Prurigo

The exact cause of prurigo is currently unknown, however it occurs in those who have chronic pruritus and is frequently associated with atopic dermatitis, also known as eczema. Eczema is a skin condition that results in inflamed, irritated, and itchy skin. Prurigo arises as a result of continuous scratching over a prolonged period of time.  Several systemic and psychiatric diseases can be associated with severe pruritus. Examples of systemic diseases associated with prurigo include diabetes mellitus, chronic renal failure, cardiovascular disease, hepatitis C, and gluten enteropathy. Prurigo is seen in some individuals with HIV infection, especially if their immune system is severely compromised. The exact role of these underlying conditions in prurigo is unclear. Anxiety and depression, as well as general emotional stress can also cause prurigo. 

Several mechanisms have been proposed for the cause of prurigo including an increased number of nerve fibers in the skin in those with prurigo. Nerve growth factor  (NGF), a neuropeptide, has been found to be overexpressed in prurigo lesions, and is involved in the release of substance P, which stimulates inflammation. Prurigo has also been thought of as a small fiber neuropathy, which is a disorder in which the small sensory cutaneous nerves are affected. Studies have found that the nerve fiber density in the skin of those with prurigo is reduced, potentially leading to sensory disturbances and itch. Lastly, Th2 cytokines have been shown to be increased in the skin of those with prurigo. Cytokines are small proteins important for cellular signaling. Th2 cytokines specifically, play an important role in the pathophysiology of allergic diseases, including asthma. Additionally, IL-31, a potent cytokine involved in pruritus, is aberrantly expressed in prurigo nodularis. 

Prurigo is diagnosed based on a thorough interview and history, along with a physical exam. The individual may report a longstanding history of severe itching. On physical examination, prurigo characteristically presents with nodular lesions that are usually symmetrically distributed. They can be excoriated, defined by loss of the upper layer of the dermis as a result of scratching. 

Treatment of prurigo includes topical and systemic therapies aimed at treating the symptoms, interrupting the scratch-itch cycle, and flattening the nodules. The individual with prurigo may also be educated on general techniques to reduce skin irritation and stop scratching. One with prurigo can adopt certain skin care habits to help alleviate the symptoms, which include using mild and gentle cleansers while bathing or showering, and applying topical emollients (i.e., moisturizers) many times a day. Those with prurigo should keep their nails short and wear gloves at night to minimize scratching, and wear occlusive bandages and dressings to prevent excoriation. Calamine lotions, and lotions containing menthol, camphor, or pramoxine hydrochloride can provide additional relief due to their ability to provide anesthetic and cooling effects to the skin. Other topical treatments that have been used with some success include topical capsaicin cream; topical calcineurin inhibitors (e.g., tacrolimus, topical pimecrolimus); and topical vitamin D analogues (e.g., calcipotriol, tacalcitol). 

Antihistamines (e.g., hydroxyzine, diphenhydramine) used at bedtime can have a sedating and anti-inflammatory effect helpful for controlling nocturnal pruritus. Selective serotonin reuptake inhibitors (e.g., sertraline, fluoxetine) and tricyclic antidepressants (e.g., amitriptyline) are also used for chronic pruritus. These are especially helpful when a component of depression is present, potentially leading to increased pruritus. 

For those with only a few large nodular lesions, topical corticosteroids, such as clobetasol dipropionate 0.05% or betamethasone valerate 0.1% ointment, can be used. These are applied under occlusion with plastic wrap at night for at least two to four weeks. In addition to topical application, injecting corticosteroids (such as triamcinolone acetonide) into the lesion once a month is also shown to be effective in reducing pruritus and flattening the nodules.