My kids used to think it was great, usually when I’m in the middle of doing something else, to throw at me the most absurd questions, such as, “Mum, what would you rather do? Jump into a pit of poisonous snakes or walk through a tunnel of deadly spiders?” Er, neither, would be my preferred answer but as that was not allowed I would have to choose one of the alternatives which somehow or other would reveal something about my character.
Another time it might be, “Mum, which is better? Being stabbed with a sword or having itchy skin for all your life?” And again, though I would prefer neither option, I don’t think I would like itchy skin for ever.
Indeed, if you ask anyone who does have itchy skin, you’ll find that it greatly affects a person’s quality of life, from being able to concentrate at school, to performance at work, confidence in social situations, to getting a good quality night’s sleep.
And though there is some research into itch, what we know about itch still feels rather limited compared to other areas of dermatology.
Why do we itch?
It is thought that the itch sensation was a trigger that alerts us to the presence of something in our environment such as an insect or parasite. Our hand then moves to the site where the insect or parasite touches our body and we scratch the skin in an attempt to dislodge the foreign object.
However, it’s not just insects or parasites that make us itchy. There are many people who have itchy skin due to an itchy skin disease eg eczema and also there are many people who have itchy skin in the absence of a skin disease.
So what do we know?
Well, we do know that there is not one cause of itch.
We do know that there are cells in the spine called inhibitory spinal interneurons that can either allow an itch sensation to travel up to the brain or not. It may be that people with chronic itch have a disorder in these cells.
[Jankowska E. Spinal interneuron systems: identification, multifunction character and reconfigurations in mammals. The Journal of Physiology 2001;533(1): 1-311.]
Sometimes itch and pain are linked.
Yes, there is a complicated relationship between pain and itch. We all know that scratching an itch can feel good but also that too much scratching can cause pain. There has been research showing that pain can temporarily relieve itch but as doctors we know that opioid painkillers can also cause itch and that opioid antagonists can also alleviate itch. There is also a hedonistic or pleasurable component to itch which may also drive compulsive scratching.
[Ward L, Wright E, McMahon SB. The comparison of the effects of noxious and innoucuous counterstimuli on experimentally induced itch and pain. Pain 1996;64(1):129-138.]
[Ikoma A, Steihnoff M, Ständer S et al. The neurobiology of itch. Nature Reviews Neuroscience 2006;7:535-547.]
Antihistamines help with itch.
That is true and there is now evidence that there are other pathways involved too which would explain why, for some people, antihistamines don’t work at alleviating their itch at all.
These other pathways involve receptors such as the protease-activated receptors (PARs) and transient receptor potential receptor vanilloid type 1 (TRPV1) – such a mouthful!
What about menthol?
Cooling of the skin or menthol containing emollients are very useful in the management of itch but we don’t really know how they work. We do know that when the skin is hot it gets itchy and that most people with itch report that their itching gets worse in the evenings or at night when the body’s temperature naturally rises.
It is now thought that menthol works via the transient receptor potential melastatin 8 or TRPM8 receptor.
[Palkar R, Ongun S, Catich E et al. Cooling relief of acute and chronic itch requires TRPM8 channels and neurons. J Invest Dermatol 2018;138:1391-1399.]
So what’s new?
It was interesting therefore to read of the use of a specific TRPM8 agonist gel called cryosim-1 in reducing itch in the June 2024 edition of the British Journal of Dermatology.
[Lee S, Wei ET, Selescu T et al. Histamine- and pruritogen-induced itch is inhibited by a TRPM8 agonist: a randomized vehicle-controlled human trial. Br J Dermatol 2024;6:885-894.]
In this study 30 healthy volunteers were skin prick tested with 5 agents known to induce itch after pre-treatment with the cryosim gel or a control. Those pre-treated with cryosim gel were found to have a statistically significant reduction in itching.
In conclusion…
It looks like we are getting somewhere with unpicking the Rubik’s cube of a puzzle that is pruritus (or itching) and hopefully of finding other treatment options for patients who have chronic itch.
Kind regards,
Sandy
Dr Sandy Flann, Consultant Dermatologist
