Itch

Itch (Latin: pruritus) is an unpleasant sensation that leads to the desire or reflex to scratch. Itch has many similarities to pain and both are unpleasant sensory experiences but their behavioral response patterns are different. Pain creates a reflex withdrawal while itch leads to a scratch reflex.Ikoma, A., Steinhoff, M., Stander, S., Yosipovitch, G., Schmelz, M. (2006). The neurobiology of itch. Nature Reviews Neuroscience, 7(7), 535-547. Nerve fibers for itch and pain both are in the skin, but information for them is sent centrally in two distinct systems that both use the same nerve bundle.Greaves, M.W., Khalifa, N. (2004). Itch: More than skin deep. Int Arch Allergy Immunol, 135, 166-172.

Historically, the sensations of itch and pain have not been considered to be independent of each other until recently where it was found that itch has several features in common with pain but has important differences.Twycross, R., Greaves, M.W., Handwerker, H., Jones, E.A., Libretto, S.E., Szepietowski, J.C., Zylicz, Z. (2003). Itch: scratching more than the surface. Q J Med, 96, 7-26. 
Causes
The feeling of itchiness can be caused by a movement of hair or the release of a chemical (histamine) from cells under the skin. Itchiness is regarded as protective, as it helps creatures remove parasites that land on their skin.

Itching can be caused by:
Food poisoning
Xerosis. This is the most common cause, frequently seen in winters.   Associated with older age, frequent bathing in hot showers or baths, and high temperature and low humidity environments.
 Skin conditions (such as psoriasis, eczema, sunburn, athlete's foot, hidradenitis suppurativa and many others). Most are of an inflammatory nature.
 Insect bites, such as those from mosquitos or chiggers.
 Allergic reactions to contact with specific chemicals, such as Urushiol from Poison Ivy or Poison Oak.
 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
 Thyroid illness
 Shaving, which may irritate the skin
 Diabetes Mellitus
 Dandruff (an unusually large amount of flaking is associated with this sensation)
 Iron deficiency anemia
 Parasitic infections
 Psychiatric
 Medication:
 Allergy - (due to reaction of an individual's immune system to certain chemical compounds)
 Photodermatitis – (sun)light reacts with chemicals in the skin, leading to the formation of irritant metabolites
 Directly (e.g. morphine and other opiates)
Cholestasis
 Related to pregnancy:
 Pruritic Urticarial Papules and Plaques of Pregnancy (PUPPP)
 Gestational pemphigoid

Treatment

A variety of over-the-counter and prescription anti-itch drugs are available.  Some plant products have been found to be effective anti-pruritics, others not.  Non-chemical remedies include cooling, warming, soft stimulation.

Sometimes scratching relieves isolated itches, hence the existence of devices such as the back scratcher.  Often, however, scratching can intensify itching and even cause further damage to the skin, dubbed the "itch-scratch-itch cycle".

References
 Andrew D, Craig AD (2001). Spinothalamic lamina I neurons selectively sensitive to histamine: a central neural pathway for itch. Nature Neuroscience Jan;4(1):9-10.
 National Cancer Institute (2003) "Pruritus" Retrieved Aug. 22, 2005.

Category:Symptoms