increased ICP.
6,7
A second consequence of in-
creased ICP is that axoplasmic flow is impaired
(as if a loose ligature had been tied around the
nerve), and there is buildup of axoplasm on the
retinal side of the disk. The swollen optic axons
obscure the disk margins, beginning at the su-
perior and inferior poles, then extending later-
ally and finally medially.
8
The size of the optic
disk increases, and this can be mapped as a
larger ‘‘blind spot’’ in the visual field. Some pa-
tients even complain of a visual scotoma in this
area. If ICP is increased sufficiently, the gan-
glion cells begin to fail from the periphery of
the retina in toward the macula. This results in a
concentric loss of vision.
Because papilledema reflects the back-
pressure on the optic nerves from increased
ICP, it is virtually always bilateral. A rare excep-
tion occurs when the optic nerve on one side is
itself compressed by a mass lesion (such as an
olfactory groove meningioma), thus resulting in
optic atrophy in one eye and papilledema in the
other eye (the Foster Kennedy syndrome). On
the other hand, optic nerve injury at the level
of the optic disk, either due to demyelinating
disease or vascular infarct of the vasa nervorum
(anterior ischemic optic neuropathy), can also
block axonal transport and venous return, due
to retrobulbar swelling of the optic nerve.
9
The
resulting papillitis can look identical to papille-
dema but is typically unilateral, or at least does
not involve the optic nerves simultaneously. In
addition, papillitis is usually accompanied by
the relatively rapid onset of visual loss, partic-
ularly focal loss called a scotoma, so the clinical
distinction is usually clear.
The origin of headache in patients with in-
creased ICP is not understood. CSF normally
leaves the subarachnoid compartment mainly
by resorption at the arachnoid villi.
10
These
structures are located along the surface of the
superior sagittal sinus, and they consist of in-
vaginations of the arachnoid membrane into
the wall of the sinus. CSF is taken up from the
subarachnoid space by endocytosis into vesi-
cles, the vesicles are transported across the
arachnoid epithelial cells, and then their con-
tents are released by exocytosis into the venous
sinus. Imbalance in the process of secretion
and resorption of CSF occurs in cases of CSF-
secreting tumors as well as in pseudotumor
cerebri. In both conditions, very high levels of
CSF pressure, in excess of 600 mm of water,
may be achieved, but rather little in the way of
brain dysfunction occurs, other than headache.
Experimental infusion of artificial CSF into
the subarachnoid space, to pressures as high
as 800 or even 1,000 mm of water, also does
not cause cerebral dysfunction and, curiously,
often does not cause headache.
11,12
However,
conditions that cause diffusely increased ICP
such as pseudotumor cerebri usually do cause
headache,
13
suggesting that they must cause
some subtle distortion of pain receptors in the
cerebral blood vessels or the meninges.
14
On the other hand, when there is obstruction
of the cerebral venous system, increased ICP is
often associated with signs of brain dysfunction
as well as severe headache. The headache is lo-
calized to the venous sinus that is obstructed
(superior sagittal sinus headache is typically at
the vertex of the skull, whereas lateral sinus
headache is usually behind the ear on the af-
fected side). The headache in these conditions
is thought to be due to irritation and local dis-
tortion of the sinus itself. Brain dysfunction
is produced by back-pressure on the draining
veins that feed into the sinus, thus reducing the
perfusion pressure of the adjacent areas of the
brain, to the point of precipitating venous in-
farction (see page 154). Small capillaries may
be damaged, producing local hemorrhage and
focal or generalized seizures. Superior sagittal
sinus thrombosis produces parasagittal ischemia
in the hemispheres, causing lower extremity pa-
resis. Lateral sinus thrombosis typically causes
infarction in the inferior lateral temporal lobe,
which may produce little in the way of signs,
other than seizures.
The most important mechanism by which
diffusely raised ICP can cause symptoms is by
impairment of the cerebral arterial supply. The
brain usually compensates for the increased
ICP by regulating its blood supply as described
in Chapter 2. However, as ICP reaches and ex-
ceeds 600 mm of water, the back-pressure on
cerebral perfusion reaches 45 to 50 mm Hg,
which becomes a major hemodynamic chal-
lenge. Typically, this is seen in severe acute
liver failure,
15
with vasomotor paralysis follow-
ing head injury, or occasionally in acute en-
cephalitis. When perfusion pressure falls below
the lower limit required for brain function, neu-
rons fail to maintain their ionic gradients due
to energy failure, resulting in additional swell-
ing, which further increases ICP and results in
a downward spiral of reduced perfusion and
further brain infarction.
92
Plum and Posner’s Diagnosis of Stupor and Coma