Multicentric Reticulohistiocytosis Progressing to Erythroderma After Long-Term Disease Progression

Multicentric reticulohistiocytosis (MRH) is a rare non-Langerhans cell histiocytosis characterized by destructive polyarthritis and papulonodular cutaneous lesions. We describe a woman who developed erythroderma 20 years after MRH diagnosis-to our knowledge, a previously unreported manifestation of this disease. The patient initially presented at age 50 with arthralgia, and MRH was diagnosed one year later based on distal interphalangeal (DIP) joint deformities, erythematous papules on the lower legs, and a skin biopsy showing dermal infiltration of CD68-positive, S-100- and CD1a-negative histiocyte-like cells with ground-glass eosinophilic cytoplasm. Despite long-term treatment with methotrexate (MTX), prednisolone (PSL), infliximab (year 11), and bone-modifying agents (alendronate, later denosumab), the disease progressed, with development of pulmonary arterial hypertension (PAH) attributable to lesions around the upper pulmonary veins. At age 71, twenty years after diagnosis, pruritic erythema appeared on the back and evolved into erythroderma despite topical corticosteroids. Biopsies from erythematous areas demonstrated CD68-positive histiocytic infiltrate identical to that of the original MRH lesions, whereas biopsies from clinically uninvolved skin showed no such infiltration, supporting a direct association with the MRH disease process. Comprehensive evaluation excluded eczematous/atopic erythroderma (normal eosinophils and IgE), drug-induced erythroderma (no new medications; infliximab discontinued 9 years previously), dermatophytosis (negative PAS staining), granulomatous mycosis fungoides (absent characteristic histological features), sarcoidosis (normal sIL-2R; no granulomas on biopsy), dermatomyositis (absent characteristic features; normal creatine kinase), and internal malignancy (contrast-enhanced CT and 2-year follow-up). MTX was discontinued because of MTX-induced interstitial pneumonia, and oral PSL was increased to 30 mg/day, with rapid improvement; the patient is currently maintained on PSL 10 mg/day without relapse. The cumulative inflammatory burden of long-standing MRH may have culminated in this erythrodermic phenotype.