Laboratory Studies
Diagnosis of an onychomatricoma is primarily based on histological analysis of the tumor. Bacterial and fungal cultures should be requested to exclude infections, as damage to the nail caused by onychomatricomas increases patient susceptibility to bacterial and fungal invasion. [22]
Imaging Studies
Less invasive techniques such as dermoscopy, MRI, ultrasound, and confocal microscopy have been reported as a potential adjuvants to diagnosis.
The most common dermoscopic features include longitudinal white lines, proximal and distal splinter hemorrhages, and holes in the distal margin. [9, 23]
MRI images are typical. The sagittal images highlight the tumor core in the matrix area and the invagination of the lesion into the funnel-shaped nail plate. The center has a low signal with a peripheral rim signal identical to that of normal epidermis. The distal filamentous extensions present a higher signal on T2-weighted images owing to the mucoid stroma with a high water content. Axial slices accurately show the holes in the substance of the nail plate filled with the filamentous extensions.
Ultrasonography can also help with the diagnosis. [24, 25] Results in a few cases have shown a hypoechoic and hypovascular growth in the matrix. Interesting findings include hyperechoic linear dots inside their hypoechoic structure, with projections into the matrix reaching into the interplate space. To date, no bone erosions or remodeling of the distal phalanx have been reported.
Recent research supports the use of reflectance confocal microscopy to assist in rapid and noninvasive diagnosis of onychomatricoma. The presence of longitudinal dark areas and bright-gray lines forming channel-like structures can help to identify onychomytricomas prior to surgical excision. [26]
Other Tests
Ordering a radiograph to exclude bony involvement by this or another condition in the differential diagnosis list is prudent.
Procedures
Complete surgical excision is the recommended therapeutic approach for onychomatricomas. Surgical excision generally involves nail plate avulsion with complete excision of the tumor back to the origin at the nail matrix. [9] Mohs micrographic surgery allows for clearance of the tumor with minimal removal of the unaffected nail matrix and conserved nail regeneration potential when the cosmetic and functional impact of complete excision are a concern. [27]
Histologic Findings
In 2002, Perrin et al [28] described the histology of onychomatricomas, establishing criteria involving 3 prerequisites, as follows:
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Fibroepithelial tumor with two anatomic zones based on proximal and distal location: The proximal zone, beneath the proximal nail folds, starts at the root of the nail and extends to the cuticle. This zone corresponds to the tumor base and is composed of deep epithelial invaginations and fibrillary stroma. The distal zone corresponds to the lunula and is characterized by multiple tumor digitations or projections lined with matrix epithelium and cavities filled with serous fluid.
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Matrical tumor in two layers: The 2 layers are (1) a superficial cellular layer with fibrillary collagen and (2) a deep, less-cellular layer filled with dense collagen bundles.
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Thick nail plate formed by a thick keratogenous zone: The nail is perforated by cavities filled with serous fluid.
Alternate histoclinical presentations and considerations
Usually, an onychomatricoma forms from the proximal nail matrix without involving other sites, such as the ventral proximal nail fold. However, reports describe formation in other sites that produce a resultant unusual presentation. These sites include the following:
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The ventral part of the proximal nail fold without matrix involvement: This tumor produces pressure on the matrix and results in a nail depression.
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The matrix, but also covering the ventral nail fold to include the angle at which it meets the dorsal epidermis: Clinically, this appears like the pterygium of lichen planus but without the histologic joining of the matrix granular metaplasia.
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Clinical mimic of nail Bowen disease, with a yellow thick nail: This is due to the pressure of the onychomatricoma on the distal differentiated matrix. The histological subungual tissue reveals minute digitations, but not the classic digitations that give the anemone look.
Because nail plates are not always available as part of the biopsy specimen (or in case reports on onychomatricomas), the diagnostic criteria have been refined based on immunohistochemistry findings. Specific staining criteria include antibody AE13 staining in a V-shaped expression pattern in the epithelium ridges of an onychomatricoma.
Nail clippings may show glove-finger epithelial projections of the nail matrix epithelium extending into the nail plate. This has recently been shown to form a pattern that could potentially assist with diagnosis via analysis of the nail clippings alone. [29] In using nail clippings, this report found multiple small holes in a frontal view of the nail clippings after hematoxylin and eosin staining. The cavities stained positively for cytokeratin and were found to have lining derived from epithelium.
Staging
Histologically, a continuum exists for fibroepithelial neoplasms of the nail in their epithelial-stromal ratio, stromal cellularity, and stromal atypia. These details have been proposed to be the basis for a more descriptive naming system for onychomatricomas, based on cellular proliferation ratios and atypia. The terms espoused are as follows:
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Unguioblastoma: This is a tumor with a predominant epithelial component. The epithelium-to-stromal proliferation is at a ratio of 3:2.
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Unguioblastic fibroma: This is a tumor with a predominant and more characteristic cellular stroma. The stromal cells have parallel spindle-to-fusiform nuclei. The epithelium-to-stromal ratio is reportedly less than 2:3.
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Atypical unguioblastic fibroma: This is a tumor in which the cellular stroma shows nuclear pleomorphism and atypia with an increase in mitotic activity. Also present may be areas of focal necrosis. Here, the stromal region displays the parallel spindle formation noted in the unguioblastic fibroma, but with much more cellularity. No ratios are reported for this class.
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Courtesy of Dr. Militello.
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Note the woodworm-like features. Courtesy of Dr. Militello.
