Most forms of acute head injury increase the volume of the intracranial contents.
Cerebral edema, hemorrhage, acute hydrocephalus, or rapidly growing
tumors all result in intracranial hypertension (see Chapter 104 and Table 104-5). The relationship
between the volume increase and the rise in intracranial pressure
is not linear. Small volumes can be accommodated either by stretch,
if the cranial sutures are still open, or by displacement of cerebrospinal
fluid (CSF) into the spinal canal (this is why in most forms of
intracranial hypertension, with the notable exception of hydrocephalus,
the ventricles appear small). To a lesser extent, the blood contained
at any time in the cerebral vessels, especially the cerebral veins,
diminishes. As the volume increase produced by the injury becomes
larger, however, the pressure increases more rapidly, a circumstance
that can be described as a volume-dependent decrease in the compliance
of the cranium. Any additional volume causes a disproportionate
elevation in intracranial pressure.
The most immediate and dangerous consequence of increased intracranial
pressure is a reduction in cerebral blood flow. Cerebral herniation
(see Chapter 104) tends to be a later development.
In most organs, blood flow is directly proportional to the difference
between arterial and venous pressure (the perfusion pressure) and
is inversely proportional to the resistance of the organ’s
vascular bed. Some organs, such as the heart and the brain, regulate
their vascular tone (and thus their vascular resistance) to maintain
constant blood flow over a wide range of perfusion pressures appropriate
for age (autoregulation). Below or above this autoregulation range,
blood flow changes in proportion to perfusion pressure. (Other organs,
such as the intestine and the skin, have their flow regulated by
neural and humoral inputs in response to a variety of physiological
circumstances, sometimes without change in blood pressure.)
The cranial enclosure introduces a new variable for consideration.
The outside surface of the cerebral blood vessels is exposed to
intracranial pressure. If this pressure exceeds venous pressure,
then the effective perfusion pressure is the difference between
arterial and intracranial pressure, the venous pressure becoming
irrelevant in defining blood flow (just as the height of a waterfall
is irrelevant in determining the flow of the river). Intracranial
hypertension can reduce cerebral perfusion pressure below the autoregulation
range of the cerebral blood vessels, resulting in cerebral ischemia.
Cerebrospinal fluid production and subsequent absorption is,
like cerebral blood flow, quite dynamic. The choroid plexus accounts for
at least 70% of the brain’s production of cerebrospinal
fluid, and the transependymal movement of fluid from the brain parenchyma to
the ventricular system accounts for the rest. The average volume
of cerebrospinal fluid in children ages 4 to 13 years is 90 mL,
and the rate of formation is approximately 500 mL per day, resulting
in an hourly turnover of about 14% of the total volume.
The rate of production remains fairly constant and declines only
slightly with increased intracranial pressure, but the rate of ...