To investigate the clinical features of transient cerebral hypoxia, sy
ncope was induced in 56 of 59 healthy volunteers through a sequence of
hyperventilation, orthostasis, and Valsalva maneuver. All events were
monitored on video by two cameras. Complete syncope with falling and
loss of consciousness was observed in 42 subjects, lasting 12.1 +/- 4.
4 seconds. Myoclonic activity occurred in 38 of these 42 episodes (90%
). The predominant movement pattern consisted of multifocal arrhythmic
jerks both in proximal and distal muscles. Superposition of generaliz
ed myoclonus was common. Additional movements such as head turns, oral
automatisms, and righting movements occurred in 79%. Eyes remained op
en throughout syncope in most subjects and initial upward deviation wa
s common. Sixty percent reported visual and auditory hallucinations. T
hirteen subjects had incomplete syncope with falls but partially prese
rved consciousness. These episodes were shorter and usually not accomp
anied by myoclonus and hallucinations. Transient amnesia and unrespons
iveness without falling occurred in 1 subject.