The rapid cycling (RC) pattern of a mood disorder is characterized by at least four affective episodes (manic, hypomanic or major depressive) during the last year; different episodes must be demarcated by a switch to an episode of opposite polarity or by a period of remission of at least 2 months. RC is very rare in unipolar patients; its prevalence, however, in bipolar patients is 10–30% with the majority being women (70–90%). Patients with RC usually suffer from bipolar II disorder with onset with a depressive episode. Genetic studies have not convincingly shown that the condition is genetically determined. Major abnormalities of thyroid function have not been shown to be related to RC, but recent studies propose that latent subclinical hypothyroidism might play a role in the acceleration of cycles. Perturbations of the circadian biological and social rhythms might influence the expression of RC. No major effect of the menstrual cycle has been found. Despite the absence of firm empirical data, the possible contribution of the kindling phenomenon on the acceleration of cycles cannot be excluded. Finally, there is evidence that RC can be induced by the use of antidepressant drugs, especially for women.
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