Itching, or pruritus, can be defined as an unpleasant sensation which elicits a desire to scratch. The causes of itching are many; in fact, any irritation of the skin and its structures can cause the ubiquitous itchy sensation in most individuals.
Itching varies in intensity and has been described as mild, ticklish, burning, prickly, pricking, crawling sensation under the skin, persistent, intermittent or severe in different diseases. Itching may be generalized or localized, and with or without skin rashes. Itchy scalp, itchy palms and soles, itchy genitals (itchy vulva and itchy scrotum), and itchy anus are the special localized types of itching.
In lower mammals, itching is of positive biological value in getting rid of insect infestations. In humans, though, the itch causes skin abrasions, and often signs of skin inflammation in the form of dermatitis.
How Does Itching Occur?
Clinically, the itchy skin may be classified as itching without skin rash and itchy skin with identifiable skin eruptions. In a 2003 article titled “Itch: Scratching more than the surface” in the Quarterly Journal of Medicine (Volume 96), Twycross et al., classified itching according to the following basic pathological causes:
- Pruritus due to irritation of itch receptors in the skin: Skin diseases like scabies, drug reactions, urticaria, dermatitis herpetiformis, insect bite reactions, eczema, and contact dermatitis cause itching through this mechanism.
- Itching due to neuropathic diseases affecting the peripheral and central nervous system, as in peripheral neuropathy or brain tumors.
- Neurogenic itchy skin due to chemical mediators affecting the central nervous system without causing structural damage to the nervous system, as in opioid peptides produced in cholestatic liver diseases.
- Psychogenic itching is diagnosed when no evident cause can be discerned after clinical and laboratory investigations and psychological disturbances are present.
How Does Scratching Relieve the Itch?
Itch impulse is induced in the skin through the release of chemical mediators, especially histamine, which stimulate special "raw" nerve endings within the skin. The itch sensation is then transmitted from the skin through specialized sensory fibers to the brain, through the thalamus, where it is perceived as an urge to scratch. This stimulates a reflex scratch impulse through the spinal axons which is also modified through the higher brain centers.
Scratching activates suppressor neurons in the cerebral cortex (in an area known as substantia gelatinosa, which is also the "gate" control of pain) resulting in the reduction of the itch sensation. An April 2009 University of Minnesota study by Davidson et. al. found that scratching resulted in stoppage of further itch signals at the spinal level of neurons rather than at the cerebral cortex.
Any mechanical (stroking, vibration), thermal (mild heat), chemical (irritants like acids and alkalies), many pharmacological agents (especially histamine, morphine, codeine, serotonin, cytokines, proteases and opioid peptides can initiate the itch sensation in the skin. In addition, any skin diseases which irritate the skin, like dry skin or inflammatory skin diseases can also be the cause of the pruritus.
In certain skin diseases like neurodermatitis and prurigo nodularis, the itch-scratch cycle produces pathological changes even in the nerve endings, causing them to enlarge and cause intense itchy sensation even on mild stimulation locally or through the psycho-neurological pathway. In such diseases, the itching and scratching could produce an incessant and vicious itch scratch cycle making life a misery for the sufferer.
The causes of itchy skin can be broadly divided into itchy skin diseases and itching caused by systemic diseases.
Reference
- Patrick Yesudian. Pruritus. IADVL Textbook and Atlas of Dermatology, 1994, Bhalani Publishing House, Mumbai.
- University of Minnesota (2009, April 29). Got An Itch? New Study Shows How Scratching May Relieve It. ScienceDaily. Retrieved October 17, 2009.
- Greaves MW. Pruritus. Rook’s Textbook of Dermatology, 7th Edition, 2004
Disclaimer
The information given in this article is for educational purpose only so that patients are aware of the options available. No diagnosis should be made or treatment undertaken without first consulting your doctor. If you do so, the author or suite101 will not be responsible for any consequences. The images provided are for illustration purpose only.
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