Having subtler symptoms, a form of epilepsy that affects only one part of the brain often goes undiagnosed long enough to cause unexpected seizures that contribute to car crashes, a new study finds.

The study, published online October 20 in the journal Epilepsia, addressed focal epilepsy, the most common form of this brain disorder. Researchers say the study is among the first to outline failure to recognize symptoms of subtle seizures as a main reason for the delay in diagnosis.