Pay close attention during your next office meeting and you will probably notice your coworkers unconsciously scratching themselves. Although it may be a bit embarrassing to be caught scratching like a barnyard animal, our constant attention to itches is normal and healthy, says Anne Louise Oaklander, a neurologist at Massachusetts General Hospital and Harvard Medical School. She should know: She is an expert on some of the world’s worst itches, one of the few doctors prepared to treat people whose lives are ruined by a chronic need to scratch. Some victims of this disorder spend years in skin-crawling misery, unable to find a cause or a cure for their problem; one patient scratched all the way through her skull to her brain. Oaklander sat down with Discover to explain why itching is actually crucial to survival, why scratching feels so good, and how, in the worst cases, an itch can become debilitating.
What exactly is it that makes us itch?
It’s a myth that there are five senses. There are an almost infinite number of different kinds of sensory receptors deployed around our body, and some specialize in receiving very specific types of input. In general, sensory stimuli are divided into noxious and innocuous—those that are potentially harmful and those that aren’t. The quintessential noxious stimulus is pain. But in the last few years it’s been recognized that there is a second set of sensory machinery focused on noxious stimuli, and that’s what produces the sensation of itch.
So is itching related to pain?
There is a lot of interaction between the two, but some aspects of these systems are separate. People once thought when pain neurons fire not so hard or so often, we perceive it as itch. But in recent years, neurons and receptors have been found in the skin and the central nervous system that are itch specific. Pain is defined as an unpleasant stimulus that makes you move away from it. Instead of withdrawing from itchy stimuli, we scratch at them. As with pain, this involves both conscious and unconscious behavior.
Since we already experience pain, why would nature set up a second pathway to defend us from harm?
Withdrawal, which is our response to pain, works very well in most circumstances, but there are a few specific kinds of harm where withdrawing won’t help, like small stimuli that cling or attach to us. Itching, which is widespread among animals, likely evolved to protect us against threats such as insects or plants like nettles. If you’re bitten by a mosquito—or worse, by a black widow spider—the thing that will save you is not jerking your hand away but swiping at that stimulus to dislodge it.
Why do mosquito bites itch, even though the insect takes only a tiny drop of blood?
Most of us think about mosquito bites as mere annoyances, but in the part of the world where early hominids evolved, mosquitoes carry several potentially lethal illnesses, including malaria and dengue fever. A mosquito bite could be a life-threatening event, so there was evolutionary pressure to develop a system to enable us to avoid mosquito bites and other insect threats. Itch is good when it’s brief and motivates you.
Why does scratching an itch feel so good?
Pretty much anybody will start noticing little things itching them while reading this article. They’re going to start scratching. In most cases scratching gives tremendous temporary relief to an itch by damping down the firing of itch neurons. At the same time, it activates reward locations in the brain. In fact, I’m just scratching my waist now. Where were we? Oh yes. It’s extremely pleasurable. Presumably, scratching is pleasurable because it has some advantage; that’s why you see not only people but animals like horses rubbing their itchy back against a fence post. Itching is intended to be felt briefly and dealt with, and then you recover. The problem comes when people begin to experience itches for prolonged periods of time. Normally if you keep scratching, at a certain point you’ll begin to damage the skin.
Why does watching someone itch make us want to scratch?
Why does watching someone drop an anvil on his foot makes us wince? There’s an emerging area of neuroscience that studies mirror neurons, which make us perceive and respond to experiences that other people are having. I’m only speculating, but there are obvious evolutionary reasons why we should respond to harmful stimuli that affect someone else.
How did you become interested in this kind of research?
I’ve worked at some of the top hospitals but never heard a lecture or read a chapter on itch during my training. When I was at Johns Hopkins, I spoke to a woman who had an itch above her eyebrow that was so severe she had scratched her eyebrow off. That certainly got my attention. Years later at Mass General I met a patient who scratched all the way through her skull and into her brain. I knew that this must be the same syndrome that the Hopkins woman had. I knew her case had to be published in order to help educate physicians. We treated her with local anesthetic to temporarily deaden the nerves and a locked helmet to prevent unconscious scratching. She survived but was left severely brain damaged.
What are the consequences of such persistent itches?
Patients forced to experience nocifensive sensations—feelings that defend us from harm—for a prolonged time can become depressed, suicidal, or scratch themselves to the point of injury. Not only is chronic itch unpleasant, but it also co-opts the functioning of the rest of our brain. Because itch sensations are designed to save lives, they have input to many parts of the brain. Not only do they affect the sensory cortex and motor cortex, but they are profound stimuli for memory, learning, attention, arousal, and emotion.
How do I know if a persistent itch is normal or a problem?
If you spend the night in my backyard in August without mosquito spray, you’ll besubjected to itch, but the cause is legitimate and it’ll be so unpleasant you’ll never do it again. The problem is when illnesses cause chronic itch. This includes three categories: One is skin conditions, like eczema, that can be treated topically with steroids. The second is problems like kidney and liver failure, which cause substances like bile salts to build up in the body and trigger itch receptors in the skin. Here the itch system is functioning normally; it’s an illness that’s to blame. Finally there’s the kind of itch I work on, called neuropathic itch, in which sensory neurons themselves are ill or damaged, firing itch signals even when there are no stimuli.
How do itch nerves get damaged?
Diabetes, AIDS, and AIDS medications can cause this. If the damage affects the neurons whose job it is to convey itch, the patient will perceive chronic itch just as if an insect were stinging him. In fact, people often describe these odd sensations as feeling as if a mosquito were biting them over and over.
What other unusual types of itch have you encountered?
Patients with dermatological parasitosis have delusions that insects are walking on or crawling through their skin. The part of the brain that processes itch sensation might be involved in some major psychiatric illnesses, and patients may have itch hallucinations just as they have auditory and visual hallucinations. The margin between what’s neurologic and psychiatric is not clear. There’s also notalgia paresthetica, in which patients have a particular spot—often on the torso, back, belly, or shoulders—that itches and itches without any dermatologic explanation. The most common cause is degenerative arthritis of the spine and a pinched nerve root. It’s the itch analogy to sciatica.
What is the prognosis for curing chronic itch?
Many patients who’ve been studied in evaluations of pain medication also have itch, so the efficacy of drugs on itch could also be assessed. I suspect that the data are already there, locked in file cabinets from past clinical trials. We need to get that information out to physicians and to the public to begin developing new, effective treatments for chronic itch.
