Stronger evidence for the role of stressful life events as proximal triggers of affective episodes in individuals with bipolar disorders comes from the more methodologically adequate prospective studies. Hall, Dunner, Zeller, and Fieve (1977) assessed 38 bipolar patients prospectively at monthly intervals for a total of 10 months. Although overall numbers of life events did not differ for patients who relapsed versus those who did not, hypomanic relapsers had greater numbers of employment-related events than did nonrelapsers. In another report from this study, Hall (1984) noted that a higher number of severe loss events, as well as work-related events, also were reported prior to manic relapse in this sample. Limitations of this study included the failure to control for medication or illness duration, as well as the lack of structured diagnostic interviews to determine relapse.
In a study of 62 bipolar patients followed for 2 years, with interviews designed to assess life events and mental state conducted every 3 months, Hunt, Bruce-Jones, and Silverstone (1992) revealed that 19% of 52 relapses were preceded by a severe event in the previous month, compared to a background rate of 5% of patients experiencing a severe event each month at other times. Manic and depressive relapses did not differ on the rate of prior life events. In contrast, using similar methods, McPherson, Herbison, and Romans (1993) found no difference in the number of moderately severe, independent events in the month preceding relapse as compared with control periods. The McPherson et al. study was limited by a high dropout rate and the absence of a required well period prior to study entrance. Johnson and Roberts (1995) reported that all studies requiring a well period or full recovery prior to study entrance have obtained a positive association between life stress and relapse.
In a prospective study of 61 bipolar outpatients followed over a 2-year period with systematic interviewing procedures to assess life events, symptoms, levels of medication maintenance, and observance of treatment regimen, Ellicott, Hammen, Gitlin, Brown, and Jamison (1990) obtained a significant association between life events and relapse of the disorder. Indeed, bipolar outpatients with high stress showed a four-and-one-half-fold greater relapse rate than those with lower stress and these findings were not accounted for by differences in levels of medication or adherence. Using similar methodology in a subsample of 52 bipolar outpatients, Hammen and Gitlin (1997) again found that patients with relapses during the 2-year follow-up period had more severe events and more total stress during the preceding 6 months, and more total stress during the preceding 3 months, than those with no episodes. In addition, inconsistent with Post's (1992) "stress sensitization" hypothesis that stressors play a larger role in precipi tating initial episodes than later episodes of mood disorder, Hammen and Gitlin reported that patients with more prior episodes were more likely to have episodes preceded by major life events and relapsed more quickly than patients with fewer prior episodes.
Johnson and Miller (1997) examined negative life events as a predictor of time to recovery from an episode of bipolar disorder. They studied 67 individuals recruited during hospitalization for bipolar disorder and conducted monthly structured interview assessments of stressful life events. Bipolar patients who experienced a severe, independent event during the index episode took three times as long to recover from the episode as those who did not experience a severe, independent event and this effect was not mediated by medication compliance.
Several investigators have considered biological mechanisms through which stressful life events may influence the onset and course of bipolar spectrum disorders. For example, some theorists (e.g., Ehlers, Frank, & Kupfer, 1988; Healy & Williams, 1988) have suggested that life events affect the course of mood disorders through their destabilizing effects on critical circadian rhythms. Consistent with this view, Malkoff-Schwartz et al. (1998) explained that bipolar patients in a manic episode had significantly more pre-onset life events characterized by social rhythm disruptions (e.g., change in the sleep-wake cycle) than did depressed bipolars.
In summary, although relatively few in number, the methodologically sound prospective studies suggest that the occurrence of stressful life events may contribute proximal risk to the onset of mood episodes in individuals with bipolar disorders. Given the more extensive literature on the role of stress as a precipitant of episodes of unipolar depression, it is not surprising that negative events may trigger bipolar depressive episodes. However, our review, as well as Johnson and Roberts' (1995) review, indicates that negative events may also contribute risk for manic/hypomanic episodes. Given that almost none of the studies on stress and bipolar disorder have investigated positive life events, future research on life events and bipolar disorder should examine whether positive events also play a role in the course of bipolar spectrum disorders. Such positive events as achievements and gains could activate bipolar individuals' engagement in goal striving, which in turn might lead to hypomanic/manic symptoms such as high activity and energy levels, racing thoughts, increased self-confidence, and risky behaviors (Harmon-Jones et al., 2002). In the next section, we review the evidence on the role of cognitive styles as distal vulnerabilities for bipolar spectrum disorders that increase the likelihood of depressive and manic/hypomanic episodes in response to stressful life events. In so doing, we address the applicability of the cognitive vulnerability-stress models of unipolar depression (Hopelessness and Beck's theories) to bipolar spectrum disorders.
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