Melanonychia Workup

If an infectious etiology is suspected, a nail plate clipping should be sent for histologic analysis and appropriate cultures.^[1]

Depending on the suspected systemic and/or dermatologic causes, appropriate laboratory tests should be ordered.

Examination of affected nails with a dermatoscope can provide useful information regarding both the etiology and location of longitudinal melanonychia.^[19] Considering the dermatoscopic appearance of subungual nevi and subungual melanoma, nevi have been reported to demonstrate regular parallel lines, while subungual melanoma has been reported to demonstrate irregular lines with some disruption of parallelism.

Longitudinal melanonychia of a single nail unit in an adult is concerning, and a biopsy of the nail unit can evaluate for the possibility of melanoma, which cannot be differentiated from benign causes of longitudinal melanonychia solely based on clinical examination.^[20]

Dermatoscopic examination of the free edge of the nail plate can help to determine the origin of longitudinal melanonychia within the nail unit and can therefore be helpful in determining which anatomic area of the nail unit requires biopsy. If the pigmentation is in the lower portion (ventral aspect) of the nail plate, the origin of pigmentation is in the distal nail matrix. Conversely, if the pigmentation is in the upper portion (dorsal aspect) of the nail plate, the origin of pigmentation is in the proximal nail matrix.^{[21, 22]} Pigmentation present throughout the nail plate indicates a lesion that extends from the proximal to distal nail matrix. An adjunct to this technique is to perform a nail clipping of the affected distal nail plate and to stain this specimen with a Fontana-Masson histologic stain. The portion of the nail plate affected by melanonychia is then easily visualized.

A variety of surgical techniques can be used to sample pigmented lesions located within the nail matrix, including the punch biopsy, matrix shave biopsy, and lateral longitudinal excision.^[23]

For bands wider than 3 mm, a 3-mm punch biopsy can be used if the origin of the band is located in the distal matrix. The risk of causing a subsequent nail plate dystrophy after the procedure is greater the further proximal the area of the matrix that is sampled.

After reflection of the proximal nailfold, to expose the nail matrix, a 3-mm punch is performed, sampling the origin of the pigmented band. The punch is pressed through the nail matrix and down to the underlying bone. Once the biopsy specimen is removed, it can be inked to help the pathology staff best orient the specimen before processing. Depending on the nature of the surgical techniques used, some control of hemostasis may be required. The proximal nailfold is returned to its initial location, and it can be secured in place by a variety of methods.

The matrix shave is useful for pigmented bands that originate in the proximal matrix, bands that are in the central portion of the nail unit, and/or those that measure 3-6 mm in diameter. This technique was originally described by Eckart Haneke and has the advantage of causing less nail dystrophy post procedure because a full-thickness wound is not created in the nail matrix.

To perform the nail matrix shave, the proximal nail fold is reflected to expose the matrix, and then the proximal nail plate is also reflected away from the biopsy site. The origin of the pigmented band in the nail matrix is scored with 1- to 2-mm margins with a scalpel blade, which is followed by removal of the area using a shave technique by gently passing the blade under the pigmented area, removing only a small thickness of epithelium. Titanium-coated scalpel blades have been reported to be particularly effective to obtain optimal results with this technique. The proximal nail plate and nail fold are then returned to their original anatomic positions.^[23] The specimen can be inked for orientation, and it may be placed on a piece of filter paper before being put in a formalin-filled container to prevent curling.

A lateral longitudinal excision is useful for pigment that is in the lateral 30% of the nail unit.^[24] In this procedure, a near elliptical excision is made. The scalpel is inserted 1-2 mm medial to the pigmented band halfway between the cuticle and distal interphalangeal crease and extended distally 3-4 mm onto the digital bed. The scalpel is again inserted proximally at the starting point and moved laterally around the matrix horn, curving medially at the hyponychium to meet the endpoint of the first incision. The tissue is removed with fine-tipped scissors with the “tips down” in a distal-proximal fashion.^[23]

In order to visualize anatomic structures within the nail unit, the nail plate is commonly removed during nail surgery. Complete nail plate avulsion is less favorable than partial nail avulsion because of a higher comparative likelihood of complications, including postoperative pain, distal plate embedding, and dorsal pterygium formation. Alternatives to complete nail plate avulsion include partial nail plate avulsion techniques, such as the trap door avulsion, longitudinal partial plate avulsion, window nail plate avulsion, partial distal nail plate avulsion, and lateral nail plate avulsion.^[22]

When clinical suspicion is high for subungual melanoma, it is important to sample the entire pigmented lesion with a full-thickness excision, because a partial biopsy may lead to a delay in diagnosis.^[25]

When possible, the source of the pigment production should be removed via biopsy to prevent recurrence of longitudinal melanonychia.

Excisions larger than 3 mm and those from the proximal matrix are more likely to produce permanent nail dystrophy.^[23]

The histopathologic findings vary based on the etiology of melanonychia.

Nail matrix nevi can have an unusual appearance histologically, and they are considered to be “special site nevi,” a subset of acral pigmented lesions. The majority of nail matrix nevi are junctional, although compound nail matrix nevi have been reported. Histologically, nail matrix nevi can be highly cellular; can have hyperchromatic, large cells that do not form discrete nests; and can possess prominent, abundant, and uneven cytoplasmic dendrites.^[26]

One study evaluated 20 subungual melanomas and 15 benign subungual melanotic macules to determine the histopathologic features that can help to differentiate these 2 entities. A statistically significant difference in the mean melanocyte count (number of melanocytes per 1-mm stretch of subungual dermoepithelial junction) was noted for invasive melanoma (mean 102; range 51-212) compared with subungual melanotic lentigines (mean, 15.3; range 5-31). Other histopathologic features seen in subungual melanoma compared with subungual melanotic macules included a confluence of melanocytes, multinucleation of melanocytes, florid pagetoid spread of melanocytes, inflammation at the epithelial stromal interface, and moderate and/or severe cytologic atypia of melanocytes.^[27]

A Fontana-Masson stain highlights melanin and thus may be useful in the determination of location of pigment within the nail matrix epithelium. Of note, 20-30% of subungual melanomas may be amelanotic,^{[15, 28]} and immunostains for melanocytes, such as S-100, Melan-A, HMB-45, and MITF, may provide important diagnostic information.