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Onychocytic matricoma (OCM) is a benign acanthoma of the nail unit that presents with localized thickening of the nail plate and melanonychia.1 This newly described entity has suggestive clinical features and distinctive histopathologic changes.
Report of a Case
A man in his 40s presented with a history of traumatic injury to the nail unit, after which he noted a dark line under the nail, which he assumed to be a splinter. It persisted for 3 years without any notable change. The patient reported removing portions of it when he would clip the nail back.
Physical examination demonstrated a 2-mm-wide black longitudinal streak extending to the distal lunula with localized nail plate thickening on the right second digit (Figure 1A and B). Dermatoscopic findings were consistent with a foreign body under the nail (Figure 1C and D). Nail clippings of the nail plate were performed to sample the distal portion of the lesion and demonstrated parakeratosis associated with pigmentation.
A, On the right second digit, there was a black longitudinal streak that involved almost the entire length of the nail unit, but stopped at the proximal nail fold. B, Localized nail-plate thickening also was observed. C, Dermatoscopic findings highlighted the pigmentation. D, The localized thickening simulated a foreign body.
A partial central nail plate avulsion was performed, as was a longitudinal excisional biopsy. Histopathologic analysis revealed a benign, pigmented epithelial proliferation of the nail matrix epithelium associated with a longitudinal collection of pigmented cells with retained nuclei below the overlying nail plate (thickened keratogenous zone) (Figure 2). Fontana staining highlighted the melanin-derived pigmentation in the epithelium and nail plate. MART-1 staining did not highlight any melanocytes. Findings of periodic acid–Schiff staining and human papilloma viral immunohistochemical analysis were negative. Based on these histopathologic findings, a diagnosis of a pigmented onychocytic matricoma with keratogenous features was made. The patient underwent definitive excision of the lesion, and at 1-year follow-up, there was no recurrence.
Excisional biopsy highlighted a proliferation of the nail matrix epithelium with pigmented cells and a thickened keratogenous zone on high power (hematoxylin-eosin, original magnification ×200).
We report a case that highlights the recently described and distinct tumor of the nail matrix, OCM. This lesion presents clinically as a localized thickening of the nail plate often with an associated longitudinal melanonychia that might simulate a foreign body, as in this case. Microscopically, it is a benign acanthoma of the nail unit with key characteristics that include endokeratinization and concentrically arranged nests of prekeratogenous and keratogenous cells with variation in the prekeratogenous and keratogenous components depending on the histopathologic subtypes (acanthotic, papillomatous, or keratogenous with retarded maturation).1 In addition, OCM can be classified by pigmentation (pigmented, melanocytic, or hypopigmented/nonpigmented variants).1,2 Our case would be classified as a keratogenous and pigmented OCM, based on the prominent keratogenous zone and pigmentation observed (Figure 2).
Various benign and malignant tumors, including melanoma, as well as lesions caused by the presence of foreign bodies can present as longitudinal melanonychia with a thickened nail plate. Detailed histopathologic analysis is needed to establish a diagnosis. Among the benign lesions that can present as longitudinal melanonychia, onychopapilloma is a nail-bed tumor characterized by acanthosis in the presence of nail-bed papillomatosis.3 Onychocytic matricoma differs by its location in the nail matrix as opposed to nail bed.
Within the nail matrix, onychomatricoma is the most common benign lesion comprising both epithelial strands and a CD34+ fibrous and cellular stroma.4 Clinically, onychomatricomas can present with longitudinal melanonychia and nail plate thickening but have a distinct honeycomb pattern after nail clipping.4,5 Onychocytic matricoma is microscopically distinct as a purely epithelial tumor, lacking the combined fibroepithelial components found in an onychomatricoma.
The most difficult differentiation may be between subungual seborrheic keratoses and OCM; the presence of prekeratogenous and keratogenous zones and thickened nail plate in OCM may help distinguish these entities.6 However, considering these both on a benign spectrum of nail-unit acanthomas may be the best technique for classification.2
We report herein a case of OCM that clinically presented as a pigmented foreign body to make clinicians aware of this benign matrical tumor and to add a new entity to the differential diagnosis of a foreign body in the nail unit.
Corresponding Author: Adam I. Rubin, MD, Department of Dermatology, Hospital of the University of Pennsylvania, 3600 Spruce St, 2 Maloney, Philadelphia, PA 19104 (Adam.Rubin@uphs.upenn.edu).
Published Online: February 5, 2014. doi:10.1001/jamadermatol.2013.6358.
Conflict of Interest Disclosures: None reported.
Additional Information: Dr Wanat is now with the Department of Dermatology, University of Iowa, Iowa City.
Wanat KA, Reid E, Rubin AI. Onychocytic Matricoma: A New, Important Nail-Unit Tumor Mistaken for a Foreign Body. JAMA Dermatol. 2014;150(3):335–337. doi:10.1001/jamadermatol.2013.6358
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