The diagnostic conundrum of Riehl melanosis and other facial pigmentary disorders: a case report with overlapping clinical, dermoscopic, and histopathological features.

Riehl melanosis (RM) is a form of pigmented contact dermatitis that often poses a diagnostic challenge due to overlaps in its clinical, dermoscopic, and histopathological features with other pigmentary disorders. This report highlights significant findings and the proper approach for diagnosis. We present the case of a 47-year-old female with progressive facial hyperpigmentation (irregular, blotchy, grayish patches on the forehead, cheeks, and around the mouth) that acknowledged applying a lightening product before her complaint. Dermoscopy revealed brownish-gray dots and pseudonetwork pigmentation. The histopathological findings were comprised of hypergranulosis, interface dermatitis, and pigment incontinence. The patient's history and clinical, dermoscopic, and histopathological examination results showed features consistent with-but not exclusive to-RM, such that it was not possible to rule out lichen planus pigmentosus (LPP) and ashy dermatosis (AD). We finally diagnosed the patient with RM and treated her with medications consisting of alpha hydroxy acid, tretinoin, hydroquinone, and sunscreen. Patch testing might help because higher positivity rates were found in RM patients compared to LPP and AD. However, positive patch testing favors RM, and, regardless of diagnosis, provides a basis for substance avoidance in the treatment plan.