Hydrocephalus
Those primarily disturbing normal homeostatic control of CSF production and CSF flow or drainage.
Those principally associated with developmental abnormalities or destructive lesions in the brain (Table 4.1).
Overproduction of CSF by a choroid plexus papilloma (Fig. 4.1), reduced propulsion consequent upon primary ciliary dysplasia, and reduced absorption due to aplasia of arachnoid granulations or raised venous pressure due to bony dysplasia (Fig. 4.2) are rare causes of hydrocephalus. More commonly hydrocephalus results from interference with CSF flow at various points in the CSF pathway (Fig. 4.3).
4.1 Choroid plexus papilloma.
These neoplasms may cause hydrocephalus by obstructing CSF flow or by oversecretion.
4.2 Hydrocephalus associated with bony dysplasia.
(a) Part of the skull base from an 8-year-old child who presented with hydrocephalus and chronic tonsillar herniation resulting from craniosynostosis, clover-leaf skull, and Klippel–Feil deformity. The bone is alternately thinned or massively thickened into irregular ridges and contains excessively narrow exit foramina, which compromise venous return and reduce CSF reabsorption, thereby causing hydrocephalus. (b) A computerized tomogram of the head of an 18-month-old infant with Crouzon syndrome shows synostosis, distortion of the skull bones, and hydrocephalus.
Many neoplasms can fill and block the ventricles or subarachnoid spaces, notably those growing close to narrow parts of the ventricular system. For example, the foramen of Monro may be blocked by a subependymal giant cell astrocytoma of tuberous sclerosis (Fig. 4.4) or a colloid cyst (see Chapters 34, 35, and 45). Otherwise, obstruction commonly occurs at the aqueduct and the fourth ventricular exit foramina.
4.4 Obstruction of interventricular foramina of Monro.
In this case the obstruction is caused by a subependymal giant cell astrocytoma in tuberous sclerosis.
OBSTRUCTION OF THE AQUEDUCT OF SYLVIUS
MACROSCOPIC AND MICROSCOPIC APPEARANCES
Stenosis may be sporadic, X-linked, or (rarely) autosomal recessive. It is characterized by a tiny lumen in the usual position, a normal ependymal lining, and an absence of gliosis (Fig. 4.5). Experimental and clinical evidence suggests that narrowing can be secondary to compression of the tectal plate by expanding hydrocephalic hemispheres.
4.5 Aqueduct stenosis.
This is characterized by a very tiny lumen, intact ependymal lining, and normal surrounding midbrain parenchyma.
In atresia (forking) a normal channel is replaced by groups of ependymal cells, small rosettes, or tiny aqueductules irregularly scattered across the midbrain tegmentum. There is no gliosis (Fig. 4.6).
4.6 Aqueduct atresia.
This is characterized by the presence of many tiny canals or aqueductules scattered through the midbrain tegmentum in the absence of overt gliosis.
With gliosis, an outline of the aqueduct is still visible. An interrupted ring of ependymal cells, rosettes, and tubules are surrounded and filled by dense fibrillary gliosis, in which there may be one or two small central channels that lack an ependymal lining (Fig. 4.7).
4.7 Aqueduct gliosis.
(a) Macroscopic view of the midbrain. Although the position of the aqueduct can be seen, it appears blocked. (b) Microscopically, the outline of the aqueduct is still visible as an interrupted ring of ependymal canals. Gliotic tissue enmeshes these tubules and fills the aqueductal lumen.
A septum is a rare variant of aqueductal gliosis. A thin translucent glial membrane is surrounded by a ring of ependymal tissue, which interrupts the aqueduct at its caudal end (Fig. 4.8).
4.8 Septum of the aqueduct.
(a) A narrow band of white tissue occludes the lower end of the aqueduct (arrow) in this sagittal hemisection of the brain. (b) Microscopically, the similarity with aqueduct gliosis is striking. A tenuous glial membrane is stretched across the lumen of the Sylvian canal, outlined by a series of ependymal tubules separated and enmeshed by astroglial tissue.
In X-linked hydrocephalus ventricular dilatation may be present with or without aqueduct stenosis (Fig. 4.9). The cerebrum shows polygyria (excessive gyration with normal histology) (Fig. 4.10), which is a common consequence of hydrocephalus in early life. Absence of the medullary pyramids (Fig. 4.11) is almost invariable. Other findings may be fusion of the thalamic nuclei and agenesis of the corpus callosum.
4.9 X-linked hydrocephalus.
(a) Coronal section through the frontal horns showing marked ventriculomegaly. (b)
