Bipolar disorder might include a distinct subphenotype characterized by externalizing symptoms and specific genetic variations, report Shanker Swaminathan, Ph.D., and his colleagues.
A study of 2,505 bipolar I disorder patients categorized participants into two groups: externalizing (those with comorbid externalizing disorders, plus a subgroup of patients with externalizing symptoms before 15 years of age) and nonexternalizing, wrote Dr. Swaminathan, formerly with the department of medical and molecular genetics at Indiana University, Indianapolis, and now with Baylor College of Medicine, Houston.
wildpixel/Thinkstock.com
Patients in the externalizing group showed significant differences in clinical features from the nonexternalizing group in multiple areas, including earlier age of onset of bipolar disorder, depression, and mania (P less than .001).
Additionally, those in the externalizing group had a higher frequency of self-harm incidents (P less than .001) and suicide attempts (P less than .008). These patients also were more likely to report a history of rapid switching and cycling (P less than .001), the authors reported.
Genetic analysis found numerous “nominally associated” SNPs, though none reached genome-wide significance, Dr. Swaminathan and his colleagues reported.
The findings “suggest the presence of an externalizing disorder subphenotype within [bipolar I disorder] warranting further investigation,” they concluded.
Read the full study here: J Affect Disord. 2015 Jun;178:206-14.