Parkinson Disease: Practice Essentials, Background, Anatomy Parkinson disease (PD) is one of the most common neurologic disorders, affecting approximately 1% of individuals older than 60 years and causing progressive disability that can be slowed, but not halted, by treatment. The 2 major neuropathologic findings in Parkinson disease are loss of pigmented dopaminergic neurons of the substantia nigra pars compacta and the presence of Lewy bodies and Lewy neurites. See the images below. Lewy bodies are intracytoplasmic eosinophilic incl Lewy bodies are intracytoplasmic eosinophilic inclusions, often with halos, that are easily seen in pigmented neurons, as shown in this histologic slide. They contain polymerized alpha-synuclein; therefore, Parkinson disease is a synucleinopathy. Signs and symptoms Initial clinical symptoms of Parkinson disease include the following: Tremor Subtle decrease in dexterity Decreased arm swing on the first-involved side Soft voice Decreased facial expression S...