Ophthalmology – Exodeviations: Incomitant

Incomitant exotropia is an outward deviation of the eyes in which the magnitude of exotropia changes in different directions of gaze. It most commonly appears as an A-pattern or V-pattern exotropia. In a V-pattern, the exotropia is greater in upgaze than in downgaze, whereas in an A-pattern the exotropia is greater in downgaze than in upgaze. Incomitance can also occur horizontally, with different measurements in right and left gaze because of extraocular muscle overaction, restriction, or paralysis.

Approximately 25% of exotropias may demonstrate some degree of incomitance. The incidence is higher in patients with cranial nerve palsies, thyroid eye disease, orbital disease, and previous strabismus surgery. Risk factors include disorders affecting the medial or lateral rectus muscles, previous eye-muscle surgery, craniofacial abnormalities, orbital trauma, and neurologic disease.

The genetic contribution is usually related to the underlying disorder rather than to the exotropia itself. Exodeviations can show multifactorial inheritance, while conditions such as craniosynostosis and congenital cranial dysinnervation disorders may have defined genetic abnormalities.

Pathophysiology and Etiology

Incomitant exotropia results when the forces controlling ocular alignment vary with the direction of gaze. This may arise from oblique muscle overaction, extraocular muscle restriction, muscle paresis, abnormal innervation, or altered orbital anatomy.

A- and V-pattern exotropias are often associated with abnormal oblique muscle function. Inferior oblique overaction is commonly associated with a V-pattern, while superior oblique overaction may be associated with an A-pattern. Abnormal extraocular muscle pulley positions may also contribute to these patterns, particularly in craniofacial disorders.

Restriction or paralysis of the horizontal rectus muscles can also cause incomitance. Important causes include cranial nerve III or VI palsy, thyroid eye disease, myasthenia gravis, orbital fractures, orbital inflammation, hemorrhage or tumor, and complications of previous strabismus surgery.

Associated systemic or ocular conditions include facial asymmetry, birth trauma, craniosynostosis, spina bifida, aberrant regeneration of the third cranial nerve, hyperthyroidism, brain tumors, intracranial aneurysms, increased intracranial pressure, head trauma, and orbital disease.

Diagnosis

The history should include details of birth and development, neurologic disease, previous trauma, and previous ocular or orbital surgery. Previous sinus surgery, orbital decompression, orbital fracture repair, retinal detachment surgery, or strabismus surgery may be particularly relevant because these procedures can affect the extraocular muscles or orbital tissues.

Patients may adopt an abnormal head position to reduce diplopia or improve binocular alignment. For example, a patient with an A-pattern exotropia may adopt a chin-down posture, while cranial nerve palsies may produce compensatory face turns.

A complete ophthalmic examination should include visual acuity, refractive assessment, and careful measurement of ocular alignment. The alternate cover test with prism measurement should be performed at distance and near, with appropriate spectacle correction in place.

To identify A- or V-patterns, alignment is measured in primary gaze, approximately 25 degrees of upgaze, and 25–35 degrees of downgaze. Measurements should also be obtained in right and left gaze to identify horizontal incomitance.

The examiner should assess for oblique muscle overaction or underaction. The vertical position of the adducting eye is evaluated during lateral gaze, and changes in vertical alignment should also be assessed with right and left head tilt.

Primary and secondary deviations should be compared when a paralytic disorder is suspected. A larger secondary deviation can support the presence of muscle paresis.

Diagnostic Testing

Routine laboratory testing is usually unnecessary. If thyroid eye disease is suspected, thyroid function studies are appropriate. If myasthenia gravis is suspected, appropriate antibody and neuromuscular testing should be obtained.

Imaging is not routinely required in uncomplicated longstanding incomitant exotropia. However, MRI of the brain and/or orbits should be considered in patients with recent-onset cranial nerve palsy, neurologic symptoms, head trauma, suspected lost extraocular muscle, or orbital disease.

Orbital CT is particularly useful when orbital fracture, bony abnormality, or traumatic muscle entrapment is suspected. High-resolution orbital MRI can sometimes demonstrate abnormal muscle paths or pulley anatomy in complex cases.

Forced-duction testing can distinguish mechanical restriction from muscle paresis. A positive forced-duction test indicates restriction, whereas relatively free passive movement favors paresis. Forced-generation testing can further help assess active muscle force.

Differential Diagnosis

The differential diagnosis includes comitant exotropia, Duane syndrome, cranial nerve III or VI palsy, internuclear ophthalmoplegia, isolated medial rectus weakness, myasthenia gravis, multiple sclerosis, and consecutive exotropia following esotropia surgery.

Craniofacial disorders can produce apparent incomitance because of abnormal orbital anatomy and extraocular muscle orientation. Unilateral ptosis may also create the appearance of ocular misalignment.

Significant anisometropia must be corrected before measurements are interpreted because alternating fixation through unequal refractive errors can produce misleading differences between primary and secondary deviations.

Treatment

Treatment begins by correcting any significant refractive error and treating associated amblyopia, particularly in children.

Prisms may help control diplopia or reduce a compensatory head posture in selected patients, although their usefulness is limited when the deviation changes substantially with gaze direction.

If an underlying systemic or neurologic disorder is identified, management should be directed toward that condition. Patients with suspected thyroid eye disease may require endocrinology evaluation, those with suspected neurologic disease may require neurology or neuro-ophthalmology consultation, and patients with craniofacial syndromes may benefit from genetics evaluation.

Vision therapy and conventional orthoptic exercises generally have limited value in most forms of incomitant strabismus, particularly when the underlying problem is mechanical restriction or muscle paralysis.

Surgery

Surgical treatment is considered when the incomitant exotropia causes significant diplopia, an abnormal head posture, functional restriction of the binocular visual field, recurrent amblyopia, or a cosmetically significant deformity.

Surgical planning is more complex than for comitant exotropia because the deviation varies with gaze. Treatment may involve horizontal rectus surgery, oblique muscle procedures, transposition procedures, or correction of restrictive abnormalities depending on the underlying mechanism.

Adjustable sutures may be used in appropriate patients to improve postoperative alignment. Botulinum toxin injections may also be considered in selected small-angle incomitant deviations.

Follow-up

Children without neurologic or cranial nerve disease are often followed approximately every 6–12 months, although more frequent review is necessary when amblyopia, progressive deviation, or diplopia is present.

Follow-up should include monitoring of visual acuity, refractive error, ocular alignment, head posture, binocular function, and amblyopia therapy.

Patients with cranial nerve palsies, thyroid eye disease, myasthenia, or evolving orbital disease require closer follow-up because the deviation may change significantly over time.

Patient Education and Prognosis

Patients and families should understand that incomitant exotropia often reflects an underlying muscular, orbital, neurologic, or craniofacial abnormality. Treatment is therefore individualized according to the cause.

In many nonparalytic cases, satisfactory alignment and improvement in head posture or binocular function can be achieved. However, complex restrictive or paralytic forms may require more than one strabismus procedure.

The main complications include amblyopia in children, persistent or recurrent diplopia, abnormal head posture, and the possibility of requiring multiple strabismus surgeries.