Three-year-old Hannah Thompson and two-year-old Haaris Kureshi appeared to be completely normal children when they were born. But both suffer from an extremely rare genetic disorder, of which there are just 30 reported cases in the world: the inability to feel pain.
No matter how badly they injure themselves, no message gets through to the brain to alert them to the danger they are in. So they mutilate themselves unaware of the harm they are doing. Hannah has bitten the tips off her fingers, bitten the tip off her tongue and last year bit off her thumb, all the while never shedding a tear.
Hannah's family could not bear to watch her destroy her body and decided to have all her teeth removed: "It was a very hard decision to make," says Charlotte Thompson, her mother. "She had lovely teeth, perfectly healthy teeth... but she was spending two out of four weeks in hospital after biting herself. The way I looked at it was once her fingers and tongue went that was it, they were gone. At least she's going to get another set of teeth when she's older."
Congenital indifference to pain is part of a group of conditions known as hereditary sensory neuropathies and was first reported in 1932, with the case of a man known as The Human Pin-cushion, who toured music-halls sticking pins into himself. The condition is thought to be caused by the failure of betaendorphin - a substance that occurs naturally in the body and modulates pain sensations - to register pain. It is incurable. Similar experiences are found in older diabetes sufferers or leprosy victims.
"Pain is probably one of the most important sensations that we have, because without it we would damage ourselves irrevocably," says Stephen Green, consultant paediatrician at St Luke's Bradford, who treats Haaris. "It is essential for the survival of the species."
Children who suffer from the condition can easily be mistaken for victims of abuse. "I found myself trying to justify myself all the time and I couldn't," says Nusrath Kureshi, Haaris's mother. "I just felt so helpless. I thought, what can I say to them that will make them think that it wasn't me or somebody else?"
As well as the risk of external injuries there is the danger that the sufferer will not register warning pain in cases such as appendicitis and peritonitis. Extra wear and tear on the joints means that arthritis is a common problem and many children are in wheelchairs by their teens.
There are also psychological difficulties for sufferers and their parents. "How do you discipline children like this?" asks Dr Green. "Many parents still administer a smack across the knuckles if their child goes near something dangerous but it doesn't work on these children. And they may develop behavioural difficulties. A child could say, `If you don't give me what I want, I'll stick my hand in the fire.' They won't feel the pain but the mother will."
Charlotte, who has spent pounds 6,000 on a safe, padded playroom for Hannah, says that looking after her daughter has taken its toll on her: "I became very depressed, became very agoraphobic." People's reactions accentuated her isolation. "I'd take Hannah out shopping and she'd be biting her tongue and have blood pouring down her mouth. She'd often be covered in bandages and I'd hear people, particularly older people talking about her saying, `Look at that poor child, she must have been battered.' "
There is as yet no treatment for these children's condition, Dr Green says , although some tests have been done in France with the drug Naloxone, which lowers pain thresholds, and is usually prescribed for those who have been taking opiates.
But little work has been done on the condition and until some sort of treatment is found, Charlotte and Nusrath must face years of worry trying to protect their children from themselves. "I think the day Haaris falls and cries from pain would be the best day of my life," says Nusrath. "It would mean I could cuddle him and when he stopped I would know he was OK."
Hannah and Haaris's story is told in `Here and Now' on BBC1 tonight at 7.30