Disorders of consciousness are conditions that affect a person’s ability to be alert and aware of one’s environment. Operationally, levels of consciousness can be thought of as being on a spectrum. A person who is fully conscious is alert and can fully perceive themselves and their environment.
‘Locked‐in’ syndrome: a person who is suffering from a ‘locked‐in’ syndrome may also be fully conscious, but has a limited ability to purposefully interact with their environment. Classically, they may have preserved volitional vertical eye movements and eye opening.
Minimally conscious state: often develops weeks to months after a severe cerebral injury wherein the person was comatose. Individuals in this state may show some subtle signs of interacting with their environment such as visually tracking, orienting to pain, or intermittently following commands. These individuals are not able to communicate their thoughts or feelings.
Persistent vegetative state: individuals who are in a ‘vegetative’ state or ‘coma vigil’ have intact circadian rhythms, eye opening, and preserved automatic functions.
Coma: patients with severe cerebral dysfunction who are unresponsive are said to be in a coma. This may result from several life‐threatening conditions including structural injuries, vascular pathology, toxicity from drugs, other metabolic disarrangement, or epileptic abnormality. Coma may be potentially reversible.
Brain death: a person who has been determined to be ‘brain dead’ has suffered irreversible cessation of all brain function. Brain death is a concept not known prior to the advent of ventilators. The determination of brain death is based on accepted medical practices and local law.
The Glasgow Coma Scale (GCS) is often used to describe a person’s level of consciousness after injury or medical event. The score is divided in to three sections (eye, verbal, and motor). Each section is given a numerical value based on the patient’s best response. The sum of all three values provides the score. The scale is graded from 3 to 15 and individuals who score 8 or less are considered to be in a coma.
Coma may result from diffuse bilateral hemispheric dysfunction, significant injury to one hemisphere, or dysfunction in the ascending reticular activating system (ARAS).
The ARAS is a network of neurons located in the midbrain and pons. These neurons have projections to the thalamus and hypothalamus, with subsequent connections to the cerebral cortex.
Decorticate posturing: flexion in the upper extremities with extension of the lower extremities. Evidence of injury at the level of the cerebral cortex.
Decerebrate posturing: extension of the upper extremities with extension of the lower extremities. Evidence of injury below the red nucleus.
Brain death is the determination of death by neurological criteria.
Components of the brain death determination process include a clinical exam documenting the complete absence of all brain stem reflexes and an apea test documenting absent respirations in the setting of hypercarbia (pCO2 >60 mm Hg).
If providers are unable to complete part of the process or there is a confounding factor (e.g. drugs, CO2 retention at baseline), the determination of brain death may be aided by a confirmatory test.
Differential diagnosis of coma or brain death
‘Locked in’ syndrome
Classically, patient with ‘locked in’ syndrome may have preserved vertical gaze and possibly eye opening
A careful history is needed and possible drug/alcohol laboratory screen is needed to exclude intoxication from the diagnosis of brain death
Guillain–Barré syndrome (GBS)
A careful history and physical exam is needed to differentiate GBS from brain death. Classically, GBS will have ascending paralysis with absent deep tendon reflexes. Deep tendon reflexes may be present in patients who are brain dead
Core temperature should always be measured before beginning the determination of brain death
A train‐of‐four should be checked if there is concern for a neuromuscular blocking agent
Clinical diagnosis of brain death
Identify proximate cause either through clinical history or CNS imaging of an acute injury compatible with brain death.
Assess the extent and potential reversibility of any damage.
Exclude factors including medical conditions that may confound clinical assessment, such as significant hypothermia (<36°C) or hypotension (SBP ≤100 mmHg, MAP ≤65 mmHg).
Coma or unresponsiveness:
No cerebral response to pain in all extremities (nail bed pressure, supraorbital pressure).
No response to bright light.
No occulocephalic reflex: eyes appear to be painted on with head movement.
No oculovestibular reflex: no deviation of the eyes to irrigation in each ear of 60 mL of cold water.
Facial sensation and motor response:
No corneal reflex.
No jaw reflex.
No grimacing to deep pressure on nail bed, supraorbital ridge, or temporomandibular joint.
Pharyngeal and tracheal reflexes:
No response after stimulation of posterior pharynx.
No cough response to endotracheal suctioning.
Core temperature >36.5°C.
Systolic blood pressure ≥100 mmHg.
Normal PCO2 (PaCO2 40 mmHg).
Normal PO2 (preoxygenate to a PO2 >200 mmHg).
Make sure patient is connected to a pulse oximeter. Disconnect from the ventilator. Deliver 100% oxygen into the trachea. This may be done via a cut nasal cannula tubing. Monitor for respiratory movements.
Obtain arterial blood gas at around 7–8 minutes and reconnect to the ventilator.
If respiratory movements are noted at any time, the test is not consistent with brain death.
If SBP is <90 mmHg or patient develops arrhythmia or significant desaturations, obtain arterial blood sample and abort the test.
Test is consistent with brain death if PCO2 rises to >60 mmHg or rises 20 mmHg from baseline and no respiratory movements are observed.
Useful clinical decision rules when diagnosing brain death
CNS depressants: if no ability to measure levels and assuming normal clearance, wait 5× half‐life.
For alcohol intoxication, wait until level is <0.80%.
Train‐of‐four should be performed in patients with history of neuromuscular blockade.
Neurologic exam is more reliable with SBP ≥100 mmHg or MAP ≥65 mmHg.
One consistent neurologic and brainstem exam along with a consistent apnea test should be sufficient to pronounce brain death (must follow local laws).
In cases where parts of the brain death determination process are unable to be performed, an ancillary test may be obtained in order to establish the diagnosis.
Laboratory diagnosis of brain death
List of diagnostic tests
EEG: used in the first brain death declarations. EEGs are affected by medication and metabolic derangements that may be reversible. No electrical activity during at least 30 minutes of recording is consistent with the diagnosis of brain death.
CTA: no intracerebral filling past the level of the circle of Willis while the external carotid circulation is patent and fills is consistent with the diagnosis of brain death.
MRA: no intracerebral filling past the level of the circle of Willis while the external carotid circulation is patent and filling is consistent with the diagnosis of brain death.
Catheter‐based cerebral angiography: this is the traditional gold standard test. No intracerebral filling past the level of the circle of Willis while the external carotid circulation is patent and fills is consistent with the diagnosis of brain death. Absence of intracranial filling requires an ICP higher than the MAP.
Nuclear medicine cerebral blood flow test: demonstration of no perfusion to the brain (‘light bulb’ sign) is consistent with the diagnosis of brain death.
Transcranial Doppler: safe, non‐invasive, and portable but requires a skilled operator. Systolic peaks with no diastolic flow leading to an oscillating or reverberating flow pattern is necessary for an exam to be consistent with the diagnosis of brain death.
Potential pitfalls/common errors made regarding diagnosis of brain death
Certain lower motor neuron motor movements are still consistent with the diagnosis of brain death, including the following:
Transient bilateral finger tremor.
Repetitive leg movements.
Cyclical constriction and dilation in light‐fixed pupils.