Atypical Dermatophyte Localization in the Stratum Lucidum: A Retrospective Histopathologic Case Series of Tinea Pedis

Dermatophyte infection extending beyond the superficial stratum corneum is an emerging challenge, particularly in immunocompromised patients. Tinea lucidum, first described in 2014, reflects this shift in pathogenesis, with dermatophytes localizing in the deeper cornified stratum lucidum and, in some cases, occurring without stratum corneum involvement. This creates diagnostic uncertainty when superficial scrapings are used for mycology testing. We retrospectively analyzed skin punch biopsies submitted to a single U.S. laboratory by dermatologists and podiatrists between January 2024 and March 2026. Specimens were obtained from the plantar and dorsal foot, representing moccasin-type tinea pedis with secondary spread to the dorsal surface. Atypical localization in the stratum lucidum occurred in 43.6% (99/227) of cases, including 13.7% (31/227) without detectable fungal elements in the stratum corneum. The dorsal foot more frequently exhibited tinea lucidum than the plantar foot (76.9% [60/78] vs. 26.2% [39/149]; p < 0.0001), potentially reflecting differences in epidermal thickness or underlying immunodeficiency. Higher densities of fungal elements and inflammatory infiltrates revealed by Periodic acid-Schiff and hematoxylin-eosin staining further suggest deeper epidermal invasion; however, the number of samples were limited, and the immune cell populations were not molecularly characterized in this study. For specimens demonstrating dermatophytes localized solely within the stratum lucidum, a survey collected de-identified patient details (responsive rate: 32.3% [10/31]), and sequencing was performed for dermatophyte identification. Trichophyton rubrum was identified in the majority of samples tested (64.5% [20/31]), and histories of immunosuppressive medication use and/or immunosuppressive conditions were noted in several patients. Further studies are needed to validate the histologic criteria and elucidate the immunopathogenesis of this phenomenon.