Subcorneal pustular dermatosis (SPD) is a rare skin disease in which pus-filled pimples or blisters (pustules) form under the top (subcorneal) layer of the skin. It is most common in middle-aged adults (particularly women) but can develop in children. Pustules usually appear over a few hours and grow together to form round or wavy patterns. They most often form in areas where the skin may touch or rub together, such as the groin area, underarms, inside the elbows, and behind the knees. The pustules may be mildly itchy or painful, but despite being pus filled, are not infected. The diagnosis of SPD is made based on the appearance of the pustules and the results of a skin biopsy (histologic findings).

The cause of SPD is not known. There is currently no evidence it is inherited (no familial cases have been reported) and it is not contagious. SPD may be associated with other diseases or health problems including several autoimmune diseases, blood (hematologic) diseases, infections, and cancers. Rarely, it has been associated with taking certain medications (drug-induced SPD).

SPD may be treated with is oral dapsone, which often improves symptoms within one month. However, the pustules may return when treatment is stopped. Other therapies have been tried with mixed results. While SPD may cause discomfort and cosmetic concerns, it typically does not affect overall health.

The main symptom of subcorneal pustular dermatosis (SPD) is an outbreak of round bumps on the skin that may resemble pus-filled pimples or blisters (pustules). The pustules appear over a short period of time (hours), usually on healthy skin, but sometimes on skin that is irritated or inflamed. Most commonly the outbreak appears in areas of the body where skin may touch or rub together through movement, including the groin area, under women's breasts, underarms, inside the elbows and behind the knees. Other areas of the trunk, the upper arms and upper legs may be affected. Pustules rarely form on the palms of the hands or soles of the feet, and the face is almost never affected. The pustules initially appear separated from each other (isolated) or be grouped together in small clusters, but may spread and join together within one or two days, forming circular, ring-shaped (faded in the center), or wavy patterns on the skin. The pustules have an unusual appearance because the pus tends to settle in the lower half of the blister leaving the upper half of the blister clear (known as hypopyon pustules).

The pustules sometimes cause irritation, pain, or mild itching. They break open easily and crust over before healing, sometimes leaving behind darkened spots on the skin (hyperpigmentation). The pustules typically do not cause scarring. SPD usually affects the skin only, but some people may have generalized (systemic) symptoms such as a general feeling of being sick (malaise), fever, or joint pain. Additional symptoms may suggest other associated diseases or health problems may be present.

The reason that a person develops subcorneal pustular dermatosis (SPD) is not known. The pustules are thought to result from a signal pulling (recruiting) neutrophils to the affected areas of skin. Neutrophils are a type of white blood cell (WBC). Skin disorders that involve an abnormal recruitment of neutrophils are called neutrophilic dermatoses (NDs). While there are known triggers of neutrophil recruitment for some NDS, no specific triggers have been found to cause the pustule formation in SPD. An exception is rare cases associated with the use of certain medications.

SPD has been associated in case reports with a number of diseases that affect other parts or systems of the body, including but not limited to:

• Autoimmune diseases such as rheumatoid arthritis, lupus, and Sjogren syndrome.
• Hematologic diseases or cancers such as monoclonal gammopathies and multiple myeloma.
• Infections such as urinary tract infections and respiratory tract infections.

Rarely, SPD has developed in association with taking certain medications (drug-induced SPD).