What can drive a person to demand one of their perfectly healthy limbs be amputated? A Scottish surgeon agreed to carry out just such an operation on two men suffering Body Dysmorphic Disorder.

This disturbing psychological problem, which causes sufferers to obsess over supposed bodily imperfections, prompted each private patient to seek the removal of their leg below the knee.

There was nothing wrong with these amputated limbs - the condition is just one of a number in which healthy people seek seemingly inapproprtiate treatment.

The best known of these is hypochondria in which sufferers do experience a range of physical symptoms, but tend to wrongly interpret them.

Excessive anxiety over their health commonly causes hypochondriacs to equate headaches, tiredness or stomach upsets with life-threatening ailments.

With physical sensation being relative, some people become convinced their everyday aches and pains are more serious than those experienced by others, heightening their panic.

Expert reassurance can often only allay such fears temporarily; while access to medical information can serve to spark new concerns.

The many thousands of health sites on the web have reputedly caused surfers to succumb to "cyberchondria" - fearing the onset of a plethora diseases of which they were previously unaware.

There are cases of people seeking medical help in the full knowledge they are fit and well - those affected by Munchausen's syndrome.

Named after Baron von Munchausen, the fictionalised 18th century liar, those with the syndrome seek to convince doctors they require treatment.

Wendy Scott admitted to gaining admission to hospital on at least 600 occasions and undergoing 42 operations in the 12 years up to 1978.

Although she claimed to have conquered the compulsion, Miss Scott complained doctors no longer took her seriously when she experienced abdominal pains.

The 50-year-old, who was thought to be the only Munchausen sufferer to successfully "cure" herself, died of stomach cancer in October 1999.

Perhaps more controversial than even this puzzling psychological condition is Munchausen's by proxy.

In such cases the sufferer does harm to someone in their care, usually a child, so that they can witness subsequent medical treatment.

Several doctors have suggested some "cot deaths" may even be linked to the problem.

Beverley Allitt, the nurse who in 1993 was convicted of killing four babies in her care and injuring nine others, was thought to have had Munchausen's by proxy.