Heterophoria refers to a muscular imbalance between the two eyes that leads to misalignment of the visual axes only under certain conditions (see below). This is in contrast to orthophoria, muscular balance with parallel visual axes. Heterophoria is typified by initially parallel visual axes and full binocular vision. The following forms are distinguished analogously to manifest strabismus:
❖ Esophoria: latent inward deviation of the visual axis.
❖ Exophoria: latent outward deviation of the visual axis.
❖ Hyperphoria: latent upward deviation of one eye.
❖ Hypophoria: latent downward deviation of one eye.
❖ Cyclophoria: latent rotation of one eye around its visual axis.
Epidemiology: This disorder occurs in 70-80% of the population. The incidence increases with age.
Etiology and symptoms: Heterophoria does not manifest itself as long as image fusion is unimpaired. Where fusion is impaired as a result of alcohol consumption, stress, fatigue, concussion, or emotional distress, the muscular imbalance can cause intermittent or occasionally permanent strabismus. This is then typically associated with symptoms such as headache, blurred vision, diplopia, and easily fatigued eyes.
Diagnostic considerations: Heterophoria is diagnosed by the uncover test.
This test simulates the special conditions under which heterophoria becomes manifest (decreased image fusion such as can occur due to extreme fatigue or consumption of alcohol) and eliminates the impetus to fuse images. In contrast to the cover test, the uncover test focuses on the response of the previously covered eye immediately after being uncovered. Once uncovered, the eye makes a visible adjustment to permit fusion and recover binocular vision.
Treatment: Heterophoria requires treatment only in symptomatic cases. Convergence deficiencies can be improved by orthoptic exercises. The patient fixates a small object at eye level, which is slowly moved to a point very close to the eyes. The object may not appear as a double image. Prism eyeglasses to compensate for a latent angle of deviation help only temporarily and are controversial because they occasionally result in an increase in heterophoria. Strabismus surgery is indicated only when heterophoria deteriorates into clinically manifest strabismus.
Fig. 17.12 Esotropia of the left eye (arrow) is only simulated by a broad dorsum of the nose. The corneal reflexes demonstrate parallel visual axes.
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