Congenital Anomalies of Central Nerve System
Dysraphism describes defects of fusion of the neural tube involving the neural tube itself, or overlying bone or skin. Dysraphism may occur in the spine or the head. Neural tube defects are among the most common congenital abnormalities.
Prenatal vitamins, especially folic acid, reduce the incidence of neural tube defects.
Spina Bifida Occulta
Spina bifida occulta is congenital absence of posterior vertebral elements. The spinous process is always missing, the laminae may be missing to various degrees, but the underlying neural tissues are not involved. Spina bifida occulta is found in 25% of the general population. and is asymptomatic unless associated with other developmental abnormalities.
Spina Bifida with Myelomeningocele
Spina bifida with myelomeningocele describes congenital absence of posterior vertebral elements with protrusion of the meninges through the defect, and underlying neural structural abnormalities. Common findings are weakness and atrophy of the lower extremities. gait disturbance, urinary incontinence. and deformities of the foot. Myelomeningoceles arising from the high lumbar cord usually cause total paralysis and incontinence, while those arising from the sacral cord may have only clawing of the foot and partial urinary function. Myelomeningocele patients often have hydrocephalus and a Chiari II malformation, an abnormal downward herniation of the cerebellum and brain stem through the foramen magnum. Patients with abnormal protrusion of meninges through the bony defect with out abnormalities of the underlying neural tissue have a meningocele. Most of these patients are neurologically normal.
Herniation of brain encased in meninges through the skull that forms an intracranial mass is referred to as encephalocele. Herniation of meninges without brain tissue is referred to as a meningocele. Most occur over the convexity of the skull. More rarely, the tissue protrudes through the skull base into the sinuses. Treatment involves excision of the herniated tissue and closure of the defect. Most patients with encephaloceles and meningoceles have impaired cognitive development. Patients with greater amounts of herniated neural tissue tend to have more severe cognitive deficits
Craniosynostosis is the abnormal early fusion of a cranial suture line with resultant restriction of skull growth in the affected area and compensatory bulging at the other sutures. Skull growth occurs at the cranial sutures for the first 2 years of life. at the end of which the skull has achieved over 90% of its eventual adult size. Fusion of the sagittal suture, or sagittal synostosis. results in a boat-Shaped head. known as scaphocephaly, Unilateral coronal synostosis results in ipsilateral forehead flattening and outward deviation of the orbit. known as plagiocephaly. The contralateral normal forehead appears to bulge by comparison. Bilateral coronal synostosis results in a broad. flattened forehead, known as brachycephaly. and is often associated with maxillary hypoplasia and proptosis. Unilateral or bilateral lambdoid synostosis results in flattening of the occiput. Occipital flattening can result from abnormal suture fusion (synostosis), or from physical remolding of the skull caused by always placing the baby in the supine position for sleep (known as positional plagiocephaly). Placing the baby in the prone position or tilted onto the contralateral side may restore near-normal skull shape in most cases of lambdoid synostosis, avoiding surgery. Treatment for synostoses in general is surgical, involving resection of the fused suture, or more complex reconstructive techniques for severe or refractory cases
Excess CSF in the brain that results in enlarged ventricles is known as hydrocephalus. The adult forms approximately 500 mL of CSF per day, much of it in the lateral ventricles. CSF flows from the
' ventricles to the subarachnoid space, and is then absorbed into the venous blood through the arachnoid granulations. Hydrocephalus may be classified as communicating or obstructive (outlined below), . and congenital or acquired. Congenital lesions associated with or causing hydrocephalus include stenosis of the cerebral aqueduct, Chiari malformations, myelomeningoceles, and intrauterine infections. Acquired hydrocephalus may result from occlusion of arachnoid granulations by meningitis or subarachnoid hemorrhage, or occlusion of CSF pathways by adjacent tumors
Obstruction at the level of the arachnoid granulations constitutes communicating hydrocephalus. This usually causes dilation of the lateral, third, and fourth ventricles equally. The most common causes in adults are meningitis and subarachnoid hemorrhage. Hydrocephalus may be transient after subarachnoid hemorrhage,
with reestablishment of normal CSF absorption after the protein content of the CSF returns to normal and the granulations reopen.
Obstruction of CSF pathways is known as obstructive hydrocephalus. Ventricles proximal to the obstruction dilate, while those distal to the obstruction remain normal in size. Typical patterns include dilation of the lateral ventricles due to a colloid cyst occluding the foramen of Monro, dilation of the lateral and third ventricles due to a tectal (midbrain) glioma or pineal region tumor occluding the cerebral aqueduct, or dilation of the lateral and third ventricles with obliteration of the fourth ventricle by an intraventricular tumor of the fourth ventricle. Obstructive hydrocephalus may present precipitously and require urgent shunting to prevent herniation
Chiari I Malformation
Chiari I malformation is the caudal displacement of the cerebellar tonsils below the foramen magnum, and may be seen as an incidental finding on MRl scans in asymptomatic patients. Symptomatic patients usually present with headache, neck pain, or symptoms of myelopathy, including numbness or weakness in the extremities. The brain stem and lower cranial nerves are normal in Chiari 1 malformations. Chiari II malformations are more severe and involve caudal displacement of the lower brain stem and stretching of the lower cranial nerves. Symptomatic patients may be treated with sub occipital craniectomy to remove the posterior arch of the foramen magnum, along with removal of the posterior ring of C I. Removal of these bony structures relieves the compression of the cerebellar tonsils and cervicomedullary junction and may allow reestablishment of normal CSF flow patterns