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Congenital brain anomalies usually cause severe neurologic deficits; some may be fatal.
Hydrocephalus
Hydrocephalus is ventricular enlargement with excessive CSF. Manifestations include an enlarged head and brain atrophy. Increased cranial pressure causes irritability and a bulging fontanelle in infants. Diagnosis is by ultrasonography in neonates and young infants with an open fontanelle and by CT or MRI in older infants and children. Treatment usually is with a ventricular shunt procedure.
Hydrocephalus is the most common cause of abnormally large heads in neonates. Hydrocephalus that develops only after the fontanelles have closed does not increase head circumference but can markedly increase intracranial pressure.
Etiology
Hydrocephalus can result from
Obstruction most often occurs in the aqueduct of Sylvius but sometimes at the outlets of the 4th ventricle (Luschka and Magendie foramina). Obstructive hydrocephalus can be caused by Dandy-Walker malformation or Chiari II type (formerly Arnold-Chiari) malformation. Dandy-Walker malformation is progressive cystic enlargement of the 4th ventricle. In Chiari II type malformation, which frequently occurs with spina bifida (see Congenital Neurologic Anomalies: Spina Bifida) and syringomyelia (see Spinal Cord Disorders: Syrinx), severe elongation of the cerebellar tonsils causes them to protrude through the foramen magnum, with beaking of the colliculi and thickening of the upper cervical spinal cord.
Impaired resorption in the subarachnoid spaces usually results from meningeal inflammation, secondary either to infection or to blood in the subarachnoid space (eg, in the premature infant who has intraventricular hemorrhage).
Symptoms and Signs
Neurologic findings depend on whether intracranial pressure is increased, symptoms of which in infants include irritability, high-pitched cry, vomiting, lethargy, strabismus, and bulging fontanelle. Older, verbal children may complain of headache, decreased vision, or both. Papilledema is a late sign of increased intracranial pressure; initial absence is not reassuring.
Consequences of chronic hydrocephalus may include precocious puberty in girls, learning disorders (eg, difficulties with attention, information processing, and memory), and impaired executive function (eg, problems with conceptualizing, abstracting, generalizing, reasoning, and organizing and planning information for problem-solving).
Diagnosis
Diagnosis is often made by routine prenatal ultrasonography. After birth, diagnosis is suspected if routine examination reveals an increased head circumference; infants may have a bulging fontanelle or widely separated cranial sutures. Similar findings can result from intracranial, space-occupying lesions (eg, subdural hematomas, porencephalic cysts, tumors). Macrocephaly may result from an underlying brain problem (eg, Alexander disease or Canavan disease) or it may be benign, such as when excessive CSF surrounds a normal brain. Children suspected of having hydrocephalus require cranial imaging by CT, MRI, or ultrasonography (if the anterior fontanelle is open). Cranial CT or ultrasonography is used to monitor progression of hydrocephalus once an anatomic diagnosis has been made. If seizures occur, an EEG may be helpful.
Treatment
Treatment depends on etiology, severity, and whether hydrocephalus is progressive (ie, size of the ventricles increases over time relative to the size of the brain). To temporarily reduce CSF pressure in infants, ventricular taps or serial lumbar punctures (if the hydrocephalus is communicating) may be used. Progressive hydrocephalus usually requires a ventricular shunt. Shunts typically connect the right lateral ventricle to the peritoneal cavity or, rarely, to the right atrium via a plastic tube with a one-way, pressure-relief valve. When a shunt is first placed in an infant or older child whose fontanelle is closed, rapid withdrawal of fluid can cause subdural bleeding as the brain shrinks away from the skull. When the fontanelles are open, the skull can decrease in circumference to match the decrease in brain size; thus, some clinicians recommend an early decision regarding shunt placement so that it can be done before fontanelle closure.
In a third ventriculostomy, an opening is created endoscopically between the 3rd ventricle and the subarachnoid space, allowing CSF to drain. This procedure is often combined with ablation of the choroid plexus and is becoming more common, particularly in less developed countries where access to emergency neurosurgical care is often limited. In certain cases (eg, hydrocephalus caused by primary aqueductal stenosis), third ventriculostomy may be adequate primary treatment.
A ventricular shunt that goes to the subgaleal space may be used in infants as a temporary measure for patients who may not require a more permanent shunt.
Although some children do not need the shunt as they age, shunts are rarely removed because of the risk of bleeding and trauma. Fetal surgery to treat congenital hydrocephalus has not been successful.
Shunt complications
The type of ventricular shunt used depends on the neurosurgeon's experience, although ventriculoperitoneal shunts cause fewer complications than ventriculoatrial shunts. Shunt complications include
Any shunt has a risk of infection. Manifestations include chronic fever, lethargy, irritability, headache, or a combination and other symptoms and signs of increased intracranial pressure; sometimes redness becomes apparent over the shunt tubing. Antibiotics effective against the organism infecting the shunt, which may include skin flora, are given, and typically the shunt must be removed and replaced.
Shunts can malfunction due to mechanical obstruction (typically blockage at the ventricular end) or to fracture of the tubing. In either case, intracranial pressure can increase, which, if sudden, can be a medical emergency. Children present with headache, vomiting, lethargy, irritability, esotropia, or paralysis of upward gaze. Seizures may occur. If the obstruction is gradual, more subtle symptoms and signs can occur, such as irritability, poor school performance, and lethargy, which may be mistaken for depression. To assess shunt function, a shunt series (x-rays of the shunt tubing) and neuroimaging studies are done. The ability to compress the bulb that is present on many shunt systems is not a reliable sign of shunt function.
After the shunt is placed, head circumference and development are assessed, and imaging is done periodically.
Other Brain Anomalies
Anencephaly
This anomaly is absence of the cerebral hemispheres. The absent brain is sometimes replaced by malformed cystic neural tissue, which may be exposed or covered with skin. Parts of the brain stem and spinal cord may be missing or malformed. Infants are stillborn or die within days or weeks. Treatment is supportive.
Encephalocele
This anomaly is a protrusion of nervous tissue and meninges through a skull defect. The defect is caused by incomplete closure of the cranial vault (cranium bifidum). Encephaloceles usually occur in the midline and protrude anywhere along a line from the occiput to the nasal passages but can be present asymmetrically in the frontal or parietal regions. Small encephaloceles may resemble cephalhematomas, but x-rays show a bony skull defect at their base. Hydrocephalus (see Congenital Neurologic Anomalies: Hydrocephalus) often occurs with encephalocele. About 50% of affected infants have other congenital anomalies. Symptoms and signs include the visible defect, seizures, and impaired cognition, including intellectual and developmental disability.
Prognosis, which depends on the location and size of the lesion, is often good. Most encephaloceles can be repaired. Even large ones often contain mostly heterotopic nervous tissue, which can be removed without worsening functional ability. When other serious malformations coexist, the decision to repair may be more difficult.
Malformed cerebral hemispheres
Cerebral hemispheres may be large, small, or asymmetric; the gyri may be absent, unusually large, or multiple and small; and microscopic sections of normal-appearing brain may show disorganization of the normal laminar neuronal arrangement. Microcephaly, moderate to severe motor and intellectual disability, and epilepsy often occur with these defects. Treatment is supportive, including anticonvulsants, if needed.
Holoprosencephaly occurs when the embryonic prosencephalon does not undergo segmentation and cleavage. The anterior midline brain, cranium, and face are abnormal. This malformation may be caused by defects of the sonic hedgehog gene. Severely affected fetuses may die before birth. Treatment is supportive.
Lissencephaly consists of an abnormally thick cortex, reduced or abnormal lamination, and diffuse neuronal heterotopia. It is caused by abnormal neuronal migration, the process by which immature neurons attach to radial glia and move from their points of origin near the ventricle to the cerebral surface. Several single-gene defects may cause this anomaly (eg, LIS1). Affected infants may have intellectual disability, muscle spasms, and seizures. Treatment is supportive, but many children die before age 2 yr.
Polymicrogyria, in which the gyri are small and overabundant, is believed to result from injuries occurring between 17 wk and 26 wk gestation. It can cause intellectual disability and seizures. Treatment is supportive.
Porencephaly
This anomaly is a cyst or cavity in a cerebral hemisphere that communicates with a ventricle. It may develop prenatally or postnatally. Porencephaly may be caused by a developmental anomaly, inflammatory disease, or a vascular accident such as intraventricular hemorrhage with parenchymal extension. Neurologic examination is usually abnormal. Diagnosis is confirmed by cranial CT, MRI, or ultrasonography. Progressive hydrocephalus occurs rarely with porencephaly. Prognosis is variable; a few children develop only minor neurologic signs and have normal intelligence. Treatment is supportive.
Hydranencephaly is an extreme form of porencephaly in which the cerebral hemispheres are almost totally absent. Usually, the cerebellum and brain stem are formed normally, and the basal ganglia are intact. The meninges, bones, and skin over the cranial vault are normal. Often hydranencephaly is diagnosed by prenatal ultrasonography. Neurologic examination is usually abnormal, and the infant does not develop normally. Externally, the head may appear normal, but when transilluminated, light shines completely through. CT or ultrasonography confirms the diagnosis. Children with this condition often have seizures and intellectual disability. Treatment is supportive, with shunting if head growth is excessive.
Schizencephaly, which many classify as a form of porencephaly, results from formation of abnormal slits, or clefts, in the cerebral hemispheres. Unlike porencephalies, however, which are thought to result from brain injury, schizencephaly is thought to represent a defect in neuronal migration and is thus a true malformation. Affected infants often have seizures and developmental delay and, depending on the location of the defect, may have focal neurologic findings such as weakness. Treatment is supportive.
Septo-optic dysplasia
This anomaly (also called de Morsier's syndrome) is a malformation of the front of the brain that occurs toward the end of the first month of gestation and includes optic nerve hypoplasia, absence of the septum pellucidum (the membranes that separate the front of the 2 lateral ventricles), and pituitary deficiencies. Although the cause may be multiple, abnormalities of one particular gene (HESX1) have been found in some children with septo-optic dysplasia.
Symptoms may include decreased visual acuity in one or both eyes, nystagmus, strabismus, and endocrine dysfunction (including growth hormone deficiency, hypothyroidism, adrenal insufficiency, diabetes insipidus, and hypogonadism). Seizures may occur. Although some children have normal intelligence, others have learning disabilities, intellectual disability, cerebral palsy, or other developmental delay. Diagnosis is by MRI. All children diagnosed with this anomaly should be screened for endocrine and developmental dysfunction. Treatment is supportive.
Last full review/revision September 2009 by Gregory S. Liptak, MD, MPH
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