Laboratory Studies
Miliaria is clinically distinctive; therefore, few laboratory tests are necessary.
When miliaria rubra occurs in people with darker skin types, dermoscopy showing large white globules with surrounding darker halos, appearing like a “white bull's eye,” may be helpful in making the diagnosis. [29]
In miliaria crystallina, cytologic examination of the vesicular contents fails to reveal inflammatory cells or multinucleated giant cells (as would be expected in herpes vesicles).
In miliaria pustulosa, cytologic examination of the pustular contents reveals inflammatory cells. Unlike erythema toxicum neonatorum, eosinophils are not prominent. Gram staining may reveal gram-positive cocci (eg, staphylococci).
Imaging Studies
High-definition optical coherence tomography has been used to help establish the diagnosis of miliaria. [30]
Histologic Findings
In miliaria crystallina, intracorneal or subcorneal vesicles communicate with eccrine sweat ducts, without surrounding inflammatory cells. Obstruction of the eccrine duct may be observed in the stratum corneum.
In miliaria rubra, spongiosis and spongiotic vesicles are observed in the stratum malpighian, in association with eccrine sweat ducts. Periductal inflammation is present.
In early lesions in miliaria profunda, a predominantly lymphocytic periductal infiltrate is present in the papillary dermis and lower epidermis. A PAS-positive diastase-resistant eosinophilic cast may be seen in the ductal lumen. In later lesions, inflammatory cells may be present lower in the dermis, and lymphocytes may enter the eccrine duct. Spongiosis of the surrounding epidermis and parakeratotic hyperkeratosis of the acrosyringium may be observed.
In the granulomatous giant centrifugal variant of miliaria profunda, biopsies show mild spongiosis and acanthosis, hypergranulosis, and hyperplasia of the acrosyringia, the eccrine ducts, and infundibula, with invagination by keratin plugs. There is a granulomatous inflammatory infiltrate within the dermis, consisting of lymphocytes and foreign body giant cells with a few neutrophils centered around the ruptured straight portion of the eccrine duct. [27]
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Miliaria crystallina in an infant. Note that the lesions are confluent. Courtesy of K.E. Greer, MD.
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Miliaria rubra in an adult. Courtesy of K.E. Greer, MD.
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Miliaria pustulosa. Courtesy of K.E. Greer, MD.
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Miliaria crystallina. Note the water-drop appearance of the lesions. Courtesy of K.E. Greer, MD.
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Miliaria crystallina in a newborn child. Courtesy of K.E. Greer, MD.
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Miliaria pustulosa. Courtesy of K.E. Greer, MD.
