Depressive residual symptoms are associated with illness course characteristics in a sample of outpatients with bipolar disorder

Rates of 50–70% of residual symptoms referring to subsyndromal manifestations between episodes that do not meet the required criteria for episode definition were reported in bipolar disorder (BD). However, the specific role of these symptoms on the course of BD patients is poorly understood; thus, we aimed to investigate factors associated with depressive residual symptoms. Overall, 255 currently euthymic BD outpatients on maintenance treatment, including 95 (37.2%) males and 160 (62.8%) females, were consecutively recruited at the Section of Psychiatry, Department of Neuroscience, University of Genoa (Italy) and underwent detailed structured interviews, comprehensive clinical interviews, and clinical record reviews for assessment/collection of relevant information concerning the course of illness and clinical status including cross-referral of all available information. After categorizing subjects according to the presence/absence of residual symptoms, groups were compared along clinical variables and variables associated with residual symptoms were analyzed using multivariate analyses. Subjects with residual symptoms were less likely to report substance abuse (χ² ₍₂₎ = 11.937, p ≤ 0.005) and lifetime psychotic symptoms (χ² ₍₂₎ = 10.577, p = 0.005), and more likely to report higher illness episodes, longer duration of illness (t₂₅₃ = 67.282, p ≤ 0.001; t₂₅₃ = 10.755, p ≤ 0.001), and longer duration of current illness episode (t₂₅₃ = 7.707, p ≤ 0.001) than those without residual symptoms. After multivariate analyses, a significant positive contribution to residual symptoms was given only by duration of current illness episode (β = 0.003; p ≤ 0.05), and lifetime psychotic symptoms (β = 1.094; p ≤ 0.005). Clinicians have to pay attention to minimize residual symptoms that may significantly impact on the course of BD and achievement of full remission between episodes.