Abstract: The present invention provides methods for defining a disease or condition with a wide range of etiologies based on clustering of genes belong to the same biological pathway and observed frequencies or prevalence of various patient factors assessed in a given patient population, including motor and non-motor symptoms, neuropathology, age of onset, genetics, and involvement of peripheral autonomic system, including the enteric nervous system of the gastrointestinal (GI) system. Also disclosed herein are methods and kits for early diagnosis and treatment of subjects for multisystem Lewy body disease (MLBD), including Parkinson's disease, and/or one or more GI conditions that indicate a propensity for MLBD.