Nystagmus

Opticokinetic nystagmus is a useful test in evaluating the eye
movements of children. A drum or tape with stripes or figures
is slowly rotated or drawn before the child’s eyes in horizontal
and vertical directions. With fixation, the child should visually
track the object in the direction the tape is being drawn, with a
rapid, rhythmic movement (refixation) of the eyes in the
reverse direction to enable fixation on the next figure or stripe.
Absence of such a response may result from failure of fixation,
amaurosis, or disturbed saccadic eye movements.
The child who appears clinically blind because of a conversion
reaction usually exhibits a normal opticokinetic nystagmus
response. Children who manifest congenital nystagmus
and have an opticokinetic nystagmus response in the vertical
plane likely have adequate functional sight. Absence of opticokinetic
nystagmus in the presence of congenital nystagmus
heralds reduced visual acuity. If asymmetry of an opticokinetic
nystagmus response is evident, lateral lesions in the posterior
half of the cerebral hemisphere are likely present. The lesion
is on the side that manifests reduced or absent opticokinetic
nystagmus reactivity. The area of involvement is generally in
the posterotemporal, parietal, or occipital areas. Hemianopic
field defects may exist.
Spontaneous nystagmus (i.e., involuntary oscillatory movements
of the eye) may be horizontal, vertical, or rotary; a patient
can exhibit all three types. The movements may consist of a slow
and a fast phase, giving rise to the termjerk nystagmus.However,
the phases may be of equal duration and amplitude, appearing
pendular.
Nystagmus, especially vertical nystagmus, is most commonly
induced by medications (e.g., barbiturates, phenytoin, carbamazepine,
benzodiazepines). Such nystagmus often has a jerk
component and is usually most prominent in the direction of
gaze. Vertical nystagmus that is not associated with medications
indicates brainstem dysfunction. A few beats of horizontal nystagmus
with extreme lateral gaze are usually normal. Persistent
horizontal nystagmus indicates dysfunction of the cerebellum or
brainstem vestibular systemcomponents; the nystagmus is coarser
(i.e., the amplitude of movements are greater) when the direction
of gaze is toward the side of the lesion. A rare condition, seesaw
nystagmus, is characterized by disconjugate (alternating) movement
of the eyes, which move upward and downward in a seesaw
motion. This type of nystagmus accompanies lesions in the region
of the optic chiasm