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The American Academy of Neurology has released updated guidelines for determining whether a person may be declared brain dead.

The first guidelines emerged more than 15 years ago to help provide physicians with a uniform method for determining brain death. Recent studies, however, have found that there is still widespread variance in the methods physicians use to diagnose brain death, HealthDay News reports. For example, variables such as low body temperature or hypothermia can affect determination of brain death, since there was no specific body temperature recommended to arrive at the most accurate diagnosis.

There are three primary causes of brain death, according to the new guidelines: coma with a known cause; absence of brain stem reflexes; and breathing has permanently stopped. The new guidelines include roughly 25 tests and criteria in the form of a step-by-step checklist for determining brain death. The guidelines appear in the June 8 issue of the journal Neurology.