Seasonal variation of manic and depressive symptoms in bipolar disorder

It is not clear whether a seasonal pattern exists in the psychopathology of bipolar disorder. The current literature is populated mainly with retrospective studies of admission data for individuals with discharge diagnoses of bipolar disorder. Mood episodes appear more likely to be exacerbated in the spring when analyzing hospital admissions as well as self-reported depression measures (1–3). The peak in admissions for mania has been found in the spring, summer, or both (4–6). Another retrospective study found a lack of seasonal pattern in hospital admissions for bipolar disorder (7). Depressive symptoms have more consistently shown peaks in the spring and autumn (8, 9). A study examining hospital admission data reported that bipolar depression shows seasonality which peaks in the months of June and July (4). In each of these studies, individuals with bipolar I and II disorder were not compared. This is significant since people with bipolar II disorder often exhibit a greater chronicity of illness than do those with bipolar I disorder (10); thus, it is plausible that the seasonality of bipolar I disorder and bipolar II disorder may differ and should be examined separately. As a result, studies examining seasonality between bipolar I and bipolar II disorder are of particular relevance. A retrospective study examining admission statistics for bipolar depression and mania found that participants with bipolar I depression were mainly hospitalized in summer and winter, whereas for bipolar II depression, most admissions for depression occurred in the spring and summer. Admission for mania peaked in spring and autumn (11). Another study that included a four-year retrospective component with prospective follow-up for two years demonstrated that relapse of mania did not show a seasonal pattern, but bipolar depression demonstrated a significant peak in autumn (12), while endogenous and neurotic depression peak in the spring (13). Manic and depressive relapses were measured with the use of two cohorts of bipolar I disorder participants selected based on a case-note search of admission to the psychiatric services at each locality from 1985–1987, with diagnosis confirmed after an initial assessment interview using the Schedule for Affective Disorders and Schizophrenia (SADS) (14). Though this is the only study that included prospective follow-up, limitations included a small sample size, with only 144 individuals fulfilling criteria and follow-up of two years (15). Retrospective studies examining seasonality of bipolar depression and mania have been limited to data on hospitalizations or retrospective report that may be subject to undue recall bias. As a result, another valuable approach to determining the seasonality of bipolar I and II disorder would be to examine prospective studies. In addition, existing studies are limited by sample size and duration of follow-up, and by reliance on admission statistics to determine a seasonal pattern. A spectrum of symptoms exists in mood disorders and most do not reach a threshold of severity wherein hospitalization would be required. Studies examining frequency of symptoms of bipolar I disorder versus bipolar II disorder have demonstrated that bipolar II disorder individuals spend a significantly higher percentage of weeks exhibiting depressive symptoms than do those with bipolar I disorder, with both disorders expressing more depressive symptoms than manic/hypomanic symptoms (10, 16). An examination of 1,000 patients from the Systematic Treatment Enhancement Program for Bipolar Disorder (STEP-BD) throughout the course of a year found that bipolar II disorder individuals were more ill year-round and exhibited a significantly greater monthly fluctuation prevalence rate of symptoms than bipolar I disorder participants (17). Subsyndromal symptoms are common and not all syndromal individuals require hospitalizations; thus, analyzing only admissions statistics may overlook less severe, but clinically significant psychopathology. Self-reported mood ratings capture these less severe, but clinically significant symptoms. Symptoms that are subsyndromal in duration or intensity are common in the longitudinal course of bipolar disorder (19, 20). One study of 360 patients with bipolar disorder, who recorded their mood daily, did not demonstrate a seasonal pattern for depressed and hypomanic/manic episodes (18). Relying solely on self-report has its limitations, such as reliance on insight into a manic episode (21). Self-reported mood studies in bipolar disorder have also been limited by duration of follow-up and sample size. We sought to assess seasonal variation in manic and depressive symptoms in bipolar disorder utilizing a long-term prospective cohort study: The Collaborative Depression Study (CDS). Our study includes prospective follow-up for at least 10 years, which is longer than any other prospective study duration. The objectives of our study were to determine if seasonal variation exists in bipolar disorder with examination of a seasonal variable in symptom burden measures, and timing of relapse for depressive as well as manic/hypomanic/mixed episodes. We additionally sought to identify differences in seasonality by bipolar subtype.