It may develop several days or weeks later in premature babies.The rash has a variable appearance, ranging from a few blotchy red spots to many yellow-white bumps and boils. The classic presentation is 1–3 mm, firm, yellow-white bumps with a surrounding red halo. The rash is often described as "flea-bitten." There may be only a few or many lesions, and they may be clustered or widespread. The rash often appears on the cheeks first and may later spread throughout the face, trunk, arms, and legs. Lesions most often appear on the thighs, buttocks, and trunk. The palms and soles are not affected, likely because the skin in these areas does not have hair follicles.Individual lesions may grow and shrink over hours or days. They usually fade within a week or two. The rash may recur within the next few weeks, but this is rare. Cause
Erythema toxicum neonatorum is related to activation of the immune system, but its exact cause is unknown. Many inflammatory factors have been detected in erythema toxicum neonatorum lesions, including IL-1alpha, IL-1beta, IL-8, and eotaxin. These molecules cause redness and swelling in the skin and attract immune cells.Eosinophils and other immune cells are found in the upper layer of the skin in erythema toxicum neonatorum lesions. Immune cells tend to cluster around hair follicles in particular. The leading hypothesis about the cause of erythema toxicum neonatorum is that bacteria activate the immune system when they enter hair follicles for the first time.
This is part of a normal process in which bacteria from the environment start to grow on a babys skin. It is unknown whether the immune response that causes erythema toxicum neonatorum is helpful to the baby. Recent research indicates an association with Demodex mites infestation (demodicosis). Diagnosis
Health professionals can diagnose erythema toxicum neonatorum with a skin exam. Most cases of erythema toxicum neonatorum can be diagnosed without further testing. If more testing is needed to make a diagnosis, the contents of a lesion can be examined under a microscope. A health professional may make a small cut into a pus-filled lesion and collect a swab of pus for testing. Lesions caused by erythema toxicum neonatorum contain eosinophils and other immune cells. These cells can be seen under a microscope when a special stain is applied to the sample.Since the appearance of erythema toxicum neonatorum varies, it may be confused with other newborn rashes. Some newborn infections cause bumps or boils, which may look like erythema toxicum neonatorum. Bacterial infections, including Staphylococcus and Streptococcus infections, almost always cause additional symptoms. These symptoms may be severe, and they are usually not limited to rash. Bacterial rashes can be diagnosed by testing pus from a lesion along with a blood sample. Bacteria can be seen under a microscope with a special stain or may be found on a culture. Fungal infection with Candida may also cause a similar rash in newborns, but it usually causes additional symptoms like thrush.