- Nucleus praepositus hypoglossi (lateral gaze)
(leftward, rightward, and downward gaze)
(irritative lesion: left, supratentorial; right, pontine) (right supratentorial lesion or left pontine destructive lesion)
Nystagmus is involuntary rhythmic movement of the eyes consisting of slow movement in one direction and rapid return movement in the other. The slow component is caused by disturbances of the motor and stabilizing systems of the eye (p. 84) or because of ocular muscle paresis; the fast component represents the rapid return movement of pontine generators. Although the slow component is the actual pathological component of nystagmus, the direction of nystagmus is conventionally said to be that of its fast component, which is easier to detect. The intensity of nystagmus increases when the patient gazes in the direction of the fast component. Nystagmus can be further classified according to the type of movement as pendular, circular, or
¡3 torsional (rotatory).
C Examination. The examiner first observes the eyes on primary gaze, then during horizontal
"g and vertical pursuit (fixation of gaze on a slowly
'c moving object) and vergence. Nystagmus of
2 labyrinthine origin is observed best with Frenzel spectacles (preventing visual fixation and giving the examiner a magnified view of the eyes). The following features of nystagmus are assessed: positional-dependence, coordination (conjugate, dissociated), direction (horizontal, vertical, rotatory, retracting, pendular), amplitude (fine, medium, coarse), and frequency (slow, moderate, fast).
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