Brain death

Brain death means that the brain has stopped working. Brain death is permanent, and cannot be cured or reversed. No treatment can help. When a person is brain dead, there is no activity in any part of their brain. Every part of the brain is dead; no oxygen flows to the brain, and there is no electrical activity in any part of the brain.
The brain controls everything we need to do to stay alive. For example, it controls breathing, body temperature, heart rate, and many other important things. When a person is brain dead, their brain cannot control any of these things. A brain-dead person cannot breathe on their own. They will never be able to wake up or be aware of what is going on around them.
Sometimes, brain-dead people are kept alive with special machines and medications. For example, doctors can put a tube down a person's throat and use a machine called a ventilator to pump oxygen into their lungs. While these machines can keep the person's other organs alive for a while, they cannot help the brain get better. Eventually, the person's other organs will stop working too.
Diagnosis.
Doctors can diagnose brain death by examining the patient. They will check for reflexes which are controlled by the brainstem, and see if the person can breathe on their own. If the person is brain-dead, the doctor will find:
Before diagnosing brain death, doctors will check for any other problem that could look like brain death (for example, a very low body temperature, or an overdose of sedatives). They will usually wait six to 24 hours before checking again to make sure the person still has no signs of activity in the brain. Then a second doctor is called in to do the same tests. If both doctors agree, the patient can officially be diagnosed "brain dead."
Immediate diagnosis.
Sometimes, doctors use other tests which can diagnose brain death right away. These tests include:
Causes.
Brain death happens because of an injury to the entire brain. The most common causes are:
Swelling or bleeding in the brain can cut off blood flow to the brain and push on the brainstem. If it gets bad enough, it can crush the brainstem.