Pruritus, the most common symptom of all skin diseases, can be simply defined as an unpleasant sensation that elicits the urge to scratch. It may present as a primary complaint, as an indication of an underlying systemic or psychiatric disorder, or as an isolated problem with no other explanation (pruritus of unknown origin).

Pruritus may result from the following:

• Common primary skin disorders such as eczema, lichen simplex chronicus, psoriasis, lichen planus, dry skin (xerosis) or, rarely, dermatitis herpetiformis.

• Exogenous causes such as drugs, cocaine abuse, contact dermatitis (e.g., poison ivy), scabies, lice, fiberglass, and aquagenic pruritus.

• Internal disorders such as chronic renal failure, acquired immunodeficiency syndrome, polycythemia vera, cholestasis, pregnancy-related disorders, primary biliary cirrhosis, diabetes mellitus, thyroid disease, and carcinoid syndrome.

• Psychogenic causes such as delusions of parasitosis, neurotic excoriations, pruritus ani, and obsessive-compulsive disorder.

• Associated malignant diseases such as Hodgkin disease, leukemia, and multiple myeloma.