Acanthosis nigricans localized to areas of healing psoriatic plaques

A 36-year-old female, a case of psoriasis on methotrexate for the past 4 months, noticed hyperpigmented areas arising from the center of the healing plaques. The patient had no history of diabetes or thyroid disorders and was not obese. On examination, there were scaly well-defined plaques of psoriasis on the upper limbs and upper back, the central area showing clearing and replaced by hyperpigmented velvety plaques [Figure 1a and b]. The patient also had acanthosis nigricans (ANs) on the nape of the neck, but not elsewhere. A skin biopsy from the center of the plaques was consistent with AN [Figure 2]. ANs in healing psoriatic plaques have been reported after adalimumab therapy, but due to methotrexate has not been reported.^[1] The localization of AN in healing psoriasis could be due to recovery of pigment signaling initiated by drugs.^[1] This localization of AN in areas of healing psoriatic plaques could also be to “locus minoris resistentiae,” the feature of certain dermatosis to appear and localized to areas of another dermatosis or scar tissue due to dermal matrix damage, and abnormal trafficking of lymphocytes.^[1]

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Figure 1:

(a) Healing psoriatic plaques with center showing hyperpigmented velvety plaques on the upper limbs and upper back, (b) Acanthosis nigricans on the nape.

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Figure 2:

Skin biopsy showing hyperkeratosis, papillomatosis, and normal pigmentation of basal layer diagnostic of acanthosis nigricans, H&E ×400.

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