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The itch-scratch cycle is an incredibly important concept that sits at the heart of atopic dermatitis and other itchy skin conditions including prurigo nodularis, lichen simplex chronicus, and neurodermatitis. The basic idea is that when there is the sensation of itch that causes people to scratch or rub at the skin. This physical act of scratching does a few things simultaneously: 1. It damages the skin barrier allowing for water to more easily escape and for allergens, irritants, and unwelcome bacteria to more easily enter; 2. It fuels some of the inflammatory pathways directly and increases blood flow to the area, also adding to the potential inflammation; and 3. It causes neuro-behavioral changes (both in the nerve endings themselves but also reinforcing behaviors) that can continue to fuel this even when there isn’t anything in particular causing an itch.
The hardest part about this vicious cycle is that once it starts, it can be very difficult to stop. And, even if some initial trigger is identified–say an allergen that touched the skin–after a certain point, the process can continue unabated without needing any exposure to an outside trigger. This is an important insight because many patients are convinced that there must be something external driving the skin disease: Could it be a food? Could it be a detergent? Could it be something in the air or water? Yes, it can of course be any (or all!) of these, but after a certain point, it could also be NONE of them. Once the itch-scratch-itch cycle begins, it truly can have a mind of its own.
But the misery of itching and the secondary skin damage are not all that the itch-scratch cycle is culpable of. We know that sleep disturbance is reported in many–probably most–patients with atopic dermatitis, and we know that the itch-scratch cycle is responsible for these sleep issues in many cases. It turns out that disrupted sleep also leads to skin barrier damage, which can in turn fuel the itch-scratch cycle in and of itself. Additionally, poor sleep leads to stress, another factor that both damages the skin barrier and feeds inflammation in the body.
The microbiome is the group of healthy bacteria, fungi, and viruses that live in and on our body. When the skin barrier is disrupted, the microbiome also changes, and this in turn can lead to further damage to the skin barrier and further inflammation. Staphylococcus bacteria in particular can produce a number of toxins that both damage the protective skin barrier and also cause inflammation in the skin.
Another important area is that of sensitization. Our skin barrier keeps water in and keeps all of the bad stuff out: allergens, irritants, pathogens, pollutants, and toxins. When that skin barrier is damaged by the itch-scratch cycle, it means that those things can now more easily gain entry. When this happens, it appears to be possible to become sensitized to certain allergens, meaning we now have the potential to have an allergic reaction to something that previously did not cause one.
The store of peanut allergy is particularly instructive here: it turns out that a damaged skin barrier seems to be a possible portal of entry for peanut protein into the skin, which can then in turn, cause the formation of antibodies against those proteins, possibly leading to true peanut allergy. Children with atopic dermatitis are much more likely to develop food allergies, including peanut allergy, and it is thought that this is at least partially due to the broken skin barrier due to the itch-scratch cycle.
What can we do for those who are stuck in such a cycle? Typically, we want to work to cool the inflammation, calm the itch, and help restore the skin barrier and the microbiome. If we can do these for a sufficient period of time, we find that we are often successful in breaking that terrible cycle and getting back into the virtuous cycle of good health with all of its benefits: better sleep, better focus and clarity, better skin appearance, little or no itch, better microbiome, and decreased inflammation overall.