Prurigo nodularis

Prurigo nodularis (PN) is a skin disease characterised by pruritic (itchy) nodules which usually appear on the arms or legs. Patients often present with multiple excoriated lesions caused by scratching.

History
It was first described by Hyde and Montgomery in 1909.

Causes
The cause is unknown, although other conditions may induce PN. These include HIV and other immunodeficiency diseases. Internal malignancies, liver failure, renal failure, and psychiatric illnesses may induce PN, although more recent research has refuted a psychiatric cause for PN. Lockshin et al recently linked PN to Becker's nevus; Torchia et al recently found PN to be linked to linear IgA disease, an autoimmune condition; and Sonkoly et al report a link between T cells and 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 subsequent 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.
 * suffer from liver or kidney dysfunctions.
 * suffer secondary skin infections.
 * have a personal or family history of atopic dermatitis.
 * have other autoimmune disorders.

Lesions and Nodules

 * Nodules are discrete, generally symmetric, hyperpigmented or purpuric, and firm. They are greater than 0.5cm in both width and depth (as opposed to papules which are less than 0.5cm).  They can appear on any part of the body, but generally begin on the arms and legs.
 * Excoriated lesions are often flat, umbilicated, or have a crusted top.
 * Nodules may appear to begin in the hair follicles.
 * Nodule pattern may be follicular.
 * In true prurigo nodularis, a nodule forms before any itching begins. Typically, these nodules are extremely pruritic and are alleviated only by steroids.

Diagnosis
Diagnosis is based on visual examination and the presence of itching. A skin biopsy is often performed to exclude other diseases. Lesion biopsies will typically show a high level of eosinophils in PN.

Treatment
Prurigo nodularis is very hard to treat, but current therapies include steroids, vitamins, cryosurgery, thalidomide and UVB light. 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 recently reported significant improvement in PN with antibiotic therapy.

Synonyms
Hyde prurigo nodularis, Picker nodules, lichen simplex chronicus, atypical nodular form of neurodermatitis circumscripta, lichen corneus obtusus. PN patients of the 21st century consider "Picker's disease" to be an offensive and medieval reference.