Scientists once saw itching as a form of pain. They now believe it to be a different order of sensation. Photograph by Gerald Slota

It was still shocking to M. how much a few wrong turns could change your life. She had graduated from Boston College with a degree in psychology, married at twenty-five, and had two children, a son and a daughter. She and her family settled in a town on Massachusetts’ southern shore. She worked for thirteen years in health care, becoming the director of a residence program for men who’d suffered severe head injuries. But she and her husband began fighting. There were betrayals. By the time she was thirty-two, her marriage had disintegrated. In the divorce, she lost possession of their home, and, amid her financial and psychological struggles, she saw that she was losing her children, too. Within a few years, she was drinking. She began dating someone, and they drank together. After a while, he brought some drugs home, and she tried them. The drugs got harder. Eventually, they were doing heroin, which turned out to be readily available from a street dealer a block away from her apartment.

One day, she went to see a doctor because she wasn’t feeling well, and learned that she had contracted H.I.V. from a contaminated needle. She had to leave her job. She lost visiting rights with her children. And she developed complications from the H.I.V., including shingles, which caused painful, blistering sores across her scalp and forehead. With treatment, though, her H.I.V. was brought under control. At thirty-six, she entered rehab, dropped the boyfriend, and kicked the drugs. She had two good, quiet years in which she began rebuilding her life. Then she got the itch.

It was right after a shingles episode. The blisters and the pain responded, as they usually did, to acyclovir, an antiviral medication. But this time the area of the scalp that was involved became numb, and the pain was replaced by a constant, relentless itch. She felt it mainly on the right side of her head. It crawled along her scalp, and no matter how much she scratched it would not go away. “I felt like my inner self, like my brain itself, was itching,” she says. And it took over her life just as she was starting to get it back.

Her internist didn’t know what to make of the problem. Itching is an extraordinarily common symptom. All kinds of dermatological conditions can cause it: allergic reactions, bacterial or fungal infections, skin cancer, psoriasis, dandruff, scabies, lice, poison ivy, sun damage, or just dry skin. Creams and makeup can cause itch, too. But M. used ordinary shampoo and soap, no creams. And when the doctor examined M.’s scalp she discovered nothing abnormal—no rash, no redness, no scaling, no thickening, no fungus, no parasites. All she saw was scratch marks.

The internist prescribed a medicated cream, but it didn’t help. The urge to scratch was unceasing and irresistible. “I would try to control it during the day, when I was aware of the itch, but it was really hard,” M. said. “At night, it was the worst. I guess I would scratch when I was asleep, because in the morning there would be blood on my pillowcase.” She began to lose her hair over the itchy area. She returned to her internist again and again. “I just kept haunting her and calling her,” M. said. But nothing the internist tried worked, and she began to suspect that the itch had nothing to do with M.’s skin.

Plenty of non-skin conditions can cause itching. Dr. Jeffrey Bernhard, a dermatologist with the University of Massachusetts Medical School, is among the few doctors to study itching systematically (he published the definitive textbook on the subject), and he told me of cases caused by hyperthyroidism, iron deficiency, liver disease, and cancers like Hodgkin’s lymphoma. Sometimes the syndrome is very specific. Persistent outer-arm itching that worsens in sunlight is known as brachioradial pruritus, and it’s caused by a crimped nerve in the neck. Aquagenic pruritus is recurrent, intense, diffuse itching upon getting out of a bath or shower, and although no one knows the mechanism, it’s a symptom of polycythemia vera, a rare condition in which the body produces too many red blood cells.

But M.’s itch was confined to the right side of her scalp. Her viral count showed that the H.I.V. was quiescent. Additional blood tests and X-rays were normal. So the internist concluded that M.’s problem was probably psychiatric. All sorts of psychiatric conditions can cause itching. Patients with psychosis can have cutaneous delusions—a belief that their skin is infested with, say, parasites, or crawling ants, or laced with tiny bits of fibreglass. Severe stress and other emotional experiences can also give rise to a physical symptom like itching—whether from the body’s release of endorphins (natural opioids, which, like morphine, can cause itching), increased skin temperature, nervous scratching, or increased sweating. In M.’s case, the internist suspected tricho-tillomania, an obsessive-compulsive disorder in which patients have an irresistible urge to pull out their hair.

M. was willing to consider such possibilities. Her life had been a mess, after all. But the antidepressant medications often prescribed for O.C.D. made no difference. And she didn’t actually feel a compulsion to pull out her hair. She simply felt itchy, on the area of her scalp that was left numb from the shingles. Although she could sometimes distract herself from it—by watching television or talking with a friend—the itch did not fluctuate with her mood or level of stress. The only thing that came close to offering relief was to scratch.

“Scratching is one of the sweetest gratifications of nature, and as ready at hand as any,” Montaigne wrote. “But repentance follows too annoyingly close at its heels.” For M., certainly, it did: the itching was so torturous, and the area so numb, that her scratching began to go through the skin. At a later office visit, her doctor found a silver-dollar-size patch of scalp where skin had been replaced by scab. M. tried bandaging her head, wearing caps to bed. But her fingernails would always find a way to her flesh, especially while she slept.

One morning, after she was awakened by her bedside alarm, she sat up and, she recalled, “this fluid came down my face, this greenish liquid.” She pressed a square of gauze to her head and went to see her doctor again. M. showed the doctor the fluid on the dressing. The doctor looked closely at the wound. She shined a light on it and in M.’s eyes. Then she walked out of the room and called an ambulance. Only in the Emergency Department at Massachusetts General Hospital, after the doctors started swarming, and one told her she needed surgery now, did M. learn what had happened. She had scratched through her skull during the night—and all the way into her brain.

Itching is a most peculiar and diabolical sensation. The definition offered by the German physician Samuel Hafenreffer in 1660 has yet to be improved upon: An unpleasant sensation that provokes the desire to scratch. Itch has been ranked, by scientific and artistic observers alike, among the most distressing physical sensations one can experience. In Dante’s Inferno, falsifiers were punished by “the burning rage / of fierce itching that nothing could relieve”: