A 10-year-old healthy boy is brought to clinic for evaluation of a rash. He has been developing asymptomatic dark brown macules over the past 3 months on his back, chest, and upper arms. On exam, the lesions are not scaly and there is no atrophy. There is no prior history of inflammation, drug intake, or preceding illness. He has Fitzpatrick type 2 skin. A punch biopsy shows increased basal layer melanin, pigment incontinence, and a mild perivascular lymphocytic infiltrate without interface changes or epidermal hyperplasia.

Which of the following is the most likely diagnosis?

A. Lichen planus pigmentosus
B. Erythema dyschromicum perstans
C. Fixed drug eruption
D. Idiopathic eruptive macular hyperpigmentation
E. Postinflammatory hyperpigmentation

Rationale: 

A structured approach to evaluating hyperpigmentation includes clinical distribution, morphology, timing, exposure history, and histopathologic findings. Key differentiating features include onset in relation to inflammation or drug exposure, presence of scaling or pruritus, and biopsy patterns such as interface changes, lichenoid infiltrates, or pigment incontinence.

Correct answer: D. Idiopathic eruptive macular hyperpigmentation

IEMH is a rare condition most often seen in children and adolescents. It presents with asymptomatic, widespread dark brown macules on the face, trunk, and proximal extremities. It occurs without preceding inflammation or drug exposure, distinguishing it from postinflammatory hyperpigmentation or fixed drug eruption. Histologically, it shows basal hypermelanosis, pigment incontinence, and mild perivascular lymphocytic infiltrate, but lacks the interface change or lichenoid pattern seen in other pigmentary disorders. IEMH typically resolves spontaneously over months to years. 

Incorrect answer choices: 

A. Lichen planus pigmentosus typically affects adults or adolescents with darker skin, often with pruritic, grayish macules on sun-exposed areas or flexures. Histology shows lichenoid interface dermatitis. 

B. Erythema dyschromicum perstans can also present with grayish macules, but usually affects older children or adults. Histology shows interface dermatitis with vacuolar change and pigment incontinence.

C. Fixed drug eruption presents with recurrent, well-demarcated violaceous plaques that resolve with residual hyperpigmentation at the same site after medication exposure. Histology shows epidermal necrosis and interface change which were not present in this patient. 

E. Postinflammatory hyperpigmentation usually occurs following a known inflammatory trigger (eczema, acne, trauma, etc.) and is often localized to areas of prior inflammation. In IEMH, no preceding inflammation or skin disorder is noted.

Additional reading at Fitzpatrick's Dermatology Chapter 77: Hypermelanoses