Treatment of Seasonal Affective Disorder: Unipolar Versus Bipolar Differences Chang-Ho Sohn, MD and Raymond W. Lam, MD, FRCPC* Address * D epar tment of Psychiatr y, U niversity of British C olumbia, 2255 W esbrook Mall, Vancouver, BC , Canada V6T 2A1. E-mail: r.lam@ ubc.ca Current Psychiatry Reports 2004, 6: 478–485 Current Science Inc. ISSN 1523-3812 Copyright © 2004 by Current Science Inc. Introduction All living organisms are influenced by the seasons and it is not surprising that there are seasonal changes in mood and behavior in humans. At the extreme end of seasonality is seasonal affective disorder (SAD), a sub- type of mood disorder characterized by regularly occur- ring depressive episodes, usually in the fall and winter, with full remission of symptoms in the spring and sum- mer. In the Diagnostic and Statistical Manual IV (DSM- IV), SAD is defined as “seasonal pattern specifier” that can be applied to major depressive disorder or to bipo- lar disorder. Although the diagnostic criteria for SAD focus on the seasonal pattern (onset and offset) of depressive episodes, many patients with SAD experience atypical vegetative symptoms such as increased sleep and appetite, carbohydrate craving, and weight gain. The other prominent feature of SAD is the good response to bright light therapy. According to DSM-III R or DSM-IV diagnostic criteria, the prevalence of SAD has been estimated as 0.5% to 3% in North American studies [1,2]. In longitudinal follow-up studies, the diagnosis of SAD shows similar stability as that of other depressive subtypes, with approximately 30% of patients with continuing with a recurrent seasonal depres- sive pattern, approximately 20% in clinical remission, and the remaining 50% experiencing more complex depressive patterns [3,4]. This paper reviews the pathophysiology and biological treatment of SAD, focusing on pharmacotherapy and light therapy, with specific clinical considerations when treating patients with bipolar disorder with a seasonal pattern (bipolar SAD). Patho physio lo gy o f Seaso nal Affective Disorder The pathogenesis and pathophysiology of SAD is uncer- tain, although there are several major hypotheses (for review, see [5•]). Initially, there was a focus on chronobi- ological theories that considered symptoms of SAD to be analogous to seasonal behavioral patterns in animals that were dependent on the light-dark cycle (photoperiod). In addition, the effects of bright light are known to affect cir- cadian rhythms in humans through the circadian pace- maker located in the suprachiasmatic nucleus of the hypothalamus. For example, the photoperiod hypothesis of SAD attributed its cause to the shorter winter photope- riod [6]. Other researchers proposed that SAD resulted from phase delayed internal circadian rhythms relative to other rhythms like the sleep-wake cycle [7,8]. Properly timed bright light would correct these phase-delayed cir- cadian rhythms [9]. However, other studies report con- flicting findings that are inconsistent with chronobiological theories [10]. There also is considerable evidence supporting major roles for monoamine neurotransmitter systems in the pathophysiology of SAD. In particular, there are consis- tent findings of serotonergic dysfunction in SAD. Given the role of hypothalamic serotonin in satiety and feeding and sleep regulation, serotonergic disturbances may partly explain the carbohydrate craving, weight gain, and Evidence-based treatments for seasonal affective disorder (SAD ) include light therapy and pharmacotherapy. W e briefly review the diagnosis and treatment of SAD, focusing on clinical and treatment differences between patients with unipolar and bipolar illness. Special considerations for the management of SAD in patients with bipolar disorder are discussed, including the need to monitor for emergence of manic and hypomanic mood switches, to use mood stabiliz- ers in patients with bipolar I disorder, and to be aware of potential interactions between bright light and medications used in treating bipolar disorder. Chronobiological treat- ments such as bright light therapy may be combined with pharmacotherapy to enhance therapeutic effects, reduce adverse side effects, and optimize treatment in patients with seasonal and nonseasonal bipolar disorder.