Second edition

07.qxd 3/10/08 9:35 AM Page 284
284 Other major syndromes
Low-flow (watershed) infarctions
‘Low-flow’ infarctions differ from embolic and thrombotic
infarctions in that the arteries serving the infarcted area are
not, in fact, occluded. Certain portions of the cerebral cortex
lie at the very periphery of the areas of distribution of
major cerebral arteries and these areas are quite vulnerable.
Perfusion pressure falls as one travels further down the
arterial tree and at these peripheries pressure is relatively
quite low. Consequently, whenever there is a substantial
reduction of pressure upstream, the pressure at the periphery
may fall below that required for tissue viability and
infarction may occur. Such upstream reductions may
occur with gradual artherosclerotic narrowing, with systemic
hypotension (as may occur with cardiac arrest or as a
side-effect of numerous drugs), or a combination of these
mechanisms. Such ‘low-flow’ infarctions are generally
referred to as either ‘watershed’ or ‘border zone’ infarctions
and have a characteristic location. Watershed infarctions at
the border zone of the anterior and middle cerebral arteries
(ACA–MCA) occur either at the dorsolateral prefrontal
cortex or extend through the central portion of the
centrum semiovale. Those occurring at the border zone of
the middle cerebral and posterior cerebral arteries
(MCA–PCA) are typically found in the parieto-occipital
cortex. Finally, there is an ‘internal’ border zone, lying
between the lenticulostriate arteries and pial branches of
the MCA: infarctions in this border zone are typically
found in the periventricular white matter. In cases of unilateral
watershed infarction, there is generally an associated
tight stenosis of the internal carotid artery; simultaneous
bilateral watershed infarcts generally only occur with dramatic
systemic hypotension, for example with cardiac
arrest.
Lipohyalinosis
Lipohyalinosis is generally considered to occur secondary
to hypertension and affects the central branches, described
above, and the penetrating branches arising from the
vertebral and basilar arteries. With occlusion of one of these
small arteries, a correspondingly small infarction, known as
a ‘lacune’ typically occurs. Occlusion of central and penetrating
arteries, while most commonly due to lipohyalinosis,
may also occur secondary to atherosclerosis
(Caplan 1989) and, rarely, to embolic occlusion.
Atherosclerosis, as noted earlier, often involves the basilar
artery, and in such a case an atherosclerotic plaque may, as
it gradually enlarges, slowly lap over the ostium of the penetrating
artery, thus occluding this innocent bystander. A
similar sequence of events may occur in the stem of the
MCA, leading to occlusion of one or all of the lenticulostriate
arteries. Embolic occlusion of a small central or penetrating
artery is unusual given that most emboli are borne
along in the mainstream of the large parent artery and simply
do not make the midstream turn required to enter central
or penetrating arteries, which generally arise at a more
or less right angle to their parent artery.
INTRACEREBRAL HEMORRHAGE
When the hemorrhage involves subcortical structures (e.g.,
putamen, caudate, or thalamus), brainstem (typically the
pons) or cerebellum, the bleeding has usually occurred secondary
to rupture of a microaneurysm on one of the central
or penetrating arteries (Cole and Yates 1967), which, in
turn has usually developed on the basis of longstanding
uncontrolled hypertension.
By contrast, when the hemorrhage is ‘lobar’, that is to say
situated in one of the lobes of the cerebrum, a variety of
causes may be at fault, including cerebral amyloid angiopathy,
hemorrhage into a tumor, rupture of a vascular malformation
(e.g., an arteriovenous malformation, a cavernous
angioma, or a venous angioma), vasculitis, or in the setting
of anticoagulant or thrombolytic treatment: patients
treated with warfarin may experience lobar hemorrhages
secondary to minor trauma and intracerebral hemorrhage
is a feared complication of treatment with tissue plasminogen
activator. Other possible causes include vasculitis or use
of sympathomimetic agents, such as cocaine.
SUBARACHNOID HEMORRHAGE
Subarachnoid hemorrhage (van Gijn and Rinkel 2001) is
most often due to rupture of a berry aneurysm. Other
causes include mycotic aneurysms, trauma, rupture of an
arteriovenous malformation into the subarachnoid space,
vasculitidies, and a condition known as perimesencephalic
hemorrhage (van Gijn et al. 1985). This last entity is characterized
by hemorrhage surrounding the midbrain and
pons; symptoms are typically mild and it is suspected that
the bleeding in this case, unlike all the other causes, is
venous.
INTRAVENTRICULAR HEMORRHAGE
Hemorrhage into a ventricle may, as noted earlier, occur
secondary to extension of an intracerebral hemorrhage.
‘Primary’ intraventricular hemorrhage may also occur: this
is rare, and generally due to bleeding from a vascular malformation
adjacent to the ventricle.
CEREBRAL VENOUS THROMBOSIS
Cerebral venous thrombosis, as noted earlier, generally is
seen as a complication of thrombosis of one of the dural
sinuses. Such thrombosis, in turn, may be due to a number
of causes (Gosk-Bierska et al. 2006), such as Behçet’s syndrome,
the anti-phospholipid syndrome, deficiencies of
anti-thrombin III or of proteins S or C, systemic lupus erythematosus,
the puerperium, paroxysmal nocturnal hemoglobinuria,
in association with certain malignancies, during
treatment with oral contraceptives, and in association with
certain infections: otitis or mastoiditis may lead to thrombosis
of the transverse sinus, and facial or sinus infection to
thrombosis of the cavernous sinus.