What are binocular vision anomalies?

Binocular vision anomalies occur when there is a problem in the co-ordinated use of the eyes as a pair. Inevitably, the prevalence of binocular vision anomalies varies according to the precise criteria
that are used to define these conditions.A conservative estimate is that binocular vision anomalies affect 5% of patients consulting primary eyecare practitioners.


The classification of binocular vision anomalies starts with two fundamental distinctions, summarized in Figure . One distinction is comitant/incomitant and the other is strabismic/heterophoric.These two approaches to classification are complementary, not exclusive.
For example, comitant deviations may be strabismic or heterophoric and strabismic conditions may be comitant or incomitant.Incomitancy  is defined as a deviation that varies in different positions of gaze and that varies depending on which eye is fixing. Incomitant deviations affect about 0.5% of the population. A strabismus , also known as a heterotropia or squint, occurs when the visual axes are deviated: where the line of sight of one of the eyes does not fall on the object of regard. Strabismus affects around 2.5–4% of the population.A strabismus can be constant or intermittent, and can be unilateral or alternating.

Young patients develop sensory adaptations to a strabismus, typically harmonious anomalous retinal correspondence (HARC) or suppression . Most people do not have a strabismus and the eyes are kept in perfect, or very close to perfect, alignment. But when one eye is covered, or the two eyes are dissociated (prevented from viewing the same scene), most people develop a latent deviation (heterophoria).A heterophoria is a normal finding: it is only apparent when the eyes are dissociated and is not present under normal viewing conditions. Occasionally, a heterophoria may decompensate, when it can cause symptoms and in some cases might break down into a strabismus.As well as being classified as compensated or decompensated, a heterophoria can also be classified according to the testing distance.

Both strabismus and heterophoria can be classified according to the direction of the deviation: eso for when the visual axes turn inwards, exo for outwards, hyper for upwards, hypo for downwards, cyclo for cyclorotation. Heterophoria is sometimes described as a latent strabismus, but this term can be confusing since a heterophoria is a normal finding which is usually not acause for concern.