Spinal and Spinal Cord Defects

Dysraphic disorders of the spine and spinal cord comprise a broad spectrum of malformations, ranging from total absence of the cord to asymptomatic bony defects and cord anomalies. Familial occurrences are common. The clinical manifestations vary from mild motor and sensory deficits to paraplegia with severe sensory impairment and loss of sphincter control.

Amyelia, total absence of the spinal cord occurs with anencephaly (see Fig. 13.2). The cord is replaced with a mixture of fibrous tissue, blood vessels, and neural elements (area myelovasculosa).

Meningocele and meningomyelocele consist of her-niations of the meninges or meninges, spinal cord, and

figure 13.4

Schematic drawing of a lumbar meningomyelocele. The cystic sac covered with skin contains the spinal cord, meninges, and nerve roots.

figure 13.4

Schematic drawing of a lumbar meningomyelocele. The cystic sac covered with skin contains the spinal cord, meninges, and nerve roots.

nerve roots through a defect in the vertebral arch. The herniated structures are covered with a thin skin or vascular membrane, which is a potential source of infection. Common sites of the malformations are the lum-bosacral and the thoracolumbar regions (Fig. 13.4). The myelocele may be associated with neuroenteric or enterogenous cysts, epidermoid cysts, and teratomas.

In tethered spinal cord, the conus is fixed to a bony defect by a fibrous band that prevents the normal upward movement of the cord.

Spina bifida occulta is the mildest dysraphic disorder; the vertebral arches are absent or unfused, but the spinal cord and meninges are normal. The skin overlying the bony defect is usually pigmented and hairy. Fatty tissue may be present under the skin or in the extra- or intradural spaces.

Diastematomyelia refers to two half cords, separated by a bony spur or a fibrous septum extending from the vertebrae into the spinal canal. The lumbosacral segments are the usual sites. The ventral horns of the hemicords face each other, and each half has ventral and dorsal horns and roots. They may be contained within two separate or a single dural sac (Fig. 13.5).

Diplomyelia refers to two whole spinal cords within a single dural sac (Fig. 13.6).

Hydromyelia results from dilatation of the central canal, usually in the lumbosacral segments. It may be associated with other dysraphic disorders (Fig. 13.7).

Syringomyelia is a tubelike cavity (syrinx) extending over several segments of the cervicothoracic cord. On transverse sections, the cavity extends from one dorsal horn to the opposite dorsal horn, crossing the midline in the anterior commissure (Fig. 13.8). It contains xanthochromic fluid that gradually expands the cavity and compresses the surrounding structures. Often

figure 13.5

Diastematomyelia. A and B. The sacral cord splits gradually into two hemi-cords, the ventral horns facing each other (LFB-CV). The hemicords, separated by a fibrous band, share a common dural sac.

figure 13.5

Diastematomyelia. A and B. The sacral cord splits gradually into two hemi-cords, the ventral horns facing each other (LFB-CV). The hemicords, separated by a fibrous band, share a common dural sac.

figure 13.6

Diplomyelia. The cord is split into two cords. Each cord has a central canal, two ventral and two dorsal horns (Weil stain).

figure 13.6

Diplomyelia. The cord is split into two cords. Each cord has a central canal, two ventral and two dorsal horns (Weil stain).

figure 13.7

Hydromyelia in a 4-year-old girl with partial cerebellar agenesis. Transverse section of the lumbar cord shows an enlarged central canal. A small slit is present in the posterior column (Weil stain).

figure 13.7

Hydromyelia in a 4-year-old girl with partial cerebellar agenesis. Transverse section of the lumbar cord shows an enlarged central canal. A small slit is present in the posterior column (Weil stain).

figure 13.8

Syringomyelia. The cavity merges with the central canal and extends across the spinal cord. It is lined with gliotic tissue (Weil stain).

figure 13.8

Syringomyelia. The cavity merges with the central canal and extends across the spinal cord. It is lined with gliotic tissue (Weil stain).

it merges with the central canal. Syringomyelia may coexist with gliomas and vascular tumors, Arnold-Chiari malformation, and kyphoscoliosis.

Syringomyelia is defined by a dissociated sensory impairment, such as loss of pain and temperature sensations with the preservation of vibratory, positional, and touch sensations in the upper extremities in a segmental distribution. As the disease progresses, the sensory deficits are associated with weakness and atrophy of the muscles and diminished or absent tendon reflexes in the corresponding segments. Later in the course, spasticity, weakness, and sensory ataxia develop in the lower extremities due to pressure on the corticospinal tracts and posterior columns. Charcot joints may also occur.

A neurenteric (enterogenic) cyst, intraspinal and extramedullary in the cervicothoracic region, is lined with epithelial cells resembling the gastrointestinal mucosa.

MALFORMATION OF THE

PROSENCEPHALON:

HOLOPROSENCEPHALY

Following the closure of the neural tube, the next major event is the formation of the three primary vesicles of the brain: the prosencephalon or forebrain, the mesen-cephalon or midbrain, and the rhombencephalon or hindbrain. The prosencephalon cleaves into the two telencephalic vesicles, which then develop into the two cerebral hemispheres and the two lateral ventricles (Fig. 13.9). A failure of this cleavage results in holoprosen-cephaly—a cerebrum that is undivided into hemispheres and has a single ventricular cavity.

Clinical features: Holoprosencephaly is associated with a broad spectrum of craniofacial anomalies, including microcephaly, hypo- or hypertelorism, flat nose, harelip, and cleft palate, and ocular anomalies ranging from cyclopia to microphthalmia (Fig. 13.10). The malformation occurs sporadically or is inherited in an autosomal dominant, autosomal recessive, or X-linked fashion. Some cases are associated with chromosomal aberrations such as trisomy 13 and 18 and triploidy. Among environmental factors, maternal diabetes mellitus, infection, irradiation, and excessive alcohol

figure 13.9

Schematic drawing of primary cerebral vesicles and cleavage of the prosencephalon.

figure 13.9

Schematic drawing of primary cerebral vesicles and cleavage of the prosencephalon.

consumption have been associated with holoprosen-cephaly. Most infants are stillborn or die in early infancy. Those who survive a few years suffer profound psycho-motor retardation, intractable seizures, apneic episodes, temperature irregularities, and diabetes insipidus. Antenatal diagnosis is possible using ultrasonography.

Pathology: Grossly, holoprosencephaly may present under three forms:

• The alobar form has no evidence of midline separation (see Fig. 13.10). The small brain displays a markedly simplified convolutional pattern, with broad and abnormally oriented convolutions. The olfactory bulbs and tracts are absent (arhinencephaly). Transverse sections reveal a single large ventricle, severely

figure 13.10

Holoprosencephaly in a 3-month-old boy. A. The head is small and the nose depressed. He has hare lip and cleft palate. Dorsal (B), lateral (C), and basal (D) views display a small alobar brain, undivided into hemispheres. The convolutional pattern is greatly simplified; the convolutions are broad and abnormally oriented. The olfactory bulbs and tracts are absent. E. Transverse section reveals a single ventricle. F. The cortex displays a four-layer lamination.

figure 13.10

Holoprosencephaly in a 3-month-old boy. A. The head is small and the nose depressed. He has hare lip and cleft palate. Dorsal (B), lateral (C), and basal (D) views display a small alobar brain, undivided into hemispheres. The convolutional pattern is greatly simplified; the convolutions are broad and abnormally oriented. The olfactory bulbs and tracts are absent. E. Transverse section reveals a single ventricle. F. The cortex displays a four-layer lamination.

reduced white matter, and fusions of the thalami and basal ganglia.

• The semilobar form presents partial midline separation and rudimentary lobes.

• The lobar form consists of two hemispheres but only a single ventricle.

Histologically, the hippocampus and the pyriform cortex are present and, with the rest of the cerebral cortex, show cytoarchitectonic anomalies. Variable cytoarchi-tectonic anomalies are also present in the subcortical gray structures, brainstem, and cerebellum.

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