A peculiar reaction to sandwiches caused a woman to suffer unexplained fainting fits, doctors reported today.

The woman, 25, was seen in hospital complaining of short 10-second episodes of feeling "light-headed, occasionally nauseous, and suddenly and alarmingly unwell".

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On more than one occasion she had collapsed. The problem had started when she was aged 15, sometimes recurring several times a week.

A battery of tests and investigations had failed to disclose anything abnormal.

The episodes tended to occur when the woman consumed certain types of food, particularly sandwiches and fizzy drinks.

She had last collapsed when eating a sandwich while driving. Fortunately, her car was stationary in a queue of traffic.

Dr Christopher Boos, from University Hospital Birmingham, described what happened when doctors gave the woman a sandwich.

She immediately suffered an electrical abnormality in her heart, which stopped beating for two seconds.

Dr Boos and three colleagues wrote in The Lancet medical journal: "The patient felt light-headed, as though she were about to faint, but did not."

The woman was finally diagnosed with "swallow syncope" - a rare condition in which the action of swallowing triggers nerve reflexes that cause the heart to stop temporarily.

The doctors wrote: "Swallow syncope, also known as deglutition syncope, is, by definition, a transient alteration or loss of consciousness during swallowing, and is usually intermittent.

"Patients with swallow syncope can languish for years because the diagnosis is little known - although a case report on it was published in The Lancet 50 years ago."

The woman was fitted with a pacemaker to overcome the problem, and when last seen in June was free of fainting.