Although itching has been extensively studied, its causes are ill understood. However any discussion of itching must refer to the physiology, so that the limitations of treatment may be understood. Itching, then, is a disagreeable sensation produced by the action of stimuli of a harmful nature on the skin surface. It is a signal of actual or potential danger to the skin. The purpose of the reflex action of scratching is to remove the causative agent from the body surface.

It is thought that a wide variety of stimuli and noxious agents may liberate chemicals in the skin which then act on peripheral nerves, eliciting the itch sensation. These chemicals include histamine, bradykine, protease, and prostaglandins. Throughout the skin there are many itch receptors. On the forearm these points lie approximately one millimetre apart. However they are more closely set in areas about orifices such as the mouth or anus. The small skin nerves then carry the impulses to the spinal cord, from where they are transmitted via the pain fibres to the brain. It is not yet understood how scratching relieves itching, but possibly it disturbs the rhythm of the impulses travelling towards the spinal cord. Scratching may also simply damage the nerve fibres which are conveying the itch.

Possible causes of itching.

1. Physical and chemical spicules, e.g. fibre glass, wool, detergent, proteases, e.g. nettles, Rhus tree drugs, e.g. opiates, quinidine.

2. Skin diseases Obvious e.g. eczema, tinea, bites. Not obvious e.g. scabies, hives, winter itch, parasitophobia.

3. Internal diseases or conditions.

Metabolic disorders, e.g. diabetes, thyroid disorders, liver disorders, kidney disorders, blood disorders, pregnancy, cancer, parasitic infestation iron deficiency.

4. Psychological disorders. Primary, secondary.

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