In the retrospective study, the team led by Masud Seyal, a professor of neurology at UC Davis Medical Centre and director of the UC Davis Comprehensive Epilepsy Program examined records of 300 seizures in 57 epilepsy patients with chronic, recurrent, unprovoked seizures.
They compared patients with severe convulsive seizures to those with milder symptoms like transient confusion, lip smacking and head turning.
They found that one-third of all seizures were associated with drops in blood-oxygen levels below 90 percent.
"Significant drops in blood oxygen levels are more common than we thought in patients with partial seizures," said Seyal.
"It may have to do with an abnormal heart rhythm or it just may be that the brain stops sending the proper signals to maintain normal breathing," Seyal added.
The scientists discovered that seizures in the temporal lobe of the brain are more often associated with significant drops in blood-oxygen levels and that males are more likely than females to experience dangerously low levels of oxygen during seizures.
Seyal said that the best strategy to reduce the likelihood of sudden death is to promptly and effectively control patients' seizures.
The study appears in journal Brain.