Parkinsonian motor characteristics in unipolar major depression

Clinical observation points to similarities between psychomotor retardation in major depression and bradykinesia in Parkinson's disease (PD). While common elements of neuropathology have been proposed to account for this, experimental investigations of this possible link have been few and inconclusive. The present study attempts to determine whether patients with depression display the characteristically Parkinsonian reliance on external cues; and if so, whether this is common to both melancholic and non-melancholic patients. Twenty three patients with unipolar major depression (11 melancholic and 12 non-melancholic) and 24 age-matched healthy controls performed a serial choice reaction time task known to be sensitive to Parkinsonian movement deficits. The melancholic patients showed a Parkinsonian pattern of impairment on the task, exhibiting a particular difficulty when initiating movements in the absence of external cues. This was largely alleviated when a moderate amount of external cueing was provided. At a high level of advance information, melancholic patients were again slow relative to controls. A base-line measure of bradykinesia and a derived measure of dependence upon external cues both correlated significantly with CORE (measure of psychomotor disturbance) ratings. The non-melancholic patients did not show any measurable motor impairment. This cue-dependent deficit may be due to an underlying basal ganglia dysfunction similar to that involved in PD, i.e. a failure of internal cueing. Difficulty with a high level of external cueing might reflect bradyphrenia or a prefrontal motor deficit of ability to plan multiple upcoming movements simultaneously. The results suggest that depression subtypes involve differing patterns of fronto-striatal impairment.