An initial report of a new biological marker for bipolar disorder: P85 evoked brain potential.

Objectives: Progress toward understanding the neurobiological and genetic underpinnings of bipolar disorder has been limited by the scarcity of potential biological markers that predict its occurrence. A measure of the integrity of brain inhibitory function, sensory gating, measured using the amplitude of the evoked potential at 50 ms to the first of two paired clicks divided by the response to the second, has been characterized as a biological marker for schizophrenia. Currently, no such biological marker exists for bipolar disorder. The goal of this research was to determine how gating of an auditory brain potential at 85 ms (P85), not previously examined in sensory gating studies, differentiated control and patient groups.

Methods: P50 and P85 auditory evoked potentials were collected from individuals diagnosed with schizoaffective disorder (n = 45), paranoid schizophrenia (n = 66), and bipolar I disorder (n = 42) using DSM-IV criteria and the Structured Clinical Interview for DSM-IV; and from 56 healthy controls.

Results: The P85 gating ratio was significantly larger in the bipolar disorder group compared to each of the other groups (F(3,204) = 5.47, p = 0.001, and post-hoc tests). The P50 gating ratio was significantly larger for the schizoaffective group than for the control group (F(3,204) = 2.81, p = 0.040), but did not differ from the ratio for the schizophrenia, paranoid type (p = 0.08) and bipolar groups.

Conclusions: The previously unstudied P85 gating ratio may provide a new marker specific to bipolar disorder. The findings will promote further studies to investigate the unique contribution of this measure as an endophenotype.