Cerebral and Cranial Malformations Associated with Hydrocephalus

Stenosis and atresia (absence) of the aqueduct of Sylvius occur sporadically; a few cases are inherited as a X-linked recessive trait affecting only boys, or as an autosomal recessive trait. In atresia, groups of tiny channels lined with ependymal cells and separated by normal tissue indicate the site of the aqueduct. Rarely, the aqueduct is occluded by a membranous septum (see Fig. 13.33).

The Dandy-Walker malformation accounts for 3% to 4% of congenital hydrocephalus. Most cases are sporadic. A few are familial, and some are linked to maternal use of isoretinoid during pregnancy.

In the Dandy-Walker malformation, the vermis is totally or partially absent, mostly its posterior portion. The fourth ventricle is transformed into a large cystic cavity with a thin, glial-vascular membranous roof lined with ependyma. The foramina Luschka and Magendie may fail to open. The posterior fossa is large, and the lateral sinuses and torcular Herophili are elevated (Fig. 13.34).

The Arnold-Chiari type 2 malformation is characterized by downward displacement of the cerebellar tonsils, inferior vermis, and the elongated medulla into the spinal canal. Subsequently, the tectum becomes beak-like, the elongated medulla kinked, and the foramen Magendie obstructed (Fig. 13.35). The posterior fossa is small. Meningomyelocele is almost invariably present, preventing the upward movement of the spinal cord, thus reversing the direction of the spinal nerve roots from downward to upward.

Clinically, the hydrocephalus in both Dandy-Walker and Arnold-Chiari type 2 malformations may either be present at birth or manifests in early infancy. If the cerebrospinal fluid (CSF) is not shunted, cerebellar signs, nystagmus, cranial nerve deficits—chiefly sixth nerve palsy—spasticity, and signs of increased ICP develop. The Arnold-Chiari malformation is further aggravated by symptoms of a lumbar or sacral meningomyelocele.

figure 13.35

Arnold-Chiari type 2 malformation in a 1.5-year-old boy. He was delivered by forceps because of his enlarged head. He had a lumbosacral meningomyelocele, club feet, and flaccid paraparesis. By age 2 months, the head circumference increased to 48 cm (normal, 40 cm). Ventral view of the cerebellum shows thickened and partially torn (artifact) basal leptomeninges and elongated tonsils adjacent to the rostral medulla.

table 13.6.

Malformations with Chromosomal Defects

Malformations

Anencephaly Dysraphic disorders Holoprosencephaly Microencephaly

Dandy-Walker malformations Congenital hydrocephalus

Agenesis of corpus callosum Arnold-Chiari type 2 malformations

Chromosomal Defects

Trisomy 9, 13, 18 Triploidy Trisomy 13, 18 Trisomy 9, 13, 18, 21

X-chromosome Trisomy 9, 18, 21 Trisomy 9, 13, 20 Triploidy Trisomy 8, 13, 20 Trisomy 18 Triploidy figure 13.35

Arnold-Chiari type 2 malformation in a 1.5-year-old boy. He was delivered by forceps because of his enlarged head. He had a lumbosacral meningomyelocele, club feet, and flaccid paraparesis. By age 2 months, the head circumference increased to 48 cm (normal, 40 cm). Ventral view of the cerebellum shows thickened and partially torn (artifact) basal leptomeninges and elongated tonsils adjacent to the rostral medulla.

figure 13.36

Chiari type 1 malformation in a 24-year-old man with chronic headache. MRI shows the cerebellar tonsils located below the foramen magnum level.

figure 13.36

Chiari type 1 malformation in a 24-year-old man with chronic headache. MRI shows the cerebellar tonsils located below the foramen magnum level.

In the Chiari type 1 malformation the cerebellar tonsils are located below the foramen magnum (chronic cerebellar herniation), possibly due to smallness of the posterior fossa (Fig. 13.36).

Clinically, infants present with nuchal rigidity and apneic spells, which may culminate in sudden death.

Young adults present with headache, cranial nerve deficits, cerebellar ataxia, stridor, sleep apnea, and symptoms of raised ICP.

Craniosynostosis results from a premature closure of the cranial sutures: sagittal, coronal, and lambdoid. The fusion may involve one or several. Subsequently, the head is abnormally shaped and the eyes are proptotic. The patients present with headache, blindness, and mental retardation.

In achondroplasia, the hydrocephalus, usually arrested, is associated with an abnormally formed occipital bone and narrow foramen magnum.

CHROMOSOMAL DISORDERS

Fetal chromosomal aberrations often result in abortions during the first trimester. Trisomies, the most common chromosomal disorders, are frequently implicated in nervous system malformations. Trisomies 13 and 18 are associated with major malformations of the nervous system, eyes, viscera, and skeleton. Affected fetuses are stillborn or short-lived. A larger group of less severe or minor malformations are associated with trisomies 21, 9, and the X chromosome. Affected individuals present with mental retardation, seizures, and variable motor deficits. Cytogenetic studies for suspected chromosomal disorders are performed on amniotic fluid or chorionic villi during the third or fourth month of pregnancy. Major malformations with chromosomal defects are listed in Table 13.6.

figure 13.37

Down syndrome. The brain of a 4-year-old boy displays a round shape, flattened occipital lobes, and simple convolu-tional pattern.

figure 13.37

Down syndrome. The brain of a 4-year-old boy displays a round shape, flattened occipital lobes, and simple convolu-tional pattern.

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