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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 bulging fontanelles in infants. Diagnosis is by ultrasonography
in neonates and by CT or MRI in older infants. Treatment usually
is with a ventricular shunt procedure.
Hydrocephalus is the most common cause of abnormally large heads in neonates. It results from either obstruction of CSF flow (obstructive hydrocephalus) or impaired resorption (communicating hydrocephalus). Obstruction most often occurs in the aqueduct of Sylvius but sometimes at the outlets of the 4th ventricle (Luschka and Magendie foramina). 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).
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.
Symptoms and Signs
Neurologic findings depend on whether intracranial pressure is increased, symptoms of which include headache (or irritability in infants), high-pitched cry, vomiting, lethargy, strabismus or decreased vision, and bulging fontanelle (in infants). Papilledema is a late sign of increased intracranial pressure; initial absence is not reassuring. Consequences of chronically increased pressure may include precocious puberty in girls, learning disorders (eg, difficulties with attention, information processing, 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 prenatal ultrasound. 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 or Canavan disease), or it may be benign, in which excessive CSF surrounds a normal brain. Patients suspected of having hydrocephalus require cranial imaging by CT, MRI, or ultrasonography. Cranial CT or ultrasonography (if the anterior fontanelle is open) is used to follow 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). Progressive hydrocephalus usually requires a ventricular shunt. To temporarily reduce CSF pressure in infants, ventricular taps or serial lumbar punctures (if the hydrocephalus is communicating) may be used.
The type of ventricular shunt used depends on the neurosurgeon's experience, although ventriculoperitoneal shunts produce fewer complications than ventriculoatrial shunts. Any shunt has a risk of infection. After the shunt is placed, head circumference and development are assessed, and imaging is performed periodically. Shunt obstruction can be a medical emergency; children present with symptoms and signs of suddenly increased intracranial pressure such as 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.
Although some children cease to need the shunt as they age, determining the appropriate time for removal (which may cause bleeding and trauma) is difficult. Thus, shunts are rarely removed. Fetal surgery to treat congenital hydrocephalus has not been successful.
Other Brain
Anomalies
Anencephaly:
Anencephaly 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:
Encephalocele 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.
Prognosis, which depends on the location and size of the lesion, is usually 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 mental retardation, and epilepsy often occur with these defects. Treatment is supportive, including anticonvulsants, if needed, to control seizures.
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 mental retardation, 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 arise from injuries occurring between 17 wk and 26 wk gestation. It can cause mental retardation and seizures. Treatment is supportive.
Porencephaly:
Porencephaly is a cyst or cavity in a cerebral hemisphere that communicates with a ventricle. It may develop pre- or postnatally. The defect 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 patients 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 ultrasound confirms the diagnosis. 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 be the result of brain injury, schizencephaly is thought to represent a defect in neuronal migration and is thus a true malformation. Treatment is supportive.
Septo-optic
dysplasia:
Septo-optic dysplasia (de Morsier syndrome) is a malformation of the front of the brain that occurs toward the end of the 1st mo 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 patients 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, mental retardation, cerebral palsy, or other developmental delay. Diagnosis is by MRI. All children diagnosed with this disorder should be screened for endocrine and developmental dysfunction. Treatment is supportive.
Last full review/revision November 2005
Content last modified November 2005
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