There’s a disease that causes people to eat their own hands!

It’s called Lesch-Nyhan syndrome, or LNS, and it has become famous for the particularly disturbing aspect of self-mutilation it causes. First officially documented in 1964, it is caused by a deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase produced by a mutation on the gene HPRT. 

It affects about one in every 380,000 births. Symptoms emerge between two and three years of age with simply strange behavior. Self-mutilation however, begins around age three. Sufferers usually begin biting their lips and tongue, and as the disease progresses, afflicted individuals will move to their hands, and in extreme cases, head banging. 

Compulsive behaviors such as vomiting, spitting, and involuntary swearing also result from the disease, and while it can be controlled in certain ways, there is no standard treatment for LNS. 

(Source)