Strabismus / ocular misalignment
Exotropia is a type of eye misalignment, where one eye deviates outward. The deviation may be constant or intermittent, and the deviating eye may always be one eye or may alternate between the two eyes.
A high percentage of normal people, display a small exophoria on clinical examination, but is within normal limits and of no concern. Newborn infants often have transient exodeviations that resolve by 2-4 months of age.
This is an exodeviation (often constant) with an onset in the first six months of life which is nonresolving. There is an increased incidence with cerebral palsy and other neurologic disorders, craniofacial disorders, and ocular albinism.
A blind or poorly seeing eye may drift outward. Infants or young children with a blind or poorly seeing eye usually develop esotropia (cross-eyes), but in children older than 2-4 years of age and adults, the eye will typically become exotropic.
This is the most common type. Onset is in childhood. There are 3 subtypes which will be discussed later in this article.
This type is often first noted in older children and teenagers F.Consecutive exotropia- a person who was formerly esotropic becomes exotropic. In some cases this occurs from a surgical overcorrection of the esotropia.
The etiology of exotropia is unknown, but on a rudimentary level there is impaired ability to maintain fusion and alignment. In the case of sensory exotropia, the poor acuity of one eye is the cause of the inability to maintain alignment.
Neurologic disorders, prematurity, and a family history of strabismus, all increase the risk of having exotropia
Intermittent exotropia, the most common type of exotropia, has a usual onset between infancy and 7 years of age. There is no specific refractive error associated with exotropia. Although there is no specific racial predilection, Asians with strabismus are more often exotropic when compared to other racial groups.
Symptoms may be mild or severe. If suppression of the deviating eye occurs, the patient can have diminished binocular vision and stereopsis. Those with later onset and milder frequency deviations can experience diplopia. Asthenopia can also occur with reading.
A newborn with a clinical exodeviation resolving by 2-4 months of age. Many normal adults will display a small exophoria on alternate-cover testing but have no symptoms.
Is an exotropia with an onset during the first 6 months of life which is usually constant and does not resolve spontaneously.
Usually is an exotropia which develops in an older child or adult with one poorly seeing eye.
Is an intermittent deviation which is more likely to be manifest when fatigued, ill, or in a lowered mental state (ex. ETOH use). It is more commonly manifest with distance fixation and also in bright sunlight. Types of intermittent exotropia:
- Basic- distance and near deviations are within 10 prism diopters (units of measurement) of each other. These patients have normal convergence.
- Pseudo-divergence excess- a larger exotropia is present for distance, but this difference is extinguished after 30-60 minutes of monocular occlusion. These patients have increased tonic fusional convergence.
- True divergence excess- a larger exotropia is present for distance, even after 30-60 minutes of monocular occlusion. More than ½ of these patients have a high AC/A ratio and are prone to postoperative overcorrections if operated on for their full distance deviation.
Deviation is worse at near, and the patient has an abnormal near point of convergence. f.Consecutive exotropia- the patient has a history of esotropia treated with glasses or surgery, and subsequently converts to displaying an exodeviation.
Pertinent Ophthalmological Examination Features for a person with exotropia: -Visual Acuity -Binocular function and stereopsis -Motility evaluation -Strabismus measurements (near, distance, extreme distance, and cardinal positions) -Measures of fusional amplitudes -Cycloplegic refraction -In some cases- strabismus measurements after prolonged monocular occlusion and with +3.00 near add test -Evaluation of anterior and posterior ocular structures
Exotropia is not a disease entity by itself, it is a motility disturbance. Other conditions can display exotropia: Duane’s syndrome 3rd nerve palsy Slipped or lost medial rectus muscle Internuclear ophthalmoplegia Orbital fibrosis
Effectiveness of these therapies varies and in some cases they are only temporizing. Nonsurgical treatments include: patching, over-minused spectacles, and convergence exercises (for convergence insufficiency). Prism glasses can be used to relieve diplopic complaints or asthenopia. Also some cases of exotropia may be improved by treating an underlying visual disturbance (ex. Cataract, refractive error, amblyopia)
Performed to preserve or restore binocular function. Also may be performed for relief of diplopia and/or cosmesis. In general, long-term surgical success is better if the exotropia is intermittent instead of constant and the patient has better binocular function at the time of surgery. There is controversy as to the optimal time for surgical intervention. Surgery consists of operating on the extraocular muscles.
AAPOS website- frequently asked questions- topic:exotropia
1.Wright KW, Spiegel PH. Pediatric Ophthalmology and Strabismus- The Requisites in Ophthalmology.1st ed.1999. Pp 246-252. Mosby.
2.Freeman RS, Isenberg SJ. The use of part-time occlusion for early onset unilateral exotropia. J Pediatr Ophthalmol Strabismus 26:94. 1989.
3.Kushner BJ. Exotropic deviations: a functional classification and approach to treatment. Am Orthoptic J 38:81-93. 1988.
4.Wright KW, De Juan E. Patch test with and without +3.00 near add. Pediatric ophthalmology & Strabismus. 1995. Mosby.
5.Wright KW. Exotropia. Color Atlas of Strabismus Surgery- Strategies and Techniques.3rd ed.2997. Pp 42-51. Springer.