Co-occurrence of parkinson disease and multiple sclerosis - a critical note.

While multiple sclerosis (MS) is associated with various movement disorders, in particular tremor and ataxia, its combination with parkinsonism is rare and co-occurrence of MS and Parkinson disease (PD) has been reported in only few definite cases. Theories about this co-occurrence range from coincidental to causal, but the true prevalence, basic features and causal relations between the two entities have not been systemically evaluated. Although there are cases of causal relationship between parkinsonism and MS related to demyelinating lesions affecting the dopaminergic nigrostriatal pathway, in a limited number of cases, PD (some gene-mediated) and MS may coexist as two separate diseases in the same patients. The prevalence of MS in LRRK2 PD, while rare, supports an important role for immune function in both disorders, while the role of PD-related PINK is still open. Furthermore, several common genes such as BACE2, CD69, CLC, CPA3 and DEFAs may play important roles in MS and PD, while MS and PD share iron accumulation in substantia nigra, which may be due to protein-protein interaction networks related to metal homeostasis. In view of the various pathogenic possibilities, the causal relationship of concurring MS and PD deserves critical consideration.