The man suffered minor burns in a brief but 'dramatic' oper' />

Flatulence is being blamed for bringing a hospital patient's operation to a fiery end.

The man suffered minor burns in a brief but "dramatic" operating theatre fire. He had gone into the Southern Cross Hospital in Invercargill to have haemorrhoids, or piles, removed and was singed in the "exceedingly rare" incident involving his own gas.

"This was thought to be flatus containing methane igniting," a health source told the Weekend Herald. "There was a sort of flashfire and that was it, but it was fairly alarming at the time."

Haemorrhoids are swollen veins in the lining of the anus. If they protrude outside the body and become troublesome, they can be removed by surgery, which in the Invercargill case employed an electrical "diathermy" machine. A hand-held tool for cutting tissue and cauterising to stop bleeding, it produces heat and can spark.

Southern Cross is releasing little detail other than confirming an "electrical fire" occurred on March 22 and that it commissioned an independent forensic scientist to investigate.

The incident follows two cases of patients being burned in Auckland hospitals in 2002 when, it was thought, diathermy machines accidentally ignited alcohol-based skin disinfectant.

A man having his appendix removed suffered burns to 5 per cent of his body and a woman undergoing a caesarean operation suffered more severe burns, although her baby boy was unharmed. After an investigation, hospitals were warned to tighten controls on the dangerous mix of alcohol disinfectants and electrical gear.

Southern Cross Hospitals chief operating officer Terry Moore said the Invercargill patient had made a good recovery from the fire, which lasted just a few seconds. He suspected a number of factors came together to cause the fire and he wanted to prevent any repeat.

The surgeon who performed the operation said of the fire: "It was dramatic." It remained unexplained and he hoped the investigation would shed more light.

"Hopefully we will get an answer because the fear is it could happen again."

Another surgeon said patients were usually given a small enema to empty the bowel before haemorrhoid surgery. Their gas exploding during surgery was recognised as a risk but was "exceedingly rare"; precautions were considered unnecessary.

"Normally the operation is done with the sphincter relaxed with some sort of medicated ointment and therefore you get a common cavity with the rectum and the atmospheric air.

"So you wouldn't normally see a problem and nor would it be considered as a contra-indication or that you need to do anything particularly."

Traditional haemorrhoid surgery is done with scissors or diathermy.