Esotropia: Incomitant

Basics

Description

Incomitant esotropia is a manifest inward deviation of one eye in which the angle of deviation changes depending on the direction of gaze. This distinguishes it from comitant esotropia, where the deviation remains approximately the same in different gaze positions.

It may be congenital or acquired and is more commonly encountered in adults. The incomitance usually reflects abnormal extraocular muscle function caused by weakness, restriction, abnormal innervation, or structural abnormalities.

Epidemiology

The exact incidence depends on the underlying cause. Overall strabismus prevalence, including both comitant and incomitant forms, is approximately 1–4%. Incomitant esotropia is more common in adults.

Risk Factors

Important risk factors include brain injury, increased intracranial pressure, thyroid eye disease, myasthenia gravis, orbital trauma, orbital tumors or infiltrative disease, previous strabismus surgery, and craniofacial abnormalities.

A family history or genetic predisposition may be present in congenital cases, particularly those associated with congenital cranial dysinnervation disorders (CCDDs).

Genetics

Several congenital disorders can produce incomitant esotropia. These include Duane syndrome, Möbius syndrome, congenital fibrosis of the extraocular muscles, and horizontal gaze palsy with progressive scoliosis.

The inheritance pattern and responsible gene depend on the specific syndrome. Genetic counseling may therefore be appropriate when a congenital or syndromic disorder is suspected.

Pathophysiology

Incomitant esotropia results from unequal functioning of the horizontal extraocular muscles. Normally, the medial and lateral rectus muscles act in a coordinated manner to maintain ocular alignment.

Weakness of the lateral rectus, as occurs in a sixth cranial nerve palsy, allows the medial rectus to pull the eye inward. Conversely, mechanical restriction of the medial rectus, such as in thyroid eye disease, can prevent normal abduction and produce a similar appearance.

Thus, incomitant esotropia can result from muscle paresis, mechanical restriction, abnormal cranial nerve innervation, muscle fibrosis, or structural abnormalities of the orbit or extraocular muscles.

Etiology

One of the most important causes is cranial nerve VI palsy, which produces weakness or paralysis of the lateral rectus muscle and consequently impaired abduction.

Thyroid eye disease can cause enlargement and fibrosis of the medial rectus, mechanically restricting abduction. Myasthenia gravis can produce variable extraocular muscle weakness and may mimic almost any pattern of strabismus.

Orbital trauma can cause muscle or soft-tissue entrapment, particularly following an orbital wall fracture. Orbital tumors, hemorrhage, inflammation, and infiltrative disorders can also interfere with normal extraocular muscle movement.

Previous strabismus surgery can occasionally produce incomitant esotropia, particularly following excessive medial rectus resection or excessive lateral rectus recession.

Congenital causes include congenital cranial dysinnervation disorders, hypoplastic or anomalous lateral rectus muscles, and craniofacial syndromes associated with absent or abnormal extraocular muscles.

Diagnosis

History

Patients with acquired incomitant esotropia commonly complain of binocular horizontal diplopia. The diplopia may be worse at distance and usually becomes more pronounced in particular directions of gaze.

For example, in a right sixth nerve palsy, the right lateral rectus is weak. The esotropia and diplopia generally become worse when the patient looks to the right, because the affected right eye cannot abduct normally.

Patients may compensate by adopting an abnormal head position or face turn to maintain binocular single vision.

The history should establish whether the deviation developed suddenly or gradually. Questions should address head or orbital trauma, previous ocular surgery, infections, neurologic symptoms, headache, symptoms of increased intracranial pressure, thyroid disease, and symptoms suggestive of myasthenia gravis.

In congenital cases, a family history of strabismus, craniofacial abnormalities, or congenital cranial dysinnervation disorders should be sought.

Physical Examination

A complete ophthalmic examination should be performed with particular attention to ocular alignment and extraocular motility.

Alignment should be measured at distance and near and in the nine diagnostic positions of gaze. The defining feature is that the amount of esotropia changes with gaze direction.

Ductions and versions should be carefully examined. An abduction deficit is especially important because it may indicate lateral rectus weakness from a sixth nerve palsy or mechanical restriction of the medial rectus.

Visual acuity and fixation preference should be assessed, especially in children, because incomitant strabismus can lead to amblyopia. Cycloplegic refraction should also be performed when appropriate.

A neurologic examination should evaluate the other cranial nerves and look for associated sensory or motor abnormalities.

The presence of proptosis and eyelid retraction raises suspicion for thyroid eye disease. Orbital trauma may be associated with restricted motility, sensory abnormalities, or other evidence of fracture.

An abnormal head position should be documented because patients frequently turn or tilt their head to obtain better binocular alignment.

Forced-Duction Testing

Forced-duction testing can help distinguish mechanical restriction from muscle weakness.

If forced ductions are positive, the examiner cannot passively move the eye normally, suggesting a mechanical restriction such as a tight or fibrotic medial rectus.

If forced ductions are negative, the eye can be moved passively despite the patient’s inability to move it actively. This favors a neurologic or muscular paresis, such as a sixth nerve palsy.

Important Congenital Findings

A congenital abduction deficit does not automatically indicate a sixth nerve palsy.

In Duane syndrome, impaired abduction is characteristically associated with globe retraction and narrowing of the palpebral fissure during attempted adduction. Upshoots or downshoots may also occur.

Congenital bilateral sixth and seventh cranial nerve dysfunction should raise suspicion for Möbius syndrome. These patients should also be examined for abnormalities involving other cranial nerves, the tongue, neck, chest, and extremities.

Diagnostic Testing

Laboratory Testing

Laboratory investigations depend on the suspected cause.

If thyroid eye disease is suspected, thyroid function studies should be obtained.

If myasthenia gravis is suspected, testing may include acetylcholine receptor antibodies and other appropriate neurologic investigations. Importantly, an initially negative test does not completely exclude myasthenia when the clinical findings remain strongly suggestive, so repeat or additional testing may sometimes be necessary.

Imaging

Neuroimaging is particularly important in acute-onset incomitant esotropia when neurologic signs or symptoms are present.

MRI of the brain and/or orbits may be indicated when an intracranial lesion, cranial nerve abnormality, brainstem disorder, or other neurologic process is suspected.

Orbital CT is particularly useful when evaluating orbital fractures, thyroid eye disease, orbital masses, hemorrhage, inflammation, or infiltrative disease.

Orbital imaging in thyroid eye disease may demonstrate characteristic extraocular muscle enlargement.

Differential Diagnosis

The major differential diagnosis is comitant esotropia. In comitant esotropia, the angle of deviation remains approximately the same in different directions of gaze and significant extraocular motility limitation is generally absent. In incomitant esotropia, the deviation changes with gaze direction and an identifiable motility deficit is much more common.

Pseudo-abduction limitation can occur in infants with large-angle infantile esotropia and cross-fixation. These children may appear unable to abduct an eye because they preferentially use the opposite eye to look toward the side. Full abduction can sometimes be demonstrated with appropriate monocular testing or the vestibulo-ocular reflex.

Other considerations include esophoria, negative angle kappa, accommodative or nonaccommodative comitant esotropia, and hyperopic anisometropia with apparent variation in deviation as fixation alternates between the eyes.

Treatment

General Principles

Treatment depends primarily on the underlying cause rather than simply on the presence of esotropia.

Any associated amblyopia should be treated, particularly in children. The underlying neurologic, orbital, endocrine, or neuromuscular disorder should also be appropriately managed.

Temporary treatment of diplopia can include Fresnel press-on prisms when the deviation is suitable for prism correction.

In patients who are beyond the amblyogenic age range, temporary occlusion of one eye may be used when diplopia cannot otherwise be controlled.

Sixth Cranial Nerve Palsy

Many acquired sixth nerve palsies, particularly those caused by microvascular disease, may improve spontaneously. Observation may therefore be appropriate after potentially serious causes have been appropriately excluded.

Botulinum toxin injection into the medial rectus has been used in selected patients, including some cases of traumatic sixth nerve paresis.

If a stable sixth nerve palsy persists, strabismus surgery may be considered. The specific operation depends on whether lateral rectus function is partially preserved or completely absent.

With complete sixth nerve paralysis, muscle transposition procedures may be required to provide abducting force to the affected eye.

Thyroid Eye Disease

Treatment should address both the systemic thyroid disorder and the orbital disease.

Strabismus measurements should generally be allowed to become stable before definitive strabismus surgery because the deviation may change during active thyroid eye disease.

Restrictive strabismus is usually treated by recession of the involved tight extraocular muscle, rather than by strengthening an opposing muscle.

Smoking cessation is particularly important in patients with thyroid eye disease because smoking is associated with more severe thyroid-associated orbitopathy.

Myasthenia Gravis

Patients suspected of having ocular myasthenia should be referred for appropriate neurologic or neuro-ophthalmologic evaluation.

Treatment is directed at the underlying neuromuscular disorder. Because ocular alignment can fluctuate considerably, definitive strabismus surgery is generally inappropriate until the disease and deviation have demonstrated sufficient stability.

Orbital Trauma

Orbital fractures can produce incomitant esotropia through extraocular muscle or orbital soft-tissue entrapment.

When clinically significant entrapment is present, surgical treatment may involve liberation of the trapped tissues and repair of the orbital fracture.

Intracranial Causes

When incomitant esotropia results from increased intracranial pressure, tumor, brain injury, or another intracranial process, treatment is directed toward the underlying neurologic disorder.

Neurology or neurosurgical referral may be required depending on the cause.

Ongoing Care

Children require particularly careful follow-up because persistent ocular misalignment can produce amblyopia and abnormal binocular visual development.

Serial examinations are useful in patients with sixth nerve palsy, thyroid eye disease, and myasthenia gravis because ocular alignment may change over time.

The deviation should generally demonstrate adequate stability before definitive strabismus surgery is undertaken.

Patients should also be monitored for changes in diplopia, ocular motility, abnormal head posture, and neurologic symptoms.

Patient Education

Patients and parents should understand that incomitant esotropia is often a manifestation of another disorder rather than an isolated alignment problem.

Parents of affected children should be educated about the risk of amblyopia and impaired stereopsis.

Adults with diplopia should be advised to report significant changes in ocular alignment or new neurologic symptoms promptly.

When a vascular sixth nerve palsy is suspected, appropriate management of systemic vascular risk factors such as hypertension and diabetes is important.

Prognosis

The prognosis depends strongly on the underlying etiology.

Many microvascular sixth nerve palsies improve spontaneously. In contrast, congenital cranial dysinnervation disorders usually represent permanent abnormalities of ocular innervation.

In congenital disorders such as Duane syndrome, treatment is generally not expected to restore completely normal ocular motility. Instead, the major surgical goals are to improve alignment in primary gaze, reduce an abnormal head posture, and improve the functional field of binocular single vision.

Thyroid eye disease, myasthenia gravis, orbital disease, and intracranial disorders have prognoses determined largely by control of the underlying condition.

Complications

The major complications are persistent binocular diplopia, abnormal head posture, loss of binocular vision, reduced stereopsis, and amblyopia in children.

A particularly important clinical principle is that acute acquired incomitant esotropia should not automatically be treated as an isolated strabismus. The possibility of a cranial nerve palsy, restrictive orbital disease, myasthenia gravis, or intracranial pathology should first be considered and investigated when clinically appropriate.