ABSTRACT
Nocturnal penile tumescence (NPT) was first described by Halverson in 1940, and it was Ohlmeyer who suggested in 1944 that abnormalities of this presumed reflex were important factors in erectile dysfunction (ED) (1,2). Normal NPT occurs three to five times per night, and 80% of these erections occur during REM sleep (3,4). The association between NPT and REM stage of sleep was first elucidated in the 1950s, and in 1965, it was suggested that the monitoring of nocturnal erections could aid in the assessment of ED (1,2,5-14). In 1970, Karacan suggested that NPT testing may be useful in distinguishing between organic and psychogenic causes of ED (11). He argued that NPT would be diminished or absent in males with ED due to organic causes such as neurological or vascular factors, since these mechanisms would be present during sleep in normal individuals. He further suggested that psychological factors that inhibit sexually induced erections would be absent during sleep. Nocturnal erections would therefore be present in men with psychogenic causes of ED. Some authors have asserted that NPT testing is the best method to distinguish between organic and psychogenic causes of ED, while others are more cautious and warn that NPT values used alone can be misleading (15,16).
