Ophthalmology – Exodeviations: Comitant

Comitant exodeviation is a horizontal outward deviation of the eyes in which the magnitude of the deviation remains approximately the same in right gaze, left gaze, and primary position at a given testing distance. It represents a spectrum ranging from exophoria, to intermittent exotropia, and eventually to constant exotropia in some patients.

The condition occurs in approximately 1 in 185 children by 10 years of age, with a prevalence of about 1% in children around 11 years old. Reported risk factors include maternal cigarette smoking during pregnancy and low birth weight. The genetic basis is usually multifactorial, although autosomal dominant inheritance has been reported in some families.

Pathophysiology

The basic mechanism is an imbalance in horizontal ocular alignment that favors divergence. The precise neurobiologic cause is not well understood. In some children, the condition may progress gradually from a latent exophoria to intermittent exotropia and ultimately to a constant exotropia.

The deviation may be modified by accommodation and convergence. This is especially important in intermittent exotropia, in which accommodative convergence may help maintain alignment at near.

Associated Conditions

Comitant exodeviations may coexist with oblique muscle dysfunction and A- or V-pattern strabismus. Amblyopia is less common than in esotropia but can occur, particularly when one eye is consistently preferred. Myopia is also relatively common.

Clinical Presentation

Parents often notice that one eye drifts outward, usually beginning in early childhood. Initially the deviation may be intermittent, particularly at distance, and may become more frequent with time.

The exotropia is often more noticeable when the child is tired, ill, daydreaming, or visually inattentive. A characteristic history is closing or squinting one eye in bright sunlight, which may help relieve diplopia or visual confusion.

Some patients report horizontal diplopia. Patients with convergence insufficiency may experience eye strain, headaches, or difficulty reading, particularly during prolonged near work.

Types of Comitant Exodeviation

Exophoria is a latent tendency for the eyes to drift outward that is normally controlled by fusion. The eyes appear aligned under ordinary binocular viewing, and stereopsis is generally good.

Intermittent exotropia alternates between periods of normal alignment and manifest outward deviation. When the eyes are aligned, fusion and stereopsis may be excellent. As control deteriorates, the deviation becomes visible more frequently.

Constant exotropia is continuously manifest. Fusion and stereopsis are usually reduced or absent, and amblyopia is more likely than in intermittent exotropia.

In divergence excess, the exotropia is larger at distance than at near. In the basic type, distance and near deviations are approximately equal. In convergence insufficiency, the near exotropia is larger than the distance deviation.

Sensory exotropia develops because longstanding poor vision in one eye disrupts binocular fusion. Consecutive exotropia occurs after previous treatment or surgery for esotropia and can be either comitant or incomitant.

Examination and Diagnosis

A complete ophthalmic examination should include measurement of visual acuity, refractive error, ocular motility, binocular function, and stereopsis.

The cover test and alternate cover test are performed at both distance and near, with the patient’s refractive correction in place when required. Prisms are used to measure the magnitude of the deviation.

The degree of control of intermittent exotropia is clinically important. With good control, the patient rapidly realigns the eyes after the cover is removed. With fair control, fusion returns only after blinking or refixation. With poor control, the eye remains exotropic for a prolonged period before fusion is regained, or the exotropia may become spontaneously manifest without dissociation.

Some patients require a prolonged occlusion or patch test, traditionally around 20 minutes, to reveal the full deviation by eliminating tonic fusional convergence.

Fixation on a very distant target may also be necessary to demonstrate the maximum distance exotropia.

In sensory exotropia, measurement may require a Krimsky test, with the prism placed over the fixing eye when standard prism cover testing is unreliable.

Differential Diagnosis

The principal distinction is between a comitant and incomitant exodeviation. In a true comitant exotropia, the angle remains approximately equal in different horizontal gaze positions and there is no major restriction or paresis of an extraocular muscle.

Significant variation with gaze should prompt consideration of cranial nerve palsy, restrictive orbital disease, Duane syndrome, previous muscle surgery, or another cause of incomitant exotropia.

Refractive errors, particularly myopia, should also be identified and appropriately corrected.

Treatment

Treatment depends on the type, severity, symptoms, control, visual acuity, and age of the patient.

Appropriate spectacle correction is important, particularly in myopic patients. Clear retinal images improve sensory fusion and can improve control of intermittent exotropia.

In selected children, over-minus lenses may be prescribed to stimulate accommodation and therefore accommodative convergence. This can temporarily reduce an exodeviation, although the potential effect on myopic progression and the need for continued treatment should be considered.

Any associated amblyopia should be treated. Part-time patching of the preferred eye may occasionally be used, especially in younger children with fixation preference or poor control.

Convergence exercises or orthoptic therapy are most useful in patients with convergence insufficiency, rather than as a universal treatment for all intermittent exotropias.

Base-in prisms may reduce diplopia and assist fusion in selected patients, but long-term use may have limitations and is usually individualized.

Surgery

Strabismus surgery is considered when there is poor or deteriorating control of intermittent exotropia, progression toward constant exotropia, troublesome diplopia or asthenopia, loss of binocular function, or a cosmetically significant persistent deviation.

Common operations include recession of one or both lateral rectus muscles, sometimes combined with resection of one or both medial rectus muscles. The surgical plan depends on the magnitude of the deviation and whether it is greater at distance or near.

Sensory exotropia is often treated with a recession of the lateral rectus and resection of the medial rectus in the poorer-seeing eye, thereby limiting surgery to that eye when possible.

A small consecutive esotropia immediately after bilateral lateral rectus recession may occur and often improves during the early postoperative period.

Follow-up

Intermittent exotropia can progress, so children are usually monitored periodically, often every 3–6 months, depending on age, control, visual acuity, and stability.

Follow-up should assess the frequency of the manifest deviation, control after dissociation, distance and near measurements, stereopsis, visual acuity, and development of amblyopia.

After surgery, patients require monitoring for recurrent exotropia, consecutive esotropia, and amblyopia.

Patient Education

Families can help by observing how often the eye drifts outward and under what circumstances it occurs. Increasing frequency of deviation, longer periods of manifest exotropia, or increasing difficulty realigning the eyes may indicate declining control.

Compliance with prescribed glasses, patching, and follow-up is important.

Prognosis

The natural course varies. Some intermittent exotropias remain relatively stable for years, while others progressively lose control and become constant. Older reports have suggested progression in a substantial proportion of untreated patients.

Surgical outcomes are generally favorable, although success rates decline with longer follow-up because recurrence is relatively common. Reported success has been approximately 80% at 1 year and around 50–70% at 5 years, depending on the definition of success and the patient population.

Complications

The most frequent long-term problem after surgery is residual or recurrent exotropia, which is also a common reason for reoperation.

Overcorrection can produce consecutive esotropia, which may result in diplopia in older patients and can create a risk of amblyopia in young children.