Melanonychia: visually differentiating the good from bad

What is Melanonychia?

Melanonychia, a term derived from the Greek "Melas" (meaning black or brown) and "Onyx" (meaning nail), describes the brownish-black discoloration of the nail plate, primarily due to melanin pigment. Although the pigmentation can occur in various morphologies it is the longitudinal stripe (Longitudinal melanonychia [LM]) running from the cuticle to the free edge that gives most concern to practitioners as a possible nail unit melanoma (NUM).

What causes longitudinal melanonychia?

The causes of melanonychia are remarkably diverse, encompassing a wide spectrum from common benign causes to less common but potentially invasive and in situ (see my earlier blog).

Causes of longitudinal melanoma

•       Exogenous pigment: varnish, henna etc.,

•       Subungual haemorrhage

•       Trauma

• Inflammatory conditions (paronychia, lichen planus, psoriasis)

• Onychomycosis

• Lentigo

• Melanocytic lesions (moles)

• Inherited conditions

•     Systemic illness (such as adrenal dysfunction, thyroid disorders)

•      Drug-induced pigmentation (many drugs are implicated)

•         Ethnicity (often involves multiple digits)

•         Bowen's disease of the nail unit

•       Subungual squamous cell carcinoma (SCC)

•       Melanoma in situ

•       Nail unit melanoma (NUM)

(Adapted from Bristow and De Berker 2010 [1])

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This broad range of causes presents a significant diagnostic dilemma for practitioners and may lead to delays in diagnosis. While the majority of melanonychia cases are benign, it is important to recognize that the most serious pathology NUM, frequently presents with LM. Nail unit melanoma is rare accounting for only 3.5% of all melanomas worldwide [2]. The condition occurs most frequently in the fifth to seventh decades and is extremely rare in children. Haneke [3] provides this significance of age table in predicting NUM (table 1):

Table 1: Significance of patients age with melanonychia [3]

Prognosis of NUM

Diagnostic difficulty often leads to initial misdiagnosis and substantial diagnostic delays, averaging two years (Andre 2007), which significantly contributes to the notably poor prognosis associated with nail apparatus melanoma (NAM), with reported 5-year survival rates as low as 40% in tumours thicker than 2.5mm [4].

Clues to Recognising Nail unit melanoma

Previous papers have highlighted pointer to suggest features suggestive of NUM, these include:

•     A longitudinal pigmented band typically wider than 3 mm

•     Most common in 1st & 2nd digits (finger and toenails)

•     A longitudinal band  exhibiting proximal widening (widening towards the cuticle) creating a triangular shape band.

•      Irregular or blurred borders of the pigmented band

•      Variegation in colour, with irregular pigmentation (light and dark brown, black), and sometimes criss-crossing patterns.

• A rapid change in size and growth rate (increase in width) over time.

• Associated nail plate dystrophy, such as thinning, splitting, cracking, distortion, destruction, or lifting of the nail from the nail bed (onycholysis).

• The presence of ulceration, bleeding, blood spots, or the formation of a nodule/mass under the nail.

• New onset of the band in an older adult

• Hutchinson's sign (extension of pigment from the nail plate onto the surrounding skin of the proximal or lateral nail folds) - see below.

Nail Unit Melanoma demonstrating pigment spread onto the periungual tissues (Hutchinson's sign)

The diagnostic challenge of early melanoma

Despite these indicators, the initial presentation of subungual melanoma as a narrow, seemingly innocuous melanonychia band allows for a period of silent progression. This often leads to the disease being diagnosed at a more advanced stage, despite the existence of well-defined "red flag" signs (above) that typically appear later in the disease course. Biopsies of the nail unit are the gold standard in diagnosis that can be painful and lead to permanent nail damage and scarring so are only carried out after careful assessment.

Can Dermoscopy help?

From the available literature it is apparent that early NUM often resembles other benign causes of LM. The introduction of dermoscopy has added an additional dimension in the assessment of pigmented streaks. A recently published paper examined a cohort of 62 patients with early LM who were monitored over time using dermoscopy [5].

At the end of the study (median 17 months of surveillance) of the 62 patients, 27 of the lesions were biopsied. Among these, six cases (9.7%) were diagnosed as melanoma in situ, nine (14.5%) as nevi and 12 (19.4%) as lentigo or melanocytic hyperplasia. Three features were highlighted as being more common in NUM during the surveillance period (table 2):

Table 2: Dermatoscopic features associated with melanonychia [5]

The study was conducted on a small, European cohort of Caucasian patients but offers new, additional insights into potential clues.

Clinical Relevance

Pigmented nail streaks are an uncommon presentation in podiatry clinics but nevertheless are a cause for concern. Running through a checklist as outlined above can be helpful to separate benign from more sinister causes. Below is a summary table which I have compiled from a range of sources highlighting characteristics of benign causes of melanonychia versus nail unit (subungual) melanoma. This list is not exhaustive but offers pointers to potential clues (table 3).

Table 3: Features of Melanonychia associated with melanoma and benign causes.

Where there is any doubt, a referral is always advisable.

Further reading:

Jefferson & Rich (2012) Melanonychia (Full text here)

Gradinaru et al., (2020) Melanonychia - Clues for a correct diagnosis (Full text here)

References

1.            Bristow, I.R., et al., Clinical guidelines for the recognition of melanoma of the foot and nail unit. J Foot Ankle Res, 2010. 3(25).

2.            Tosti, A., B.M. Piraccini, and D.C. de Farias, Dealing with melanonychia. Seminars in Cutaneous Medicine and Surgery, 2009. 28(1): p. 49–54.

3.            Haneke, E., Important malignant and new nail tumors. JDDG: Journal der Deutschen Dermatologischen Gesellschaft, 2017. 15(4): p. 367–386.

4.            Banfield, C. and R. Dawber, Nail melanoma: a review of the literature with recommendations to improve patient management. British Journal Of Dermatology, 1999. 141(4): p. 628–632.

5.            Moscarella, E., et al., Digital dermoscopy follow‐up for acquired longitudinal melanonychia. Journal of the European Academy of Dermatology and Venereology, 2025.