Exotropia refers to outward deviation of an eye. Although exotropia commonly begins from shortly after birth to 4 years, it can appear at any age. Known causes of exotropia include a third cranial nerve palsy, exotropic Duane syndrome, Brown syndrome (in upgaze), and previous eye muscle surgery. Exotropia is also associated with craniosynostosis, multiple sclerosis, Parkinson’s disease, and less predictably myasthenia gravis, among other diseases.
Intermittent exotropia is the most common kind of exotropia, although there is no known cause. This condition does not typically resolve spontaneously. Individuals with intermittent exotropia often do not have amblyopia.
Signs of Exotropia
Noticeable outward deviation of one or the other eye is usually the primary sign. Initially, intermittent exotropia may only be seen when a child is tired, ill, or daydreaming. The deviation is more often noted when the child looks in the distance than at near. A child with exotropia may close, squint, or rub one eye (usually the deviating eye), particularly in bright, sunny environments. Adults may experience blurry vision, double vision, eye strain, or and/or headaches.
If a patient is myopic, glasses sometimes help control the outward turning of an eye because the lenses stimulate accommodation. Moreover, contact lenses provide more control of exotropia at near for the same magnitude of myopic correction in spectacles because a myopic eye needs to accommodate more on a near object when corrected with a contact lens than when corrected with spectacles.
Convergence insufficiency is a type of exotropia characterized by a deficiency of the eyes to converge on a near object, e.g. while reading. Instead of the eyes converging together on a near object, one of the eyes deviates outward. This sometimes causes the affected person to see double, lose his/her place while reading, and/or close/cover one eye while looking at near, in addition to blurry vision, tired eyes (asthenopia), and headaches. Convergence insufficiency can be intermittent or constant and may respond to eye muscle exercises. It is more commonly associated with Parkinson’s disease and opiate use.
Adult patients may consider prism for small angles of deviation. Prisms ground into reading glasses seem to be better tolerated than stick-on prisms.
Patching to eliminate a suppression scotoma (or ignored part of the visual field) may be tried although it is typically ineffective.
Eye Muscle Surgery
Eye muscle surgery to address outward turning of the eye(s) is generally recommended if one or more of the following criteria are present:
- The exotropia occurs more than 50% of the day.
- The frequency of the exotropia is increasing.
- Significant exotropia occurs when the individual intently views objects at near.
- The adult has trouble maintaining binocular vision (brain’s ability to use both eyes together as a single unit)/depth perception, or
- The patient is bothered by diplopia.
Alignment of the eyes promotes or restores stereoacuity.
Exotropia caused by a Third Nerve Palsy
Third nerve palsy refers to a weakness of the nerve that supplies impulses to four of the six extraocular muscles, to a muscle that elevates the eyelids, and to the pupil. This may be congenital or acquired following head trauma, brain tumor, stroke, or cerebral aneurysm. The affected eye is generally turned outward (exotropia) and downward (hypotropia). At times, there is an associated droopy eyelid (ptosis) or enlarged pupil.
Occasionally a congenital or acquired third nerve palsy can “regenerate” spontaneously over the course of six months. While we wait to see if the third nerve palsy resolves without intervention, the patient may occlude one lens in glasses or cover one eye to help alleviate diplopia. If the strabismus arising from a third nerve palsy remains stable but significant, eye muscle surgery can be performed to eliminate ocular misalignment and restore single vision, at least in primary gaze. A droopy upper lid can confound the patient’s ability to use the realigned eye. In that case, ptosis repair following strabismus surgery is then warranted.