The Activation of Itch

Todd Schlesinger (00:07):

Hi, I'm Dr. Todd Schlesinger. I'm a board certified dermatologist and member of the American Academy of Dermatology. I practice in Charleston, South Carolina. And I founded the Clinical Research Center of the Carolinas. And I also work with Epiphany Dermatology as my practice location.

(00:23):

When we're thinking about itch in the clinic and we have patients that have both atopic dermatitis and prurigo nodularis, or one or the other, the question comes to mind, and of course it has been written about quite a bit, is what's the difference between the mechanism of itch in atopic dermatitis versus prurigo?

(00:40):

Of course, prurigo nodularis, we think in basic terms of two forms of itch, inflammatory and neurogenic itch. Of course, I think those two factors play a role in atopic dermatitis as well. I think with atopic dermatitis, it's more in the inflammatory side. So thinking of those receptors and those cytokines that we know we have drugs targeting, we understand this pretty well now.

(01:00):

So for atopic dermatitis, I think a little bit less about the antigens that are directly... The pathogens that are directly affecting the nociceptors in the skin, the pain receptors along type C fibers. And I think more about the inflammation along the IL-4, IL-13, IL-31 pathways, and other ones that we know about. And atopic dermatitis, I think more about barrier dysfunction with atopic dermatitis than I do about prurigo nodularis. So I think it's important on both sides.

(01:29):

Oftentimes, the atopic dermatitis have fissuring, lichenification, maybe early on not as much thickening of the skin, but they do get that as well. And then those common plaques that we see, of course, in the common areas for atopic dermatitis versus prurigo with a nodule.

(01:46):

So I just remember reading a lot about how sensitive our nerve fiber endings are in the skin. They're tiny, very tiny, very sensitive. So even the smallest scratch, even when you just scratch your skin, when you have intact skin and you just scratch an itch, you're really activating a lot on the neurogenic pathway for itch. And you can actually even damage those receptors so they get damaged easily. And then when it gets damaged easily, you end up with a thickening of the skin and the lesions that we see.

(02:16):

Even when you scratch your skin when there's no lesion there or just an itch, you're actually activating your nociceptors in the skin quite sensitively. So they're very sensitive to external stimuli. And you can think about that in terms of if you just take a Q-tip or a cotton-tipped applicator and very gently touched against the skin, you can feel how intense that sensation can be in certain parts of the body. So the nociceptors, the receptors in the skin, the itch receptors and pain receptors are very sensitive. So you could imagine how much damage is occurring in patients with atopic dermatitis and prurigo nodularis with all the itch and scratch they're doing.

(02:54):

So just remember that it's not that hard to get those activated to create that inflammatory cycle in the skin, which will create keratinocyte hypertrophy, thickening of the skin, leading to not only lesions of prurigo nodularis, but also atopic dermatitis.