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It has long been recognized that aspects of bipolar disorder (BD) have a seasonal pattern (1); however, studies have focused nearly exclusively on hospital admissions for mania (2–8). A few retrospective chart reviews have examined bipolar depression or mood episodes and have found the prevalence of a seasonal subtype of BD in clinic samples to be 15% to 18% (9–11). To our knowledge, there have been no reports of the prevalence of the BD seasonal subtype in a community sample. Latitude has been examined for the effect on seasonality in recurrent major depressive disorder (12–16), but there are limited data with respect to the effect of latitude on BD. This study reports the prevalence of the seasonal subtype of BD in a community sample in the province of Ontario. To address the impact of latitude on the prevalence of BD, the study gave particular attention to latitude in the sampling technique. Patients and MethodsThis study used the telephone-administered Depression and Seasonality Interview, Ontario Version (DSI-O). Levitt and others provide a detailed description of this interview (17). For the purposes of sampling, the province was divided into 8 strata (1º latitude each) from 41.50ºN to 49.49ºN. An equal number of residential telephone numbers was randomly selected for each stratum. The selected residences were then randomly assigned for contact in each of the 4 seasons of the year. Households were eligible if at least 1 member of the household was aged 20 years or over and had been residing at the current abode (or within 150 km) for more than 6 months in each of the past 3 years. The person in the household with the next birthday who was aged 20 years or over was then identified as the “target respondent.” If the target respondent agreed to participate, we obtained verbal informed consent. Goldfarb Consultants, a market research company involved in developing the DSI-O and experienced in its use, conducted all interviews. The Institutional Research Ethics Board approved the procedures for this study. The DSI-O uses DSM-III-R criteria to define mood episodes and to diagnose BD. Seasonal mania was defined as occuring when subjects reported that 50% or more of their episodes of mania began and ended at the same time of the year each year. Seasonal bipolar depression was defined in an analogous manner. Subjects with BD who met the criteria for seasonal mania or seasonal bipolar depression were defined as having a seasonal subtype of BD.
ResultsSample Description Prevalence and Lifetime Course of BD
Seasonal BD Figure 1 displays the months of the year in which mood episodes began for the 14 subjects with a seasonal subtype of BD. Of note, the total number of months is greater than 14, because episodes may have been defined as occurring during a particular season rather than during a specific month. The small number of subjects with seasonal BD did not allow for meaningful comparisons between the frequency of onset of mood episodes at different months of the year. Effect of Latitude Figure 1 Month of the year in which seasonal mania or  DiscussionThis study examined the influence of season and latitude in a community sample of subjects with BD. This is the first study we know of that examines these factors among subjects who were not recruited at treatment centres, and as a result, may be more generalizable to community populations. Seasonal mania or seasonal bipolar depression was identified in 22.6% of the BD subjects. This is somewhat higher than previously reported prevalence rates of seasonal BD (15% to 18%) (9–11). One potential explanation is that our use of telephone interviewing may overestimate the prevalence of the seasonal subtype. However, because we sampled a nontreatment group of individuals, who are less affected by treatment or sampling bias, the estimated prevalence rates identified in this study may be more representative of the true community prevalence rates. The seasonal subtype was numerically more frequent among subjects with BD II (30%) vs BD I (15.6%). This result is consistent with the original findings of Rosenthal and others, who noted that most cases of seasonal depression are found in subjects with BD II (18). The fact that BD patients appear to be more likely to experience an onset of a mood episode during certain times of the year (Figure 1) raises an interesting methodological issue with respect to the timing of recruitment for BD treatment studies. This seasonal influence makes it less likely that subjects with seasonal BD who are recruited in August will maintain their response to treatment, compared with subjects recruited in April. One way to examine this potential bias would be for treatment studies to document the proportion of BD subjects with a seasonal pattern of illness and then report on rates of recruitment according to month. Latitude does not seem to be an important factor in the occurrence of BD or in the proportion of BD subjects with a seasonal subtype. These results support the indirect findings of the available literature examining BD in different locations, which has failed to identify a clear pattern with respect to the effect of latitude on seasonal variations in BD. The findings of this study must be considered in the context of several limitations. First, this study relied on telephone interviewing, which may be less accurate than face-to-face interviews. Notably, however, our group’s previous experience in comparing telephone administration of the DSI-O with face-to-face interviews using the Schedule for Affective Disorders and Schizophrenia Lifetime Version (SADS-LV (19) found a sensitivity of 83%, a specificity of 100%, a negative predictive value of 93%, and a positive predictive value of 100% (17). The advantage of using telephone interviews in this study is that it allowed us to sample subjects across a large geographic area. A second potential limitation is that the study was limited to the province of Ontario, which is entirely above 41.5ºN. We cannot rule out the possibility that studying a larger area, or an area below 41.5ºN or above 49.5ºN, would demonstrate evidence of an effect of latitude. A third limitation of this study is the relatively small number of subjects with BD (n = 62). This is relatively large, compared with other studies of seasonal BD, but it is small in terms of epidemiologic surveys. Thus, the reported prevalence rates of BD and the seasonal subtype of BD must be interpreted with appropriate caution. A final limitation of the study is that 2397 telephone contacts refused to participate in the study before hearing any details. This is a universal issue when conducting general population studies and necessitates caution when interpreting the results. Given the consistent evidence for a seasonal pattern of illness in some BD patients, determining the influence of season should be part of assessing any patient with BD. Future studies are needed to evaluate the influence of season on treatment response in BD. References1. Goodwin FK, Jamison KR. Manic-depressive illness. New York: Oxford University Press; 1990. 2. Symonds RL, Williams P. Seasonal variation in the incidence of mania. Br J Psychiatry 1976;129:45–8. 3. Eastwood MR, Stiasny S. Psychiatric disorder, hospital admission, and season. Arch Gen Psychiatry 1978;35:769–71. 4. Mulder RT, Cosgriff JP, Smith AM, Joyce PR. Seasonality of mania in New Zealand. Aust N Z J Psychiatry 1990;24:187–90. 5. Sayer HK, Marshall S, Mellsop GW. Mania and seasonality in the southern hemisphere. J Affect Disord 1991;23:151–6. 6. Partonen T, Lonnqvist J. Seasonal variation in bipolar disorder. Br J Psychiatry 1997;170:483–4. 7. Whitney DK, Sharma V, Kueneman K. Seasonality of manic depressive illness in Canada. J Affect Disord 1999;55:99–105. 8. Clarke M, Moran P, Keogh F, Morris M, Kinsella A, Larkin C, and others. Seasonal influences on admissions for affective disorder and schizophrenia in Ireland: a comparison of first and readmissions. Eur Psychiatry 1999;14:251–5. 9. Hunt N, Sayer H, Silverstone T. Season and manic relapse. Acta Psychiatr Scand 1992;85:123–6. 10. Faedda GL, Tondo L, Teicher MH, Baldessarini RJ, Gelbard HA, Floris GF. Seasonal mood disorders: patterns of seasonal recurrence in mania and depression. Arch Gen Psychiatry 1993;50:17–23. 11. Avasthi A, Sharma A, Gupta N, Kulhara P, Varma VK, Malhotra S, and others. Seasonality and affective disorders: a report from North India. J Affect Disord 2001;64:145–54. 12. Rosen LN, Targum SD, Terman M, Bryant MJ, Hoffman H, Kasper SF, and others. Prevalence of seasonal affective disorder at four latitudes. Psychiatry Res 1990;31:131–44. 13. Williams RJ, Schmidt GG. Frequency of seasonal affective disorder among individuals seeking treatment at a Northern Canadian Mental Health Center. Psychiatry Res 1993;46:41–5. 14. Ozaki N, Ono Y, Ito A, Rosenthal NE. Prevalence of seasonal difficulties in mood and behavior among Japanese civil servants. Am J Psychiatry 1995;152:1225–7. 15. Blazer DG, Kessler RC, Swartz MS. Epidemiology of recurrent major and minor depression with a seasonal pattern: the National Comorbidity Survey. Br J Psychiatry 1998;172:164–7. 16. Saarijarvi S, Lauerma H, Helenius H, Saarilehto S. Seasonal affective disorders among rural Finns and Lapps. Acta Psychiatr Scand 1999;99:95–101. 17. Levitt AJ, Boyle MH, Joffe RT, Baumal Z. Estimated prevalence of the seasonal subtype of major depression in a Canadian community sample. Can J Psychiatry 2000;45:650–4. 18. Rosenthal NE, Sack DA, Gillin C, Lewy AJ, Goodwin FK, Davenport Y, and others. Seasonal affective disorder: a description of the syndrome and preliminary findings with light therapy. Arch Gen Psychiatry 1984;41:72–80. 19. Endicott J, Spitzer RL. A diagnostic interview: the schedule for affective disorders and schizophrenia. Arch Gen Psychiatry 1978;35:837–44. Author(s)Manuscript received September 2002 and accepted October 2002. 1. Assistant Professor, Department of Psychiatry, University of Toronto, Toronto, Ontario; Deputy Head, Mood Disorders Program, Sunnybrook and Women’s College Health Sciences Centre, Toronto, Ontario. 2. Associate Professor, Department of Psychiatry, University of Toronto, Toronto, Ontario; Psychiatrist-in-Chief, Sunnybrook and Women’s College Health Sciences Centre, Toronto, Ontario. 3. Professor, Department of Psychiatry and Behavioural Neurosciences, McMaster University, Hamilton, Ontario. Address for correspondence: Dr A Schaffer, Sunnybrook and Women’s College Health Sciences Centre, 2075 Bayview Avenue, Room FG46, Toronto, ON M4N 3M5 e-mail: ayal.schaffer@swchsc.on.ca
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