Differential diagnosis
Kaposi’s varicelliform eruption is a disseminated eruption of HSV on skin already affected with another dermatitis; eczema herpeticum refers specifically to the occurrence of an eruption on skin affected by atopic dermatitis. Thus, other types of Kaposi’s varicelliform eruption should be considered in the differential diagnosis. These include HSV infections on skin affected by Darier-White disease, pemphigus foliaceus, and mycosis fungoides. A good history taken from the patient regarding coexisting skin disorders makes the difference clear.
Other generalized vesicular eruptions such as Varicella should also be considered before making the diagnosis. Since distinct vesicles are not often present by the time the patient presents for care, the rash may also be confused with impetigo or other bacterial infections.
Laboratory tests
Several tests are available to detect the presence of HSV in the skin lesions of eczema herpeticum. These include polymerase chain reaction (PCR), immunofluorescence, and electron microscopy. Electron microscopy is not widely available and PCR can take several days, so often it is helpful to do direct fluorescent antibody testing while PCR results are pending. Light microscopy can be used to do a Tzanck test, which looks for multinucleated giant cells in blister fluid.
Serologies are often ordered, but results are nonspecific. Viral cultures are not very sensitive and take a while to get results. Aerobic and anaerobic bacterial cultures should be done because superinfection is common.