nystagmus is a disorder of eye movements, when they show involuntary, rhythmic oscillations. The name is from the Greek, depicting the slow nodding of drowsiness accompanied by irregular, quick raising of the chin. Clinically, most nystagmus is horizontal in direction, but vertical and torsional forms can occur. It usually presents a diagnostic challenge and special tests are used to induce nystagmus and elucidate the neurological cause. Two major groups are recognized: jerk nystagmus, with oscillations that are faster in one direction than in the other, creating a jerky rhythm; and pendular nystagmus, with oscillations that are roughly equal in speed to either side.

Jerk nystagmus is named according to the direction of the fast phase, although the slower, return movement that regains and holds ocular fixation is more important functionally. Such movements are easily seen in someone looking out of a moving train who is trying to count the railway sleepers in the adjacent track. The fast phase is in the direction of travel of the train. Such opto-kinetic nystagmus (OKN) can be demonstrated by rotating a cylindrical drum painted with black vertical stripes in front of the subject: the eyes will move in the direction of drum rotation, followed by a quick return to fixate on the next moving stripe. The urge to follow these movements is so powerful that OKN can even be used to prove vision in someone claiming to be blind.

Jerk nystagmus can also result from stimulation of the semicircular canals of the vestibular system. There are two groups of three canals that lie in three planes at right angles to each other in either side of the skull. Stimulation of these canals by head movement causes ocular movements that maintain the eyes' positions in space and so stabilize the field of view. If the head movement causes the eyes to reach the limit of comfortable sideward gaze the eyes make a fast, compensatory movement to the central position. Careful testing with OKN and vestibular-induced nystagmus can be used to pinpoint the site of neurological defects in some disease conditions.

Pendular nystagmus is found with loss of central, detail vision, such as occurs with bilateral macular lesions present from birth in albinism, aniridia (absence of the iris), or total colour blindness. There are rapid, pendular eye movements in miners' nystagmus' and the condition was attributed to defective illumination in mines. This occupational nystagmus has now been effectively eliminated by adequate lighting underground.

Congenital nystagmus can occur without other defects. The nystagmus appears pendular in straight-ahead gaze and becomes jerky on side gaze. Although visual acuity in the distance is always reduced, and usually to levels below the legal requirements for driving, reading can be surprisingly good, provided that the patient is allowed to hold the book in the preferred position. This may be closer than normal and with the head turned to one side. Parents of a child with congenital nystagmus may gain some comfort from the descriptive term ‘dancing eyes’ and from the knowledge that, with understanding teachers, education at a normal school followed by university is achievable.

Peter Fells

See also eyes; eye movements; vestibular system.