Bipolar disorders are characterized by mood disturbances with alternating periods of depression and mania or hypomania. Bipolar disorder can often occur alongside other mental illnesses including substance abuse, eating disorders and conduct disorder. Anxiety disorders, such as panic disorder, post-traumatic stress disorder, and obsessive-compulsive disorder are the most frequent types of co-occuring conditions and a study of 44 treatment-resistant bipolar patients in Seattle compared those with and without anxiety disorders. The researchers found that 'on several measures, bipolar patients with comorbid anxiety disorders were more significantly ill than bipolar patients without comorbid anxiety disorders'. Those bipolar patients with anxiety disorders had an earlier onset of illness, were more depressed and had a lower level of functioning. They had a more frequent history of substance abuse and were more likely to have suicidal thoughts.
Lee, John H. and Dunner, David L. - The effect of anxiety disorder comorbidity on treatment resistant bipolar disorders Depression and Anxiety 2008, 25(2), 91-97