Mild cognitive impairment in Parkinson's disease.

Parkinson's disease (PD) has initially been described as a clinical syndrome, although its exact definition has changed over the past centuries. The identification of the pathological changes added another level of complexity, with Lewy bodies, synuclein deposits and neuronal loss in the substantia nigra being used alternatively as criteria. A third level of complexity was added with the recognition of genetic mutations resulting in Parkinsonism, sometimes with and sometimes without Lewy bodies or synuclein deposition. Lastly, frequent additional important pre-motor manifestations, particularly depression, anosmia and sleep-associated phenomena have been described. These different points of view on the definition of PD have important implications on the study of the etiology and even the therapy of PD. Cognitive impairment is also an important feature of PD, while the spectrum of deficits ranges from none to severe dementia. The no-man land in-between normal cognition and dementia has been termed mild cognitive impairment in PD. At present, this term lacks heuristic value or clinical utility, and remains a target for scientific research.