Mechanisms of itch are both peripheral and central. Impulses pass along the same nerve fibres as those of pain, but the sensation experienced differs qualitatively as well as quantitatively from pain. In the CNS endogenous opioid peptides are released and naloxone can relieve some cases of intractable itch. Local liberation of histamine and other autacoids in the skin also contributes and may be responsible for much of the itch of urticarial allergic reactions. Histamine release by bile salts may explain some, but not all, of the itch of obstructive jaundice. It is likely that other chemical mediators, e.g. serotonin and prostaglandins, are involved.
In the absence of a primary dermatosis it is important to search for an underlying cause, e.g. iron deficiency, liver or renal failure and lymphoma, but there remain patients in whom the cause either cannot be removed or is not known.
Antihistamines (Hj receptor), especially chlor-phenamine and hydroxyzine orally, are used for their sedative or anxiolytic effect (except in urticaria); they should not be applied topically over a prolonged period for risk of allergy.
In severe pruritus, a sedative antidepressant may also help. The itching of obstructive jaundice may be relieved by androgens but they may increase the jaundice. If obstruction is only partial, colestyramine and phototherapy can be useful. Naltrexone offers short-term relief of the pruritus associated with haemodialysis.
Scratching or rubbing seems to give relief by converting the intolerable persistent itch into a more bearable pain. Firm pressure with a finger may relieve the itch. A vicious cycle can be set up in which itching provokes scratching and scratching leads to skin lesions which itch, as in lichenified eczema. Covering the lesion or enclosing it in a medicated bandage so as to prevent any further scratching or rubbing may help.
Topical corticosteroid preparations are used to treat the underlying inflammatory cause of pruritus, e.g. in eczema.
A cooling application such as 0.5-2% menthol in aqueous cream is antipruritic, probably by weak local anaesthetic action.
Calamine and astringents (aluminium acetate, tannic acid) may help. Local anaesthetics do not offer any long-term solution and since they are liable to sensitise the skin they are best avoided; lignocaine is least troublesome in this respect. Topical doxepin can be helpful in localised pruritus, but extensive use induces sedation; like other topical antihistamines it induces allergic contact dermatitis.
Crotamiton, an acaricide, is reputed to have a specific but unexplained antipruritic action, although it is irritant.
Pruritus ani is managed by attention to hygiene, emollients, e.g. washing with aqueous cream, and a weak corticosteroid with antiseptic/anticandida application used as briefly as practicable (some cases are a form of neurodermatitis). Secondary contact sensitivity, e.g. to local anaesthetics, is common.
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