A new study [media summary] by itch guru Gil Yosipovitch and colleagues at Wake Forest University has shown that the intensity of itch, and the pleasure of relieving it by scratching, depends on the precise location on the body that a standard itch stimulus is applied.
In standard studies of itch (and, one would presume, creams and ointments designed to relieve it) the usual site of focus has been the underside of the forearm. But Yosipovitch and colleagues, who compared itch on the forearm, the back (slightly to one side of the spine, near the middle) and the ankle, find that itch is sensed more intensely on both ankle and back, and the pleasure of scratching lasts longer on the ankle--even after the itch has dropped off--than on the forearm.
They used "cowhage spicules," which I think are akin to nettle spines, to cause the itch, and a standard laboratory scratching brush to relieve it in 18 volunteers. The paper, which is open-access, doesn't mention whether any of the volunteers had eczema. This matters, I think, because you'd expect eczema patients to perceive and respond to itch differently than "normal" people.
The authors, in their discussion section, speculate on why itch intensity and scratch relief differ across the body. They discuss various types of nerve fibers that might be responsible, but come to no conclusion. Really the paper just poses a question: why are these differences there? And we could expect, in following work, that they might narrow the answer down to the presence or absence of nerve types. It's clear that the mechanism of itch and relief is far from understood.
[It occurred to me later that maybe this research points to the need for different anti-itch treatments for different parts of the body. Whether they work or not, I can imagine companies claiming that various ointments and creams are optimized for your hands or whatever.]
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