Abstract
Onychomatricoma is a tumor of the nail matrix which often presents with alterations in the nail plate while the tumor itself is concealed beneath the nail plate. It is a benign, biphasic fibroepithelial tumor which has to be differentiated from other subungual and periungual tumors. We report a rare case of onychomatricoma and describe a three-dimensional histopathological analysis and immunohistochemical patterns of onychomatricoma.
Keywords: Histopathology, immunohistochemistry, nail tumors, onychomatricoma
What was known?
Onychomatricoma is an onychogenic fibroepithelial tumor arising from the nail matrix.
Introduction
Tumors of the nail apparatus often present a challenge to diagnosis as well as treatment due to the specialized nail anatomy. The nail plate may conceal the tumor as well as alter the tumor growth pattern. However, alterations in the color, thickness, curvature of the nail plate may often provide a clue.
Case Report
A 40-year-old male presented with yellowish discoloration, thickening and altered curvature of the right great toe nail for the past 2 years. There was history of mild pain on walking and application of pressure for the past 2 weeks. The patient had been adequately treated with oral antifungals but without relief. There was no history of prior trauma.
On examination, there was a markedly thickened nail plate, increased proximal transverse curvature and yellowish band-like discoloration of medial two thirds of the nail (xanthonychia) [Figure 1]. However, there was no friability of the nail plate. There were no splinter hemorrhages. Cuticle was intact. There was swelling and brownish discoloration of medial two thirds of the proximal nail fold. There was minimal pain on applying pressure on medial part of nail and nail fold. Love's pin test was negative. All other nails of feet and hands were normal. KOH mount and culture of the nail clipping did not reveal any fungal elements. X-ray of the foot did not show any bony erosions.
Figure 1.
Markedly thickened nail plate, increased proximal transverse curvature and yellowish band like discoloration (Xanthonychia) of medial two thirds of the right great toe nail. Also, note swelling of medial two thirds of proximal nail fold
The patient underwent a total nail avulsion which revealed a well-defined tumor extruding from below the proximal nail fold toward the distal end of the nail bed [Figure 2a]. Two longitudinal incisions were made laterally on the proximal nail fold. The proximal nail fold was everted and the tumor was excised in entirety. The excised tumor (3 cm × 2 cm × 0.5 cm) was firm in consistency with parallel ridges corresponding to the ridges on the nail plate [Figures 2b–d]. A differential diagnosis of onychomatricoma, fibrokeratoma of the nail matrix, superficial acral fibromyxoma, fibroma and glomus tumor was considered. Histopathology of proximal transverse section [Figure 3] revealed a polypoid fibroepithelial tumor with a foliated pattern, acanthosis, papillomatosis and deep epithelial invaginations of multilayered basal and suprabasal cells with elongated nuclei oriented perpendicular to the basement membrane, absent granular layer with clear V-shaped clefts. At places, remnants of the keratogenous zone were visible surmounting the prekeratogenous zone. The stroma had a superficial cellular, fibrillary and vascular layer and a deep, relatively, acellular layer with denser collagen. There were numerous mast cells in the stroma. A distal longitudinal section [Figure 4a and b] showed a different pattern of the tumor: Glove finger-like/monodigitate pattern with marked papillomatosis and multiple thick and deep epithelial ridges. On immunohistochemical analysis, the fibrous stroma showed diffuse staining for CD34 and negative staining for CD99 [Figures 5a and b]. Thus, a final diagnosis of onychomatricoma was made.
Figure 2.
(a) Post nail avulsion; a well-defined tumor extruding from below the proximal nail fold toward the distal end of the nail bed. (b) The excised tumor showing parallel ridges (R) corresponding to the ridges on the nail plate. (c) Dorsal view of the avulsed nail plate showing thin proximal portion and thickened yellowish distal portion (d) Ventral view showing ridges on the nail corresponding to the tumor
Figure 3.
Histopathology: Proximal transverse section H and E (×10 and ×40) (a) A polypoid fibroepithelial tumor with a foliated pattern; acanthotic, papillo matous epithelium with V-shaped clefts (V) corresponding to the avulsed nail. The stroma had a superficial cellular, fibrillary and vascular layer and a deep relatively acellular layer with denser collagen. (b) The tumor was lined by mature malpighian epithelium resembling normal matriceal epithelium; absent stratum granulosum with multilayered basal and suprabasal cells with elongated nuclei oriented perpendicular to the basement membrane. At places, remnants of the keratogenous zone were visible surmounting the prekeratogenous (P) zone. (c) Numerous mast cells (M) in the stroma
Figure 4.
Immunohistochemistry: (×10) Fibrous stroma showing diffuse staining for CD34 (a) and negative staining for CD99 (b)
Figure 5.
Histopathology (H and E) a (×10), b (×40). A distal longitudinal section showing a glove finger-like/monodigitate pattern with marked papillomatosis and multiple thick and deep epithelial ridges
Discussion
Onychomatricoma (OM) is a rare onychogenic tumor of the nail matrix, described mostly in Caucasians. The usual clinical presentation is a thickened nail with xanthonychia, increased transverse curvature and proximal splinter hemorrhages (due to vascular stroma).[1,2] Rare clinical features include longitudinal melanonychia and cutaneous horn.[1,3]
Perrin et al. have defined two types of OM based on tumor and nail characteristics: (1) onychomatricoma of ventral matrix (OM type I): Characterized by a single large fibroepithelial tumor. The nail plate is thinned out proximally and thick distally giving the appearance of a porch roof.[4] (2) Onychomatricoma of the proximal nail fold (OM type II): Characterized by a tumor with multiple fibroepithelial projections.[4] The nail plate shows a woodworm-like appearance with multiple cavities. OM type II often presents with unusual clinical features such as pterygium, fibrokeratoma like OM, total onychodystrophy and verrucous band-like pattern (suggesting wart or Bowen's disease).[4,5]
Histopathologically, OM type I shows two different architectural patterns in transverse and longitudinal sections.[4] On proximal transverse sections, a lobulated/foliated pattern is observed while a glove finger monodigitate pattern is observed on longitudinal distal sections. Awareness of these different patterns is important when only fragmented specimens are available for histological analysis. A three-dimensional model using proximal and distal transverse and longitudinal sections helps in better understanding and visualization of the morphology of this rare tumor.[4]
OM is characterized by deep epithelial invaginations with a thick V-shaped keratogenous zone overlying the prekeratogenous zone. On avulsion of the nail plate, this is seen as an optically empty V-shaped zone. The epithelium corresponds to the matriceal epithelium with absent granular layer, basal cells with thin elongated nuclei and multi-layered suprabasal cells with oval nuclei. Keratin markers, K5 and 17, are positive through the epithelium (indicative of matriceal differentiation), while K 85 stains the prekeratogenous zone.[6]
The stroma contains several mast cells and is arranged in two layers: A superficial cellular, vascular layer with fibrillary collagen and a deeper relatively less cellular layer with dense collagen bundles.[7] The two-layered stroma is more frequently observed in the proximal portion than in the distal zone of the tumor. Based on the proportion of epithelial and mesenchymal components and presence/absence of atypia in the stroma, Ko et al. have proposed a different nomenclature with three categories: (1) unguioblastoma (predominant epithelial component) (2) unguioblastic fibroma (predominant stromal component), and (3) atypical unguioblastic fibroma (nuclear pleomorphism, atypia in the stroma).[8]
Differential diagnosis of OM includes fibrokeratoma, fibroma, superficial acral fibromyxoma (SAF), onycholemmal horn, malignant proliferating onycholemmal cyst, subungual warts, subungual keratoacanthoma, subungual squamous cell carcinoma and Bowen's disease.[7] All of these are easily distinguishable by histopathology except fibrokeratoma, fibroma and SAF, which need more careful differentiation. In distal longitudinal sections, the structure of OM tends to resemble a fibrokeratoma. However, absence of a horny corn, multiple fibroepithelial digitations and two layered stroma (on transverse section) rules out fibrokeratoma.[1] The stroma of OM may resemble a fibroma, which can however be excluded by the hyperplastic and onychogenic epithelium of OM.[1] SAF has a close resemblance to OM due to highly vascular collagenous stroma, presence of mast cells and epidermal hyperplasia with papillomatosis.[4] Also, a single case of OM with myxocollagenous stroma (superficial acral fibromyxoma like OM) has been reported.[4] The immunohistochemical analysis of the stroma helps in distinguishing OM from SAF. SAF shows diffuse CD34, CD99 expression, while OM shows diffuse expression of CD34, but is CD99 negative.[4]
Treatment of OM is complete surgical excision. Despite the atypical nature of stroma reported in some cases, no frank malignant variant has been described to date.[9]
In conclusion, onychomatricoma is a rare fibroepithelial tumor mostly arising from the nail matrix. It needs to be distinguished from other subungual/periungual tumors; immunohistochemical markers CD34 and CD99 are useful adjuncts in the diagnosis of OM. A three-dimensional spatial reconstitution of OM helps in better understanding of its variable morphological characteristics.
What is new?
Histopathological patterns of onychomatricoma vary in proximal and distal sections; foliated pattern on proximal transverse sections and a glove finger-like pattern on the distal longitudinal section.
Study of onychomatricoma in multiple sections and planes is important for a three-dimensional spatial reconstitution of the tumor and differentiation from other subungual/periungual tumors.
Patterns of stromal immunohistochemical markers, CD34 and CD99 are especially useful in differentiating onychomatricoma from superficial acral fibromyxoma, a subungual tumor with several similar histological features as onychomatricoma.
Footnotes
Source of support: Nil
Conflict of Interest: Nil.
References
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