Reversible Melanonychia Revealing Nutritional Vitamin-B12 Deficiency (2024)

A 10-year-old boy presented with blackening of multiple fingernails and toenails. He was a strict vegetarian by diet. The child was active with no past history of any serious illness. The general examination was unremarkable. Mucocutaneous examinations revealed multiple closely set longitudinal melanonychia. It was numerous in the first three nails imparting a bluish-black discoloration to the entire nail plate, [Figure 1]. Increased pigmentation of proximal nail folds (Pseudo-Hutchinson's sign), knuckles and bony prominences were also noted. No fungal pathogen was demonstrated on microscopy of nail clippings. Blood investigations revealed a hemoglobin level of 117 g/L, the mean corpuscular volume of 84.2 fl, and a peripheral smear demonstrating normocytic-normovolemic red cells with an adequate number of platelets. The iron profile was normal. His vitamin-B12 level was 143 ng/L (normal 180–914). His serum hom*ocysteine level (23.65, normal 5–15) and methylmalonic acid levels (0.6 nmol/mL, normal <0.4) were raised. The boy was euthyroid, his morning serum cortisol level was normal and the serology for HIV was negative. A provisional diagnosis of melanonychia due to vitamin-B12 deficiency was made.

We advised intramuscular cyanocobalamin (1000 μg/week) for the boy. After 8 weeks, his serum vitamin-B12 levels normalized along with improvement in melanonychia. Monthly intramuscular cyanocobalamine administration (100 μg/month) was continued for another three months and there was further improvement in melanonychia [Figure 2].

Melanonychia, especially longitudinal melanonychia is a constitutional finding in dark-skinned adults. Other causes include melanocytic activation due to either systemic (pregnancy, hyperthyroidism, HIV infection, Addison's disease, nutritional deficiencies, certain drugs) or local (chronic paronychia, onychomycosis, psoriasis, lichen planus) aetiologies. Melanocyte hyperplasia as in benign conditions like lentigines and naevi or malignant conditions like melanoma may also present as melanonychia. Isolated involvement of a single nail, asymmetry in the distribution of pigments, and extension of pigmentation to the nail folds (Hutchinson's sign) helps to clinically suspect melanoma. Laugier–Hunziker syndrome, Peutz- Jegher and Touraine syndromes are some syndromic causes for melanonychia. Apart from melanonychia, these conditions also present with mucosal pigmentation (in all three conditions) and gastrointestinal malignancies (in Peutz-Jegher and Touraine syndrome). Nonmelanocytic causes such as subungual hematoma and certain exogenous pigments can also present as pigmentation in the nails. A subungual hematoma is a common differential diagnosis for melanonychia and can be differentiated by its globular pattern and uniform brown-black pigmentation in the absence of longitudinal lines on dermoscopy. The colored substances causing exogenous pigmentation adhere to the nail plate and the proximal margins following the border of the cuticle. Common causes of exogenous pigmentation are henna, colored nail lacquer, nicotine, dirt, and potassium permanganate.[1]

Reversible melanonychia has been reported with vitamin-B12 deficiency, especially in dark-skinned individuals.[2] Reduced glutathione levels in B12 deficiency causes disinhibition of tyrosinase, the major enzyme involved in melanogenesis.[3] Strict vegetarian diet, knuckle pigmentation, low serum vitamin-B12 levels, and a raised serum hom*ocysteine along with raised methylmalonic acid levels (c.f. normal methylmalonic acid levels in folate deficiency) corroborated a diagnosis of vitamin-B12 deficiency in the patient. Reversal of melanonychia along with normalization of serum B12 levels after supplementation of the same reinforced the deficient vitamin-B12 level as the cause of melanonychia in the boy.

Vitamin-B12 deficiency can present with glossitis, pigmentary changes of nails, hairs, and skin or more serious features like megaloblastic anemia and neuropsychiatric symptoms.[2] Neuropsychiatric features, though rare, can become irreversible in severe and prolonged deficiency.[4,5] Timely diagnosis of vitamin-B12 deficiency based on general examination findings like melanonychia helps in the early institution of replacement therapy and hence the prevention of hematological and neurological complications.

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