Asymmetry in parkinsonism, spreading pathogens and the nose

Parkinson's disease, as well as many other parkinsonisms, including most toxic, neurodegenerative and familial types are typically asymmetric. No explanation for this phenomenon exists. A summary of the frequency of asymmetry in a spectrum of parkinsonian disorders is provided. Evidence against asymmetry being the result of normal asymmetries of the substantia nigrais reviewed. Asymmetry either results from a greater susceptibility on one side or a spreading pathology entering or starting on one side of the CNS. With the increasing evidence for spreading pathologies (toxins, viruses, α-synuclein), knowledge of neuroanatomical connections, and literature implicating spreading pathogens from the enteric and olfactory nerves, potential explanations can be theorized and explored, including the possibility of a pathogen preferentially entering or originating in the olfactory bulb on one side, with subsequent involvement of the other side.