Itch overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
An itch (Latin: pruritus) is a sensation felt on an area of skin that evokes the desire or reflex to scratch that area. Itching can be related to anything from dry skin to cancer. 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.[1] Unmyelinated nerve fibers for itch and pain both originate in the skin, however information for them are conveyed centrally in two distinct systems that both use the same peripheral nerve bundle and spinothalamic tract.[2]
Historical perspective
Until recent times, sensations of itch were considered to be largely the same as those of pain. Currently, pruritus is considered to be an autonomous, pain-independent sensation. Itch, also known as medically pruritus, The first documented definition of itch was given by the German physician Samuel Hafenreffer as an “unpleasant sensation that elicits the desire or reflex to scratch” about 350 years ago. In recent years, pruritus has been determined to be an autonomous, pain-independent sensation. Pruritus is a frequently misspelled word, often misspelled as "pruritis". 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 exhibits notable differences. Pruritic stimuli mostly create the same reactions as noxious stimuli in experimental animals. As a result, it is difficult to study the physiologic mechanisms of itch in animal models as they lack the ability to discern the differences between both stimuli. Therefore human studies have provided most of the information regarding the processing of pruritic stimuli. It was initially thought histamine was the major chemical involved in mediating pruritus through binding to H1R and activating phospholipase Cβ3 (PLCβ3) and phospholipase A2 (PLA2). Bell et al. demonstrated that histamine promotes the influx of calcium at the axon terminals of the spinal cord neurons via a transient receptor vanilloid 1 (TRPV1) receptor activation and then, through the promotion of a series of intracellular signal activation, eventually lead to the generation of itching sensation. However, the current consensus is that there are other chemical mediators that are highly involved in the promotion of pruritus.
Pathophysiology
There are five main causes of itching:
- Dermal/pruritoceptive which originates in the skin,
- Neuropathic which is due to damage along the afferent pathway of a nerve,
- Neurogenic itch which originates centrally without the involvement of the peripheral nerves,
- Psychogenic itch which is associated with psychiatric disorders and phobias, and
- A mixed picture which is due to two or more causes of itching