Bruce Schnall

BASICS

DESCRIPTION

A horizontal divergence of the eyes that measures the same in left and right gaze as in the primary position for a given testing distance.

EPIDEMIOLOGY

Incidence

1 in 185 children by age 10 years.

Prevalence

Approximately 1% of all children <11 years old.

RISK FACTORS

Maternal cigarette smoking during pregnancy and low birth weight.

Genetics

• Multifactorial inheritance

• Autosomal-dominant inheritance has been reported

PATHOPHYSIOLOGY

• Horizontal eye muscle imbalance that results in divergent horizontal strabismus. May progress from exophoria to intermittent exotropia to constant exotropia.

– Neurobiologic pathophysiology unknown

ETIOLOGY

Unknown

COMMONLY ASSOCIATED CONDITIONS

• May be associated with oblique muscle dysfunction, A or V patters

• Amblyopia is infrequent but may occur

• More common if refractive error is myopic

DIAGNOSIS

HISTORY

• A turning outward of the eyes, which is usually noticed in early childhood. At first it may be intermittent in the distance and then become more constant with time. It may be noticed more often during illness or fatigue

• Closing one eye outdoors in bright sunlight.

• Horizontal diplopia.

• Asthenopia with reading is common with convergence insufficiency.

PHYSICAL EXAM

Types:

• Exophoria: A tendency for the eyes to turn outward, which is controlled by fusion. This is usually not noticed. Fusion and stereopsis are good.

• Intermittent exotropia: The deviation is latent at times and manifest at others. This type of deviation may be noticed and may cause diplopia. The variability is related to a high AC/A ratio and accommodative convergence, which modify the distant exodeviation and lessen it at near. Fusion and stereopsis are good when the eyes are aligned.

• Constant exotropia: The eyes turn out constantly. Amblyopia is more common in this group. Fusion and stereopsis are lacking.

• Divergence excess: The deviation is greater at distance.

• Basic: The measurements at distance and near are the same.

• Convergence insufficiency: The near deviation is greater than distance.

• Sensory exotropia: Exotropia develops as a result of longstanding poor vision in one eye.

• Consecutive exotropia: An exotropia that occurs following treatment for esotropia. May be comitant or incomitant.

DIAGNOSTIC TESTS & INTERPRETATION

Diagnostic Procedures/Other

• Cover testing should be performed with glasses being worn, if necessary. The distant and near deviations are measured with prisms.

• An assessment of the control of an intermittent exotropia should be made. Good control-fusion is rapidly realigned after cover testing. Fair control-fusion is regained after blinking or refixation. Poor control-eye remains exotropic for an extended period of time.

• May require prolonged cover (20 minute patch test) to elicit deviation.

• Sensory exotropia is measured by Krimsky with the prism being placed over the fixing eye.

– Fixation on a far distant target (e.g., out a window) may be required to elicit maximum deviation.

Pathological Findings

Normal extraocular muscle

DIFFERENTIAL DIAGNOSIS

• Rule out incomitant deviation (see chapter on Incomitant Exodeviation)

– Myopia

TREATMENT

ADDITIONAL TREATMENT

General Measures

• Glasses should be prescribed when indicated, especially if myopic. Some children may be offered additional minus lens power to stimulate accommodative convergence. Can add up to 3.00 diopters of minus to cycloplegic refraction

• Amblyopia if present should be treated.

• Part-time patching of preferred eye.

• Convergence training or orthoptics are most effective in treating convergence insufficiency

• Base-in prisms can help to maintain fusion and prevent diplopia. Long-term use of base-in prism is likely to result in a reduction of fusion vergence amplitudes.

Issues for Referral

• Deviations not controlled by glasses or patching or the recurrence of amblyopia despite medical treatment are indications for surgery

– Reconstruction of a normal appearance is an indication for surgery, provided medical treatment has failed and amblyopia treated. Surgery is indicated when there is poor control of an intermittent exotropia, progression toward constant exotropia, diplopia, or asthenopia.

SURGERY/OTHER PROCEDURES

• Surgery involves recession of one or both lateral rectus muscles, which may be combined, with resection of one or both medial recti muscles. The decision on which muscles to recess or resect is based upon the size of the deviation and whether the exotropia is greater at the distance or near.

• Sensory exotropia is treated with recession of the lateral rectus and resection of the medial rectus in the deviated eye.

• Immediately following recession of both lateral recti for intermittent exotropia, a consecutive esotropia is commonly seen which should resolve within 2 weeks.

ONGOING CARE

FOLLOW-UP RECOMMENDATIONS

Patient Monitoring

• Intermittent exotropia may progress and patients may need to be monitored at 3– 6 month intervals to look for progression.

• Following strabismus surgery, patients need to be monitored for recurrence of exotropia, consecutive esotropia, and amblyopia.

– Amblyopia monitoring as indicated based on age, depth of amblyopia (See Amblyopia chapter)

PATIENT EDUCATION

• Families should keep track of frequency of deviation

– Importance of patching and glasses; wear as indicated

PROGNOSIS

• Untreated, up to 75% may progress over time.

• Surgical outcome is dependent upon length of follow-up. Reported success rate at 1 year is approximately 80% and 50–70% at 5 years.

COMPLICATIONS

• Surgical overcorrection of intermittent exotropia can result in diplopia and amblyopia

– Most common reason for reoperation is residual exotropia, although consecutive esotropia may also occur

ADDITIONAL READING

• Pediatric Ophthalmology and Strabismus, Basic and Clinical Science Course, Section 6, American Academy of Ophthalmology, 2010–2011.

• Haggerty H, Richardson S, Hrisos S, et al. The Newcastle Score; a new method of grading the severity of intermittent exotropia. Br J Ophthalmol 2004;88(2):233–235.

• Mohney BG, Huffaker RK. Common forms of childhood exotropia. Ophthalmology 2003;110(11):2093–2096.

• Hunter DG, Ellis FJ. Prevalence of systemic and ocular disease in infantile exotropia: Comparison with infantile esotropia. Ophthalmology 1999;106(10):1951–1956.

CODES

ICD9

• 378.10 Exotropia, unspecified

• 378.20 Intermittent heterotropia, unspecified

CLINICAL PEARLS

• Amblyopia is uncommon in intermittent exotropia.

• Serial follow-up is critical as early intervention can prevent the development of constant exotropia.

• Myopic correction may be a useful therapy prior to surgery.

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