Phasic motor activity reduction occurring with horizontal rapid eye movements during REM sleep is disturbed in infantile spasms

Single polysomnography was performed before treatment in 15 patients with infantile spasms (IS) (13 with a cryptogenic type and 4 with a symptomatic one). Their sleep components during rapid eve movement(REM) sleep wen compared with those in 22 age-matched controls, The tonic muscle atonia during REM sleep was observed in all IS patients as in controls. The amount of REM sleep in IS patients was significantly lower, while the incidences of gross movements, phasic chill muscle activity:, and bursts of horizontal rapid eye movements were identical with those in controls. The phasic inhibition index (PII), i.e., the rate of simultaneous occurrence of phasic chin muscle activity and bursts of horizontal rapid eye movements, was significantly higher in IS than in controls. The PII value was the only parameter that reflected our patients' prognosis among the obtained REM sleep parameters. We presume that the elevated PII in IS reflects the weakness of the phasic motor activity reduction occurring with horizontal rapid eye movements, and attribute this disturbance to a functional instability of the rostral pontine tegmentum. We propose that PII is a useful parameter for assessing the prognosis of IS. Considering the neutral basis for elevated PII in IS, this index is expected to provide a clue for explaining the pathophysiology of IS.