From The Guardian:
One of our first patients, just 17 years old, was brought to us in a wheelchair,” says Professor Christer Lindquist, a pioneer in the use of a brain surgery technique for people with Obsessive Compulsive Disorder (OCD), known as Gamma Knife. “This boy would set himself maths problems, which he had to solve before he could eat. His OCD had become so severe, and the maths problems he set himself so complex, that he couldn’t solve them any more, so he couldn’t eat.
(With thanks to muriel)