Cognitive deficits in patients with daytime sleepiness.

A chief complaint of subjects with daytime sleepiness is the disturbance of cognitive functions like concentration, learning and memory. Since sleepiness interferes with the regulation of vigilance, one may assume that a disturbance of this basic dynamic variable causes deficiencies in information processing which in turn reduce the capacity for learning and memory. In two studies the time course of vigilance was measured by means of the critical flicker fusion (CFF) test in patients with narcolepsy or with an obstructive sleep apnea syndrome (OSAS). The CFF test was applied at 15 min intervals. The total test duration was ten hours in the study with narcoleptic patients and three hours in the study with OSAS patients. The mean level of performance was similar in healthy subjects and those with narcolepsy, while the latter displayed a three- to four-fold increase in temporal variability. Such an increase in variability of performance was not seen in subjects with OSAS. These data suggest that clinically distinguishable groups of patients with daytime sleepiness differ also in the pattern of performance impairment.