Onychomatricoma: Benign sporadic nail lesion or much more?
Published Web Locationhttps://doi.org/10.5070/D34b54s6rt
Onychomatricoma: Benign sporadic nail lesion or much more?1. Loyola University Medical Center, Maywood, IL
Rashid M. Rashid1 and James Swan2
Dermatology Online Journal 12 (6): 4
2. Department of Dermatology, Loyola University Medical Center, Maywood, IL
Onychomatricomas (OM) are rare subungual tumors first described just over a decade ago. Surprisingly, these lesions still rarely are reported in the literature. This may be because of a lack of awareness, understanding, or interest in these perceivably benign lesions. In this review, we discuss the current literature and understanding of OM. We attempt to ascertain predilections and patterns in an attempt to establish a stronger appreciation of OM, and to encourage future reports. We emphasize the importance of reporting such lesions; with a larger literature base, prevalence, etiology, and pathogenesis can be better determined.
Onychomatricoma (OM) is a relatively new topic in the dermatologic literature, first described in 1992 by Baran et al. . Since the original finding of this nail matrix lesion, fewer than 30 cases have been reported in the English literature. However, new strides have been made in developing a better understanding of OM. In the following review we hope to provide a comprehensive and clinically relevant examination of the available literature on this unique and rare pathology.
The terminology of OM has slowly been adjusted over time. The original 1992 description  termed this nail lesion as an onychomatrixoma based on the description of a filamentous tufted tumor in the matrix. Subsequently, the term onychomatricoma was first applied, in the English literature, by Haneke et al. in 1995 .
However, a 2004 report by Ko et al. proposed further elaboration on the terminology of this lesion . Based on histological analysis, discussed below, more descriptive terms have been proposed and include unguioblastoma, unguioblastic fibroma, and atypical unguioblastic fibroma.
We reviewed the English literature to develop a comprehensive picture of OM lesions (Table 1). The male to female ratio is approximately 1:1, with a mean age of presentation at 51 years old. Although the majority of cases reported are in the elderly, OM has presented in patients as young as 24 years of age.
Occurrence is predominantly (75 %) in the fingers. The percent of lesions on the first, second, third, and fourth finger were 5%, 16%, 61%, and 16% respectively. Only two cases were found containing multiple digit occurrences in a single patient. The fifth finger and fifth toe have been spared. The involved location was almost twice as often on the right side of the body versus the left. One case occurred in a black patient, while all other reports are in Caucasians . On the basis of current reports, OM must now be a serious consideration in the differential diagnosis of suspicious subungual lesions in elderly Caucasian patients, regardless of sex.
The above data also bring to light multiple interesting, but unanswered, points. For example, it may be of great benefit for dermatologic studies to focus on and develop a better understanding of finger lesion locations. Although studies commonly note finger versus toe lesions frequencies, emphasis is rarely placed on which finger or which toe is most likely involved . This may be a major oversight, as certain diseases have been noted to have a predilection for certain digits, as in the case of pemphigus vulgaris . Appreciating lesion location detail may further our grasp on the management of digit pathologies such as OM.
Clinically, these patients present with rather unique features that suggest the diagnosis of OM. Patients may have a thickened nail with longitudinal yellow discoloration and/or dystrophy of the entire nail plate. The thickened nail may exhibit ridging. There may be longitudinal melanonychia and proximal nail swelling . Capillaroscopy reveals proximal splinter hemorrhages in the nail . Although it is rare, bleeding may occur when the distal edge of the nail is cut .
Avulsing the nail plate and turning over the proximal nail fold exposes the tumor of the matrix . The avulsed nail has a funnel-shaped deformity with transverse curvature. The thickened portion of the nail is made up of small hole-like cavities penetrated by small filamentous tufted digitations rising from the matrix tumor. The vascular stroma of these digitations is believed to be the source of the splinter hemorrhages . Despite these interesting clinical characteristics, diagnosis may be difficult and must ultimately be confirmed by histology .
Histology and diagnostic criteria
Diagnosis of OM is primarily done via histological analysis, although less invasive techniques such as MRI scanning have been discussed as a potential adjuvant . Perrin et al. elaborated on the histopathology findings of OM and described criteria with 3 prerequisites :
- A 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/projections lined with matrix epithelium and cavities filled with serous fluid. The distal digitations are oriented in a transverse plane.
- A matrical tumor in 2 layers. A superficial cellular layer with fibrillary collagen, and a deep less cellular layer filled with dense collagen bundles.
- A thick nail plate formed by a thick keratogenous zone. The nail is perforated by cavities filled with serous fluid.
Noting that nail plates are not always available for analysis, Perin et al. revisited, and further refined, the diagnostic criteria using immunohistochemistry in the absence of a nail plate . Specific staining criteria include antibody AE13 staining in a V-shaped expression pattern, in epithelium ridges of OM .
Ko et al. further expanded on these reports with 3 cases found to be clinically consistent with OM . Histologically there is a continuum of fibroepithelial neoplasms of the nail in their epithelial-stromal ratio, stromal cellularity, and stromal atypia. These details were proposed to be the basis for a more descriptive naming system for OM based on cellular proliferation ratios and atypia. The nomenclature chosen was based on a similar naming pattern used in bone and tooth neoplasms . The terms espoused are as follows:
- Unguioblastoma: Was chosen as the term for tumors with a predominant epithelial component. The report noted an epithelium and stromal proliferation at a ratio of 3:2. Ko et al. believed this term to best apply to the classic OM based on descriptions in the literature.
- Unguioblastic fibroma: Was chosen as the term for tumors with a predominant and more characteristic cellular stroma. The stromal cells had parallel spindle to fusiform nuclei. The epithelium to stromal ration was reported as less than 2:3.
- Atypical unguioblastic fibroma: Was chosen as the term to describe the lesion in which the cellular stroma shows nuclear pleomorphism and atypia with an increase of mitotic activity. Also noted, were areas of focal necrosis. Here the stromal region displayed the parallel spindle formation noted in the unguioblastic fibroma, but with distinctively much more cellularity. No ratios were reported.
It is important to be aware of the potential benefits of such terminology in future reports. For although OM is a benign lesion, to date, it is common for detailed analysis to eventually suggest a previously benign lesion may be part of a continuum with malignant forms. As an example of such a continuum, is the recent debate with the pilomatricoma and pilomatrix carcinoma . And, since Ko et al. noted an OM may have atypia and mitotic activity, malignant potential is a strong possibility .
Additionally, it is equally important for future reports and longer term studies to focus on these subtle, but distinctive feature, as they may allow us to develop a better understanding of the etiology, pathogenesis, and prognosis of these nail lesions. Finally, as a fundamental point, although Ko et al. offered interesting and detailed descriptions of OM in a small study, larger and more expansive works should be undertaken to establish a stronger foundation on these distinguishing histologic characteristics .
Onychomycosis and bacterial infections should be on the list of any differential of nail dystrophy or discoloration. To rule out these possibilities, cultures are the best initial step. However, positive cultures that do not resolve with treatment should lead one back to the diagnostic possibility of OM 
In the differential of OM, physicians must also consider other tumors and lesions with similar anatomic location. A brief discussion of a few of these and their differentiating features follows:
- Verrucae vulgaris' clinical picture does not usually resemble that of OM and histopathology is reminiscent to that of virus papilloma .
- Longitudinal melanonychia may result secondary to numerous etiologies including secondary to infection or various therapies [14, 15, 16]. Detailed history, and histologic analysis, is important in differentiating this lesion.
- Acquired fibrokeratomas are acanthotic with a horny corn over dense connective tissue and further lack fibroepithelial digitations. Histology also reveals a tumor with hyperkeratotic epidermis . This is compared to an OM which has epithelial-lined invaginations, a stroma organized in 2 layers, and the absence of horny corn.
- Onycholemmal horns, a variant of the trichilemmal horn, are histologically unique due to the accumulation of keratotic material with tricholemmal keratinization that is located on the nail groove [19, 20].
- Periungual fibromas are clinically distinguished by thinned nail plate with a longitudinal groove. Also, fibromas histologically lack the hyperplastic and onychogenic features of OM [10,18].
- Bowen's disease has a variant reported to clinically resemble an OM. However, histopathology of this OM-like Bowen's disease shows atypical keratinocytes and clear cell changes. Other exclusionary features noted in this OM-like variant of Bowen's include a lack of stroma, proximal deep invaginations, and small numerous cavities in the thickened nail plate . In the case of subungal squamous cell carcinoma, keratin pearl formation, chronic paronychia, or oozing may be noted.
- Subungal keratoacanthoma grow rapidly, tend to be painful, and cause onycholysis. Besides these clinical distinctions, these lesions do not have histologic similarities to OM, and may retain a keratin plug .
- Yellow nail syndrome presents with systemic symptoms such as upper and lower respiratory tract infection, and patients are further believed to have a humoral immune response deficit .
The etiology of OM is not yet known. Patients usually deny any history of trauma. Predisposing factors such as onychomycosis have been suggested to play a role . Onychomycosis may be an especially crucial pre-requisite for toe OM; almost half the reported cases had fungal association. Considering the lack of reports on OM and presenting cofactors, this may or may not be a coincidental finding. Furthermore, most OM appear on the fingers and most onychomycosis cases are reported on the toes . Lateur et al. addressed the location of onychomycosis in toes versus fingers and in children versus adults. They attributed the discrepancy to children having less trauma history to the nails, faster nail growth, and a decreased surface area; findings less likely to be associated with fungal infection . Many of these factors may also play a role in OM, although further studies would be needed to advance such a theory. An interesting pattern noted from our review of the literature, is the predominant involvement of OM in the most protruding/exposed finger (table 1).
Another theory espoused is an OM being fibroproliferative in origin as an epithelial and connective tissue hamartoma simulating the nail matrix [9, 21, 24]. This theory is further supported by electron microscopy study that tied OM to a nail matrix origin . Still, with so few reports on OM, it is important to keep an open mind, encourage theories and debate, and continue to seek out new cases.
Management and prognosis
Complete surgical excision has been the recommended therapeutic approach for OM . Limited (but optimistic) prognostic data are available.
Surgical removal by Van Holden et al. on one case, reported no re-occurrence on 2 year follow up . Similar 2 year follow up was noted in other reports as well . Nail re-growth also occurs without incident. However, physicians must not let their guard down; for as discussed earlier, a continuum with potentially malignant OM may exist.
Studies on OM are of particular importance with our aging population. This lesion has a tendency to appear in elderly patients; and although OM is seemingly benign, larger and more expansive studies have yet to be completed. Furthermore, with many of the OM reports produced from the authors that originally described the lesion, we believe the rarity of such reports is partly due to the lack of OM awareness. This lack of awareness further highlights the importance of encouraging continued reports, reviews, and studies on OM.
In this review, we examined the OM reports in the English literature and attempted to establish initial predilection numbers. We further accentuated that future studies should focus on the potential importance of digit location, and the possible reasons for such preferential lesion appearances. Finally, we summarized the current understanding and evolving perspective of OM. We hope this work will encourage new thoughts and debate on this fascinating, recently described, and possibly underreported, subungal tumor.
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