General Medical Information

Motility

Jean Edwards Holt

Six muscles surrounding each eye are responsible for ocular motility. Several terms are used to describe various eye movements. The movement of one eye from one position to another is called duction. The simultaneous movement of both eyes from the primary, straight-ahead position to a secondary position (up, down, right, left) is called version. Vergence is the term applied to simultaneous rotation of both eyes inward (convergence) or outward (divergence). Evaluation of the extraocular muscle function is begun with general inspection to find any gross deviation of either eye (heterotropia). The patient is asked to look up, down, right, and left to reveal whether the deviation is the same in all fields of gaze (concomitant) or varies (nonconcomitant and usually neurologically significant). During these gaze movements, involuntary eye jerks, called nystagmus, also can be detected.

The flashlight used to evaluate pupillary reflexes also can be used to assess the corneal light reflex. The light should be symmetrically reflected in each pupil. If there is deviation, the degree of abnormality can be estimated by the asymmetry of the light reflex.
The cover test is used to evaluate motility. The patient is instructed to fix on an object. If both eyes appear straight (orthotropia), cover either one. If under cover the eye deviates, phoria, or latent deviation that becomes evident only when vision is interrupted, has been elicited. Usually the eye resumes fixation when uncovered. If one eye is obviously deviated, the straight-ahead eye is covered. If the deviated eye rapidly moves to resume fixation, it most likely has good visual potential. The deviation can be eso (inward), exo (outward), hyper (upward), or hypo (downward).