Itch is a poorly understood compon...

Itch is a poorly understood component part of the nociceptive system that can be stimulated at systemic conditions as well as those of the skin. Like pain, itch is an unpleasant sensory experience that can impair quality of life. However, instead of eliciting the withdrawal introspective that pain does, itch elicits a scratch replication Two distinct responses to an itch stimulus have been described: a for the use of all localized itch that does not persist lengthy after the stimulus is remov and a diffuse, poorly localized area around the stimulus site that corresponds to any subsequent stimulus, including light touch, with intense itching. Yosipovitch and colleagues reviewed the etiology, evaluation, and treatments for itch.

Itch is known to be transmitted from specific C neurons that have moderate conduction velocities, lasting response to histamine, and extensive terminal branching. Nevertheless, no specific "itch center" appears to exist in the brain. Studies exhibit that itch activates several areas of the forebrain, with significant overlap with the activation areas for pain. Pain is known to inhibit itch by dint of central and peripheral mechanisms. Scratching and rubbing skin stimulates myelinated A neuron by the agency of mechanoreceptors that activate spinal suppression of itching. In addition to neuronal mechanisms, the proximity of mast confined apartments to afferent C neuron terminals in the skin indicates a neurochemical mechanism for several aspects of itch. Stimulated mast lonely dwellings could release tryptase, substance P and other active mixs that could cause direct irritating tenors or stimulate C neurons.

Four clinical forms of itch have been described; clinically significant itch may have components of one or more patterns Pruritoceptive itch results from skin inflammation, drynes or damage; this is the familiar itch resulting from an insect bite or urticaria. Neuropathic itch happens with any disease along the afferent neuronal pathway. Several neurologic diseases, as it is as postherpetic neuralgia, multiple sclerosis, and central nervous theory neoplasms, can cause this form of itching. Neurogenic itch is a ensue of the central action of opioid neuropeptides and come into one's heads in patients with cholestasis and renal failure. Psychogenic itch has no physiologic basis; it may be an independent behavior or associated with parasitophobia.

A detailed history and thorough physical examination may establish an underlying cause of itch. couple questionnaires assessing itch as a sensory disturbance and quantifying its weight on quality of life have been validated. Itch associated with renal failure or cholestasis is cogitation to be a result of a central action of opioid peptides. In these patients, itch can be debilitating, and treatment with opioid antagonists like as naloxone or naltrexone is commended For hepatic causes of itch, like as hepatitis C infection, cholestyramine also is useful. Itch can be an early primary symptom of human immunodeficiency virus (HIV) infection, or it may be secondary to HIV-associated skin conditions or systemic complications of that kind as lymphoma or hepatic disease. When the CD4 reckon falls below 300 per [mmsup3] (300 3 [10sup6] by L), intensely pruritic eosinophilic folliculitis may fall out Central neuropathic itch that awakens the patient may be an early symptom of multiple sclerosis. Neuropathic itch also can be found with pain in herpes zoster Central and peripheral mechanisms have been implicated in itching associated with atopic eczema. Treatment of drynes and infection may form itching, and sedating antihistamines are especially useful in preventing the characteristic nocturnal scratching. stern itch may be reduced at modifying inflammation with topical steroids or on using phototherapy or photochemotherapy. Oral azathioprine, cyclosporine, and tacrolimus also have been used for treatment of plain atopic eczema.

While the treatment of itch should be directed at the underlying cause, the cause may not be apparent in pruritoceptive itch, and topical modalities may be effective. Capsaicin desensitizes nociceptive manhood endings; concentrations of 0.025 to 0075 percent can be useful in the treatment of atopic eczema and other forms of itch. Local irritation can be reduc by way of use of topical anesthetic (EMLA). Topical doxepin can be an effective antipruritic agent, still even topical application can cause sedation. Topical aspirin also is a useful antipruritic agent, especially in patients with lichen simplex chronicus. Clinical trials of just discovered antipruritic agents are in progress

Yosipovitch G et al. Itch. Lancet February 22 2003;361: 690-4

COPYRIGHT 2003 American Academy of Family Physicians

COPYRIGHT 2003 Gale Group