Subthreshold depressions: clinical and polysomnographic validation of dysthymic, residual and masked forms.

We summarize clinical and polysomnographic findings in support of the existence of a broad and prevalent spectrum of less than syndromal or subthreshold depressive conditions that constitute subeffective disorders. Many of these conditions were previously subsumed under such rubrics as 'neurotic,' 'characterological,' and 'existential' depressions. Prospective follow-up studies of neurotic depressions (defined by a predominance of the psychological features of, in most instances, less than syndromal depression) have demonstrated their transformation into moderate to melancholic or psychotic depressive, and even bipolar, disorders. Many characterological depressives (outpatients with early insidious onset and fluctuating chronicity of subthreshold manifestations falling short of full syndromal depression), were shown to have shortened REM latency, increased REM%, redistribution of REM to the first part of the night, classic diurnality, high rates of family history for mood disorders, positive response to antidepressants and sleep deprivation, and prospective follow-up course leading to major affective episodes. Shortened REM latency and related sleep neurophysiological disturbances have also been reported to characterize so-called 'borderline' personality disorder even when examined in the absence of concomitant major depression. Finally, among primary care referrals to a sleep disorders center, short REM latency was found in a large number of patients without subjective mood change but with somatic manifestations of depression (meeting Probable Feighner Depression and/or lesser subacute manifestations). Rather than being incidental, the REM disturbances in the foregoing studies appear consistently on consecutive nights of polysomnography in the subthreshold affective group; this was not the case for patients with non-affective personality and anxiety disorders. The findings overall tend to support a common neurophysiological substrate for subthreshold and melancholic depressions and, interpreted in the context of clinical observations, family history and follow-up course, uphold the validity of dysthymic, intermittent and subsyndromal depressions.