Prurigo nodularis

Prurigo nodularis (PN), also known as nodular prurigo, is a skin disorder characterized by pruritic (itchy), nodular lesions, which commonly appear on the trunk, arms and legs. Patients often present with multiple excoriated nodules caused by chronic scratching. Although the exact cause of PN is unknown, PN is associated with other dermatologic conditions such as untreated or severe atopic dermatitis and systemic causes of pruritus including liver disease and end stage kidney disease. The goal of treatment in PN is to decrease itching. PN is also known as Hyde prurigo nodularis, or Picker's nodules.

Signs and symptoms

 * Nodules are discrete, generally symmetric, hyperpigmented and firm. They are greater than 0.5 cm in both width and depth (as opposed to papules which are less than 0.5 cm).
 * The nodules of PN can appear on any part of the body, but generally are found in areas where patients are able to reach to scratch. Patients can exhibit a 'butterfly sign' in which nodules are absent in the mid upper back.
 * Nodular lesions are often excoriated from persistent scratching.
 * The nodules in PN are extremely itchy, this sensation can have an impact on patients perceived quality of life
 * Nodule pattern can be follicular.

Causes
The exact cause of prurigo nodularis is unknown, however, it is thought to be induced by other dermatologic conditions such as severe atopic dermatitis, Becker's nevus, and linear IgA disease. PN is also associated with systemic causes of pruritus such as liver disease, cholestasis, thyroid disease, polycythemia vera, uremia, Hodgkins lymphoma, HIV and kidney failure diseases. Psychiatric illnesses have been considered to induce PN, although later research refuted a psychiatric cause for PN. Patients report an ongoing battle to distinguish themselves from those with psychiatric disorders, such as delusions of parasitosis and other psychiatric conditions.

Pathophysiology
Chronic and repetitive scratching, picking, or rubbing of the nodules may result in permanent changes to the skin, including nodular lichenification, hyperkeratosis, hyperpigmentation, and skin thickening. Unhealed, excoriated lesions are often scaly, crusted or scabbed. Many patients report a lack of wound healing even when medications relieve the itching and consequent scratching.

Patients often:
 * seek treatment during middle-age, although PN can occur at any age.
 * have a history of chronic severe pruritus.
 * have a significant medical history for unrelated conditions.
 * develop liver or kidney dysfunctions.
 * develop secondary skin infections.
 * have a personal or family history of atopic dermatitis.
 * have other autoimmune disorders.
 * have low vitamin D levels.

Diagnosis
Diagnosis is based on visual examination and the presence of itching for greater than 6 weeks duration.

A skin biopsy is often performed to exclude other diseases. Lesion biopsies usually show light inflammation, sometimes with increased numbers of eosinophils. A culture of at least one lesion will rule out staphylococcus infection, which has been significantly linked to atopic dermatitis.

Treatment
Prurigo nodularis is very hard to treat, but current therapies include steroids, dupilumab, vitamins, cryosurgery, thalidomide and UVB light. In the event that staphylococcus or other infection is present, antibiotics have proven effective, but tend to cause more harm than good for this particular disease. A physician may administer a strong dose of prednisone, which will almost immediately stop the itch/scratch cycle. However, cessation of steroids allows relapse to occur, usually within a few weeks. Horiuchi et al in 2006 reported significant improvement in PN with antibiotic therapy.

Azathioprine, an immunosuppressive drug used in organ transplantation and autoimmune diseases belonging to the chemical class of purine analogues, also known by the brand name Imuran, has been reported to be effective.

Dupilumab (Dupixent) was approved by the US FDA in September 2022 to treat prurigo nodularis, the first such medication.

History
Prurigo nodularis was first described by Hyde and Montgomery in 1909.