As far as party tricks go, Kay Underwood's is hard to beat. At least that's how many of her friends see it. Frequently introduced as the woman who "collapses when she laughs", this 20-year-old architecture student has cataplexy, which causes an abrupt relaxation of all her muscles the moment she begins to giggle.

"Sometimes I only have to think about something funny to collapse - and sometimes it happens again just as I'm coming round because I think about the joke that made me fall over in the first place," she says. "Once I tried to calculate how often it happened in one day, but I lost count at 40."

Like all people with cataplexy, Underwood wasn't born with these symptoms. "It first happened one day shortly after I went to university. I was sitting in my kitchen, my flatmate made me laugh and I hit the floor. I could hear and see everything going on, but I couldn't move a muscle."

Cataplexy is not a condition in its own right. After excessive daytime sleepiness, which Underwood has also experienced since she was 15, it is the second most common symptom of narcolepsy (which is caused by the lack of a neurotransmitter called hypocretin, which controls wakefulness and sleep). Its causes are as yet unknown. Some believe it may be genetic, while others think it could be the result of an auto-immune disorder or brain damage caused by a severe infection. There is also no known cure for this lifelong illness, which affects around 20,000 people in the UK. It is estimated that 60-90% of people with narcolepsy also have cataplexy.

Laughter is the most common trigger but some people have cataplexic attacks when they feel other strong emotions - anger, excitement, embarrassment, fear, or surprise, for example. There are even people who collapse when they think about sex or say, "I love you." "I had one patient who it happened to whenever he felt smug," says Dr Andrew Hall, a consultant in anaesthesia, intensive care and sleep disorders at Leicester General Hospital, where Underwood is being treated.

"If anger set me off, I'd be fine," says Underwood. "I'm always laughing. Sometimes my mum and I don't even have to be talking about something funny to laugh."

No sooner have we sat down in Underwood's parents' garden in Leicestershire than the two of them chuckle about the biscuits we're eating. It's difficult to anticipate when comedy will strike and for the past two years, Underwood has risked hurting herself every time it does - she once almost drowned in a swimming pool. She has developed a fear of escalators and can't walk down stairs without someone in front of her.

"It's always at the forefront of your mind because you constantly have to assess an area for danger. Is there glass around? Any sharp corners? Even having a cup of tea in your hand can be dangerous," she says. "I had to stop cycling and driving. At one time, I thought I might not be able to have a family because what kind of a mother would I be, collapsing all the time? Then there's the embarrassment factor. All of a sudden, you can fall into your dinner or your drink starts pouring straight out of your mouth."

However, Underwood does not collapse over the biscuit joke. "I'm on new medication that started working three months ago," she explains. Having taken it for a year, and aware that there are no long-term solutions, she won't disclose the name of the drug.

In the United States and the rest of Europe sodium oxybate, which replicates the activity of hypocretin, is a first-line treatment for cataplexy. Few patients receive it in the UK because it costs around £9,000 per year.

More commonly prescribed here are anti-depressants which can help to alleviate excessive tiredness, which is thought to decrease attacks. However for many, including Underwood, they not only fail to work but cause a range of unpleasant side-effects.

"I'm constantly wrestling with the difficulty of getting sodium oxybate prescribed for patients," says Adam Zeman, professor of cognitive and behavioural neurology at Peninsula Medical School in Exeter. "There's an incredibly elaborate procedure for every patient and the prescribing boards stall as long as they can every time. I think there's a feeling that cataplexy is a condition that doesn't need a pricey drug."

"A lot of people don't take it seriously," says Underwood. "It's not like you're in pain with it and because it happens when you laugh, people think it's funny."

Underwood adds that she had enough trouble getting her cataplexy diagnosed at all. "The first doctor said it was anxiety and the second said it was my blood-sugar levels.

The third one sent me to a specialist, but only because I'd found out what it was myself by looking up my symptoms on the internet."

The average time between first experiencing symptoms and being diagnosed is six years. Zeman believes that this is partly because patients with mild symptoms may assume that everyone gets weak when they laugh and partly because doctors are poorly educated about narcolepsy and cataplexy.

"It's almost certainly under-recognised and it usually tends to get picked up first by the patient themselves rather than a doctor. I see so many people who have been dismissed time and time again. It can be a disabling condition. One study found that its effects can be at least as great as epilepsy. Some people try to keep their emotions at bay, which can affect relationships and the ability to socialise, for example, and others find it difficult to get a job."

Many people with narcolepsy experience other symptoms besides cataplexy - notably automatic behaviour (being unaware of what you are doing), sleep paralysis (the inability to move when waking up or falling asleep) and hallucinations. "You can think you're going mad," says Underwood, whose hallucinations have included seeing chairs lift off the ground, pirates entering the house and a bomb being dropped through the letterbox. "I've had two patients who were sent to a psychiatrist and wrongly diagnosed with psychosis," says Zenobia Zaiwalla, a consultant neurophysiologist at John Radcliffe Hospital, Oxford.

For Underwood, a friend caused her longest and scariest cataplexic episode, tickling her in order to bring on the famous "party trick".

"I know it's just ignorance," she says. "One day I hope to set up a charity to raise awareness and money for further research".