Pigmented onychomatricoma: Avoid getting nailed by misdiagnosis

By Warren R. Heymann, MD
Dec. 2, 2020
Vol. 2, No. 47

Rightfully, monodactylous linear melanonychia should make any dermatologist nervous, with or without a Hutchinson sign. No nail unit melanoma should be left behind.

Longitudinal melanonychia (LM) is defined as a longitudinally oriented band of brown-black pigment in the nail. LM may be caused by exogenous pigments, blood or melanin. The most common cause of LM in adults and children is melanocytic activation and benign melanocytic nevi, respectively. Regardless, the diagnosis of nail unit melanoma must come to mind first as two-thirds of nail melanomas present clinically as LM. (1)

In 1992, Baran and Kint recognized a distinct nail tumor which they called “onychomatrixoma,” summarized by this abstract: “Identical lesions of the nails were observed in three patients. The main signs were a yellow colouration along the entire length of the nail plate with splinter haemorrhages in its proximal portion, a tendency towards transverse overcurvature of the affected nails, and exposure of a matrix tumour after the nail had been avulsed and the proximal nail fold turned back. The nail signs were striking enough to lead to the clinical suspicion of a filamentous tufted tumour in the matrix of a funnel-shaped nail, an entity not previously described.” (2)

Onychomatricoma (OM) is a rare, benign, fibroepithelial tumor of the nail matrix. Beneath the proximal nailfold, the neoplasm consists of multiple digitated fibroepithelial projections that grow longitudinally within the nail plate toward the distal edge. At the edge of the nail plate multiple honeycomb-like perforations that have lost the digitations may be apparent. When the classic clinical findings are present, particularly those found at the free edge of the nail plate along with the corresponding nail clipping changes, the diagnosis of OM can be established with confidence without an invasive nail matrix biopsy specimen. (3,4) Although OM may be misdiagnosed as onychomycosis, the two may occur concurrently. (5)

A 65-year-old man presented with a right index finger lesion that had been present for 10 years without any history of trauma or appreciable changes (see images). I was overjoyed at finally seeing what I had only read about in the literature — the honeycombed distal nail plate — but was concerned about the pigmentation. I was previously unaware of the diagnosis of a pigmented OM (POM). He is not the first case; it is rare, with fewer than a dozen reported cases. (After careful discussion of his options, based on the clinical findings and stable long-standing history, the patient prefers to follow the lesion rather than have it removed and biopsied.)

The differential diagnosis of POM is broad as lesions can mimic malignant melanoma, melanotic macule, pigmented squamous cell carcinoma, pigmented onychopapilloma, pigmented onychomycosis, onychocytic matricoma, or hematoma. POM preferentially affect men and involve the toenails in 73% of reported cases. (6) Even experts can be fooled, as exemplified by the case reported by Baran and Perrin, in which a 39-year-old man presented with what they presumed was a pigmented onychomatricoma but proved to be Bowen’s disease. (7)

The need for biopsy in any equivocal cases cannot be overstated. When evaluating all forms of longitudinal melanonychia it is essential to orient the tissue longitudinally ensuring that all aspects of the nail can be evaluated. (6) Histologically, POM parallel those of the classic lesion, including a papillomatous matrix epithelium, also known as ‘gloved‐finger’ digitations, with a fibrous dermal proliferation. There are two separate anatomical zones with a papillomatous matrix epithelium overlying a cellular, fibrous stroma. (8) CD34⁺ staining of the fibromucinous stroma may simulate a cellular digital fibroma or acral fibromyxoma, representing common diagnostic pitfalls. (3) The only definitive treatment is surgical removal.

The lesson is clear — while most cases of monodactylous LM are benign, including POM, ruling out nail unit melanoma remains paramount. One final comment: do not rely on a Hutchinson sign (pigmentation that has spread onto periungual skin). As observed by Baran and Kechijian, such pigment is present in a variety of benign disorders and, therefore, may lead to overdiagnosis of subungual melanoma. The term “Pseudo-Hutchinson’s sign” has been coined to encompass these three simulants of Hutchinson's sign. (9) When assessing patients clinically, I do not believe that the term “Pseudo-Hutchinson sign” should be utilized, as that rendering can only be made after histologic confirmation ruling out melanoma.

Point to Remember: Despite the long differential diagnosis for monodactylous linear melanonychia, including the rare pigmented onychomatricoma, it is incumbent on the dermatologist to consider the diagnosis of a nail unit melanoma first and foremost. Not every lesion must be biopsied, but if there is any diagnostic equivocation, it should be encouraged.

Our Expert’s Viewpoint

Adam Rubin, MD
Associate Professor of Dermatology
Hospital of the University of Pennsylvania

Recognition of pigmented onychomatricoma (POM) is increasing, as evidenced by more publications on this entity, including the publication here by Dr. Heymann. The presence of a pigmented, thickened nail plate (pachymelanonychia) brings to mind mainly two diagnoses, a pigmented onychomatricoma, as well as an onychocytic matricoma. Of note, we are now shifting toward the term longitudinal subungual acanthoma, which was introduced by Robert Baran and is an inclusive term which encompasses nail unit tumors of longitudinal keratosis, ungual seborrheic keratosis, as well as onychocytic matricoma. Of course, the main concern when evaluating a patient with monodactylous longitudinal melanonychia is to consider the possibility of nail unit melanoma, and the differential diagnosis (depending on the age of the patient) might also include a nail unit melanocytic nevus, subungual lentigo, as well as melanocytic activation / melanotic macule of the nail unit. It is important to remember that other entities in the nail may present with longitudinal melanonychia as well, such as a pigmented onychopapilloma, and fungal melanonychia. Subungual hemorrhage occasionally can also present in a linear pattern in a nail, and truly mimic a melanocytic lesion or other pigmented nail unit tumor. A nail unit biopsy is the method to establish an unambiguous final diagnosis. Nail unit dermoscopy (onychoscopy) can be helpful in offering clues to narrow the differential diagnosis. In the case of a pigmented onychomatricoma, the telltale “honeycomb” pattern or cavities present at the free edge of the nail plate will be present. A nail clipping from the free edge of the nail plate submitted for histology can also demonstrate these cavities in the nail plate. Fungal infection is common with onychomatricomas, so discussion with your dermatopathologist regarding the presence of cavities in the nail plate, and where the fungus is located, could provide useful information. It is interesting to know that the size of the holes in the nail plate can also give clues to the identity of the tumor which is present. Onychomatricoma has the largest cavities, onychocytic carcinoma has the smallest cavities, and onychocytic matricoma has cavities which are intermediate between these two tumors. To date, a malignant variant of onychomatricoma has not been described. Depending on the size of an onychomatricoma, surgical removal may be challenging. Remember that when submitting specimens from an onychomatricoma, it is important to include both the nail plate as well as areas of the matrix to make the correct diagnosis and identify the key features. If soft tissue is only submitted, it is easy for a dermatopathologist to make the misdiagnosis of a superficial acral fibromyxoma, as onychomatricoma and superficial acral fibromyxoma are both CD34⁺ spindle cell tumors that occur in the nail unit and have a varying amount of mucin deposition in them. In fact, acral dermatofibrosarcoma protuberans also needs to be considered histologically. The characteristic cavities in the nail plate and papillary epithelial projections of matrix epithelium (as well as clinical images) will help differentiate onychomatricoma from other nail tumors considered clinically and histologically in the differential diagnosis.

1. Khan K, Mavanur AA. Longitudinal melanonychia. BMJ Case Rep 2015 Dec 10;2015. pii: bcr2015213459. doi: 10.1136/bcr-2015-213459.

2. Baran R, Kint A. Onychomatrixoma. Filamentous tufted tumour in the matrix of a funnel-shaped nail: A new entity (report of three cases). Br J Dermatol 1992; 126: 510-515.

3. Okon LG, Saeid N, Schwartz L, Lee JB. A case of onychomatricoma: Classic clinical, dermoscopic, and nail clipping histologic findings. J Am Acad Dermatol 2017; 76 (2S1): S19-S21.

4. Miteva M, de Farias DC, Zaiac M, Romanelli P, Tosti A. Nail clipping diagnosis of onychomatricoma. Arch Dermatol 2011; 147: 1118-1119.

5. Kallis P, Tosti A. Onychomycosis and onychomatricoma. Skin Appendage Disord 2016: 1: 2019-212.

6. Nguyen CV, Moshiri AS, Council ML, Rosman IS, Rubin AI. Pigmented onychomatricoma mimicking nail unit melanoma. J Cutan Pathol 2019; 46: 895-897.

7. Baran R, Perrin C. Bowen’s disease clinically simulating an onychomatricoma. J Am Acad Dermatol 2002; 47: 947-949.

8. Isales MC, Haugh AM, Bubley J, Zarkin S. Pigmented onychomatricoma: A rare mimic of subungual melanoma. Clin Exp Dermatol 2018; 43: 623-626.

9. Baran R, Kechijian P. Hutchinson’s sign: A reappraisal. J Am Acad Dermatol 1996; 34: 87-90.

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