The official ruling from the medical examiner's office stated that Vasquez died of "complications from blunt trauma to the head with subdural hemorrhage." Due to a significant backlog of cases at the medical examiner's office, it took four months to complete the necessary tests to determine definitively the cause of death.

A subdural hemorrhage occurs when a blood vessel bursts and blood builds up between the brain and its outer lining, which exerts pressure on the brain.

Most subdural hemorrhages are triggered by impact to the head, resulting from a car accident or severe fall. It's an injury rarely seen in mixed martial arts.

"This type of injury is common in the 'regular world,' say when a person falls and hits their head, especially someone on a blood thinner. But in combat sports, this injury is very, very rare," said Dr. Johnny Benjamin, chief of orthopedic surgery at Indian River Medical Center in Vero Beach, Fla., and a sports-medicine specialist who's consulted with a number of fighters.

Benjamin says fighters put themselves at increased risk for brain injuries by taking anti-inflammatory drugs, even over-the-counter (OTCs) medications such as ibuprofen and aspirin. This class of drugs thins the blood, which heightens the chance that a severe blow to the head could cause a subdural hemorrhage, which is bleeding on the brain.