Subcorneal pustular dermatosis

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).^[1][2] The pustules appear over a short period of time (hours), usually on healthy skin, but sometimes on skin that is irritated or inflamed.^[2][4] 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. ^[2][3][4] Pustules rarely form on the palms of the hands or soles of the feet, and the face is almost never affected.^[2][4] 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.^[1][2][3][5] 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).^[1][2] 

The pustules sometimes cause irritation, pain, or mild itching.^[2][3] 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.^[2][4] 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.^[2] Additional symptoms may suggest other associated diseases or health problems may be present. ^[2][3][4]

The reason that a person develops subcorneal pustular dermatosis (SPD) is not known.^[2] 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.^[2][5]

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:^[2][3][4]

• 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).^[2]

There are currently no official diagnostic criteria for subcorneal pustular dermatosis (SPD). The disease is diagnosed based on the appearance of the pustules,  a skin biopsy which identifies certain characteristics when examined under the microscope (histologic findings), and ruling out other diseases that cause similar pustules.^[3] There are no blood tests that can confirm a diagnosis of SPD.^[3] Pustular psoriasis can be difficult to distinguish from SPD, and some people initially diagnosed with SPD have later been found to have pustular psoriasis.^[3] Identifying a family history of psoriasis may influence whether one or the other is diagnosed.^[5] Some cases of SPD have been considered a variant of pustular psoriasis.^[5] At this time, the exact relationship between SPD and pustular psoriasis is not understood.^[2][3][4][5]

The main goals of treatment for subcorneal pustular dermatosis (SPD) are to minimize discomfort and improve quality of life.^[3] Treatment usually begins with oral dapsone, which often clears the pustules within one month.^[2][3] Although there have not been randomized clinical trials studying its use for SPD, clinical experience has suggested oral dapsone is effective and well-tolerated.^[3] When treatment with dapsone is stopped, the pustules may return, so continued therapy at a lower dose may be recommended to prevent recurrence.^[2] Other treatments have been tried with variable success, and data on their use are limited to reports of individual cases. These treatments are generally only tried when dapsone is not effective or not well-tolerated.^[3] Possible treatment options include topical or oral corticosteroids, oral retinoids, phototherapy, immunosuppressive oral therapies, and biologic agents.^[2][3] In some cases, a combination of therapies may be used. Since SPD does not cause serious health issues, it is important to consider the possible side effects of treatment options.^[3]

Subcorneal pustular dermatosis (SPD) generally is a long-lasting (chronic) disease where the pustules go away for a while and then come back. In medical terms, this is called a relapsing and remitting course.^[3] Some people do have long-term or permanent remission of SPD. The course of the disease in each person cannot be predicted.^[3] While SPD can cause discomfort, itching, and concerns about appearance, overall health is typically not affected. In people with SPD who have associated diseases, the long-term outlook may depend upon the other disease present.^[3][5]

The number of people who have subcorneal pustular dermatosis (SPD) is not known. Based on the limited number of cases that have been reported, it is considered a rare disease.^{[1][2][3][4][5]} In the United States, a rare disease is defined as one that affects fewer than 200,000 people.^[6] SPD most commonly develops in people between 40 and 70 years of age. It is four times more common in women than in men and is extremely rare in children.^[2] There are no ethnic or geographical factors known to increase a person’s chance of having SPD.^[3]