DESCRIPTION: Involuntary, rhythmical, repeated oscillations of one or both eyes, in any or all fields of gaze; may be pendular (with undulating movements of equal speed, amplitude, and duration, in each direction) or jerky (with slower movements in one direction, followed by a faster return to the original position). Movements may be horizontal, vertical, oblique, rotary, circular, or any combination of these. Generally, the faster the rate, the smaller the amplitude (and vice versa). The defect is classified according to the position of the eyes when it occurs. Grade I occurs only when the eyes are directed toward the fast component; grade II occurs when the eyes are also in their primary position; grade III occurs even when the eyes are directed toward the slow component. The cause of nystagmus is unknown. Reduced acuity is caused by the inability to maintain steady fixation. Head-tilting may decrease the nystagmus and is usually involuntary (toward the fast component in jerky nystagmus, or in such a position to minimize pendular nystagmus). Head nodding often accompanies congenital nystagmus. Dizziness or vertigo may be experienced if oscillopsia (illusory movements of objects) occurs. Nystagmus may be induced with an optokinetic drum or through the stimulation of the semicircular canals. Congenital nystagmus of the pendular type usually accompanies congenital visual impairment (e.g., corneal opacity, cataract, albinism, aniridia, optic atrophy, chorioretinitis). Nystagmus may also accompany a number of neurological disorders, and may be a reaction to certain drugs (including barbiturates).
TREATMENT: There is no known treatment, however, certain types of jerky nystagmus (commonly Grade I types) show spontaneous improvement in childhood (up to age 10). This type may also be amenable to muscle surgery (essentially, a repositioning of muscles to take advantage of the point of least nystagmus, or position of relative rest).
IMPLICATIONS: With the exception of brief experiences of oscillopsia, most individuals with nystagmus perceive objects as being stationary. It is believed that the brain is responsible for the perceptual adjustment.
Educationally, children with nystagmus (who may tend to lose their place in beginning reading instruction) may be helped through the use of a typoscope (card with a rectangular hole, to view one word or line at a time) or an underliner (card or strip of paper to "underline" the line being read). As children with nystagmus mature, they seem to need these support devices less often.
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