The problem with having skin is that it’s not blemishless and can burn very badly and is liable to develop rashes or sores, not to mention cancers, and — at once more benign and, in the moment, just as miserable as any of these things — it can itch. Some itches, of course, are perfectly pleasant, particularly those that can be easily scratched. But all itches, low-level or consuming, present the same question: why? Why do we itch? What is its evolutionary purpose? What, over millions of years, was itching honed to accomplish, and where does this itch-sensation come from? For this week’s Giz Asks, we reached out to a number of experts to find out.
Associate Biologist (Dermatology) at Massachusetts General Hospital and Head of the Lerner Lab, whose goal is to understand the mechanisms that underlie the sensation of itch to develop effective anti-itch therapies
The traditional understanding of itching is that we have the sensation in order to remove bugs, or whatever other environmental stimuli are interacting with our skin. There’s a lot of support for this concept, although I do take some issue with it, given that one of the most common itches is from mosquito bites, and by the time you know a mosquito is there, it’s gone.
A number of us feel the reason we itch is to help stimulate the immune system to become activated and to recognise something in the environment that shouldn’t be there — it helps turn on the immune system to fight off insults. It’s a nervous system thing, an example of the tight connection between the nervous system and the immune system and the barrier of the skin, which otherwise protects us, to keep things out.
Then there’s the question of why scratching an itch often feels quite good. Not all itches are pleasurable, of course, but they often are — it can feel really good, if you have dry skin, to jump in the shower and start scratching. One of the reasons it’s pleasurable, I think, is to get you to scratch, to further activate the immune system and make sure it gets the right signal.
Assistant Professor, Dermatology, Massachusetts General Hospital
100% of the population experiences acute itch at some point in their lives, but it’s quite startling when you consider the number of people suffering from chronic itch, defined as experiencing itch most days of the week for over 6 weeks. Anywhere from 5 to 10 per cent of the general population, and up to 40 per cent of certain medical populations, experience chronic itch — it’s a real problem.
Why we itch is an interesting question and involves communication between multiple cell types in the skin or mucosa and the nervous system. At a high level, we feel itch when cells in our skin release signalling molecules that activate peripheral nerve fibres that in turn relay that itch signal to the spinal cord and up to the brain. In the brain, these chemical signals are interpreted as itch. Skin cells that can release these itch-inducing signals include the cells that make up the skin, called keratinocytes, but also the immune cells that populate and guard your skin. Sometimes these cells are triggered to release factors in response to exogenous (outside) agents like chemicals, irritants, plants or insects, and other times they are driven by endogenous (internal) factors released by the immune system when you have inflammatory skin diseases (like eczema or psoriasis), fungal infections, diseases of other organs (like liver or blood diseases), or by medications you take.
Occasionally, your itch nerves can be damaged or diseased and will simply trigger unsolicited itch sensations. This type of mistaken signal can happen anywhere along the neural pathway, from the skin to the spinal cord to the brain. In this case, people feel itchy even without a rash.
Director of Dermatopathology and Assistant Professor of Dermatology at UC Irvine
The neurobiology of itch is complex and is still not well understood. Itch is similar to pain in the sense that both are considered noxious, or unpleasant, stimuli. Both pain and itch are mediated by sensory nerves which communicate through the spinal cord to various parts of our brain. However, our behavioural response to each is different. To pain, we respond with withdrawal. To itch, our response is to scratch. That’s the definition of itch: any sensation that causes a desire to scratch.
So what causes itch? Well, anything that affects the itch pathway can cause the sensation of itch — from the skin to the spinal cord to the brain itself.
More practically, dermatologists broadly think of itch in two categories: “itch with rash” and “itch without rash.” The former is easier to identify and treat: These would be skin problems like hives, bug bites, eczema, psoriasis, etc. The primary cause of itch in “itch with rash” is usually in the skin itself, so we are able to directly address those causes.
In “itch without rash,” it can be more difficult to identify the underlying cause, and thus may also be more difficult to treat. In these cases, it can be an issue of the sensory nerves or the central nervous system itself. It can also be a systemic problem — there are certain systemic disorders that can cause itch, including medication side effects, thyroid disease, kidney dysfunction, biliary obstruction, and certain forms of cancers, among other things.
Itch that is purely related to nerve problems is not uncommon. Multiple sclerosis is a disease of the nervous system, and these patients can experience itch due to the dysfunction of the nerves without any skin causes. There is a phenomenon termed pruritus of the elderly. (Pruritus is the clinical term for itch.) In these cases, there’s nothing wrong with the skin, and it is thought that the itch is due to age-related dysfunction of the nerves in your skin. Another example is spinal cord abnormalities leading to the sensation of itch. For example, if you’ve gotten into a car accident and injured your spine, or if you developed a disc herniation, these traumas to your spinal cord can lead to the misfiring of the sensory nerve pathway, leading to a sensation of localised itch. The abnormality impinging on the nerves will cause the nerve to inappropriately fire, sending a message to your brain that you’re itchy in one spot even though there’s nothing in that area of the skin that is truly stimulating the ends of those nerves.
Itch is hugely disruptive to the lives of those who suffer from it — can you imagine night after night of not being able to sleep because you’re too itchy? A recent study revealed how chronic itch can cause insomnia, mood disorders, disruptive thoughts, stress and even suicidal thoughts, and can severely impact personal relationships.
Finally, an interesting phenomenon is something called contagious itch — that is, when you encounter someone who is itching and scratching themselves, you start to feel itchy and compelled to scratch yourself as well. We don’t really understand it, but it’s really interesting. A lot of work still needs to be done to understand the various pathways of itch, and how the brain processes it.
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