Throughout most of medical history, doctors thought that itch was a mild form of pain - then in 1987, scientists found out that it's a completely separate form of sensation. Still, itching can be extremely aggravating, especially if the feeling won't quit.
Here's a fascinating though a bit long article at the New Yorker by Atul Gawande about itching. It talks about a woman called "M." who suffered a persistent scalp itch that, of course, led to constant scratching, and a man called "H." who suffered from itching and pain in his arm after a spinal cord surgery.
“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 At a later office visit, her doctor found a silver-dollar-size patch of scalp where skin had been replaced by scab.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.
(Okay, that may be a stretch - but the rest of the article is very good)