A 26-year-old Thai man was first seen in our department for the evaluation of an intermittent, pruritic, papular eruption that had appeared over the previous 2 months. He noticed that the lesions usually arose following exposure to heat or excessive sweating. The patient was otherwise in good health and did not take any medication. His family history did not reveal any similar disease.
On examination, there were multiple 2-3 mm, clustered, erythematous papules widely distributed on the chest, abdomen, buttock, and thighs (Fig. 1). They were purpuric and blanched slightly when palpated. All lesions appeared to be at the same stage of development. Some papules had coalesced to give edematous plaques with satellite margins. Healing lesions showed striking reticulated hyperpigmentation. The clinical differential diagnosis included allergic contact dermatitis and herpes simplex. Histology of a punch biopsy from the back revealed squamous metaplasia of the eccrine ductal epithelium of the epidermis and the upper dermis. The metaplastic ducts were lined by large polygonal epithelial cells. Focal necrosis of the ductal epithelium, obliteration of the lumen, and parakeratosis were also observed. A slight perivascular infiltrate of lymphocytes with some neutrophils and eosinophils was noted in the upper dermis. The coils in the deeper dermis were not involved. A diagnosis of primary squamous syringometaplasia was made. Routine laboratory tests were normal.