The study, published in the journal Epilepsia, looked at 440 patient records from Kesennuma City Hospital, in a city that was devastated by the massive tsunami touched off by the 9.0 magnitude earthquake.

Thirteen patients were admitted with seizures in the eight weeks after the disaster, but only one had been admitted in the two months before March 11.

Previous research has linked stressful life-threatening disasters with an increased risk of seizures, but most case reports lacked clinical data with multiple patients.

"We suggest that stress associated with life-threatening situations may enhance seizure generation," wrote lead author Ichiyo Shibahara, a staff neurosurgeon at Sendai Medical Center in northern Japan.

But he added that stress itself is not a universal risk factor for seizures.

"Most of the seizure patients had some sort of neurological disease before the earthquake," he said.

His team examined medical records from patients admitted to the neurosurgery ward in the eight weeks before and after the March 11 disaster and compared them to the same time period each year between 2008 and 2010.

In 2008, there were 11 seizure patients admitted between January 14 and May 15. In 2009, there were seven and in 2010, just four.

Of the 13 admitted after the disaster, 11 had preexisting brain disorders that included epilepsy, head injuries or stroke. All the patients lived independently, and eight took anti-convulsive medication.

Shibahara noted that of the five patients admitted just days after the tsunami, it was "not because of a lack of anticonvulsants, but because of the stress."

One later patient, though, was unable to refill his medication weeks after the devastation.

"This is interesting, but I'm not 100 percent convinced," said William Theodore, senior investigator of the clinical epilepsy section at the National Institute for Neurological Disorders and Stroke in Bethesda, Maryland.

Theodore, who was not involved in the study, told Reuters health that because the number of patients was so small, random variation could explain the surge in seizures. Upset patients may also have forgotten to take, or weren't able to find, anticonvulsant drugs.

There are also various ways that natural disasters might cause seizures, including head trauma, infections from polluted water or a lack of sleep, he added.

But the study did have a practical take home message, he said: "If you already have seizures and you're taking medication, always make sure you have a decent supply just in case some natural disaster occurs."

(Reporting from New York by Trevor Stokes at Reuters Health; editing by Elaine Lies)

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