Differing illness course, rather than the illness itself, is the greatest determinant of the differential risk of suicide attempts between patients with bipolar disorder and those with major depressive disorder (MDD), study findings indicate.
Although patients with bipolar disorder had a higher prevalence of suicide attempts than those with MDD, the incidence during comparable illness phases did not differ between the groups, report Erkki Isometsä (University of Helsinki, Finland) and colleagues in Bipolar Disorders.
The higher prevalence among patients with bipolar disorder can be attributed to a lower age at onset, longer duration of illness, a higher number of illness episodes and thus a greater proportion of time spent in major depressive episodes (MDEs) and mixed illness phases, say the researchers.
This suggests that “[d]iminishing the relative time spent in high-risk phases appears crucial for prevention of suicide acts”, they write.
The findings arose from a study of pooled data from 176 patients with bipolar disorder and 249 patients with MDD who were participating in the Jorvi Bipolar Study and the Vantaa Depression Study, respectively.
Prior to study baseline, 51% of patients with bipolar disorder had attempted suicide, compared with 33% of those with MDD. During the first 18 months of follow-up, the respective suicide attempt rates were 20% and 10%.
The researchers found that the median length of preceding illness was over 7 years longer among patients with bipolar disorder compared with patients with MDD (11.6 vs 4.1 years), and their age of onset was 10 years younger (median 21 vs 31 years). Furthermore, patients with bipolar disorder spent more time in MDEs than those with MDD (33.5 vs 22.5%) but less time with subthreshold depression (16.6 vs 30.3%).
Rates of suicide attempts did not differ significantly among patients with bipolar I disorder, bipolar II disorder or MDD overall, but were highly dependent on mood phase.
Using euthymia as a reference, Isometsä and team found that the incidence of suicide attempts increased fivefold during subthreshold depression, 25-fold during MDEs and 65-fold during combined mixed episodes, regardless of illness type. They note, however, that the time spent in mixed phase (4.1% of total time) was relatively short and would therefore have a smaller influence on the total risk of a suicide attempt compared with MDEs.
Cox proportional hazards models showed that in addition to MDE and subthreshold depression, female gender, age below 40 years and poor social support were associated with a significantly increased risk of suicide attempts.
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