Page 889 - Textbook of Pathology, 6th Edition

malformations of the CNS but only a few important and Arnold-Chiari Malformations 873
common ones are mentioned here. Congenital hydrocephalus Arnold-Chiari malformation is the term used for a group of
is considered separately along with other types of hydro- malformations of the brain involving the brainstem and
cephalus. cerebellum. The primary defect is elongation of the medulla
and part of the vermis of the cerebellum resulting from failure
Spinal Cord Defects of the pontine flexure to form. Approximately 50% of children
with hydrocephalus have the Arnold-Chiari malformation.
Spina bifida is the term applied to the malformations of the Four types are described, of which type II malformation is
vertebral column involving incomplete embryologic closure the most common and is most frequently associated with
of one or more of the vertebral arches (rachischisis), most congenital hydrocephalus. Most patients of Arnold-Chiari
frequently in the lumbosacral region. The vertebral defect is malformation have, in addition, meningomyelocele. The
frequently associated with defect in the neural tube structures major components of type II Arnold-Chiari malformation are
and their coverings. The bony defect may be of varying as follows:
degree. The least serious form is spina bifida occulta in which 1. Elongation of the medulla with part of fourth ventricle
there is only vertebral defect but no abnormality of the spinal
cord and its meninges. The site of bony defect is marked by in the cervical canal.
a small dimple, or a hairy pigment mole in the overlying 2. Distortion of the medulla forming a characteristic S-
skin. The larger bony defect, however, appears as a distinct shaped bend at the junction with the cervical spinal cord.
cystic swelling over the affected site called spina bifida 3. Lengthening and herniation of the cerebellar vermis and
cystica. This is associated with herniation of the meninges cerebellar tonsils through the foramen magnum resulting in
or the spinal cord, or both. formation of a mass over the upper cervical cord.
Herniation of the meninges alone through the bony Combination of these abnormalities results in stenosis of
defect, meningocele, is a less common variety. The herni- the aqueduct or obstruction of the foramina of Luschka and
ated sac in meningocele consists of dura and arachnoid. Magendie causing internal hydrocephalus (discussed below).
The commonest and more serious form is, however, HYDROCEPHALUS CHAPTER 30
meningomyelocele in which the spinal cord or its roots also
herniate through the defect and are attached to the posterior Hydrocephalus is the term used for increased volume of CSF
wall of the sac. In this defect, the dura and the skin in the sac within the skull, accompanied by dilatation of the ventricles.
are deficient. A more serious variant of meningomyelocele In majority of cases of hydrocephalus, there is increased
is associated with hydrocephalus and Arnold-Chiari intracranial pressure. This type of hydrocephalus involving
malformation. ventricular dilatation is termed internal hydrocephalus. A
A rare form of the defect is myelocele or syringo- localised collection of CSF in the subarachnoid space is called
myelocele in which there is defective closure of the spinal external hydrocephalus. For better understanding of causes and
mechanisms involved in the hydrocephalus, it is essential to
canal so that the sac consists of an open flat neural tissue briefly review the source and circulation of CSF.
plate without skin covering and the CSF leaking through it. The Nervous System
Meningomyelocele and myelocele are frequently asso- Source and Circulation of CSF
ciated with neurologic defects of varying degree which CSF is mainly produced by choroid plexus in the lateral, third
include bladder and bowel dysfunction, motor and sensory and fourth ventricle, and a small part is formed on the surface
defects, and paraplegia. of the brain and spinal cord. The total volume of CSF is about
The existence of defect in bony closure in the region of 120-150 ml. CSF formed in the lateral ventricles flows through
occipital bone or fronto-ethmoid junction may result in the foramina of Munro to the third ventricle and from there
cranial meningocele and encephalocele. by the aqueduct of Sylvius to the fourth ventricle. The fluid
then passes through the foramina of Magendie and Luschka
Syringomyelia and Syringobulbia of the fourth ventricle to reach the subarachnoid space of the
These are congenital malformations which manifest clinically brain. It then spreads through the subarachnoid space over
later in life and often develop in association with certain the surface of the spinal cord. It is absorbed into the blood
acquired lesions involving the CNS. Syringomyelia and by the arachnoid villi present along the dural venous sinuses
syringobulbia are characterised by development of a syrinx (Fig. 30.2).
or a tubular cavity in the spinal cord and medulla Types and Etiopathogenesis
respectively. The cavity may be fusiform or irregular. It
usually begins in the grey matter of the spinal cord dorsal to Hydrocephalus is classified into primary and secondary
the central canal. The syrinx is usually surrounded by glial types, the former being much more common, both types have
tissue. If the cavity communicates with the spinal canal, it is distinct etiology and pathogenesis.
lined by ependymal cells. Since the fibres of lateral spino- PRIMARY HYDROCEPHALUS. Primary hydrocephalus is
thalamic tract are frequently involved in the cavity, the defined as actual increase in the volume of CSF within the
clinical effects include loss of pain and temperature sensation skull along with elevated intracranial pressure. There are 3
in the affected region. possible mechanisms of primary hydrocephalus: