A review of the gastrointestinal, olfactory, and skin abnormalities in patients with Parkinson's disease.

Parkinson's disease (PD) is a complex neurodegenerative motor disorder caused by the loss of dopaminergic neurons in the substantia nigra pars compacta. The substantia nigra is neither the first nor the only brain region affected by PD. Recent and old studies have shown that PD does not only affect the CNS; in fact, autonomic innervation in the GIT, skin, and olfactory system was found to be affected by α-synuclein pathology outside the CNS, affecting patients' quality of life. In the gastrointestinal system, dysphagia, constipation, and bacterial overgrowth in the small intestine are common in patients with PD. In addition, several skin conditions were reported in PD, including seborrheic dermatitis, rosacea, melanoma, and others. Finally, olfactory system dysfunction, such as reduced touch sensation and smell, was associated with motor abnormalities. Further high-quality studies are needed to develop reliable tests that could help in the early diagnosis of PD.