Melanonychia is the term used to describe any brown/black discolouration of the nail. Most often, it presents as a longitudinal band running from the cuticle to the free edge of the nail [1]. It has many causes but rarely it may be a result of a malignant process such as melanoma. For the clinician, diagnosis can be difficult. In the foot, friction and trauma are often overlooked as frequent causes of pigmented nail bands – as this blog explores.
Longitudinal melanonychia is frequently a cause for concern for podiatrists when confronted with it in the clinic but it is important to stress it has many causes – including melanoma of the nail unit, but this is a rare diagnosis. Benign causes are much more common, particularly in patients with darker skin types. To help understand melanonychia it is pertinent to review the physiology of the condition.
What causes melanonychia?
The nail matrix is the generative part of the nail unit and contains melanocytes (around 200 per mm2) within its basal layer – like other parts of the skin. For the most part, these melanocytes are quiescent and produce no discernible pigmentation. When skin becomes damaged or inflamed it is not uncommon to observe temporary changes in the skin’s pigmentation (known as post-inflammatory hyperpigmentation). This is a result of increased melanocyte activity. Melanocytes become activated and increase production of melanin and consequently localised skin can be seen to darken as a result at the site of injury.
A similar process occurs in the nail unit. Activation of the nail unit melanocytes causes production of melanin which is incorporated into the emerging nail causing a longitudinal grey-brown line of melanonychia to develop. This arises due to the release of prostaglandins and leukotrienes which stimulate the nail unit melanocytes to increase transfer of melanosomes along the dendritic processes of the melanocyte into the developing nail keratinocytes (onychocytes) resulting in a black/grey line within the nail. This process of melanocyte activation is known to occur more readily in those with darker skin types [2] and in multiple nails [3].
A case of “frictional” melanonychia
A recent paper [4] published in the International Journal of Dermatology highlights the case of a patient who presented with a bilateral, symmetrical melanonychia in both halluces along with a hyperpigmented patch on the lateral side of her talar head on the dorsum of the foot. The patient had been wearing shoes with a metal buckle pressing on the talar head and narrow toe box. Examination of the pigmented bands showed a consistent, black-grey colour, with homogeneous structure and regular borders.
A case of frictional melanonychia Osaka et al., (2023)
Pressure on the dorsum of the foot (talar head) and on both first nails, had likely caused trauma and stimulated inflammation and subsequent melanocytic activity resulting in hyperpigmentation in the skin and in the nail unit, resulted in a longitudinal melanonychia in the nails. Other inflammatory disorders known to trigger melanonychia include lichen planus and psoriasis. Longitudinal melanonychia can also be seen in nails which are traumatised by regular nail biting, for example [5].
Implications for podiatrists
Brown streaks in the nail are a common cause for concern. It should be remembered that most causes are benign, and that melanoma is rare. According to a review of 163 patients with discoloured nails [1], melanonychia was the most common documented cause, with trauma and inflammatory conditions being the most frequently presumed cause of the condition. With the foot, being a common recipient of trauma from footwear, injury and activity, it is important to consider “mechanical” causes particularly as they are so common. A future blog will look at additional causes of melanonychia.
Whilst rare, it is important to remain alert to the possibility of nail unit melanoma and refer on any patients with nails that have suspicious features.
What are the clues to nail melanoma? [6]:
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Single nail affected, particularly in the first and second digit.
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Patients over the age of 50.
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A progressive enlargement/widening/evolution of the streak.
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Accompanying nail plate splitting or dystrophy.
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More than 40% of the nail plate affected.
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Spread of pigmentation onto surrounding soft tissue.
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Variegation of colour within the pigmented streak.
Where there is doubt, a second opinion should be sought.
References
1. Bae SH, Lee MY, Lee JB: Distinct Patterns and Aetiology of Chromonychia. Acta Derm Venereol 2018, 98:108-113.
2. Kaufman BP, Aman T, Alexis AF: Postinflammatory Hyperpigmentation: Epidemiology, Clinical Presentation, Pathogenesis and Treatment. Am J Clin Dermatol 2018, 19:489-503.
3. Tosti A, Piraccini BM, de Farias DC: Dealing with melanonychia. Semin Cutan Med Surg 2009, 28:49-54.
4. Oska C, Morrison B, Tosti A: Postinflammatory pigmentation and friction melanonychia. Int J Dermatol 2023, 62:e474-e476.
5. Lee DK, Lipner SR: Update on Diagnosis and Management of Onychophagia and Onychotillomania. Int J Environ Res Public Health 2022, 19.
6. Levit EK, Kagen MH, Scher RK, Grossman M, Altman E: The ABC rule for clinical detection of subungual melanoma. J Am Acad Dermatol 2000, 42:269-274.
