Heterophoria

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Heterophoria is an eye condition in which the eyes at rest position (when not performing binocular fusion) do not point in the same direction. This condition can be esophoria, where the eyes tend to cross inward in the absence of fusion; exophoria, in which they diverge; hyperphoria, in which one eye points up or down relative to the other; or cyclophoria, in which one eye is rotated differently around its line of sight from that of the other. Phorias are known as 'latent squint' because the tendency of the eyes to deviate is kept latent (hidden) by fusion. A person with two normal eyes has single vision (usually) because of the combined use of the sensory and motor systems. The motor system acts to point both eyes at the target of interest; any offset is detected visually, and the motor system corrects it. Heterophoria occurs only during dissociation of the left eye and right eye, when fusion of the eyes is absent. If one eye is covered (e.g., with a hand), the sensory information about the eye's position in the orbit is lost. Without this information, there is no stimulus to binocular fusion, and the eye will move to a position of "rest". The difference between this position and where it would be were the eye uncovered is the heterophoria. The opposite of heterophoria, where the eyes are straight when relaxed and not fusing, is called orthophoria. In contrast, fixation disparity is a very small deviation of the pointing directions of the eyes that accompanies binocular fusion. Heterophoria is usually asymptomatic. This is when it is said to be "compensated." When fusional reserve is used to compensate for heterophoria, it is known as compensating vergence. In severe cases, when the heterophoria is not overcome by fusional vergence, sign and symptoms...