Onychomatricoma Micropapilliferum, a New Variant of Onychomatricoma: Clinical, Dermoscopical, and Histological Correlations (Report of 4 Cases)
Authors: Perrin Christophe 
Published: August 2019

Abstract:
This is a report of a previously undescribed type of onychomatricoma (OM) with an unusual clinical presentation as a thickened free edge of the nail plate without discernible cavities and distinguished histologically from the ordinary OM by 3 features: (1) the lack of cavitation at the proximal border of the nail plate and the small sizes of the cavities at the free edge of the distal nail plate; (2) a papillated epithelial hyperplasia pattern very different from the digitate pattern of the ordinary OM; and (3) a special pattern of matrical keratinization with pseudohorn cysts that mirror closely those found in onychocytic matricoma (OCM). Furthermore, the sex ratio and sites of the lesion seem different than those of conventional OM with the caveat that the numbers in this series are small. A practical approach to the diagnosis of onychogenic tumor mainly involves consideration of tumors that clinically present as localized longitudinal pachyonychia including melanoma and Bowen disease. Whether pachyonychia is caused by a thickened nail plate or by a localized band of subungual hyperkeratosis may not be clinically and dermoscopically obvious, and leucoxanthonychia or melanonychia is observed in OCM, OM, and onychocytic carcinoma. Therefore, the definitive diagnosis of these 3 onychogenic tumors is made by histopathology on nail clipping specimen or nail biopsy. OM is easily diagnosed as a fibroepithelial tumor keeping in mind its micropapilliferum variant which can simulate trichoblastoma or basal cell carcinoma on biopsies without nail plate. In these biopsies, the fibroepithelial portion of OM micropapilleferum resembles trichoblastoma including trichoepithelioma, or keratotic basal cell carcinoma, whereas the pseudohorn cysts may be mistaken for seborrheic keratosis. As previously indicated in the seminal report of OCM and perfectly demonstrated in this series, the pseudohorn cysts of both OCM and OM micropapilleferum have 2 distinct layers with a ring pattern, the prekeratogenous and keratogenous zone, and the transitional eosinophilic onychocytes become progressively clear with shadow cells. By contrast, horn cysts with hyaline and trichilemmal keratinization have rounded or irregular shapes, a thin inner layer of eosinophilic cells with large, oval, pale, vesicular nuclei, and are filled with compact keratinous masses without transition to onychocytic shadow cells. The squamous eddies of irritated seborrheic keratosis are easily differentiated from the pseudohorn cysts of OM by their inner layer of eosinophilic flattened squamous cells, and their loose or compact eosinophilic keratinous masses without transition to onychocytic shadow cells. To avoid confusion with the pseudohorn cysts of seborrheic keratosis which present a thin granular layer and laminated cornified cells, we propose to designate the pseudohorn cysts of both OM and OCM as keratogenous spheres. The papillae of the latter end as a tip without keratogenous zone explaining the microcavities. The microcavities getting in touch with the surface of the nail plate are responsible for the white dots (the so-called milia cysts) observed by dermoscopy both in OCM and OM micropapilliferum. The low, projecting ridges separated by the irregular longitudinal furrows explain the clinically irregular white line. The evenly thickened free edge of the distal nail plate is explained either by the small size of the cavities or the presence of a keratogenous zone at the tip of the papillae which manufacture a homogeneous thick nail plate. This free edge nail wall-like pattern (with or without a pitted wall) is in stark contrast to the usual honeycomb-like cavity pattern seen in conventional OM. It is inferred that these dermatoscopic findings could be clinical clues to differentiate both OCM and OM micropapilliferum from conventional OM. In the initial description of OCM, this entity was clearly differentiated from seborrheic keratosis. From time to time, these 2 lesions continue to pose problems in the histological differential diagnosis, and OCM with its various clinical presentations as leucoxanthonychia or melanonychia has been described using different names as subungual seborrheic keratosis, nail unit acanthoma, or longitudinal subungual acanthoma. These new superfluous synonymies add confusion in nail tumors. In the estimation of the author, these so-called new entities are OCM, if the histologic criteria of keratogenous spheres defined in this article are used. In sum, there are 2 clinicopathological variants of OM: macropapilliferum and micropapillerum. As OM micropapillerum has small cavities, the main differential diagnosis on nail clipping is onychocytic carcinoma.