What are the essential components in assessing death by neurological criteria?
The Academy of the Medical Royal Colleges UK (AoMRC) code for the diagnosis of death by neurological criteria outlines three essential components:
1. Evidence of irreversible brain damage of known aetiology
2. Exclusion of reversible causes of coma and apnoea
3. Formal demonstration of coma, apnoea, and the absence of brainstem reflex activity
What are the reversible causes of coma and apnoea that must be excluded in brain death?
Coma
Specific criteria are laid out in guidance from the AOMRC
- Metabolic or endocrine disturbance
- Circulatory disturbance
- Sedative/depressant drugs
- Hypothermia
Apnoea
Rare but should be actively sought if the history or clinical examination suggests their presence
- Severe neuromuscular weakness of any cause
- Spinal cord injury following trauma – should be excluded through x-ray, CT or MRI imaging
- Residual neuromuscular blockade - it is sensible to routinely use a nerve stimulator to confirm the absence of drug-related block
In which situations may ancillary testing be required for diagnosis of brainstem death?
- Ancillary tests are not routinely required for diagnosis of brainstem death
- There are several circumstances where brain death cannot be confirmed according to clinical testing alone:
- Inability to exclude the influence of residual sedative drugs
- High cervical cord injury
- Severe maxillofacial injury limiting brainstem death testing
Can a single ancillary test be used to confirm brainstem death?
- No single test can be used to confirm brainstem death:
- Brain death remains a clinical diagnosis
- Ancillary testing cannot replace a clinical determination
- However, in situations where full brainstem death testing is not possible, one additional test may help to confirm diagnosis:
- May reduce any element of uncertainty and foreshorten period of observation prior to formal testing
- No data exists to support the superiority of one test over others
- Using multiple ancillary tests is not advisable – increases the odds of indeterminate or false-positive results due to artefact
Which ancillary tests are available for diagnosis of brainstem death?
Measures of brain Electrical Activity
Measures of brain Electrical Activity
EEG
- Most popular and validated test worldwide, though there has been a recent move away
- Isoelectric EEG may be mimicked by conditions such as hypothermia, barbiturates, or other central nervous system (CNS) depressants - hence it is of little value when these are suspected
Somatosensory or Auditory Evoked Potential (SSEP)
- A peripheral stimulus given (i.e. median nerve} and a response is measured at the contralateral primary sensory cortex
- Absence of transmission measured 20 ms (N20 response) after stimulation suggests brainstem dysfunction
- May be useful where coma of toxic aetiology is suspected - short-latency responses that are absent in brain death but preserved in toxic and metabolic disorders
Measures of Blood Flow
Measures of Blood Flow
Cerebral Angiography
(4 vessel)
(4 vessel)
- Contrast medium is injected in the aortic arch under high pressure to reach both anterior and posterior circulations
- Confirmatory testing demonstrates absence of intracerebral filling beyond the carotid or vertebral arteries' entry to the skull
Transcranial Doppler
- Useful only if a reliable waveform is found
- Complete absence of flow may not be reliable if inadequate windows exist
- Confirmatory testing should demonstrate either reverberating flow or small systolic peaks in early systole
Cerebral Scintigraphy
- Non-invasive and safe measure of cerebral blood flow
- No patient transport required if a portable gamma camera is available
- Technetium 99 m is given by intravenous bolus with images obtained by a gamma camera every 3 seconds for a total of 60 seconds
- External carotid flow is either digitally subtracted or excluded by forehead tourniquet
- Confirmatory testing demonstrates no radionuclide localization in the middle cerebral artery, anterior cerebral artery, or basilar artery territories of the cerebral hemispheres
Spiral CT or MRI angiography
- Increasingly available and investigated as ancillary tests
- Confirmatory testing demonstrates absence of intracerebral filling beyond the carotid or vertebral arteries' entry to the skull