Death is defined in the United States by the Uniform Determination of Death Act, proposed in 1981, as 

1.  Irreversible cessation of circulatory and pulmonary functions
2.  Irreversible cessation of all functions of the whole brain, which means brain death. 

The definition of brain death or irreversible coma as "loss of brain functions" was released by the Ad Hoc Committee of the Harvard Medical School in 1968. The American Academy of Neurology (AAN) guidelines of brain death determination ascertained this definition and released its first version in 1995. According to the AAN guidelines, brain death is clinically equivalent to the irreversible loss of all brain stem functions. Irreversibility in the definition refers to the impossibility of recovery, regardless of any medical intervention, which required clear elaboration. As with the advancement of mechanical ventilation and life support technologies during the 20th century, patients who suffered severe brain damage could be maintained physiologically for prolonged periods in the intensive care units (ICUs).[1][2][3]

It is crucial to differentiate brain death from other forms of severe brain damage, which can cause vegetative states when some of the brain functions are maintained, and recovery can occur even after prolonged periods, especially in patients with traumatic brain injuries. Also, it is important to distinguish the term “brain death” from “coma” to the public, as a coma may imply a limited form of life. Understanding that brain death is equivalent to death helps both the physicians and patients’ families to decide about the withdrawal of care and prevents the unnecessary expenditure of resources. Another essential topic that evolved in parallel with brain death is the need of obtaining organs for transplantation. According to the “dead donor rule,” organ procurement can occur only after death. So for patients who are brain dead, the procurement of viable organs is allowed, even if they still have some circulatory and pulmonary functions. This concept is still causing an ongoing debate and controversy.[4][5][6]

Brain death occurs as a result of an acute catastrophic brain injury. Abruption of cerebral perfusion occurs if a concomitant elevation of intracranial pressure is more than mean arterial pressure (Cerebral perfusion pressure= mean arterial pressure -intracranial pressure). This process was studied by monitoring brain tissue oxygenation in patients with brain death, and can occur in 2 different mechanisms:    

1. Extracranial brain injury, as seen in patients post cardiopulmonary arrest with delayed resuscitation causing prolonged cessation of brain blood flow. Resultant brain anoxia causes neuronal damage, leading to cellular membrane pump failure and disturbed osmoregulation ending with severe brain edema. As a result of the confined space of the skull, intracranial pressure increases compromising cerebral perfusion and may cause brain herniation. It’s important to note that in some circumstances, as patients with decompressive craniotomy or ventricular shunts, an increment of intracranial pressure (ICP) may not be high enough to cause a complete cessation of cerebral blood flow. However, cerebral oxygenation is still severely impaired by the catastrophic injury at the cellular level leading to brain death.     
2. Intracranial brain injury, as seen in traumatic brain injury and intracerebral hemorrhage, which can cause an elevation in intracranial pressure and impairment of brain oxygenation as mentioned above.[4][7][8]

A recent study of brain death determination protocols over the world in 91 countries showed high variability in the protocols in different countries and even the absence of protocols in most low-income countries. Another study in the United States showed variability in the protocols between various health institutions, and the performance of the apnea and the ancillary tests. Also, different states have different requirements for the examiner, including if one or two physicians are required to determine brain death. Some states (as Alaska) allow the authority to nurses to assess brain death with further certification by a physician. Another common requirement is if the patient is potential for organ procurement, the physician who declares brain death should not be a member of the procurement team.[9][10]

Certain pre-requisites should be present before the determination of brain death, including: 

1. Evidence of an etiology of coma should be known. Confounding conditions should be excluded, including severe metabolic, endocrinal, and acid-base derangements. If a drug intoxication is suspected, five half-lives of drug clearance should be waited, with adjustment to renal and hepatic functions.  
2. Core body temperature should be more than 36 degrees C. Warming blankets are an option.        
3. Achieve systolic blood pressure (SBP) more than 100 mmHg, often accomplished by using vasopressors or vasopressin.[11]

Brain death can be assessed by doing a physical examination, the apnea test, and the ancillary tests.

I. Physical examination: which includes the response to pain and assessment of brain stem reflexes.   

Loss of response to central pain occurs in brain death. Central pain assessment can be by the application of noxious stimuli to certain areas as the supraorbital notch, the ankle of the jaw, upper trapezius, the anterior axillary fold, and the sternum. Neither eye response nor motor reflexes are detectable in brain death. It’s important to note that some spinal reflexes can be present in patients with brain death. Saposnik et al. studied spinal reflexes in 107 patients with brain death and noted the following reflexes:     

• Undulating toe flexion response, which is repetitive flexion and extension of toes triggered by plantar tactile stimulation
• Triple flexion reflex, which is flexion of thigh, leg, and foot triggered by plantar tactile stimulation
• The plantar response, which is plantar flexion triggered by plantar stimulation
• Pronator extension reflex triggered by head-turning
• Quadriceps flexion triggered by local noxious stimuli
• Facial myokymia, which is repetitive twitching of facial muscles
• Lazarus sign, which is bilateral arm flexion, shoulder adduction, and hand raising to the chest/neck, triggered by head flexion and sternal stimulation
• Myoclonus of arm and leg
• Muscle fasciculations

Brain death can be confirmed if brain stem reflexes are lost, including[12][13]:        

• CN II: Loss of pupillary reflex (light reflex), pupils should be mid dilated 4 to 9 mm, and not reactive to light.    
• CN III, IV, VI: Eye motion is lost in reaction to head movement (Doll’s eyes).    
• CN V, VII: Loss of corneal reflex.  
• CN VIII: Loss of oculovestibular reflex (Caloric test): With irrigation of each by 60 ml of ice water, the eye won’t move toward the irrigated ear.        
• CN IX: Loss of gag reflex.          
• CN X: Loss of cough reflex

II. Apnea test:

The apnea test is used to assess the brain's ability to drive pulmonary function in response to the rise of CO2. Before the performance of the apnea test, the mechanical ventilator should be adjusted to obtain PCO2 within 35 to 45 mmHg and PO2 above 200 mmHg, with using a positive end-expiratory pressure (PEEP) of 5 to 8 cm H2O. During the test, oxygen should be supplemented using a cannula connected to the endotracheal tube at 6 L/min, or T piece at 12 L/min or using CPAP of 5 to 10 cm H2O. In the case of loss of respiratory drive, CO2 is expected to rise 5 mmHg every minute in the first 2 minutes, then by 2 mmHg every minute after. Repeat arterial blood gas (ABG) after 8 to 10 minutes showing CO2 of 60 mmHg or the rise of CO2 more than 20 mmHg above baseline is consistent with brain death. If the patient develops hypotension with SBP below 90 mmHg or cardiac arrhythmias, the test should terminate, and arterial blood gases are drawn. For patients on extracorporeal membrane oxygenation (ECMO) machines, oxygenation can be maintained while performing the apnea test by decreasing the gas sweep flow rate to 0.5 to 1.0 L/min and using an oxygenation source through the endotracheal (ET) tube.

III. Ancillary tests:

These tests are considerations if there is any uncertainty of diagnosis of brain death, or if the apnea test can’t be performed (as in cases of chronic CO2 retainers).

*Ancillary tests used for detection of cessation of cerebral blood flow:   

• Cerebral angiography: Four vessel angiography is considered the gold standard for tests that evaluate cerebral blood flow. It can confirm brain death when it shows cessation of blood flow to the brain. Limitations include invasiveness of the test and transferring the patient to the radiology suite. Also, the contrast may induce nephrotoxicity affecting the donor's kidney. False-negative tests can occur when ICP becomes lowered by surgery, trauma, or ventricular shunts.         
• Transcranial ultrasound: Can be used to assess pulsations of middle cerebral arteries, vertebral and basilar arteries bilaterally, also anterior cerebral arteries or ophthalmic arteries if possible. The transcranial US can confirm brain death by showing small peaked systolic pulsations or absence of diastolic pulsations. Test limitations include the performer expertise, presence of unsuitable windows due to thick temporal bones, as well as lowered ICP by surgery or ventricular shunts giving false-negative tests. 
• Computed tomogram (CT) brain angiography and MR angiography showing cessation of cerebral blood flow.                  
• Radionuclide brain imaging: Can be done using a 99mTc-labeled hexamethyl propylene amine oxime (HMPAO) isotope tracer then imaging by single-photon emission computed tomography (SPECT) brain scintigraphy. The absence of a tracer in the brain circulation (the hollow skull phenomenon) is consistent with brain death. The test may show false-positive results if imaging is done in one plane only instead of 2 planes (anterior and lateral).

*Ancillary tests used for detection of loss of bioelectrical activity of the brain:  

• Electroencephalogram (EEG): Can confirm brain death when it shows no electrical activity more than 2 microvolts, at a sensitivity of 2 microvolts/mm for at least 30 seconds. Also, it should show no reactivity to intense somatosensory stimuli. Limitations include affection by drug intoxication and hypothermia. Doing an electrocardiogram at the same time helps to identify any associated artifacts. Reports exist of false-positive tests in patients with brain death in the past.  
• Somatosensory evoked potentials: Patients with brain death show no somatosensory evoked potentials in response to bilateral median nerve stimulation, and no brain stem evoked potentials in response to auditory stimuli. SSEPs can confirm EEG findings, as it is less affected by drug intoxication; however, it still can be affected by hypothermia.[11][14][15]