Seborrheic dermatitis, increased sebum excretion, and Parkinson's disease: a survey of (im)possible links.

The concept of skin as a mirror of parkinsonism, dates back at the beginning of the last century. Since then, much evidence has been accumulated supporting a causal association between the neurological disturbance and changes detectable on areas of the integument with the richest sebaceous gland supply, namely seborrheic dermatitis and/or seborrhea. However, the many persisting sources of perplexity weighing on some general aspects of the skin condition itself (lack of standardized diagnostic criteria, high variability of estimates of prevalence, and controversies on etiology and pathomechanisms) must have to date hampered assessment of the real nature, and significance (if any) of the links observed. The three major pathogenical pathways so far conceived in this context will be critically reviewed on the basis of evidences provided in the literature.