DP09 Pleomorphic variant of onychomatricoma of the toenail bed

Abstract A 93-year-old man presented with a 12-month history of nail discoloration in both big toes. The right toe had brown discoloration, while the left showed similar changes with bleeding and pain. He had a 30-year history of chronic nail fungal infection, self-treated with over-the-counter antifungals. His medical history included heart disease and prostate issues. On examination, the right big toe displayed dystrophic changes, including thickening, yellow-brown discoloration, onycholysis, keratosis and an unpleasant odour. The left big toe had green-yellow discoloration, dystrophy and foul odour, suggestive of Pseudomonas infection. Intertoe fungal infection was also present. The initial diagnosis was onychomycosis with secondary Pseudomonas infection. Nail clippings were sent for fungal analysis, and a biopsy from the right nailbed was taken. Fungal tests were negative. Histology revealed a squamoproliferative lesion consistent with onychomatricoma, likely the pleomorphic variant. Features included hyperkeratosis, acanthosis, papillomatosis and irregular downward invaginations. The dermis showed lymphoplasmacytic infiltrate and epithelioid to spindle cells with bizarre nuclei but no necrosis or mitosis. Immunohistochemistry was positive for CD34, p16 and CD10. Further expert opinion from St Thomas’ Hospital confirmed the diagnosis, with Rb1 loss supporting its benign nature. Differential diagnoses included fibrokeratoma, acral fibromyxoma and pleomorphic fibroma. Despite pleomorphism, there was no indication of aggressive behaviour, although close follow-up was advised. The main challenge was differentiating it from more aggressive conditions like squamous cell carcinoma. Initial treatment included topical antifungals (terbinafine and amorolfine). Due to persistent symptoms, the patient opted for nail removal with punch biopsy under local anaesthesia. Histopathology confirmed onychomatricoma. No further treatment was required, and 3-year follow-up was advised. The patient later passed away due to cardiac disease, discontinuing further monitoring. Onychomatricoma, particularly the pleomorphic variant, is a rare and often under-recognized cause of nail dystrophy. Although benign, its clinical presentation can mimic malignancies such as squamous cell carcinoma or melanoma, making histological evaluation crucial. This case underscores the importance of thorough histopathological analysis and expert consultation. Immunohistochemistry played a key role in confirming the diagnosis and ruling out malignancy. Close monitoring is recommended for recurrence or clinical changes.