Introduction
Background
Encephalocele represents one end of the spectrum of open neural tube diagnoses.1 With ultrasonography (US) scanning, the diagnosis is based on the herniation of a spherical, fluid-filled structure, more correctly diagnosed as a meningocele or brain parenchyma (encephalocele) beyond the calvarial confines.2,3,4,5,6,7,8 The herniation occurs through a calvarial defect. The earliest reported ultrasonographic diagnosis was made at 13 weeks' gestation. Once an encephalocele is diagnosed, a thorough search for associated abnormalities should be performed.9,10
An encephalocele results from failure of the surface ectoderm to separate from the neuroectoderm. This leads to a bony defect in the skull table, which allows herniation of the meninges (cranial meningocele) or of brain tissue. The occiput is the most common site of this type of neural tube defect (75%) in the United States and Western Europe. Approximately 90% of cases involve the midline.
Other malformations and/or chromosomal anomalies are noted in at least 60% of patients with encephalocele.9,10,11 Currently, most cases are diagnosed prenatally.2,3,4,5,6,7,8,12,13 Maternal serum alpha-fetoprotein levels are elevated in only 3% of patients, because most encephaloceles are covered with skin. Postnatally, infants may have associated cerebrospinal fluid (CSF) rhinorrhea and recurrent meningitis.14
See also the following related eMedicine topics:
Neural Tube Defects
Neural Tube Defects in the Neonatal Period
See also the following related Medscape topic:
Resource Center Spinal Disorders
Pathophysiology
The primary abnormality in the development of an encephalocele is a mesodermal defect resulting in a defect in the calvarium and dura that is associated with herniation of CSF, brain tissue, and meninges through the defect. The root cause of an encephalocele is the failure of surface ectoderm to separate from the neuroectoderm early in embryonic development. In the calvarium, induction of bone formation may be defective, or pressure erosion from an intracranial mass may occur. Defects at the skull base may be related to faulty closure of the neural tube or to failure of basilar ossification. Encephaloceles may be occipital (75%), fronto-ethmoidal (13-15%), parietal (10-12%), or sphenoidal. Fronto-ethmoidal encephaloceles are most common in Asia.15,16
In terms of patient survival, the absence of brain tissue in the herniated sac is the single most favorable prognostic feature.17 Brain tissue in the herniated sac is usually apparent. However, it may be difficult to confidently exclude incorporated brain tissue in sacs that appear to be filled with CSF alone. Diagnosis is thought to be impossible before skull ossification, which starts at 10 weeks' gestation.
The earliest reported diagnosis was made at 13 weeks' gestation. The ultrasonographic appearance of an encephalocele is variable in the first trimester. Once an encephalocele is diagnosed, a search for associated anomalies, including intracranial and extracranial abnormalities (60-80%), should be performed.9,10,15 The risk of chromosomal abnormalities is 13-44%; therefore, karyotyping should be offered to the mother.11
Associated anomalies include findings associated with various genetic syndromes, such as Meckel-Gruber, von Voss, Chemke, Roberts, and Knobloch syndromes.9,10 In addition, findings may be related to nongenetic anomalies, such as cryptophthalmos syndrome, the presence of an amniotic band, warfarin use, maternal rubella, and diabetes. Other associated brain abnormalities that may occur in isolation or as part of genetic or nongenetic syndromes include spina bifida, agenesis of the corpus callosum, Arnold-Chiari II malformation, Dandy-Walker malformation, and brain migrational anomalies. The most common associated chromosomal anomaly is trisomy 18.
Occipital encephalocele is the most common form of encephalocele in the Western Hemisphere (71% in the United States). It is often associated with Dandy-Walker malformation and Arnold-Chiari II malformation. An occipital encephalocele may be high, above the foramen magnum, or it may involve the upper cervical spine and occipital bone. (The Chiari III malformation is a cervico-occipital encephalocele that contains most of the cerebellum.)
The frontoparietal encephalocele is the most common type of encephalocele in Southeast Asia. It is associated with midline craniofacial dysraphism.9,10,18 Sphenoidal encephaloceles are often clinically occult and usually become apparent at the end of the first decade of life.
See also the following related eMedicine topics:
Chiari I Malformation
Chiari II Malformation
Dandy-Walker Malformation
Meckel-Gruber Syndrome
Spinal Dysraphism/Myelomeningocele
Frequency
United States
There are 1-4 cases of encephalocele per 10,000 live births. In fetuses that have been spontaneously aborted before 20 weeks' gestational age, it is the predominant neural axis anomaly.5,15,19
International
The worldwide incidence of encephalocele is not known.20
Mortality/Morbidity
The development of an encephalocele reduces the chance of live birth to 21%, and only half of these live births survive. Approximately 75% of survivors have a mental deficit. Encephalocele recurs in 3% of patients after surgical repair, whereas the recurrence rate is higher in Meckel-Gruber syndrome (25%). The absence of brain tissue in the herniated sac is the single most favorable prognostic feature for survival.15,17
The prognosis for and treatment of a patient with an encephalocele depend on the site, size, and contents of the encephalocele.21 The ultimate outcome depends on the patient's karyotype and on his/her associated syndromes, as well as on the ease of surgical correction. A good prognosis is indicated for a patient who has an anterior encephalocele containing no brain tissue and who has no associated anomalies. Poor prognostic indicators include a large or posterior encephalocele and systemic anomalies. Patients with an anterior encephalocele have a 100% survival rate, but the survival rate decreases to 55% in persons with a posterior encephalocele. A nasal meningo-encephalocele is a rare cause of recurrent meningitis.
A prenatal diagnosis of an encephalocele should prompt a thorough search for other abnormalities.9,10 An accurate diagnosis is critical in determining the prognosis and in providing appropriate genetic counseling.
Race
Encephaloceles have a multifactorial etiology, and genetic and geographic factors have been implicated. Frontal encephaloceles are far more common in the Far East, particularly in the Chinese population, and are associated with a more favorable prognosis.
Sex
Encephaloceles occur more commonly in females than in males.
Age
Currently, most encephaloceles are diagnosed prenatally and present at birth.2,3,4,5,6,7,8,12,13 Some, particularly sphenoidal encephaloceles, may become apparent later in childhood.
Anatomy
An encephalocele results from a defective closure of the embryologic neural tube. The defect is believed to occur because of a failed closure of the rostral end of the neural tube during the fourth week of gestation. The defect causes an abnormality of the skull and underlying meninges. A spectrum of anomalies is seen, with the most mild being cranium bifidum occultum; this is analogous to spina bifida occulta.
A dermal defect is frequently present. An example is a dermal sinus leading from the cranial defect, which may also be associated with an intracranial dermoid cyst. Herniation of the meninges through the cranial defect is called a meningocele, whereas herniation of brain tissue and meninges through the bony defect is called an encephalocele.
Presentation
Most encephaloceles are diagnosed through routine prenatal US scanning.2,3,4,5,6,7,8 Maternal serum alpha-fetoprotein levels are elevated in only 3% of patients, because most encephaloceles are covered with skin. Postnatally, infants may present with CSF rhinorrhea and recurrent meningitis. Postnatal presentation also depends on the associated malformations and the size and contents of the defect. As previously stated, approximately 75% of survivors have a mental deficit.
Preferred Examination
US scanning remains the mainstay of fetal imaging; there is evidence, however, that fetal magnetic resonance imaging (MRI) may provide superior detail of central nervous system (CNS) anomalies.
Differential Diagnoses
Branchial Cleft Cysts
Cystic Hygroma
Hemangioma
Myelomeningocele
Other Problems to Be Considered
Normal fetal hair
Scalp edema
Iniencephaly
Epidermal scalp cyst22
Cloverleaf skull
Iniencephaly
Teratoma (In a newborn, a mature cystic teratoma in the pineal region may mimic a parietal encephalocele.)23
In addition, a pituitary adenoma in association with a sphenoidal meningoencephalocele, although extremely rare, should be considered whenever a sellar tumor with cystic extension is encountered.
Of these, encephalocele is the only condition associated with a calvarial defect. In most encephaloceles, other associated intracranial anomalies are present.
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Further Reading
Keywords
open neural tube, meningocele, brain parenchyma
