SUBARACHNOID POSTERIOR
FOSSA LESIONS
Subarachnoid blood, infection, or tumor usually
occurs in the posterior fossa in association with
similar supratentorial lesions. Exceptions in-
clude subdural or parenchymal posterior fossa
lesions that rupture into the subarachnoid space
and posterior fossa subarachnoid hemorrhage.
150
Posterior fossa subarachnoid hemorrhages are
caused either by aneurysms or dissection of
vertebral or basilar arteries or their branches.
Unruptured aneurysms of the basilar and ver-
tebral arteries sometimes grow to a size of sev-
eral centimeters and act like posterior fossa ex-
tramedullary tumors. However, they generally
do not cause coma unless they rupture. When
a vertebrobasilar aneurysm ruptures, the event
is characteristically abrupt and frequently is
marked by the complaint of sudden weak legs,
collapse, and coma. Most patients also have sud-
den occipital headache, but in contrast with an-
terior fossa aneurysms in which the history of
coma, if present, is usually clear cut, it some-
times is difficult to be certain whether a patient
with a ruptured posterior fossa aneurysm had
briefly lost consciousness or merely collapsed
because of paralysis of the lower extremities.
Ruptured vertebrobasilar aneurysms are often
reported as presenting few clinical signs that
clearly localize the source of the subarachnoid
bleeding to the posterior fossa. In Logue’s 12
patients,
151
four had unilateral sixth nerve weak-
ness (which can occur with any subarachnoid
hemorrhage), one had bilateral sixth nerve weak-
ness, and only two had other cranial nerve ab-
normalities to signify a posterior fossa localiza-
tion. Duvoisin and Yahr
152
reported that only
about one-half of their patients with ruptured
posterior fossa aneurysms had signs that sug-
gested the origin of their bleeding. Jamieson
reported 19 cases with even fewer localizing
signs: five patients suffered third nerve weak-
ness and two had sixth nerve palsies.
153
Our own experience differs somewhat from
the above. We have had eight patients with
ruptured vertebrobasilar aneurysms confirmed
at arteriography or autopsy, and six had pu-
pillary, motor, or oculomotor signs indicating a
posterior fossa lesion (Table 4–8).
The diagnosis is usually obvious on CT.
Blood isolated to the fourth ventricle suggests
a ruptured posterior inferior cerebellar artery
aneurysm.
150
Perimesencephalic hemorrhage
is characterized by subarachnoid blood ac-
cumulating around the midbrain. While this
often presents with a headache and loss of con-
sciousness, it has a relatively benign progno-
sis.
154
Unlike most subarachnoid hemorrhage,
the bleeding is usually venous in origin
155
; ce-
rebral angiograms are negative and bleeding
rarely recurs.
INTRAPARENCHYMAL
POSTERIOR FOSSA MASS
LESIONS
Intraparenchymal mass lesions in the posterior
fossa that cause coma usually are located in the
cerebellum. In part this is because the cere-
bellum occupies a large portion of this com-
partment, but in part because the brainstem is
so small that an expanding mass lesion often
does more damage by tissue destruction than
as a compressive lesion.
Cerebellar Hemorrhage
About 10% of intraparenchymal intracranial
hemorrhages occur in the cerebellum. A cer-
ebellar hemorrhage can cause coma and death
by compressing the brainstem. Increasing num-
bers of reports in recent years indicate that if
the diagnosis is made promptly, many patients
can be treated successfully by evacuating the
clot or removing an associated angioma.
156,157
However, for those patients who are comatose,
mortality is high despite prompt surgical inter-
vention.
156,158
Approximately three-quarters of
patients with cerebellar hemorrhage have hy-
pertension; most of the remaining ones have
Table 4–8 Localizing Signs in Six
Cases of Ruptured Vertebrobasilar
Aneurysms
Occipital headache
5
Skew deviation of the eyes
3
Third nerve paralysis
2
Cerebellar signs
3
Acute paraplegia before loss
of consciousness
2
Specific Causes of Structural Coma
145
cerebellar angiomas or are receiving anticoag-
ulant drugs. In elderly patients, amyloid angio-
pathy may be the culprit.
159
On rare occasions,
a cerebellar hemorrhage may follow a supra-
tentorial craniotomy.
160
Among our own 28 pa-
tients,
161
five had posterior fossa arteriovenous
vascular malformations, one had thrombocyto-
penic purpura, three were normotensive but re-
ceiving anticoagulants, and the remainder, who
ranged between 39 and 83 years of age, had
hypertensive vascular disease. Hemorrhages in
hypertensive patients arise in the neighborhood
of the dentate nuclei; those coming from angi-
omas tend to lie more superficially. Both types
usually rupture into the subarachnoid space
or fourth ventricle and cause coma chiefly by
compressing the brainstem.
Fisher’s paper in 1965
162
did much to stim-
ulate efforts at clinical diagnosis and encour-
aged attempts at successful treatment. Subse-
quent reports from several large centers have
increasingly emphasized that early diagnosis is
critical for satisfactory treatment of cerebel-
lar hemorrhage, and that once patients become
stuporous or comatose, surgical drainage is a
near-hopeless exercise.
156
The most common
initial symptoms of cerebellar hemorrhage are
headaches (most often occipital), nausea and
vomiting, dizziness or vertiginous sensations,
unsteadiness or an inability to walk, dysarthria,
and, less often, drowsiness. Messert and asso-
ciates described two patients who had unilat-
eral eyelid closure contralateral to the cere-
bellar hemorrhage, apparently as an attempt to
prevent diplopia.
163
Patient 4–3, below, is a
typical example.
Patient 4–3
A 55-year-old man with hypertension and a his-
tory of poor medication compliance had sudden
onset of severe occipital headache and nausea
when sitting down with his family to Christmas
dinner. He noticed that he was uncoordinated
when he tried to carve the turkey. When he arrived
in the hospital emergency department he was
unable to sit or stand unaided, and had severe
bilateral ataxia in both upper extremities. He was
a bit drowsy but had full eye movements with end
gaze nystagmus to either side. There was no weak-
ness or change in muscle tone, but tendon reflexes
were brisk, and toes were downgoing. He was sent
for a CT scan, but by the time the scan was finished
the CT technician could no longer arouse him.
The CT scan showed a 5-cm egg-shaped hem-
orrhage into the left cerebellar hemisphere, com-
pressing the fourth ventricle, with hydrocephalus.
By the time the patient returned to the emergency
department he had no oculocephalic responses,
and breathing was ataxic. Shortly afterward, he
had a respiratory arrest and died before the neu-
rosurgical team could take him to the operating
room.
Table 4–9 Presenting Clinical Findings in 72 Patients With
Cerebellar Hemorrhage
Symptoms
No. Patients
(%)
Signs
No. Patients
(%)
Vomiting
58 (81)
Anisocoria
10 (14)
Headache
48 (67)
Pinpoint pupils
4 (6)
Dizziness/vertigo
43 (60)
Abnormal OCR or EOM
23 (32)
Truncal/gait ataxia
40 (56)
Skew deviation
6 (8)
Dysarthria
30 (42)
Nystagmus
24 (33)
Drowsiness
30 (42)
Absent/asymmetric CR
9 (13)
Confusion
8 (11)
Facial paresis
13 (18)
Dysarthria
18 (25)
Limb ataxia
32 (44)
Hemiparesis
8 (11)
Babinski sign
36 (50)
CR, corneal reflex; EOM, extraocular movements; OCR, oculocephalic reflex.
Modified from Fisher et al.
162
146
Plum and Posner’s Diagnosis of Stupor and Coma