Samantha Davies was 13 when she began to develop the deformities that would transform her into what she described as ‘the most ugly person in the world’. Her nose began to spread into a formless lump across her face, her cheeks inflated to three times their normal size and her head became square and masculine.

First, she tried hiding behind make-up. She would use so much foundation that ‘her face was just orange, like a mask’, her mother says. She would apply six or seven layers of mascara. She would straighten her hair (to cover her face) with hair irons to the point of singeing it.

After three months she decided she was too monstrous to be seen. She confined herself to her bedroom and refused to go to school. She would agree to be taught at home only if the tutor couldn’t see her face. ‘She would sit on the bed with a quilt over her head. The woman would teach her through the quilt,’ her mother remembers. Finally, aged 13 and a half, Samantha had had enough. She took an overdose. And if this weren’t evidence enough of her state of mind at that point, her reaction when she came to in hospital underlined it. ‘My first thought was, “What do I look like?”’ And the feelings of self-revulsion returned.

The strange thing is that Samantha looks perfectly normal. It was all in her head. In October 2009, after months of anguish and three suicide attempts, she was finally treated for severe body dysmorphic disorder.

Body dysmorphic disorder (BDD) is driven by intense anxiety about appearance, and its exceptional force lies in its delusional quality. The focus can be any body part, but typically it is the head - hair, nose, ears, skin, the size and shape of the jaw - which sufferers see as ugly, “not right”.

“I remember a colleague from the States who was treating a soldier in the American Army. He had been on the front line in Iraq and had been shot at and all he could think about was the size of his nose; that is how intense and consuming the preoccupation can be,” says Dr David Mataix-Cols, a professor and consultant clinical psychologist at the Institute of Psychiatry and at the Maudsley Hospital, London, which has ramped up its service for young people with BDD in the past year.

BDD is relatively common - it affects about one in 100 people (significantly more than schizophrenia; slightly more than anorexia). It typically starts in early teens and affects boys as frequently as girls. The causes of BDD are still unknown. “We know it runs in families and that there is a strong genetic component, but that doesn"t explain the whole picture,” Dr Mataix-Cols says, listing such factors as “appearance-related teasing” and bullying. “It"s not clear if they simply trigger a pre-existing vulnerability or whether they have a proper causal effect,” he adds.

“It"s an obvious thing to blame our image-obsessed society, but I don"t think it really has such a major role,” Dr Mataix-Cols says. However, Dr Katharine Phillips, a professor of psychiatry and human behaviour at the Alpert Medical School, Brown University, in Rhode Island, and a leading expert on the disorder, is not so sure. “It"s possible that the rate of BDD is increasing as women get bombarded with media images of perfection. Lots of studies have shown that the more you see images of perfection around you, and the more you compare yourself with those images, the worse you tend to feel about yourself.”

The point on which experts agree is its seriousness. Sufferers are compelled to take steps to mitigate the disaster they see in the mirror. Samantha was fixated on hair extensions and make-up. “I used to put a lot of make-up on, wipe it off, start again, for hours and hours,” she remembers. Obsessive exercising is another compulsion. Photographing themselves on mobile phones and digital cameras is another. Sufferers repeatedly ask if they look fat/disgusting/ugly; overfocus on the appearance of other people (“comparison rituals”); pick at their skin. At her worst Samantha was checking her face in the mirror 80 times a day, sometimes for up to two hours at a time.

People with BDD seek cosmetic surgery - about a quarter of BDD sufferers have actually had it - and avoid other people. Sometimes children drop out of school. “It is terrible for the young person"s development because being around other people is extremely painful and anxiety-provoking, and in severe cases they can become housebound,” Dr Mataix-Cols says. BDD also raises the spectre of suicide, with a rate approximately 45 times higher than in the general population.

And yet, despite it being such a serious problem, BDD is under-reported and undertreated. “People with BDD are afraid they will be considered superficial or vain,” Dr Mataix-Cols says. “It is hard to go to your GP and say, I think my nose is too big. So what often happens is they report an associated problem such as low mood or anxiety, but they won"t say what the reason for that is.”

The result is that the Maudsley treats only a “handful” of new cases a year, with patients ranging in age from 12 to 18, typically the more extreme cases from across the country. Less severe cases tend to be treated at local Child and Adolescent Mental Health Services (CAMHS) - if they"re lucky. “It doesn"t get picked up as quickly as we would like,” Dr Mataix-Cols says. “We know that if you live outside a big city your chances of getting access to good psychological treatment may be less.”