by Gina B. Rainwater, M.D.

Melanocytic lesions on acral sites, particularly nails, pose significant diagnostic and management difficulties. In this handout, we discuss clinical presentation, evaluation, and biopsy

techniques for melanocytic lesions of nail unit. We will also discuss common mimickers of melanocytic lesions and their management.

  • Nail unit melanoma comprises 1.5-2% of all melanomas in Caucasians, 17% in Hong Kong Chinese, 23% in Japanese, and 25% in patients of African descent with peak incidence in 60th-70th decade.
  • Acral melanomas arise on the feet more than 75% of the cases, but subungual melanomas are more common on the fingers than on the toes.
  • Any finger or toe can be affected, but the thumb and hallix are disproportionately involved.
  • 25% of nail melanomas are amelanotic (devoid of pigment) [Fig. 2A].
  • Mortality depends on the depth of invasion, with nail melanomas less than 2.5 mm associated with 88% 5-year survival, while tumors greater than 2.5 mm associated with only 44% 5-year survival.
  • There is no clear-cut relationship with UV exposure
  • Acral melanoma is genetically more akin to melanoma arising in non-sun-exposed skin
  • There is a possible association with nail trauma (reported history of trauma in 23-44% cases)
  • Trauma to an already clinically evident subungual melanoma has been associated with a worse prognosis.

Challenges in Diagnosing Nail Unit Melanoma

  1. Clinically mimics multitude of nail diseases (nail hemorrhage, pigmented onychomycosis, lentigo/nevus, melanocytic activation due to trauma, medication-induced hyperpigmentation, ethnic/racial hyperpigmentation, pregnancy, Peutz-Jegher’s syndrome, endocrinopathies, etc)
  2. May be present for many months, years, even decades
  3. Often asymptomatic and amelanotic, thus low motivation to seek medical advice
  4. Physicians are reluctant to perform nail biopsies due to time constraints in the clinic, lack of familiarity with procedure, lack of proper instrumentation, and, often, insufficient sampling of the lesion.

Evaluation of Melanocytic Lesions of the Nail Unit and Decision to Biopsy

  • The widespread adoption of “ABCDE criteria” for detection of cutaneous melanoma has helped to increase public and physician awareness, and thus helped increase early detection of melanoma. But, the same criteria cannot be applied to the diagnosis of nail unit melanoma. Levit EK, et al., developed the “ABCDEF” approach to evaluating subungual melanoma,” which was published in JAAD in 2000 [Table 1].
  • In brief, a pigmented nail lesion that is clinically changing in an older patient, particularly of darker skin type, that is greater than 3mm in width, affecting a single nail, especially of the thumb or hallux, warrants a biopsy.
  • Two clinical signs are particularly useful is evaluating melanocytic lesions of the nail:
  1. Hutchinson’s sign – an extension of the pigment onto the lateral and/or proximal nail fold [Fig. 3].
  2. Melanonychia exhibiting wide base with distal tapering signifies rapidly growing lesions and warrants a biopsy [Fig. 4].

Biopsy Techniques

  • It is critically important to obtain an adequate sample of nail matrix for accurate and timely diagnosis of melanocytic lesions of the nail [Fig. 5]. If the nail matrix is not available for histopathological examination, the biopsy report is often inconclusive, requiring a repeat procedure and delaying the final diagnosis and treatment.
  • Several biopsy techniques can be implemented to adequately sample nail matrix.

  • Matrix shave biopsy [Fig. 6]
  • Matrix punch biopsy [Fig. 7]
  • Lateral longitudinal excision [Fig. 8]

Conclusions

  • Melanocytic lesions of nail apparatus is a difficult clinical diagnosis and requires a high clinical suspicion
  • It is important to maintain a low threshold to biopsy/excise nail abnormalities involving a single nail, particularly thumb or hallux.
  • For accurate histopathological diagnosis, it is essential to obtain sufficient tissue including nail matrix given a high risk of sampling error

References

  1. R. K. Scher and C.R Daniel III. Nails: Diagnosis, Therapy, Surgery. 3rd edition. 2005
  2. Jellinek N. Nail matrix biopsy of longitudinal melanonychia: diagnostic algorythm. J Am Acad Dermatol. 2007: 56 (5), 803-10.
  3. Elston D. Practical advice regarding problematic pigmented lesions. J Am Acad Derm 2012, 67 (1), 148-155
  4. Madankumar R. et al. Acral melanocytic lesions in the United States: prevalence, awareness, and dermoscopic patterns. J Am Acad Derm 2016, 74 (4), 724-730
  5. Massi G. and LeBoit P., Histological Diagnosis of Nevi and Melanoma. 2nd ed.