Is response to prophylactic lithium a familial trait?

Background: Selecting a drug according to the treatment response in a relative has been widely accepted advice in the management of mood disorders. However, this recommendation has not been adequately substantiated in the literature. We tested the hypothesis that response to long-term lithium treatment is a familial trait. Method: We compared response to long-term lithium treatment in bipolar relatives of bipolar lithium responders and bipolar controls. Twenty-four relatives with bipolar disorder (as determined using the Schedule for Affective Disorders and Schizophrenia-Lifetime version [SADS-L] and Research Diagnostic Criteria [RDC]) were identified in families of 106 patients with lithium-responsive bipolar disorder. A consecutive series of 40 lithium-treated patients in a bipolar clinic (meeting RDC and DSM-IV criteria for bipolar disorder) served as a comparison group. Lithium response was evaluated on a rating scale reflecting the quality and quantity of available data. Results: The prevalence of unequivocal response among the relatives was 67%, as compared with the response rate of 35% in the comparison group (X²= 6.04, df = 1, p = .014). Conclusion: This highly significant difference in response between relatives and the control group supports the view that the response to lithium prophylaxis clusters in families.