Hyperpigmented plaques over forearms in a teenage girl with atopic dermatitis
Adrian KC Cheng 鄭嘉俊

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Clinical history:

A teenage girl with a history of atopic dermatitis complained of hyperpigmentation over her upper limbs for several months. She was unable to remove the pigmentation by soap and water. Examination revealed coalescing brownish thin plaques distributed symmetrically over her antecubital fossae and volar forearms. Dermoscopy showed polygonal brownish scales.

What is the diagnosis?

Answer:
B. Terra-firma forme dermatosis

The diagnosis is B. Terra-firma forme dermatosis. Terra-firma forme dermatosis is a benign disorder of keratinization that commonly affects children and adolescents. It presents as brown, dirt-like plaques that cannot be removed with ordinary soap and water. Dermoscopy typically demonstrates polygonal brown scales arranged in a mosaic pattern, as seen in this patient. The diagnosis can be confirmed by firmly wiping the lesion with 70% isopropyl alcohol, which removes the pigmentation and is both diagnostic and therapeutic. This distinguishes it from post-inflammatory hyperpigmentation and acanthosis nigricans, which do not disappear with alcohol wiping. Confluent and reticulated papillomatosis typically affects the central trunk and has a papillomatous, reticulated appearance.

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The slide and the question was prepared by:

Dr. Adrian KC Cheng, ^{MBBS(HK), MRCP(UK), MSc GEOR (CUHK), FHKCP, FHKAM (Medicine)}

Specialist in Dermatology & Venereology;

Associate Consultant,

Division of Dermatology, Department of Medicine, Queen Mary Hospital, Hong Kong SAR