A common observation in neurological practice is
parkinsonism with concomitant
cognitive decline, an association that usually arises from various underlying degenerative or vascular conditions, most of which are untreatable. An elderly woman with no history of
psychiatric disease presented complaining of memory and
cognitive impairment serious enough to interfere with daily life activities over the preceding year. She soon developed a predominantly left-sided
tremor, rigidity and
bradykinesia. She had had only 2 years of formal education. Neuropsychological assessment showed poor performance on Wechsler memory scale sub-items, as well as constructional
apraxia,
dyscalculia, reasoning difficulties and gross information deficits. A 3-month trial course of
levodopa was followed by dramatic improvement in both parkinsonian symptoms and cognitive performance, including a 7-point gain in the Mini-Mental Status Examination score. At the same time, the Beck Depression Inventory score fell from 27 (baseline) to 18. Over the 10-year follow-up period the patient developed late
levodopa syndrome and a persistent but mild
dysthymic disorder, but never manifested
dementia as defined by DSM-III-R criteria. This patient's case illustrates three important principles. First, although
parkinsonism is known to be preceded by depressive episodes, particularly in a subgroup of younger patients, the symptoms of the elderly patient whose
Parkinson's disease is foreshadowed by depression can mimic depressive
pseudodementia, potentially leading to diagnostic
confusion. Second, impaired motivation and disturbances in cognitive function are different from
mood disorders, as the former involve the mesolimbic/mesocortical
dopamine system, explaining the beneficial effect of
levodopa on motivation and cognition in this patient even as mood was unaffected. Finally, depressive
pseudodementia in
Parkinson's disease does not necessarily herald the development of organic
dementia in the long term.