The Itch: Mind Over Matter?

    Imagine an army of ants crawling up your back. Slowly they move up your spine, their short legs scurrying along. Eventually they reach your neck, briefly touching your skin before they move up the back of your head. Does your skin feel tingly? Did your head start to itch? Did you give in to the temptation to scratch? If so, you aren’t alone; I had those same sensations as I was writing this. That inexplicable sensation of itching just by reading about it is not uncommon. Atul Gawande, in The Itch, a treatise on itching published in The New Yorker, described a similar experience. Using Gawande’s article, I want to show how that “phantom” itch, in the absence of real stimulation, is at the center of a fascinating theoretical controversy over the origin and nature of itching. In this paper I will explore the mechanisms behind that “itchy” feeling and show how the itch is not simply a function of nerves. There is good evidence that the brain is very involved in the experience of itching.

    What is not controversial is the definition of itch, which originated in the 1660’s and has not changed much since then. It is described as “an unpleasant sensation that provokes the desire to scratch” and has been ranked as one of the most distressing physical sensations one can experience [2]. It was long thought to be on a continuum with pain; however, this theory was disproved in the 1980’s when studies showed pain and itch to be separate sensations that are transmitted along different neural pathways [2]. The next discovery came along in 1997, when scientists found a nerve specific for itch. A single itch nerve fiber can detect an itch sensation up to three inches away, and these fibers have incredibly slow conduction speeds, which is why itching is slow to build and slow to dissipate. Despite this newfound understanding of itch, it still remains somewhat of a mystery. The human body has as many itch receptors as other receptors (i.e. pain, touch), so why then does the brush of a feather sometimes itch and sometimes tickle? Why, when you read the first few sentences of this paper, did you start to itch merely at the suggestion of itching? And could this sensation be attributed to nerves? Gawande explores these questions and more in his article, which begins with his recounting of some horrifying stories of people with chronic itching.
    

   First he describes M., a woman who developed a persistent itch on the right side of her scalp after a shingles episode. After treatment for the shingles, the area became numb and the pain was replaced with constant itching. Despite multiple appointments with doctors of all specialties, no cause or source of the itching was determined. One night, the itching was so intense that while she was sleeping, she scratched through her scalp and into her brain. She was tested for OCD and other psychological disorders, but the results were negative. Eventually, a neurologist named Dr. Oaklander approached M. and said she knew that M. was not crazy, and told her she had seen this kind of thing before. After conducting a few tests, Dr. Oaklander developed two theories to explain the itching. The first suggested that the “few remaining nerve fibers were itch fibers, and with no other fibers around to offer competing signals, they had become constantly active” and the second was the opposite, suggesting that “the nerves were dead, but perhaps the itch system in M.’s brain had gone haywire, running on a loop all its own” [2]. While the rest of the doctors favored the first, Dr. Oaklander believed the second theory was true. However, M. followed the advice of the other doctors because she was so desperate for some relief. She allowed them to sever her supraorbital nerve (right above the eye). The itching disappeared, only to return full force over a larger area a few weeks later.

    Next Gawande described the case of Mr. H., a man who had a tumor removed from his spinal cord. After the operation, he developed a list of bizarre sensations: his left hand felt “cartoonishly large;” he had this intense, constant burning pain extending from the left side of his neck, all the way down his arm; and an itch crawled along the same path, one that could not be relieved with scratching. He tried a large number of treatments, to no avail, and eventually had to retire. His life had become one of solitude, save for when his wife returned from work. He busied himself with projects, attempting to ignore the troublesome sensations, of which the itching was by far the worst [2].

    The two people described in Gawande’s article are victims of an intense suffering caused by itching of some unknown source. Their treatments have been futile and their discomfort continues, affecting their lives in every aspect. So what is the cause of all this misery if it is not a nerve problem? In recent years, a new understanding of perception has come to light. The old theory of perception is that what we see is what we perceive, that the world is “picked up by our nerve endings, transmitted through the spinal cord like a message through a wire, and decoded by the brain” [2]. However, it is becoming increasingly clear through phenomena such as phantom limbs, dreams that are indistinguishable from reality, or itching that comes from nowhere, that this is not the mechanism by which we perceive the world. Gawande explains that scientists analyzing the signals of someone viewing a tree found them to be extremely lacking: “Given simply the transmissions along the optic nerve from the light entering the eye, one would not be able to reconstruct the three-dimensionality, or the distance, or the bark- attributes that we perceive instantly” [2]. Our perception of the world is extremely rich and full of detail. But, as we discussed in class, the incoming information is poor. To get from the input to perception, the brain fills in the gaps. Much of vision, and perception in general, is a construction of the unconscious part of the nervous system.

    How does the brain know what to fill in? In class we talked about the “brain’s best guess” and how there isn’t necessarily a right answer for what to fill in. Gawande expands this claim stating, “…visual perception is more than ninety percent memory and less than ten percent sensory nerve signals” [2]. This process of filling in the blanks means perception is inference. Our brain “integrates scattered, weak, rudimentary signals from a variety of sensory channels, information from past experiences, and hard-wired processes, and produces a sensory experience full of brain-provided color, sound, texture, and meaning” [2]. It is this process that allows for the creativity and diversity in how we perceive the world we live in and how we can advance in it.

    Despite this advantage, this system has its flaws and can backfire. This is the case for M. and H., whose perceptual systems seem to have gotten confused. For M. the shingles destroyed the nerves on her scalp, and her brain, using the little input it had, concluded that something was terribly itchy, like ants crawling up the back of your head. Without any contradictory signals, the brain continued to tell M. that her head was itchy and she should scratch it. So she did. A similar chain of events may have occurred for H. as well, after the operation to remove the spinal tumor.

    This and other neurological-type symptoms, for which testing has not revealed a hardwire explanation, are not sensory nerve phenomena but have to do with brain confusion. There are many chronic conditions (i.e. chronic back pain, fibromyalgia, tinnitus) that are treated as nerve or tissue problems. But the conundrum is that surgical solutions, such as snipping nerves, do not fix the sensory pain; it is still there [2]. The constantly ringing sensor for itching is similar to the phantom limb pain sensation, for which possible treatments are emerging.

    An exciting example of these is mirror-box treatment, which is used to treat phantom limb pain of amputees. The set-up is quite simple, making is easy to perform in one’s own home. The patient stands or sits with their chest against the side of the mirror, so that one limb is in front and the other, affected limb is behind the mirror. The patient moves the unaffected limb and sees the mirror image of that limb moving, giving the illusion that both limbs are unaffected. When the mirror-box treatment was used for H., his left hand (the cartoonishly large one) was behind the mirror. He tipped his head so that the image of his right arm seemed to occupy the same position as his left, and he started to wave both of his arms. Mr. H reported that the symptoms in his left arm subsided. He also had success, over time, with relieving the itching sensations. When they got to be too much to bear, he simply pulled the mirror out and that did the trick [2].

    A study reported in the New England Journal of Medicine reports similar findings in patients with an amputated foot. The study had three testing groups, with six patients per group: one that viewed a reflected image of their foot, one that viewed a covered mirror, and one that was trained in mental visualization. The patients completed their assigned therapy for fifteen minutes daily for four weeks. After four weeks, 100% of the people in the first group (mirror group) reported a decrease in pain, compared to 17% in the covered mirror group and 33% in the visualization group [1]. Although I do not think these results definitively state the success of the mirror treatment, it is a good starting place to encourage further studies that can accurately evaluate the efficacy of this kind of therapy.

    How does this magic mirror work? I think the mechanism is still a little vague, but it is possible that mirror neurons, in the hemisphere of the brain that is contralateral to the amputated limb, have been activated. Another possibility is that “visual input of what appears to be movement of the amputated limb reduces the activity of systems that perceive protopathic pain” [1]. I believe it is also possible that the mirror is confusing the already confused brain and perceptual system, and so it creates a new but false reality. In other words, the therapy confuses the brain again to resolve the conflict, and thus the symptoms. If this chronic, unexplained itching sensation is similar to the mechanisms that cause phantom limb pain, then it is possible that mirror therapy could alleviate itching.

    Are you still itchy? Do you feel as though those ants are crawling up your back again? If you told someone else about your symptoms, would they start itching too? This contagiousness of itching is still quite puzzling. How is it that a sensation that seems to be so personal can be so universal and infectious at the same time? These are questions that I hope can be answered in the future with more studies in this field of neurobiology. Sensations such as itching and phantom limb pain do not have obvious chemical or mechanical causes. But they are just as debilitating, if not more, because successful, long-lasting treatments do not always exist. I feel that the research on itching expands the new explanations of how we perceive and how we might manipulate those perceptions.  




References
[1]  Chan, B, et al. "Mirror Therapy for Phantom Limb Pain." The New England    Journal of Medicine 357.21 (2007): 2207-2206. PubMed. Web. 10 May 2010.    <http://content.nejm.org/cgi/reprint/357/21/2206.pd>.

[2] Gawande, Atul. "The Itch." The New Yorker 30 June 2008: n. pag. Web. 30 Apr.    2010. <http://www.newyorker.com/reporting/2008/06/30/080630fa_fact    gawande>.