Intermittent Exotropia

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Definition

Exodeviations (from Greek εξοτρὀπια, εξο "exo" meaning "to exit" or "move out of”) are either manifest (exotropia) or latent (exophoria). This topic will focus specifically on intermittent exotropia, which is the most common type of manifest exodeviation.

Disease

Intermittent exotropia comprises about 75-90% of the cases of exotropia and is usually preceded by a stage of exophoria. It affects about 1% of the general population.[1][2]

Etiology

There are several theories as to why intermittent exotropia develops. These include:

Innervational and mechanical factors

Most theories on the etiology of exodeviations suggest that exodeviations are caused by combination of mechanical and innervational factors.[3] [4][5] Other theories include innervational imbalance in the divergence centers in the areas of the tegmentum of the brainstem in humans and divergence burst cells in the area of the mesencephalic reticular formation [6]or abnormal position of the extraocular muscle pulley position.[7][8][9]

Defective fusion

An older theory suggests that the essential cause of exotropia is a defect of the fusion faculty and indeed is a congenital total absence of the fusion faculty.[10]

Abnormal AC/A ratio

Several authors suggest that patients with intermittent exotropia have a high AC/A ratio.[11][12]

Refractive errors

In a patient with uncorrected myopia, less than normal accommodative effort is required during near vision thus causing decreased accommodative convergence. According to Donders, this constant under-stimulation of convergence may cause an exodeviation to develop.[13] Similarly in patients with a high degree of uncorrected hypermetropia, no effort is made to overcome the refractive error by an accommodative effort.[14]

Genetics

Although heredity appears to play a role in exodeviations, the etiology of this disorder is thought to be multifactorial. A positive family history is often noted. Children born with craniofacial anomalies and those with neurologic defects are more likely to exhibit exotropia. Maternal smoking during pregnancy and low birth weight are significant and independent risk factors for the development of horizontal deviations.[15][16]

Clinical Features

Age of Onset

Intermittent exotropia typically begins before the age of 5 years old. In a series of 472 patients with intermittent exotropia, the deviation was present at birth in 204 individuals and appeared in 16 individuals at 6 months of age and in 72 individuals between 6-12 months of age.[17]

Characteristics

The deviation may become manifest during illness, fatigue or daydreaming. Often, exposure to bright light or photography may elicit eye closure of the non-dominant eye. The tropia is usually larger at distance than near. Over time, the deviation can remain stable, resolve or progress, however patients maintain normal retinal correspondence and good binocular function when they are ortho.[18] Von Noorden found that 75% of 51 untreated patients showed progression over an average follow up period of 3.5 years while 9% did not change, and 16% improved.[19]

Classification

Intermittent exotropia has been divided into four groups according to the classification system proposed by Burian.[20]This system is based upon the concept of fusional convergence and divergence and relies on measurements of the distance and near deviations.

  1. Basic: distance deviation is within 10 prism diopters of the near deviation.
  2. Divergence excess: distance deviation is 10 prism diopters greater than the near deviation.
  3. Convergence insufficiency: near deviation is 10 prism diopters greater than the distance deviation.
  4. Simulated or pseudo-divergence excess: distance deviation at distance is greater than near, but the near deviation increases to within 10 prism diopters of the distance deviation after 30-60 minutes of monocular occlusion.


Diagnosis

History

A thorough history should be obtained with regard to the age of onset, frequency of deviation, duration, and whether the deviation is increasing in frequency and/or duration. Additionally it is helpful to ascertain whether the deviation is more evident at distance, near or both.

Physical Examination

Assessing Control of Intermittent Exotropia

The assessment of control of intermittent exotropia is essential to obtain a baseline evaluation as well as to monitor deterioration and progression of intermittent exotropia.

Assessment Methods for Control

  • At home: The parents may be told to keep a chart noting the control of deviation at home in terms of the percentage of waking hours the manifest deviation is noted at home.
  • In the office:
    • Good Control: Patient “breaks” only after cover testing and resumes fusion rapidly without need for a blink or refixation.
    • Fair Control: Patient blinks or refixates to control the deviation after disruption with cover testing.
    • Poor Control: Patient who breaks spontaneously without any form of fusion disruption, or who does not spontaneously regain alignment despite blink or refixation.
  • Alternative office method:
    • 5 = Constant exotropia
    • 4 = Exotropia > 50% of the exam before dissociation
    • 3 = Exotropia < 50% of the exam before dissociation
    • 2 = No exotropia unless dissociated, recovers in > 5 seconds
    • 1 = No exotropia unless dissociated, recovers in 1–5 seconds
    • 0 = No exotropia unless dissociated, recovers in < 1 second (phoria) [21]

Stereopsis

  • Distance stereoacuity: Distance stereoacuity provides an objective assessment of both control of the deviation and deterioration of fusion. Normal distance stereoacuity usually indicates good control with little or no suppression.
  • Near stereoacuity: In a study it was shown that near stereoacuity does not correlate well with the degree of control in intermittent exotropia.[22] However, near stereoacuity can be used as a marker for deterioration of the disease; if near stereoacuity is deteriorating, it is likely that the patient’s control is not sufficient to prevent amblyopia from developing.

Angle of Deviation

  • Alternate cover testing – Due to the variable angle of deviation, measurement in a patient with intermittent exotropia can be difficult. A prolonged alternate cover testing should be used to suspend tonic fusional convergence.
  • Patch Test - The patch test is used to control the tonic fusional convergence to differentiate pseudo-divergence excess from true divergence excess and to reduce the angle variability. Patching one eye for 30 minutes will suspend the tonic fusional convergence and thus reveal the actual amount of deviation.
  • +3.0 D near add test (lens gradient method) - This test can distinguish pseudo-divergence excess from a high AC/A ratio rather than true divergence excess. This test should be used in patients who have a deviation of 10 prism diopters or more at distance than near after the patch test.


Management

Nonsurgical

Management options include the following:

  • Observation: can be an option for patients who are well-controlled and asymptomatic with good binocular fusion.
  • Correction of refractive errors: Refractive errors can impair fusion and result in a manifest deviation. Myopes, in particular, will often improve control of their intermittent exotropia if given corrective lenses.
  • Over-minus lenses: This technique is based on the principle that stimulating accommodative convergence can reduce an exodeviation.[23] Overcorrection with minus lenses may improve the quality of fusion and occasionally may even decrease the angle of deviation so that surgery may be deferred. This is particularly useful in patients who have a high AC/A ratio.
  • Part time occlusion: This technique has found some use in very young children. Part time patching of the non-deviating eye may convert an intermittent exotropia to a phoria by treating suppression and amblyopia. Although the benefit may be temporary, occlusion can be used to postpone surgical intervention in responsive patients.[24] Alternate occlusion may be used in patients with equal fixation preferences.
  • Prisms: base-in prisms can promote fusion but may reduce fusional vergence amplitudes. For this reason, they are rarely used for long-term management.
  • Orthoptics: Convergence exercises such as pencil push-us or computer orthoptics may be helpful in patients with convergence insufficiency who have a remote near point of convergence, or in whom poor fusional convergence amplitudes are demonstrated.

Surgical

Indications for surgery

In intermittent exotropia, one of the important indications for surgery is an increasing tropia phase, since this indicates deteriorating fusional control. Signs of progression include:

  • Increased frequency of the manifest phase
  • Increase in magnitude of deviation
  • Difficulty regaining fusion after dissociation during cover testing
  • Decrease of stereoacuity

Timing for Surgery

There is no defined optimal age or timing of surgery. Most studies have failed to show that age at time of surgery makes any difference in outcome. [25][26][27]

Type of Surgery

Lateral Rectus Recession: Bilateral lateral rectus recession is one option for surgery. In patients with small deviations, unilateral lateral rectus recession on the non-dominant eye may be effective.

Recession-Resection Procedure: Another option is a unilateral lateral rectus recession with medial rectus resection. In patients with amblyopia, the surgeon may choose a unilateral recess-resect surgery to avoid surgery on the eye with good vision. Large angle exotropia of more than 50 prism diopters may require bilateral lateral rectus recessions combined with resection of one or more medial rectus muscles, even in the presence of amblyopia.

Medial Rectus Resection: Bilateral medial rectus resections are thought to be useful for the treatment of the convergence insufficiency type of intermittent exotropia because of the increased effect of this surgery at near.

A-and V-patterns: Oblique overaction: Intermittent exotropia may be associated with inferior or superior oblique overaction and thus A- and V-pattern. For inferior oblique overaction with a significant V-pattern, the inferior oblique muscles may be weakened at the time of horizontal surgery. If significant superior oblique overaction and an A-pattern is present, one may consider a superior oblique weakening procedure at the time or horizontal muscle surgery.


Prognosis

Due to lack of a standard definition for a successful outcome, variability in classification systems, multiple treatment approaches, and a paucity of long-term data it is difficult to determine the true outcome of currently available treatments for intermittent exotropia. The success rate of intermittent exotropia is dependent on the length of follow-up. In recent studies, the reported success rate in all types of intermittent exotropia have been about 60-70%.[28][29][30][31]


Additional Resources

References

  1. Govindan M, Mohney BG, Diehl NN, Burke JP. Incidence and types of childhood exotropia: a population-based study. Ophthalmology. 2005 Jan;112(1):104-8.
  2. Noorden GK von. Exodeviations. In: Binocular Vision and Ocular Motility 5 th ed., 1996 Mosby, pg 343.
  3. Duane A. A new classification of the motor anomalies of the eyes based upon physiological principles together with their symptoms, diagnosis and treatment. Ann. Ophthalmol. Otolaryngol. 1896,5:969; 18976:84,247.
  4. Burian HM. Pathophysiology of exodeviation. In: symposium on horizontal ocular deviation. St. Louis, Ed. Manley DR. 1971 Mosby-Year Book, Inc., pg 119
  5. Costenbader FD. The physiology and management of divergent strabismus. In: Strabismic Ophthalmic Symposium I, St. Louis, Ed Allen JH. 1950 Mosby-Year Book, Inc.
  6. Lee SA, Sunwoo IN, Kim KW. Divergence paralysis due to a small hematoma in the tegmentum of the brainstem. Yonsei Med J 1987:28:326.
  7. Clark RA, Demer JL, Rosenbaum AL. Heterotropic rectus extraocular muscle pulleys simulate oblique muscle dysfunction. Presented at the 22nd annual meeting of the American Association for Pediatric Ophthalmology and Strabismus 1997.
  8. Demer JL, Miller JM, Poukens V. Surgical implications of the rectus extraocular muscle pulleys. J Pediatr Ophthalmol Strabismus 1996:33:208.
  9. Demer JL, Miller JM, Poukens V, et al. Evidence for fibromuscular pulleys of the recti extraocular muscles. Invest Ophthalmol Vis Sci 1995;36:1125.
  10. Worth C. Squint, its causes, pathology and treatment ed. 6. London 1929, Bailliere, Tyndall and Cox.
  11. Cooper J, Medow N. Intermittent Exotropia, basic and divergence excess type. Binoc Vis Eye Muscle Surg 1993;8:185-216.
  12. Kushner BJ: Exotropic deviations: A functional classification and approach to treatment. Am Ortho J 1988;38:81-93.
  13. Donders FC. An essay on the nature and the consequences of anomalies of refraction. Ed. Oliver CA. Philadelphia 1899. P. Blakiston’s Son and Co., pg 59.
  14. Noorden GK von, Avilla CW. Accomodative convergence in hypermetropia. Am J Ophthalmol. 1990;110-287.
  15. Burian HM, Spivey BE. The surgical management of exodeviations. Am J Ophthalmol 1965;59:603-620
  16. Chew E, Remaley NA, Tamboli A, et al: Risk factors for esotropia and exotropia. Arch Ophthalmol 1994;112:1349-1355.
  17. Costenbader FD. The physiology and management of divergent strabismus. In: Strabismic Ophthalmic Symposium I, St. Louis, Ed Allen JH. 1950 Mosby-Year Book, Inc.
  18. American Academy of Ophthalmology. Basic Clinical Sciences Course, Pediatric Ophthalmology and Strabismus. “Intermittent Exotropia.” 2020, 99-103.
  19. Noorden GK von. Exodeviations. In: Binocular Vision and Ocular Motility 5 th ed., 1996 Mosby.
  20. Burian HM, Spivey BE. The surgical management of exodeviations. Am J Ophthalmol 1965;59:603-620
  21. Mohney BG, Holmes JM. An Office-based Scale for Assessing Control in Intermittent Exotropia. Strabismus. 2006 Sep; 14(3): 147–150.
  22. Zanoni D, Rosenbaum AL: A new method for evaluating distance stereoacuity. J Pediatr Ophthalmol Strabismus 1991;28:255.
  23. Caltreider M, Jampolsky A: Overcorrecting minus lens therapy for treatment of intermittent exotropia. Ophthalmology 1983;90:1160-1165.
  24. Freeman RS, Isenberg SJ: The use of part time occlusion for early onset unilateral exotropia. J Pediatr Ophthalmol Strabismus 1989;26:94-96.
  25. Dunlap EA. Over correction in esotropia surgery. In: Arruga A. ed.: International strabismus symposium, Basel 1968, S. Karger AG 319.
  26. Parks MM. Metchell P. Concomitant exodeviation. In: Duane TD ed. Clinical Ophthalmology, Vol. 1.Philadelphia 1988, JB lippin cott Co. p 1.
  27. Pratt Johnson JA, Barlow JM & Tilson G. Early surgery for Intermittent exotropia. Am J Ophthalmol. 1977;84:689.
  28. Ing MR, Nishimura J, Okino L: Outcome study of bilateral lateral rectus recession for intermittent exotropia in children. Trans Am Ophthalmol Soc (XCV): 1997:433-452.
  29. Richard JM, Parks MM: Intermittent exotropia: Surgical results in different age groups. Ophthalmology. 1983;90:1172-1177.
  30. Scott WE, Keech R, Mash AJ: The post-operative results and stability of exodeviations. Arch Ophthalmol. 1981:1814-18.
  31. Olitsky SE: Early and late postoperative alignment following unilateral lateral rectus recession for intermittent exotropia. J Pediatr Ophthalmol Strabismus 1998;35:146-148.
  1. Jampolsky A, Flom BC, Weymouth FS, Moster LE. Unequal corrected visual acuity as related to anisometropia. Arch Ophthalmol. 1955;54:893
  2. Hall IB. Primary divergent strabismus. Br Orthopt J. 1961;18:106.
  3. Manley DR. Classification of the exodeviations. In: Manley D ed.: Symposium on horizontal ocular deviations. St. Louis. 1971. Mosby-Year Book Inc. pg.128
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