Use of Nail Dermoscopy in the Management of Melanonychia: Review

The term melanonychia describes a black-brown-gray pigmentation of the nail plate that, in most cases, appears as a longitudinal band. Melanonychia can be observed at any age, affecting one digit or several digits, in both fingernails and toenails. Its causes can be difficult to differentiate clinically and depend on the number of bands and on their color, edge, and width. For this reason, especially in adults and when the pigmentation is localized in one single digit, biopsy and histopathological examination still represent the gold standard for a definitive diagnosis. Dermoscopy should be used routinely in the evaluation of a pigmented nail, as it provides important information for the management of melanonychia and can help avoid unnecessary nail biopsies. In cases of melanonychia it is important to establish whether the pigment is melanin or not and to determine whether the pigmentation of melanin is due to activation or proliferation and whether it is benign or malignant.


Introduction
The term melanonychia describes a black-brown-gray pigmentation of the nail plate. Although nail matrix melanocytes are normally quiescent, these cells can eventually become activated with or without proliferation, thus inducing the pigmentation of the nail plate. Melanonychia can present itself in many different ways, but most frequently it appears as a longitudinal band (longitudinal melanonychia) that starts from the proximal margin of the nail and extends to the distal margin, following the growth of the nail.
Less commonly, the pigment can be transverse (transverse melanonychia) [1] or even involve all of the nail plate (total melanonychia). Melanonychia can be observed at any age, affecting one digit or several digits, in both fingernails and toenails.
The causes of longitudinal melanonychia can be difficult to differentiate clinically and, first of all, depend on the number of the bands and on their color, edge, and width.
Biopsy and histopathological examination is recommended when a longitudinal melanonychia occurs in an adult and it is localized in one single digit, even more so in the absence of local or systemic causes that may explain its onset. The real problem of how to distinguish whether the lesion is benign or malignant remains, as an early diagnosis of melanoma can be crucial for the prognosis.
If the pigmentation is due to melanin produced by nail matrix, the second step of melanonychia management is to identify whether the pigmentation is due to an activation or proliferation. The number of digits involved is an important diagnostic clue: if more than one digit is affected, it is likely to be a melanocytic    in childhood melanonychia [11].
Nonetheless nail matrix nevi may be present at birth; they can also occur with age. Nail matrix nevi affect fingernails more frequently than toenails, most often the thumb. Usually, the nail has one or more longitudinal heavily pigmented bands whose size can vary from a few millimeters to the whole nail and whose color may not be homogeneous and more or less dark [3]. Dermoscopic    (Figure 7). However, this rule is not always reliable, as it is possible to find lines that are irregular in width or color also in benign lesions [5,18].  indicate decreased melanocytic activity from nevus cells [14].
The diagnostic accuracy of longitudinal melanonychia has improved over  lines. Nevertheless, a delayed diagnosis of melanoma remains common and a poor prognosis is often attributable to longstanding disease. The misdiagnosis rate has been found to be higher than 20% [22].
Intraoperative dermoscopy enables visualization of the origin of the pigmentation, revealing aspects not observed when the nail plate is interposed between the pigmented lesion and the dermatoscope [23]. This type of dermoscopy is more reliable and accurate than plate dermoscopy and has high sensitivity and specificity in differentiating pigmented  There are no evidence-based studies that determine the ideal frequency of dermoscopic follow-up in patients with longitudinal melanonychia nor any that make precise dermoscopic criteria to decide when the pigmented lesions require biopsy [3]. One suggestion is to monitor the patient every 6 months with dermoscopy in cases of benign melanocytic lesions and to have a strict follow-up every 3 months in cases of irregular pigmentation without other signs of malignancy [25]. Nevertheless, the gold standard for a definitive diagnosis of nail pigmentation still remains histopathology [26,27].

Conclusions
Dermoscopy of the nail (onychoscopy) is nowadays used routinely, as it provides important information, giving specific criteria for the dermoscopic assessment of nail pigmentation.
The causes of melanonychia vary from subungual hematoma, to a fungal and/or bacterial infection, to a melanocytic lesion (nevus melanoma, etc), and the differential diagnosis is not easy for the nonexpert dermatologist. In addition, the management is often a reason for concern. Dermoscopy helps to identify the type of pigment and to determine the origin of the pigment. It is an important technique for the evaluation of melanocytic pigmentation; however, it has to be added to clinical data and, in cases of suspected melanoma, to biopsy for pathology.