Parkinson's disease and depression. A critical re-evaluation

The possibility of an 'organically' based depression intrinsic to the pathophysiology of Parkinson's disease (PD) and comparable to endogenous depression (Major Depressive Episode) has been raised. It has also been argued that signs of depression observed in PD are merely the natural reaction of the patients to their progressive and inevitable physical limitations and loss of independent function. Because conventional depression rating scales are limited in scope, a psychometric investigation of depression in PD was pursued. Based on the known impairment of short-term memory (STM) in endogenous depression, which was confirmed in a group of psychiatric patients in the present study, measures of STM were also obtained in groups of depressed and nondepressed PD patients and in 15 normal control subjects. Regardless of depression severity, PD patients performed as well as control subjects and both these groups consistently obtained scores significantly better than those of the endogenously depressed patients. A relative weakness in the PD patients on order-dependent STM tests was further explored and interpreted as an indication of mild frontal lobe dysfunction. It was concluded that PD patients are frequently depressed when confronted with their behavioural limitations and that this reaction may be exacerbated by a form of emotional liability related to pathophysiological processes which may involve prefrontal cortical areas.