perfusion pressure may fall below the thresh-
old for autoregulation, and blood flow may be
diminished below the level needed to support
neurologic function. Such patients may show
improvement in neurologic function when the
head of the bed is flat. Conversely, in cases of
head trauma where there is increased intra-
cranial pressure, it may be important to raise
the head of the bed 15 to 30 degrees to im-
prove venous drainage to maximize cerebral
perfusion pressure.
6
Similarly, it is necessary
to remove tight neckwear and ensure that a
cervical spine collar is not applied too tightly to
a victim of head injury to avoid diminishing
venous outflow from the brain.
In a patient with impaired consciousness,
the blood pressure can give important clues to
the level of the nervous system that has been
damaged. Damage to the descending sympa-
thetic pathways that support blood pressure
may result in a fall to levels seen after spinal
transaction (mean arterial pressure about 60 to
70 mm Hg). Blood pressure is supported by a
descending sympathoexcitatory pathway from
the rostral ventrolateral medulla to the spinal
cord, and so damage along the course of this
pathway can result in spinal levels of blood
pressure. The hypothalamus in turn provides
a descending sympathoexcitatory input to the
medulla and the spinal cord.
7,8
As a conse-
quence, bilateral diencephalic lesions result
in a fall in sympathetic tone, including mei-
otic pupils (see below), decreased sweating re-
sponses, and a generally low level of arterial
pressure.
9
However, persistent hypotension below these
levels in a comatose patient is almost never
caused by an acute neurologic injury. One of
the most common mistakes seen in evaluation
of a comatose patient with a mean arterial pres-
sure below 60 mm Hg is the assumption that a
neurologic event may have caused the hypo-
tension. This is almost never the case. A mean
arterial pressure at or above 60 mm Hg is gen-
erally sufficient in a supine patient to support
cerebral and systemic function. On the other
hand, acute hypotension, due to cardiogenic or
vasomotor shock, is a common cause of loss of
consciousness and a threat to the patient’s life.
Thus, the initial evaluation of a comatose pa-
tient with low blood pressure should focus on
identifying the cause of and correcting the
hypotension.
On the other hand, lesions that result in stim-
ulation of the sympathoexcitatory system may
cause an increase in blood pressure. For exam-
ple, pain is a major ascending sympathoexcit-
atory stimulus, which acts via direct collaterals
from the ascending spinothalamic tract into the
rostral ventrolateral medulla. The elevation of
blood pressure in response to a painful stim-
ulus applied to the body (pinch of skin, ster-
nal rub) is evidence of intact medullospinal
connections.
10,11
In a patient who is still semi-
wakeful after subarachnoid hemorrhage, blood
pressure may be elevated as a response to head-
ache pain. Each of these conditions is associ-
ated with a rise in heart rate as well.
Direct pressure to the floor of the medulla
can activate the Cushing reflex, an increase in
blood pressure and a decrease in heart rate.
12
In children, the Cushing reflex may be seen
when there is a generalized increased intracra-
nial pressure, even above the tentorium. How-
ever, the more rigid compartmentalization of
intracranial contents in adults usually prevents
this phenomenon unless the expansile mass is
in the posterior fossa.
Activation of descending sympathoexcita-
tory pathways from the forebrain may also ele-
vate blood pressure. Irritative lesions of the hy-
pothalamus, such as occur with subarachnoid
hemorrhage, may result in an excess hypotha-
lamic input to the sympathetic and parasym-
pathetic control systems.
13
This condition can
trigger virtually any type of cardiac arrhythmia,
from sinus pause to supraventricular tachycar-
dia to ventricular fibrillation.
14
However, the
most common finding in subarachnoid hemor-
rhage is a pattern of subendocardial ischemia.
Such patients may in fact have enzyme evi-
dence of myocardial infarction, and at autopsy
demonstrate contraction band necrosis of the
myocardium.
15
Sympathoexcitation is also seen in patients
who are delirious. The infralimbic and insular
cortex and the central nucleus of the amygdala
provide important inputs to sympathoexcit-
atory areas of the hypothalamus and the me-
dulla.
8
Activation of these areas due to misper-
ception of stimuli in the environment causing
emotional responses such as fear or anger may
result in hypertension, tachycardia, and en-
larged pupils.
Stokes-Adams attacks are periods of brief
loss of consciousness due to lack of adequate
44
Plum and Posner’s Diagnosis of Stupor and Coma
cerebral perfusion. These almost always oc-
cur in an upright position. In recumbent po-
sitions, when the head is at the same height as
the heart, it takes a much steeper fall in blood
pressure (below 60 to 70 mm Hg mean pres-
sure) to cause loss of consciousness. The fall in
blood pressure during a Stokes-Adams attack
may reflect a failure of the baroreceptor reflex
arc on assuming an upright posture (in which
case it can be reproduced by testing orthostatic
responses). Alternatively, hyperactivity of the
baroreceptor reflex nerves may occasionally
cause hypotension (e.g., in patients with carotid
sinus hypersensitivity or glossopharyngeal neu-
ralgia, where brief bursts of activity in barore-
ceptor nerves trigger a rapid fall in heart rate
and blood pressure).
16,17
In other patients, the
fall in blood pressure may be caused by an in-
termittent failure of the pump (i.e., cardiac ar-
rhythmia). Thus, careful cardiologic evaluation is
required if a neurologic cause is not identified.
PATHOPHYSIOLOGY
The brain ordinarily tightly controls the cir-
culation to provide an adequate level of cere-
bral perfusion. It does this in two ways. First,
across a wide range of arterial blood pressures,
it autoregulates its own blood flow.
18–21
The
mechanism for this remarkable stability of
blood flow is not entirely understood, although
it appears to be due to intrinsic innervation of
the cerebral blood vessels and may also be
regulated by local metabolism.
20,22
In general,
local increases in CBF correspond to increases
in local metabolic rate, allowing the use of blood
flow (in positron emission tomography [PET]
imaging) or local blood volume (in functional
magnetic resonance imaging [MRI]) to approx-
imate neuronal activity. However, there are also
neuronal networks that regulate cerebral perfu-
sion distinct from metabolic need. The two sys-
tems normally act in concert to ensure sufficient
blood supply to allow normal cerebral function
over a wide range of blood pressures but are
dysregulated following some brain injuries.
Second, the brain acts through the auto-
nomic nervous system to acutely adjust sys-
temic arterial pressure in order to maintain a
pressure head that is within the range that al-
lows cerebral autoregulation. Blood pressure
is the product of the cardiac output times the
total vascular peripheral resistance. Cardiac
output in turn is the product of heart rate and
stroke volume. Both heart rate and stroke vol-
ume are increased by beta-1 adrenergic stim-
ulation from sympathetic nerves (or adrenal
catechols), which play a key role in regulating
cardiac output. Heart rate is slowed by mus-
carinic cholinergic action of the vagus nerve,
and hence, increased vagal tone decreases car-
diac output. Peripheral resistance is regulated
mainly by the level of alpha-1 adrenergic tone
in small arterioles, the most important resis-
tance vessels. Therefore, the blood pressure is
regulated by the balance of sympathetic tone,
which increases both cardiac output and vaso-
constrictor tone, versus parasympathetic tone,
which slows heart rate and therefore decreases
cardiac output. The cardiac vagal tone is main-
tained by the nucleus ambiguus in the medulla,
which contains most of the cardiac parasym-
pathetic preganglionic neurons.
23
Sympathetic
vascular and cardiac sympathetic tone is set by
neurons in the rostral ventrolateral medulla
that provide a tonic activating input to the sym-
pathetic preganglionic neurons in the thoracic
spinal cord.
24
When in a lying position, the brain is at the
same level as the heart, but as one rises, the
brain elevates to a position 20 to 30 cm above
the heart. This drop in perfusion pressure (ar-
terial pressure minus intracranial pressure) is
equivalent to 15 to 23 mm Hg, and it may be
sufficient to cause a drop in cerebral perfusion
pressure that would make it difficult to main-
tain CBF necessary to allow conscious brain
function.
To defend against such a precipitous fall in
perfusion pressure, the brain maintains reflex
mechanisms to compensate for the hydrody-
namic consequences of gravity. The level of
arterial pressure is measured at two sites, the
aortic arch (by the aortic depressor nerve, a
branch of the vagus nerve) and the carotid bi-
furcation (by the carotid sinus nerve, a branch
of the glossopharyngeal nerve). These two
nerves terminate in the brain in the nucleus of
the solitary tract, which is the main relay for all
visceral sensory information in the brain.
25,26
The nucleus of the solitary tract then provides
an excitatory input to the caudal ventrolateral
medulla.
27
The caudal ventrolateral medulla in turn pro-
vides an ascending inhibitory input to the tonic
vasomotor neurons in the rostral ventrolateral
Examination of the Comatose Patient
45