vessels of the circle of Willis. Hence, even
small degrees of displacement may stretch and
compress important feeding vessels and re-
duce blood flow. In addition to accounting for
the pathogenesis of coma (due to impairment
of the ascending arousal system at the dience-
phalic level), the ischemia causes local swelling
and eventually infarction, which causes further
edema, thus contributing to gradually progres-
sive displacement of the diencephalon. In se-
vere cases, the pituitary stalk may even become
partially avulsed, causing diabetes insipidus,
and the diencephalon may buckle against the
midbrain. The earliest and most subtle signs
of impending central herniation tend to begin
with compression of the diencephalon.
Less commonly, the midbrain may be forced
downward through the tentorial opening by a
mass lesion impinging upon it from the dorsal
surface. Pressure from this direction produces
the characteristic dorsal midbrain or Parinaud’s
syndrome (loss of upgaze and convergence, re-
tractory nystagmus; see below).
Rostrocaudal deterioration of the brainstem
may occur when the distortion of the brainstem
compromises its vascular supply. Downward
displacement of the midbrain or pons stretches
the medial perforating branches of the basi-
lar artery, which itself is tethered to the circle
of Willis and cannot shift downward (Figure
3–4). Paramedian ischemia may contribute to
loss of consciousness, and postmortem injec-
tion of the basilar artery demonstrates that the
paramedian arteries are at risk of necrosis and
extravasation. The characteristic slit-like hem-
orrhages seen in the area of brainstem dis-
placement postmortem are called Duret hem-
orrhages
53
(Figure 3–7). Such hemorrhages
can be replicated experimentally in animals.
54
It is also possible for the venous drainage of the
brainstem to be compromised by compression
of the great vein of Galen, which runs along the
midline on the dorsal surface of the midbrain.
However, in postmortem series, venous infarc-
tion is a rare contributor to brainstem injury.
55
Tonsillar herniation occurs in cases in which
the pressure gradient across the foramen mag-
num impacts the cerebellar tonsils against the
foramen magnum, closing off the fourth ven-
tricular outflow and compressing the medulla
(Figures 3–7 and 3–8). This may occur quite
suddenly, as in cases of subarachnoid hemor-
rhage, when a large pressure wave drives the
cerebellar tonsils against the foramen magnum,
compressing the caudal medulla. The patient
suddenly stops breathing, and blood pressure
rapidly increases as the vascular reflex pathways
in the lower brainstem attempt to perfuse the
lower medulla against the intense local pressure.
A similar syndrome is sometimes seen when
lumbar puncture is performed on a patient
whose intracranial mass lesion has exhausted
the intracranial compliance.
56
In patients with
sustained tonsillar herniation, the cerebellar
tonsils are typically found to be necrotic due to
their impaction against the unyielding edge
of the foramen magnum. This problem is dis-
cussed further below.
Upward brainstem herniation may also occur
through the tentorial notch in the presence of
a rapidly expanding posterior fossa lesion.
3
The
superior surface of the cerebellar vermis and
the midbrain are pushed upward, compressing
the dorsal mesencephalon as well as the adja-
cent blood vessels and the cerebral aqueduct
(Figure 3–8).
The dorsal midbrain compression results in
impairment of vertical eye movements as well as
consciousness. The pineal gland is typically
Figure 3–6. Bilateral occipital infarction in Patient 3–1.
Hemorrhage into a large frontal lobe tumor caused trans-
tentorial herniation, compressing both posterior cerebral
arteries. The patient underwent emergency craniotomy to
remove the tumor, but when she recovered from surgery
she was cortically blind.
102
Plum and Posner’s Diagnosis of Stupor and Coma
displaced upward on CT scan.
57
The compres-
sion of the cerebral aqueduct can cause acute
hydrocephalus, and the superior cerebellar ar-
tery may be trapped against the tentorial edge,
resulting in infarction and edema of the superior
cerebellum and increasing the upward pressure.
Clinical Findings in Uncal
Herniation Syndrome
EARLY THIRD NERVE STAGE
The proximity of the dorsal surface of the ocu-
lomotor nerve to the medial edge of the tem-
poral lobe (Figure 3–5) means that the earliest
and most subtle sign of uncal herniation is of-
ten an increase in the diameter of the ipsilat-
eral pupil. The pupil may respond sluggishly
to light, and typically it dilates progressively as
the herniation continues. Early on, there may
be no other impairment of oculomotor func-
tion (i.e., no ptosis or ocular motor signs). Once
the herniation advances to the point where the
function of the brainstem is compromised, signs
of brainstem deterioration may proceed rap-
idly, and the patient may slip from full con-
sciousness to deep coma over a matter of min-
utes (Figure 3–9).
Patient 3–2
A 22-year-old woman was admitted to the emer-
gency room with the complaint of erratic behavior
‘‘since her boyfriend had hit her on the head with a
gun.’’ She was awake but behaved erratically in the
emergency room, and was sent for CT scanning
while a neurology consult was called. The neurol-
ogist found the patient in the x-ray department and
the technician noted that she had initially been
uncooperative, but for the previous 10 minutes she
had lain still while the study was completed.
Figure 3–7. Neuropathology of
herniation due to a large brain
tumor. A large, right hemisphere
brain tumor caused subfalcine
herniation (arrow in A) and
pushed the temporal lobe against
the diencephalon (arrowhead).
Herniation of the uncus caused
hemorrhage into the hippocampus
(double arrowhead). Downward
displacement of the brainstem
caused elongation of the brainstem
and midline Duret hemorrhages
(B). Downward displacement of the
cerebellum impacted the cerebellar
tonsils against the foramen mag-
num, infarcting the tonsillar tissue
(arrow in C).
Structural Causes of Stupor and Coma
103