The Itch That Rashes

The Itch That Rashes

In caring for so many with eczema or contact dermatitis, in both children and adults, we often hear that the itch comes before the rash.  Researchers wondered if the scratching really did make the rash worse or was that timing perception just folklore, so they devised an experiment to give a definitive answer.  Apparently, the act of scratching does increase inflammation and bring in more immune cells to worsen the situation in the skin.  So, if we could just stop the itching before the scratching ensues.

Like the researchers, we all wonder why scratching feels so good if it is bad for us?  After a full trunk episode with poison ivy years ago, I can attest to the joy of relief provided by scratching until the itch subsided even for a few seconds.

With the help of some mice, the researchers induced eczema like symptoms on the ears of mice you can feel an itch and a group whose itch nerves were absent.  Furthermore, in the mice who could feel an itch, some were given little collars to prevent scratching.

For those mice who felt the itch and scratched away, the ears became more swollen and immune cells migrated to the areas in greater numbers.  In those who did not feel an itch or were prevented from scratching, there was less swelling and immune cell inflammation.

Correlation could explain this were it not for another step in the experiment.  They found that pain-sensing neurons released a chemical called substance P by scratching.  This chemical would then activate mast cells that further coordinate inflammation.  While mast cells are directly activated in contact dermatitis, this demonstrates that they are further activated by the physical process of scratching.

As we also care for many mast cell activation syndrome patients, we can appreciate the identification of pathway by which physical stimuli can trigger mast cells directly.  Many of our patients will experience hives or histamine symptoms from either physical stimuli like pressure or temperature changes or, in the syndrome of multiple chemical sensitivity, from random odors.

In helping our patients live healthier, more abundant lives, understanding the mechanisms of a disease process like eczema and contact dermatitis allows us to target therapies more effectively and give better advice on how they can overcome their symptoms and get back to regular life.

 

Original Article:

Andrew W. Liu, Youran R. Zhang, Chien-Sin Chen, Tara N. Edwards, Sumeyye Ozyaman, Torben Ramcke, Lindsay M. McKendrick, Eric S. Weiss, Jacob E. Gillis, Colin R. Laughlin, Simran K. Randhawa, Catherine M. Phelps, Kazuo Kurihara, Hannah M. Kang, Sydney-Lam N. Nguyen, Jiwon Kim, Tayler D. Sheahan, Sarah E. Ross, Marlies Meisel, Tina L. Sumpter, Daniel H. Kaplan. Scratching promotes allergic inflammation and host defense via neurogenic mast cell activation. Science, 2025; 387 (6733) DOI: 10.1126/science.adn9390

 

Thanks to Science Daily:

University of Pittsburgh. “Why you shouldn’t scratch an itchy rash: New study explains.” ScienceDaily. ScienceDaily, 30 January 2025. <www.sciencedaily.com/releases/2025/01/250130161631.htm>.

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Sanctuary Functional Medicine, under the direction of Dr Eric Potter, IFMCP MD, provides functional medicine services to Nashville, Middle Tennessee and beyond. We frequently treat patients from Kentucky, Alabama, Mississippi, Georgia, Ohio, Indiana, and more... offering the hope of healthier more abundant lives to those with chronic illness.