Reticular Erythematous Mucinosis Mimicking a Vascular Dermatosis: A Clinicopathologic Diagnostic Challenge.

Reticular erythematous mucinosis (REM) is a rare primary cutaneous mucinosis characterized by dermal mucin deposition and a chronic, indolent clinical course. Because of its nonspecific clinical presentation, REM may mimic a broad spectrum of inflammatory, connective tissue, and vascular dermatoses, frequently resulting in delayed diagnosis and inappropriate management. We report the case of a 57-year-old man with no significant past medical history who presented with a four-year history of a slowly progressive, occasionally pruritic eruption involving the upper back. The lesions gradually enlarged before stabilizing and failed to respond to multiple courses of topical corticosteroids. Clinical examination revealed grouped erythematous macules and papules coalescing into a reticulated erythematous plaque with a pseudo-vascular appearance. Dermoscopic examination performed with a polarized DermLite DL5 (3Gen Inc., San Juan Capistrano, CA, USA) demonstrated a diffuse pink background with fine irregular telangiectasias and multiple whitish structureless areas. Routine laboratory investigations, including antinuclear antibody testing, were unremarkable. Histopathological examination revealed a superficial and deep perivascular and periappendageal lymphohistiocytic infiltrate, while Alcian blue staining demonstrated dermal mucin deposition. Immunohistochemical analysis showed a predominantly T-cell infiltrate with scattered B lymphocytes, rare CD30-positive cells, and a few plasma cells, supporting a reactive inflammatory process. Integration of the clinical, dermoscopic, histopathological, and immunohistochemical findings established the diagnosis of REM. As the eruption was minimally symptomatic and clinically stable, the patient declined treatment, and no disease progression was observed during follow-up. This case expands the clinical spectrum of REM by illustrating an uncommon pseudo-vascular presentation in a male patient. It underscores the pivotal role of clinicodermoscopic-pathologic correlation in establishing the diagnosis of atypical presentations and avoiding unnecessary investigations, inappropriate treatments, and diagnostic delay.