Genetic, Lipid, Fungal Microbiome and Neuroinflammatory Links Between Seborrheic Dermatitis and Parkinson's Disease: A Narrative Review.

Background: Parkinson's disease (PD) is a progressive neurodegenerative disorder with a well- documented prodromal phase during which non-motor symptoms appear years before motor onset. Seborrheic dermatitis (SD) is a chronic inflammatory skin disease that is significantly more prevalent in PD patients than in the general population. The biological mechanisms underlying this association have not been integrated into a single framework. Objectives: This narrative review aims to synthesise the available evidence on the molecular, genetic, and pathophysiological mechanisms linking SD and PD. Methods: A literature search was conducted across PubMed/MEDLINE, Scopus and ScienceDirect, supplemented by manual searches in OMIM and GeneCards. Results: Four major biological links between SD and PD have been proposed. First, alpha-synuclein deposits are detectable in cutaneous nerve fibres and autonomic fibres innervating sebaceous glands in PD patients and their pattern has been associated with disease subtype and progression. Second, dysfunction in ceramide and sphingolipid metabolism, associated with variants in GBA1, LRRK2, and ZNF750, has been linked to impaired lysosomal function and skin barrier integrity in both brain and skin tissue. Third, Malassezia, a commonly associated organism and possible trigger in susceptible hosts, produces metabolites that activate neuroinflammatory pathways relevant to PD and PD patients show altered Malassezia species composition on their skin. Fourth, NLRP3 inflammasome activation, which can be triggered by ceramide accumulation and fungal metabolites, has been proposed as a shared inflammatory mechanism that may operate in both keratinocytes and dopaminergic neurons. Conclusions: SD and PD appear to share overlapping genetic, lipidomic, microbial, and neuroinflammatory features. These associations are consistent with, but do not yet prove, a common pathological basis rather than a coincidental one. SD may therefore represent a potential prodromal feature or risk marker of PD. Future research should examine whether targeted intervention in high-risk SD patients can delay or modify PD onset.