Do symptoms of bipolar disorder exhibit seasonal variation? A multisite prospective investigation.

OBJECTIVES

Evidence that symptoms of bipolar disorder (BD) vary seasonally is inconclusive. Here, a multisite prospective investigation of patients with BD was used to test the hypothesis that, on average, depressive symptoms peak in autumn/winter and hypo/manic symptoms peak in spring/summer. Secondary analyses explored gender and diagnosis [bipolar I disorder (BD-I) versus bipolar II disorder (BD-II)] effects on seasonality.

METHODS

A sample of 429 patients with BD (61.6% female; 56.2% BD-I) were recruited from 12 sites across Canada. Clinician-rated measures of manic [Young Mania Rating Scale (YMRS), n = 4,753 total observations] and depression symptoms [Montgomery-Åsberg Depression Rating Scale (MADRS), n = 4,691 observations] were taken at scheduled three-month visits as well as any unscheduled visits. At scheduled visits only, Hamilton Depression Rating Scale (Ham-D) assessments (n = 3,153 observations) were also made. Multi-level modeling (MLM) analyses were conducted separately for the three dependent variables and three definitions of Time: calendar month, nominal season, and harmonic analysis.

RESULTS

Primary analyses of the whole sample found that for manic symptoms (YMRS), neither calendar month nor nominal season were significant, and harmonic analyses found an unpredicted frequency two sinusoid, with peaks at 4th December and 4th June (p < 0.018). Secondary analyses found that this sinusoid approximately fit the YMRS data for females and those diagnosed with BD-II. For depression symptoms measured on the MADRS and Ham-D, no significant seasonal patterns were found in primary analyses of the whole sample. Secondary analyses found a significant increase in MADRS scores in November/December among females, but this pattern was not corroborated in nominal season or harmonic analyses.

CONCLUSIONS

No evidence of systematic seasonal variation in symptoms was found in the sample as a whole. Primary analyses found no evidence that hypo/manic symptoms peaked in the lighter months and depressive symptoms peaked in the darker months. The present findings align with broadly negative conclusions from three earlier prospective investigations, and provide the strongest evidence to date that seasonal changes do not in fact cause coordinated variation in BD symptoms.