Nails in Skin of Color
Acral lentiginous melanoma (including the nail unit) is the most common melanoma subtype in black and Asian patients. Melanoma of the nail unit can be difficult to diagnose in persons of color as they are also more likely to have benign nail hyperpigmentation.
Longitudinal melanonychia (LM) (also called melanonychia striata) is a pigmented band in the nail plate resulting from melanin deposition. This may result from activation or proliferation of nail matrix melanocytes. Melanocytic proliferation of the nail matrix is caused by nevi and melanoma and melanocytic activation produces ethnic pigmentation and nail lentigo. The nail bed does not normally have melanocytes.
Hyperpigmented nails or LM in persons of color has been given many names:
- ethnic melanonychia (racial melanonychia)
- ethnic-type melanonychia
- ethnic melanosis
- ethnic hyperpigmentation
- ethnic pigmentation
In each case the term racial is sometimes used instead of ethnic, but it is our preference to use ethnic as the concept of race can be murky and fraught with bias. The terms benign and nail can also be added to the labels above.
Hutchinson sign is the extension of hyperpigmentation to the nail folds or hyponychial skin as seen in melanoma. Pigmentation in the hyponychial skin may be associated with a parallel ridge pattern on dermoscopy.
Micro-Hutchinson sign is pigmentation of the cuticle that is not visible to the naked eye but visible by dermoscopy. It is very concerning for melanoma when found. It has also been described in congenital nevi in children. Pseudo-Hutchinson sign is the presence of dark pigment around the proximal nail fold secondary to benign conditions such as ethnic melanosis and not melanoma. Another cause of pseudo-Hutchinson sign is a translucent cuticle below which the pigment of LM is visible. Trauma and drug-induced pigmentation can also produce a pseudo-Hutchinson sign.
Benign nail pigmentation is more common in persons of color. In one study, pigmented bands occurred in 77% of 200 African Americans older than age 20 years.  and in almost 100% of those older than age 50 years. 
It also occurs in 10% to 20% of persons of Japanese descent. LM is unusual in whites, occurring in only approximately 1% of the population. 
Approach to nail lesion diagnosis
Causes of nail hyperpigmentation
- Subungual melanoma and SCC of the nail unit must be considered first.
- Ethnic melanonychia is due to benign melanocyte activation that often involves several nails and is more common in skin of color. 
- Chronic trauma, especially in the great toes may cause melanocytic activation and nail darkening. It may occur in the fingernails from trauma to the proximal nail fold and cuticle such as in habit tic deformity of the nail.
- Skin diseases with nail involvement such as psoriasis, lichen planus, amyloidosis, scleroderma and Darier’s disease may result in nail color changes.
- Drugs causing melanonychia include chemotherapy agents, antimalarials and psoralens. Drug-induced nail pigmentation typically and affects multiple nails.
- Endocrine disorders, such as Addison disease, Cushing syndrome, hyperthyroidism, and acromegaly, can be responsible for LM.
- Subungual hematoma/hemorrhage has a distinct dermatoscopic pattern of proximal globules with distal streaks. The color may range from red to brown to black.
- Nail infections - trichophyton rubrum and some non-dermatophyte molds (particularly Neoscytalidium species) produce pigmented hyphae that can cause green or brown nail discoloration. The cloud-like appearance with dermoscopy can help to make this diagnosis along with KOH, culture or PAS stain.
In a retrospective observational study performed by the IDS, nail melanoma cases were significantly associated with a pigmented band involving greater than 2/3 the width of the nail plate, grey and black colors, irregularly pigmented lines, Hutchinson and micro-Hutchinson signs, and nail dystrophy. Granular pigmentation was found in 40% of melanomas and only in 3.5% of benign lesions. 
Other dermoscopic features of nail melanoma: longitudinal brown to black lines with irregular color, spacing, and thickness. Lines usually show loss of parallelism and may vary within single lines. Other reasons to be concerned for nail melanoma include: abrupt onset of LM after middle age, personal or family history of melanoma, rapid growth, darkening of a melanonychia band, pigment variegation, blurry lateral borders, irregular elevation of the surface, a band width >3 mm, proximal widening, associated nail plate dystrophy, and single rather than multiple digit involvement. 
Nail squamous cell carcinoma
- Leyden et al.: Diffuse and banded melanin pigmentation in nails. Arch Dermatol 1972;105:548-50. PMID: 5017264.
- Ruben: Pigmented lesions of the nail unit: clinical and histopathologic features. Semin Cutan Med Surg 2010;29:148-58. PMID: 21051008. DOI.
- Baran & Kechijian: Longitudinal melanonychia (melanonychia striata): diagnosis and management. J Am Acad Dermatol 1989;21:1165-75. PMID: 2685057. DOI.
- Astur et al.: Reassessing Melanonychia Striata in Phototypes IV, V, and VI Patients. Dermatol Surg 2016;42:183-90. PMID: 26845538. DOI.
- Benati et al.: Clinical and dermoscopic clues to differentiate pigmented nail bands: an International Dermoscopy Society study. J Eur Acad Dermatol Venereol 2017;31:732-736. PMID: 27696528. DOI.
- Leung et al.: Melanonychia striata: clarifying behind the Black Curtain. A review on clinical evaluation and management of the 21st century. Int J Dermatol 2019;58:1239-1245. PMID: 31006857. DOI.