itch - Mechanism, Causes

An irritating sensation in the skin, which may be extremely distressing, also known as pruritus [prooriytuhs]. It may occur in one area as a result of a local irritant or skin disease (eg lice, scabies, dermatitis, psoriasis). It may also be a generalized symptom of an underlying disorder (eg lymphoma, jaundice). Occasionally the complaint is the result of a psychoneurosis.

ICD-9	698
DiseasesDB	25363
MedlinePlus	003217

An itch (Latin: pruritus) is a sensation felt on an area of skin that causes a person or animal to desire to scratch that area.

Mechanism

The central chemical involved in itching is histamine, a molecule released by mast cells in the skin. Histamine is the chemical that causes the itch and reddening when bitten by insects. An itch caused by histamine is transmitted to the brain by a different neural pathway, described in 2001 by Andrew et al. As with pain, a histamine-induced itch travels via the spinothalamic tract, but in fibres specific for itch.

Causes

The feeling of itchiness can be caused by a movement of hairs or the release of a chemical (histamine) from cells under the skin.

Common causes for itching can be:

Skin conditions (such as psoriasis, eczema, sunburn and many others). Hodgkin's disease Jaundice (bilirubin is a skin irritant at high concentrations) Polycythemia, which can cause generalized itching due to increased histamine Scabies or infection with lice or worms Medication: Allergy Photodermatitis – (sun)light reacts with chemicals in the skin, leading to the formation of irritant metabolites Directly (e.g. Often, however, scratching can intensify itching and even cause further damage to the skin, dubbed the "itch-scratch-itch cycle". For itch caused by mosquito bites, stinging nettles, or poison ivy in the woods, one may use the juice of the Jewelweed plant.