Pruritus and itch are synonymous. Itch is defined as a sensation that elicits scratching or the desire to scratch. Many skin diseases, such as psoriasis, atopic dermatitis, and scabies, can be intensely pruritic. In the setting of itching without a rash – either generalized or localized – secondary skin changes such as scratch marks and infection may occur but are not the primary diagnosis. Although there are a number of symptomatic treatments for pruritus, there is no single medication for itch as effective as aspirin is for pain.

Generalized itching without a rash can be an important symptom of "internal" conditions such as biliary obstruction, renal failure (acute, chronic), hyperthyroidism, Hodgkin disease, other lymphomas, and other myeloproliferative diseases. The connection, if any, between pruritus and solid malignancies is tenuous and exceedingly rare except when, for example, a tumor causes biliary obstruction. Medications can also cause generalized pruritus without a rash. 

Diffuse itch with a prickling character that develops immediately after any water contact and lacks associated visible skin changes is called aquagenic pruritus and may be a sign of polycythemia vera. It has also been associated with myelodysplastic syndrome, hypereosinophilic syndrome, and juvenile xanthogranuloma. Rarely it has been reported as occurring with myelofibrosis. Some medications have been reported to induce the condition as well.

Localized itching, often with a burning or stinging character, can occur as a consequence of isolated peripheral sensory neuropathies. These abnormal sensations may be described as dysesthetic, paresthetic, or neuropathic. Itch localized to the forearms may represent brachioradial pruritus. Dysesthesia of the scalp can occur as a consequence of cervical spine disease. Intense itching of the scalp also occurs in dermatomyositis. On occasion, a localized neuropathic itch such as brachioradial pruritus may trigger generalized pruritus.

Generalized pruritus in the setting of immunocompromise can be a sign of lymphoma, drug reactions, and all of the other conditions associated with generalized itch.

Pediatric Patient Considerations: In the pediatric age group, congenital disorders such as biliary atresia and Alagille syndrome can cause itching. The diagnosis of scabies can be easily missed in children, where locations such as the scalp and feet may have subtle diagnostic burrows.

Related topic: HIV/AIDS-related pruritus