Onychocytic Matricoma Presenting as Longitudinal Melanonychia in a Skin of Color Patient

Abstract

Introduction

Onychocytic matricoma (OCM) is a benign acanthoma of the nail matrix that presents with longitudinal melanonychia and nail thickening. Only 18 previously reported cases of OCM are in the literature since it was first described in 2012.

Case Presentation

The purpose of this case report was to report a unique presentation of OCM in the toenail of a Black patient as well as to review the clinical presentation, histologic features, and management of this rare entity. Previously described cases presented on the fingernails and were predominantly in white males.

Conclusion

OCM is a benign entity that may mimic a nail unit melanoma or squamous cell carcinoma especially when pachyonychia is present. Despite some clinical clues to suggest a diagnosis of OCM, a nail matrix biopsy is often required to rule out malignancy.

Established Facts

• •Onychocytic matricoma (OCM) is a benign nail lesion.
• •There are only a few reported cases in the last 10 years.

Novel Insights

• •This is the first reported case of OCM in a patient with skin of color.
• •It can present as a longitudinal melanonychia.
• •Cases are often found in digits, but our case was found in a toenail.

Introduction/Literature Review

Onychocytic matricoma (OCM) is a benign tumor of onychocytes in the nail matrix causing localized nail thickening and melanonychia, also referred to as longitudinal pachymelanonychia [1]. This epithelial tumor or acanthoma has been reported in 18 cases. Clinically, the differential includes subungual seborrheic keratoses, onychopapilloma, onychomatricoma, onychocytic carcinoma, subungual melanoma, or a foreign body [2, 3]. Histologically, OCM shows acanthotic, papillomatous, or keratogenous features with minimal melanocyte changes [4, 5]. Of the reported cases, OCM has been found in 11 males [1, 2, 4–8], 7 females [1, 6, 8–11], with 1 case in late childhood [11]. Additionally, there were recent findings in one South Korean female, one Chinese male, one Taiwanese male, and one Taiwanese female [7–9]. There are no reported cases of OCM in Black individuals as well as no reported cases found in toenails. The epidemiology of OCMs is currently limited. This case serves to contribute additional data for this rare condition. We present a case of OCM and review the clinical and pathologic features as well as differential diagnosis, prognosis, treatment, and guidance for clinicians regarding OCM.

Case Report

An 81-year-old Black female presented to the dermatology clinic for an asymptomatic “dark line” on her right great toenail. The “line” was present for many years but recently changed over the last 6 months. There was no history of skin cancer or trauma to the affected toe. Physical exam revealed a brown-black longitudinal band with localized distal onycholysis on the right great toenail as seen in Figure 1a. Additionally, there was slight nail thickening or pachyonychia present where the pigment presented. See Figure 1b for free edge under dermoscopy. Based on clinical findings, a nail matrix biopsy was recommended to rule out melanoma.

Fig. 1.

a This is a photo of our patient’s right great toe on presentation. She described it as a “dark line” that was present for many years and recently changed over the last 6 months. The patient denied symptoms of tenderness, bleeding, irritation, or trauma to the site. b This is an image of the free edge of the toenail under dermoscopy. There is slight nail thickening around the area of pigmentation.

Histopathologic examination revealed acanthosis of the nail matrix with matrical onychocytes characterized by small monomorphic nuclei, scant cytoplasm, and absence of cytologic atypia focally displaying concentric nests as shown in Figure 2a. A Fontana-Masson stain in Figure 2b is performed and showed increased melanin deposition. SOX10 and MART-1 immunohistochemical stains failed to demonstrate a definitive melanocytic lesion. The constellation of histologic and immunohistochemical findings supports the diagnosis of OCM. In the differential diagnosis, matrix seborrheic keratosis was included. In this patient, however, their distinctive nests with a central squamous eddy surrounding by a whorl of light-staining cells were not observed. Unfortunately, nail plate biopsy was not collected to assess for irregular cystic spacing as typically seen in OCM.

Fig. 2.

a H&E stain section: High power evaluation of the matrix biopsy shows nest with concentric arrangement and absence of cytologic atypia AuthorAnonymous, b Fontana-Masson stain of biopsy at ×40 – the special stain demonstrates the presence of melanin in the lesion.

Discussion

There are 18 cases of OCM reported to date. All prior reports were on the fingernails of people who were White or Asian [11]. This is the first case to our knowledge of OCM reported in a Black patient. Differentiating features between OCM and other possible diagnoses include longitudinal erythromelanonychia for subungal seborrheic keratosis; verrucous band of thickened nail plate with a yellowish discoloration in onychocytic carcinoma; and xanthonychia with splinter hemorrhages mimicking onychomycosis seen in onychomatricoma [4, 9, 12, 13]. Differential diagnosis for longitudinal melanonychia may also include melanoma and pigmented Bowen’s disease [14]. Additionally, longitudinal pachyonychia with or without melanonychia includes onychomatricoma, onychocytic carcinoma, and pigmented squamous cell carcinoma [15].

OCMs can present similarly to various benign and malignant nail tumors. Histopathologic examination is required to establish the diagnosis and to exclude malignancy. The histopathology is characterized by concentrically arranged nests of prekeratogenous and keratogenous cells. There are four distinct subtypes of this tumor: the acanthotic type, which exhibits an acanthotic growth pattern; the acanthotic and papillomatous type, which exhibits an increased proportion of basaloid cells and spheres of prekeratogenous and keratogenous cells in association with superficial epithelial papillae; and the keratogenous type, which displays a prominent keratogenous zone associated with delayed maturation. The presence or absence of pigmentation can also be used for classification purposes [1, 5, 6]. Nail clippings may benefit clinically in differentiating between causes of longitudinal pachyonychia although it is not diagnostic. We did not collect a nail clipping in this case because the primary differential diagnosis was a nail unit melanoma which led to the nail matrix biopsy.

Onychomatricoma, a possible mimicker of OCM, clinically consists of four features: thickening of nail plate, transverse or longitudinal over curvature, xanthonychia, and splinter hemorrhages [9]. Histologically, it consists of a fibroepithelial tumor with two distinct areas: a proximal zone with deep epithelial invaginations with overlying ungual protrusions and a distal zone with epithelial digitations originating from matrix epithelium [9]. Onychocytic carcinoma differs from OCM by having malignancy-indicating traits including cellular pleomorphism or proliferation [12]. Pigmented squamous cell carcinoma usually has atypical and focal pigmented keratinocytes that expand the nail matrix [15]. Subungual seborrheic keratoses, when irritated, can closely resemble OCMs histologically. The presence of squamous eddies and lack of concentric layers composed of prekeratogenous and keratogenous zones in OCM help to differentiate it from irritated subungual seborrheic keratoses [13].

Although long-term outcome data is limited, OCMs are generally thought to follow a benign course. The underlying etiology is currently unknown but appears to occur sporadically in patients without a family history of the condition. Thus far, there is no evidence to suggest that patients with one lesion have an increased risk of developing another one. Treatment includes observation or surgical excision. Surgical excision of nail plate alone may not be curative as OCM originates from the matrix, but there are no reports of recurrence to date after nail plate removal [5]. In our case, we recommended monitoring the area as the lesion was asymptomatic and not cosmetically bothersome.

OCM is a rare acanthoma of the nail matrix. Knowledge of this entity is gained from a small number of reported cases over the past 10 years. Here, we report the first case of an OCM presenting on the toenail of a Black patient. Clinicians should include this entity on the differential for benign and malignant nail tumors and be aware of the need for clinicopathological correlation to avoid misdiagnosis.

Statement of Ethics

This study protocol was reviewed and granted exemption by the Institutional Review Board at Duke University Hospital, approval number [Pro00110847]. Written informed consent was obtained from the patient for publication of details of their medical case and any accompanying images.

Conflict of Interest Statement

The authors have no conflicts of interest to declare.

Funding Sources

The authors received no funding.

Author Contributions

Ms. Alvarez contributed to this case by researching the patient’s case, reviewing existing literature, and writing the report. This is Dr. Fresco’s and Dr. Kha’s patient from clinic. They contributed to the editing process of the report and gave their own insight. Dr. Selim read the original pathology slides and provided the pathology images as well as contributed to the pathology descriptions.

Funding Statement

The authors received no funding.

Data Availability Statement

All data generated or analyzed during this study are included in this article. Further inquiries can be directed to the corresponding author.