Diagnosis
A provocative test for positional nystagmus may be performed. The patient is first seated on an examining table; then with his head turned to the side, he quickly assumes the supine position with his head dependent over the end of the table. After a latent period of several seconds, severe vertigo occurs; it is likely to last 15 to 20 sec and is accompanied by rotary nystagmus. If the left ear is affected, the nystagmus is clockwise when the head is turned to the left; if the right ear is affected, the nystagmus is counterclockwise. When the patient sits up, the response recurs, but the nystagmus is rotary in the opposite direction and is milder. The response fatigues, so that if the test is repeated immediately, the response is diminished.

Positional nystagmus may occur with end-organ or CNS lesions. The latency and fatigability of the response, the severe subjective sensation, the limited duration, and the direction of the rotary nystagmus distinguish benign paroxysmal positional vertigo from a CNS lesion. The positional nystagmus of CNS lesions lacks latency, fatigability, and severe subjective sensation and may continue for as long as the position is maintained. Nystagmus due to a CNS lesion may be vertical or change direction, and if rotary, it is likely to be perverted (ie, not in the anticipated direction).

This nystagmus usually showed poor fatigability with the maneuver repetition,
and its features remained unchanged with repeated observations. ...
www.laryngoscope.com/pt/re/laryngoscope/ fulltext.00005537-199706000-00016.htm



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Note added at 1 hr 25 mins (2005-10-27 05:56:06 GMT)
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Sorry should be "NYSTAGMUS".
It is still to erarly in the morning.