MED-3
CENTRAL NERVOUS SYSTEM
Pathology Laboratory
Hydrocephalus
Hydrocephalus refers to the accumulation of excessive CSF within the ventricular system. Most cases
occur as a consequence of impaired flow or impaired resorption of CSF; in rare instances (e.g., tumors of
the choroid plexus), overproduction of CSF may be responsible. When hydrocephalus develops in infancy
before closure of the cranial sutures, there is enlargement of the head. Hydrocephalus developing after
fusion of the sutures, in contrast, is associated with expansion of the ventricles and increased intracranial
pressure, without a change in head circumference.
If there is an obstacle to the flow of CSF within the ventricular system, then a portion of the ventricles
enlarges while the remainder does not. This pattern is referred to as noncommunicating hydrocephalus
and is most commonly seen with masses at the formamen of Monro or aqueduct of Sylvius.
In communicating hydrocephalus all of the ventricular system is enlarged; here the cause is most often
reduced resorption of CSF.
The term hydrocephalus ex vacuo refers to dilation of the ventricular system with a compensatory
increase in CSF volume secondary to a loss of brain parenchyma, as may occur after infarcts or with a
degenerative disease.
Intraparenchymal Hemorrhage
Spontaneous (nontraumatic) intraparenchymal hemorrhages occur most commonly in mid to late adult life,
with a peak incidence at about 60 years of age. Most are caused by rupture of a small intraparenchymal
vessel. Hypertension is the most common underlying cause, and brain hemorrhage accounts for roughly
15% of deaths among individuals with chronic hypertension. Hypertensive intraparenchymal hemorrhages
typically occur in the basal ganglia, thalamus, pons, and cerebellum.
Subarachnoid hemorrhage
The most frequent cause of clinically significant subarachnoid hemorrhage is rupture of a saccular (berry)
aneurysm. Hemorrhage into the subarachnoid space may also result from vascular malformation, trauma
(in which case it is usually associated with other signs of the injury), rupture of an intracerebral
hemorrhage into the ventricular system, hematologic disturbances, and tumors.
About 90% of saccular aneurysms occur in the anterior circulation near major arterial branch points
multiple aneurysms exist in 20% to 30% of cases. Although they are sometimes referred to as congenital,
they are not present at birth but develop over time because of underlying defects in the vessel media.
Besides an association with disorders of extracellular matrix proteins, there is an increased risk of
aneurysms in individuals with autosomal dominant polycystic kidney disease.
While saccular aneurysms are the most common type of intracranial aneurysm, other types include
atherosclerotic (fusiform, mostly of the basilar artery), mycotic, traumatic, and dissecting aneurysms.
These latter three, as with saccular aneurysms, are most often found in the anterior circulation. They
usually present with cerebral infarction from vascular occlusion instead of subarachnoid hemorrhage.
Epidural Hematoma
Vessels that run in the dura, most importantly the middle meningeal artery, are vulnerable to injury,
particularly with skull fractures. In children, in whom the skull is deformable, a temporary displacement of
the skull bones may tear a vessel in the absence of a skull fracture. Once a vessel has been torn, the
accumulation of blood under arterial pressure can cause separation of the dura from the inner surface of
the skull. The expanding hematoma has a smooth inner contour that compresses the brain surface.
Subdural Hematoma
The rapid movement of the brain that occurs in trauma can tear the bridging veins that extend from the
cerebral hemispheres through the subarachnoid and subdural space to empty into dural sinuses. These
vessels are particularly prone to tearing, and their disruption leads to bleeding into the subdural space.
Subdural hematomas most often become manifest within the first 48 hours after injury. They are most
common over the lateral aspects of the cerebral hemispheres and are bilateral in about 10% of cases.