covariation

Covariation is the ability to maintain harmonious anomalous retinal correspondence despite changes in the objective angle of strabismus.¹ As the objective angle changes, the angle of anomaly changes with it.²

In harmonious anomalous correspondence the angle of anomaly equals the objective angle, the subjective angle is zero, and the sensory adaptation is complete.³ Covariation keeps that match as the deviation changes.¹ In intermittent strabismus the patient has normal retinal correspondence with the visual axes straight and harmonious anomalous correspondence with the eyes deviated. The change between the two can be sudden or gradual.² Covariation is likely to place extra neural demands on the visual system, so constant strabismus is more likely than intermittent or variable strabismus to develop harmonious anomalous correspondence.²

During treatment of the motor deviation, a patient with harmonious anomalous correspondence may show covariation without changing vergence.⁴ The practitioner should confirm normal correspondence with the cover test and watch for appropriate vergence movements.⁴