2.2 Brainstem Testing

Question 1

Definition BS death

Answer 1

Brain death describes the situation in which a patient has undergone the
irreversible loss of any capacity for consciousness,

together with the irreversible loss of the ability to breathe.

Question 2

Preconditions:

Answer 2

Before testing can be considered,

there are preconditions that must be satisfied,

the most important of which is that there must
be a definitive diagnosis of the cause of the brain damage.

The patient should also be in an apnoeic coma, with a Glasgow Coma Score of 3
(no eye opening, no verbal response and no localization of pain).

Question 3

Children

Answer 3

Children:
theoretically, the clinical criteria are the same in children,

although there are enough concerns about their applicability to make this a very difficult area.

In neonates, for example,
CNS immaturity raises doubts about the validity of brain stem
death tests,

and there is much anecdotal evidence of children who have recovered
substantial neurological function despite severe insult and prolonged coma.

Question 4

Exclusions:

Answer 4

1. Temperature: this must be at least 35 C.
2. Sedatives:
3. Neuromuscular blockade
4. Metabolic Derangement
5. Normocapnia

Question 5

2. Sedatives

Answer 5

There should be no residual depressant drugs in the system,
which in practice may mean substantial delay until clearance can be assured.

Such patients are usually sedated with short-acting agents
whose elimination can be predicted with some confidence.

(Observation over four elimination half-lives is commonly recommended).

If, however, they have received longer-acting drugs, such as barbiturates (e.g. thiopental) to
control convulsive activity,
or if there is a suspicion of illicit drug use then the situation can be more difficult.

Plasma determinations may be indicated, but if the intracranial catastrophe is obvious and extreme, some clinicians do not believe them to be necessary.

Question 6

Neuromuscular blockade

Answer 6

Neuromuscular blockade: This should be excluded (where appropriate)
by using a peripheral nerve stimulator.

Question 7

Metabolic derangement:

Answer 7

There must be no endocrine or metabolic disturbance that may contribute to continued coma,
and there should be no possibility that impaired circulatory function is compromising cerebral perfusion.

Question 8

Normocapnia

Answer 8

Normocapnia: A high PaCO2 can obtund cerebral function
and so must be kept normal (for that patient).

Question 9

The tests:

Answer 9

these are carried out by two doctors,

both of whom have been registered for more than 5 years,

and one of whom must be a consultant.

Two sets of tests are performed, although there is no set interval between them.

In practice, they are usually done a few hours apart.

There has never been a reported case of a patient who
initially satisfied the criteria for brain stem death and who subsequently failed to do
so.

The tests aim to confirm the absence of brain stem reflexes and examine those
cranial nerves which are amenable to testing.

Question 10

Nerves tested

Answer 10

II

V

VII

VIII

IX

X

Question 11

The cranial nerve reflexes

Answer 11

I: the first nerve (olfactory) cannot be tested.

— II: the second nerve (optic),
together with the parasympathetic constrictor outflow,
is tested by pupillary responses to light (direct and consensual).
Pupillary size is not important.

— III, IV, VI:
the third, fourth and sixth nerves
(oculomotor, trochlear and abducens)
are not tested.

Question 12

CN V VII

Answer 12

V, VII:

the fifth (trigeminal) and seventh (facial) nerves are tested

first by the corneal reflex,

and then by the response to painful stimuli applied to the face
(supraorbital or infraorbital pressure),

to the limbs (nail bed pressure) and to the
trunk (sternal stimulation).

It is because of the possibility of tetraplegia that a stimulus should be applied above the neck.

Question 13

VIII

Answer 13

VIII: the eighth nerve (auditory/vestibular)
is examined by caloric testing.

It is important to establish that both drums are visible and intact,

after which 30 ml of ice-cold water is instilled via a syringe.

Nystagmus is absent if the patient is brain-dead.

The assessment of doll’s eye movements,

to test whether the eyes move with the head (which is abnormal)
instead of maintaining central gaze, is
not part of the brain stem death tests as performed in the UK.

Question 14

IX X

Answer 14

IX, X: the ninth (glossopharyngeal) and tenth (vagus) nerves are tested by stimulating
the pharynx, larynx and trachea.

The patient should neither gag nor cough

Question 15

XI, XII:

Answer 15

the eleventh (accessory) and twelfth (hypoglossal) nerves are not tested.

Question 16

Apnoea testing

Answer 16

after ventilation with 100% oxygen for 10 minutes,

the patient is disconnected from the ventilator.

Oxygen saturation is maintained thereafter by
apnoeic oxygenation via a tracheal catheter.

In the apnoeic patient, arterial CO2 rises at a rate of about 0.40–0.80 kPa per minute, depending on the metabolic rate,
and so it may take some time to reach the arterial blood gas level of 6.65 kPa
required by the testing criteria.

Question 17

Potential Pitfalls

Answer 17

There are a number of lesions of the brain stem which may closely mimic irreversible
brain death.

These include
severe Guillain–Barré
+ Miller–Fisher syndromes,

Bickerstaff’s brain stem encephalitis

ventral pontine infarction associated with the ‘locked-in syndrome’.

Question 18

Brain stem encephalitis

Answer 18

Brain stem encephalitis is characterized by acute progressive
cranial nerve dysfunction associated with ataxia, coma and apnoea.

There is no structural abnormality of the brain,
but the picture is one of brain stem death. It is
reversible.

Question 19

Bilateral ventral pontine lesions

Answer 19

Bilateral ventral pontine lesions may involve both corticospinal and
corticobulbar tracts,

leading to tetraplegia and the locked-in syndrome

Patients are unable to speak or produce facial movements.

They can usually blink and move their eyes vertically,
and because the tegmentum of the pons is spared, they remain sensate,

fully conscious and aware.

It is the stuff of nightmares, and meaningful recovery from the locked-in syndrome is extremely rare.

Question 20

Further Confirmatory Tests That Can Be Undertaken

Answer 20

Auditory, visual and somatosensory evoked potentials can be used,

as can the EEG
and cerebral angiography.

None of these is required in the UK.

Question 21

Management of the ASA 6 patient for organ retrieval.

Answer 21

Clearly the potential donor organs must be oxygenated and well perfused,

and this may require some haemodynamic manipulation.

The problem arises with the question of ‘anaesthesia’.

The legal time of death occurs when brain stem death is confirmed,

and so logically a dead patient cannot require anaesthesia

except perhaps for muscle relaxants to prevent spinal reflexes).

There are, however, those who believe brain stem death testing to be little more than a
pragmatic way of providing donor organs for transplant,

and some anaesthetists appear to share enough residual unease
about the process to make them give a general anaesthetic.

The philosophical questions that this raises are interesting and important,
but the clinical science oral is probably not the best place to explore them.

Question 22

Pupillary Signs

Answer 22

Pupillary signs:

lateral herniation of the tentorium as a result of increased intracranial pressure (ICP)

can compress the oculomotor (III) nerve with ipsilateral pupillary dilatation.

This may also be accompanied by ptosis and motor paralysis of the extraocular muscles (apart from the superior oblique and lateral rectus muscles
which are supplied by cranial IV + VI)

Central tentorial herniation can cause miosis
(due to diencephalic damage).

If there is midbrain compression,
the size of the pupils may remain in the mid-range,
but they are unresponsive.

Pinpoint and unreactive pupils may signify pontine haemorrhage.

Question 23

Eye signs

Answer 23

Eye signs:
raised ICP obstructs cerebrospinal fluid (CSF) flow in the optic nerve
sheath with the development of papilloedema.

The lateral rectus is also affected because of the
displacement of the sixth cranial nerve (abducens) during its long
intracranial course.

(As it leaves the posterior margin of the pons,
it is crossed by the anterior inferior cerebellar artery.
Cerebellar displacement may cause compression of the nerve,
paresis and failure of lateral gaze.)

Question 24

CN XIII XIV

Answer 24

Cranial nerves XIII and XIV: it is unlikely to come from the examiners, but they
may well be impressed if you impart information about these two extra cranial nerves
which have long been identified but which have not routinely been described in
medical texts. (You could mention, e.g., that ‘cranial nerves XIII and XIV are of
course not tested as part of brain stem function’.) The thirteenth cranial nerve, also
known as Nerve Zero (because it lies more rostrally than the other cranial nerves) or
the Nervus Terminalis, has its origins close to the olfactory bulb but is not part of it.
It appears to mediate the release of luteinizing hormone and is thought therefore to
have some role in the regulation of reproductive behaviour. This hypothesis is
reinforced by its projections to septal nuclei and the preoptic areas of the brain,
which in mammals are associated with sexual behaviour, including the response to
pheromones.

Question 25

CN XIV

Answer 25

The fourteenth cranial nerve is also known as the Nervus Intermedius
and has usually been considered to be part of the 7th cranial nerve, the facial nerve. It
lies between this nerve and the superior part of the 8th cranial nerve, the vestibulocochlear.

However, it has a different origin in the brain and subserves different
functions, including taste; the sensory innervation of part of the outer ear, the nose
and the mouth; and lacrimation and salivation. Its motor functions include contraction
of the orbicularis oris muscle.