Pigmented Malignant Onychopapilloma of the Fingernail: Description of the "String Sign".

Introduction: Onychopapilloma is a benign nail tumor of the nail bed and distal matrix that typically presents as isolated longitudinal erythronychia on clinical examination. In 2021, Haneke et al. [J Cutan Pathol. 2021;48(1):174-9] described the 1st case of a malignant variant of onychopapilloma.

Case Presentation: We present the case of a 34-year-old female patient, Fitzpatrick type V, who presented to our specialty outpatient nail clinic with a 10-year history of a pigmented growth on the lateral nail fold of the left 3rd fingernail that resembled a string. On histopathology, the patient's biopsy showed significant onycholemmal squamous dysplasia with intracytoplasmic melanin pigment deposition in a background of onychopapilloma; concurrent abnormal suprabasilar nuclear staining amidst keratinocytes for Ki67 was observed.

Conclusion: The proportion of onychopapilloma cases that develop malignant potential is currently unknown, as onychopapillomas were believed to be entirely benign until 2021. Malignant onychopapilloma often presents with longitudinal melanonychia in patients with skin of color and with longitudinal erythronychia in patients with lighter phototypes. Our case highlights the importance of performing a biopsy in patients who present with atypical features of onychopapilloma, including changes in pigmentation and growth, the latter potentially presenting with the "string sign."