Is "Parkinson's disease" one disease?

Consideration is given to how and why categories of ill health are divided into diseases. Aetiology is a fundamental criterion for the delineation of individual diseases. The same clinical and pathological picture may have many different causes; for example meningococcal meningitis and pneumococcal meningitis are distinct diseases that may display the same symptoms and signs. On the other hand, a single aetiology may lead to quite separate clinical and pathological phenomena; for example, neurosyphilis is a disorder that can present with general paresis or tabes dorsalis (or any combination of the two). In attempting to find a nosological placement for Parkinson's disease, we must take into account the extensive overlap with idiopathic dementia (Alzheimer's disease). Present evidence raises the possibility of several causes for Parkinson's disease, some of which may also be responsible for idiopathic dementia. A classification in accord with its position is desirable, and as a first step it would be helpful to replace "Parkinson's disease" with a term that is not saddled with implications of a single causal mechanism. "Idiopathic Parkinsonism" is suitable nomenclature for what is really a syndrome of unknown origin.