Heterophoria

Signs and symptoms

When fusional vergence can no longer compensate, heterophoria becomes manifest. Vertical heterophoria can present as seemingly unrelated issues such as chronic headaches, dizziness, and reading difficulties, even with 20/20 vision corrected. Eye strain from correcting misalignment produces symptoms that may mimic chronic fatigue or motion sickness, complicating diagnosis.

Diagnosis

The cross-cover test, or alternating cover test, is used to detect heterophoria. In this test, one eye is covered, then the cover is moved immediately to the other eye; the uncovered eye is seen moving from a deviated point. Unlike heterotropia, where a correcting movement appears with a simple cover test, heterophoria requires the cross-cover test to break fusion and reveal the latent deviation.

Quantitative assessment can be performed with the Lancaster red-green test. The Maddox rod test and double Maddox rod test, developed by Ernest Maddox, are especially sensitive to eye deviations and must be conducted in low light with a dim point source, as even minimal light stimulates fusion. These tests faced criticism during WW1 for inaccuracy, but this was later attributed to excessive room light and an overly bright light source.

Did You Know?

• Heterophoria may lead to squint, also known as strabismus.