Acral Acanthosis Nigricans with Concurrent Vitamin B12 Deficiency in an Indian Patient: An Atypical Presentation

Acanthosis Nigricans (AN) is characterised by velvety, papillomatous, brownish-black, hyperkeratotic plaques, typically on the intertriginous areas and neck. It is more commonly seen in the African American descent, rarely occurs in the Indian population. The present case includes 28-year-old male, who presented to the Outpatient Department with asymptomatic hyperpigmention over skin of both feet and knuckles of both hands since three months. The distribution pattern over the knuckles was suggestive of vitamin B12 deficiency however the atypical distribution over the feet leads to suspicion of a differential diagnosis of lichen planopigmentosus, vitamin B12 deficiency and acral AN. Dermoscopy was done to aid in the diagnosis and revealed linear crista cutis and sulcus cutis which are features suggestive of AN. On investigation, serum vitamin B12 levels were below 100 μg/mL and histologic sections showed hyperkeratosis with papillomatosis and mild acanthosis. The patient was treated solely with 1500 mcg mecobalamine daily for 10 days and followed-up. The dermoscopic and histopathological findings were consistent with the diagnosis of acral AN and thus a final diagnosis of acral AN superimposed with vitamin B12 deficiency was made. This case is being reported due to its rare occurrence in the Indian population, the atypical clinical presentation of AN in form of macules and to highlight dermoscopy as an essential non invasive tool aiding in the diagnosis. The diagnosis of acral AN should not be overlooked despite the finding of low serum vitamin B12 levels in such a case and the authors wish to create awareness among the clinicians to investigate the patient further.