Is there a link between atypical and early-onset "unipolar" depression and bipolar II disorder?

The aim of the present study was to determine whether there is a link between "unipolar" depression with atypical features and early onset, and bipolar II disorder, using atypical features and early onset as markers of bipolarity. A total of 158 consecutive unipolar and 234 bipolar II major depressive episode (MDE) outpatients were interviewed using the Structured Clinical Interview for DSM-IV (SCID). Patients were divided into those with and without atypical features, and into those with and without early onset. Comparisons were made on variables reported to distinguish bipolar from unipolar: age of onset, recurrences, atypical features, depressive mixed state (MDE plus three or more concurrent hypomanic symptoms [DMX3]), and bipolar II family history. Compared to bipolar II patients, patients with atypical unipolar were not significantly different regarding age of onset, DMX3, recurrences, and bipolar II family history. Compared to non-atypical unipolar patients, atypical unipolar patients had a significantly different age of onset. Nonatypical unipolar patients, versus bipolar II patients, were significantly different regarding age of onset, recurrences, DMX3, and bipolar II family history. Early onset unipolar, versus bipolar II, were not significantly different regarding atypical features, recurrences, DMX3, and bipolar II family history. Later onset unipolar patients, versus bipolar II patients, were significantly different regarding atypical features, recurrences, DMX3, and bipolar II family history. These results support a link of atypical and early-onset "unipolar" depression with bipolar II disorder, and support Pages and Dunner's suggestion to combine bipolar II and recurrent unipolar into a single group.