Esotropia

Types

Concomitant esotropia, an inward squint that does not vary with gaze direction, mostly appears before 12 months of age (constituting 40% of all strabismus cases) or at age three or four. Most early-onset patients are emmetropic, while most later-onset patients are hyperopic. This is the most frequent type of natural strabismus in humans and monkeys. Concomitant esotropia is subdivided into constant esotropia, present all the time, and intermittent esotropia, which is not always present.

In very rare cases, intermittent esotropia may occur in cycles of one day on, one day off (cyclic esotropia), but most are accommodative in origin. A patient may have constant esotropia for reading but intermittent for distance. Accommodative esotropia, also called refractive esotropia, results from efforts to focus, often in moderate hyperopia. The hyperopic person over-converges as part of the accommodation reflex, which can disrupt binocular control.

The likelihood of developing esotropia depends on the degree of hyperopia: small errors allow clear vision through over-accommodation but may disrupt control, while large errors prevent clear vision and thus reduce incentive for over-accommodation. Only about 20% of children with hyperopia greater than +3.5 diopters develop strabismus.

Diagnosis

A person with esotropia squints with either the right or left eye, never both simultaneously. In left esotropia the left eye squints; in right esotropia the right eye squints. Alternating esotropia allows the patient to switch fixation between eyes, so one eye fixates while the other turns inward; this alternation is mostly spontaneous but may be voluntary. Consistent fixation with one eye and squinting with the other increases the risk of amblyopia in the squinting eye, whereas alternating squint rarely leads to amblyopia because both eyes receive equal stimulation.

Occlusion or patching of the dominant eye can encourage alternation. Esotropias are classified as concomitant, where deviation size does not vary with gaze direction, or incomitant, where gaze direction affects the deviation. Most esotropias are concomitant and begin in early childhood, typically between ages 2 and 4. Incomitant esotropias occur in childhood and adulthood due to neurological, mechanical, or myogenic problems.

Concomitant esotropias may be primary (initial problem), secondary (due to vision loss), or consecutive (following overcorrection of exotropia); the vast majority are primary.

Treatment

Management depends on the origin and classification of the condition. The general course includes identifying and treating any underlying systemic condition, prescribing glasses and allowing time for adjustment, and using occlusion to treat amblyopia and encourage alternation.

Orthoptic exercises may restore binocularity, and prismatic correction can relieve double vision temporarily or permanently. In specific cases, especially in adults, botulinum toxin can be used as a permanent therapy or to prevent muscle contracture before surgery. Extra-ocular muscle surgery, such as strabismus surgery, can improve cosmesis and occasionally restore binocularity.