabismus, 1983, 20:185.
12. Wright, K.W. et al: Double Elevator Palsy, ptosis and Jaw Winking, Am. Orthopt. J., 1989, 39:143.
13. Ziffer, A.J. and Rosenbaum, A.L. et al: Congenital double elevator palsy: vertical saccadic velocity utilizing the scleral search coil technique, J. Pediatr. Ophthalmol. Strabismus, 1992, 29:142.
14. Metz, H.S.: Double elevator palsy, J. Pediatric Ophthalmol. Strabismus, 18:31, 1981.
15. Bucke, J.P., Ruben, J.B. and Scott, W.E.: Vertical transposition of horizontal recti (Knapp's procedure) for the treatment of double elevator palsy: Effectiveness and long term stability, Br. J. Ophthalmol. 76:734, 1992.
16. Knapp, P.: The surgical treatment of double elevator palsy, Trans. Am. Ophthalmol. Soc. 67:304, 1969.
17. Callahan, M.A.: Surgically mismanaged ptosis with double elevator palsy, Am. J. Ophthalmol. 99:108, 1981.
18. Cooper, E.L. and Greenspan, J.A.: Operation for double elevator palsy, J. Pediatr. Ophthalmol. 8:8, 1971.
19. Dunlap, E.A.: Vertical displacement of horizontal recti, In: Symposium on Strabismus, St. Louis, C.V. Mosby Co., !971, p. 307.
20. Dunlap, E.A.: Discussion: Double elevator palsy, Trans. Am. Ophthalmol. Soc., 67:322, 1969.
21. Ziffer, A.: Clinical Strabismus Management: Principles and Surgical Techniques, editors: Rosenbaum, A.L. and Santiago, A.P., 1999, Philadelphia, PA, W.B. Saunders Company, p. 279.