Version May 2024
INTRODUCTION — An encephalocele is a protrusion of the brain and/or meninges through a defect in the skull (cranium bifidum) that is "closed" or covered with skin. Encephalocele is one of the three most common neural tube defects (NTDs). The epidemiology, clinical features, diagnosis, and management of encephaloceles are discussed in this topic review.
The other main types of NTDs are anencephaly and myelomeningocele, which are discussed in separate topic reviews:
●Anencephaly, which is an open NTD as the affected region of the cranial neural tube is exposed to the body surface, is a severe defect and is not compatible with survival. (See "Anencephaly".)
●Myelomeningocele, which is characterized by a cleft in the vertebral column, with a corresponding defect in the skin so that the meninges and spinal cord are exposed. (See "Myelomeningocele (spina bifida): Anatomy, clinical manifestations, and complications" and "Myelomeningocele (spina bifida): Management and outcome".)
CLASSIFICATION — In this topic review we will use the term "encephalocele" to describe lesions that include brain and/or meninges. Some authors use the more general term "cephalocele" and reserve the term "encephalocele" for lesions that include brain and "meningocele" for those that include only meninges [1].
Classification of encephalocele is based on the cause and anatomical location of the skull defect.
Primary encephalocele — Primary encephaloceles are congenital and present at birth. Primary encephaloceles are divided into four major types:
●Sincipital (fronto-ethmoidal, nasofrontal, nasoethmoidal, and naso-orbital)
●Basal (trans-sphenoidal (image 1), spheno-ethmoidal, trans-ethmoidal (image 2), and spheno-orbital)
●Occipital (picture 1 and image 3)
●Parietal
Secondary encephalocele — Secondary encephaloceles are acquired and commonly due to trauma or a postsurgical defect [2-5].
EMBRYOLOGY — An encephalocele is a type of malformation in which there are calvarial and dural defects with extracranial herniation of leptomeninges and sometimes of brain and cerebrospinal fluid.
The central nervous system appears as a plate of thickened ectoderm called the neural plate at the beginning of the third week of embryonic life. The lateral edges of the neural plate become elevated to form the neural folds and fuse to form the neural tube; the fusion begins in the cervical region and proceeds in both the rostral and caudal directions (figure 1). The rostral neuropore closes on the 25th day after conception, and the caudal neuropore closes two days later [6]. Neural tube defects result from failure of the neural tube closure between 25 and 27 days after conception. The embryology of the neural tube is discussed in a separate topic review. (See "Closed spinal dysraphism: Pathogenesis and types", section on 'Normal cord development in humans'.)
The mechanism causing primary (congenital) encephalocele is uncertain, although it involves defective closure of the anterior neural tube. Onset of the most severe lesions likely occurs prior to 26 days after conception; less severe lesions that primarily involve the skull or meninges may occur later [7]. Nasofrontal lesions are thought to result from defective separation of neural and surface ectoderm at the site of final closure of the rostral neuropore [8].
Anterior encephaloceles (sincipital and basal) probably result from defective development of the prosencephalic neural crest tissue. This mechanism is suggested by the patency of the midline foramina, which are open only transiently during normal craniofacial development [9].
By contrast, the embryology of occipital encephaloceles suggests defective segmentation of the bones of the posterior cranium (supraoccipital, exoccipital, and basioccipital, and the posterior part of the parietal bones). These bones are not of neural crest origin [10].
INCIDENCE — Encephalocele is less common than other neural tube defects. Its prevalence has been estimated at 0.8 to 5 per 10,000 live births. In a report from the Metropolitan Atlanta Congenital Defects Program, 115 cases were identified from 701,458 live births during 1979 to 1998, a prevalence of 1.64 per 10,000 live births [11]. These cases included 91 liveborn infants, 18 stillbirths, and 6 elective terminations (likely an underestimate). Males and females were equally affected.
Occipital encephaloceles are the most frequent type in North America and Western Europe, where approximately 85 percent of encephaloceles take this form [12]. Their incidence ranges between 1 in 3000 to 1 in 10,000 live births. On the other hand, in Southeast Asia, parts of Russia, and central Africa, anterior encephaloceles are more frequent (1 in 3500 to 1 in 5000) than the occipital type. The true incidence of secondary encephaloceles is not known.
INHERITANCE — Isolated encephaloceles (ie, those that are not associated with other congenital anomalies) have generally not been shown to be familial. One exception is an autosomal dominant form of occipital encephalocele with incomplete penetrance that was reported in a Vietnamese family [13]. Most of the affected individuals in this family were developmentally normal and had normal neurologic examinations except for skin-covered occipital bulges. Therefore, there was no evidence for any of the known genetic syndromes associated with encephaloceles in this family.
Encephaloceles may be part of specific genetic syndromes if there are associated anomalies. The inheritance pattern for these syndromes is usually autosomal recessive, in which case the risk of recurrence in subsequent pregnancies is substantial. As an example, the risk of recurrence for an encephalocele with Meckel-Gruber syndrome is 25 percent. (See 'Associated anomalies' below.)
CLINICAL FEATURES — The clinical features of encephalocele are variable depending upon the type (location) and severity:
●Sincipital (fronto-ethmoidal) encephaloceles may be occult lesions that are not noticeable or may present with marked craniofacial deformities (hypertelorism, telecanthus, orbital dystopia, or unilateral micro/anophthalmos).
●Basal encephaloceles may or may not be apparent on external inspection, but there may be a broadened nasal bridge, hypertelorism, or other midfacial anomalies. Affected patients may present as a nasal or epipharyngeal mass, difficulty breathing, recurrent upper tract infections, nasal discharges, recurrent meningitis, or cerebrospinal fluid leaks [12].
●Occipital encephaloceles usually are obvious at the time of birth and many are diagnosed prenatally by ultrasonography. The neural tissue usually is covered by skin (picture 1). Those of relatively large size may be associated with cranial nerve deficits, poor sucking and feeding, spasticity, blindness, seizures, or developmental delay. Neurologic deficits may progress after birth if hydrocephalus develops. Occipital encephalocele also may be associated with hind-brain anomaly (Chiari III malformation) in which herniating occipital/cerebellar tissues distort the posterior fossa structures (image 3). (See "Chiari malformations".)
●Parietal encephaloceles range in size from atretic to large. Large vertex encephaloceles consist of herniating brain and meninges through a large cranium defect. Giant encephalocele is the term used when the head size or occipitofrontal circumference is smaller than the herniating sac. Atretic encephaloceles are small, abortive, rudimentary skin-covered encephaloceles, consisting of meningeal and vestigial neural/glial tissues herniating through a small cranium defect.
The protruding tissue of an encephalocele may consist of normal brain or fibrous atrophic gliotic tissue, which has little or no function. In a nasofrontal encephalocele, most of the herniating mass consists of nonfunctional gliotic neural tissue [14,15]. On the other hand, occipital encephaloceles may include a variety of tissues. In a series of occipital encephaloceles, 32 percent contained cerebral tissue, 21 percent included cerebral and cerebellar tissue, 5 percent had cerebellar tissue, and 37 percent had glial nodules or dysplastic neural tissue [16].
ASSOCIATED ANOMALIES — Infants with encephalocele frequently have other malformations that may be part of recognized syndromes [11,17,18]. In a series from Atlanta, United States, at least one other major structural congenital defect was present in 17 of 83 (20 percent) of affected liveborn infants with encephalocele [11]. This is a smaller proportion than in other series due to exclusion of infants with chromosomal abnormalities, other major central nervous system lesions, and amniotic bands.
The most common of the associated syndromes is Meckel-Gruber syndrome (Meckel syndrome, MIM #249000), which includes occipital encephalocele, microcephaly, microphthalmia, polycystic kidneys, ambiguous genitalia, polydactyly, cleft lip and palate, and other malformations [19]. (See "Kidney cystic diseases in children" and "Clinical manifestations, diagnosis, and treatment of nephronophthisis", section on 'Meckel-Gruber syndrome'.)
Other cerebral malformations are often associated with encephalocele. These include deformities of the tentorium, complete or partial agenesis of the corpus callosum, and myelomeningocele. Individuals with occipital encephaloceles frequently have hydrocephalus (30 to 50 percent), corpus callosal abnormalities (18 percent), and cerebral dysgenesis (13 percent) [19,20].
DIAGNOSIS — The diagnosis is apparent at birth in most of the occipital encephaloceles. Basal encephaloceles may present as a midline mass in the nose or may not be visible. An encephalocele may be mistaken for a nasal polyp if it is located within the nose or for a soft tissue tumor if it is covered with skin and anterior to the nose. These basal encephaloceles, either ethmoidal or sphenoidal, tend to present with meningitis.
Imaging — Neuroimaging should be performed to evaluate the intracranial components of the malformation and identify any associated brain or vascular anomalies. Computed tomography scans are effective for detection of the extent of cranial defects. Magnetic resonance imaging (MRI) is effective for detection of the extent of neural herniation. It is especially useful for basal encephaloceles. Neuroimaging will also detect hydrocephalus, if present.
Prenatal diagnosis — Prenatal diagnosis is accomplished by maternal screening of serum alpha fetoprotein levels and ultrasonography. (See "Neural tube defects: Overview of prenatal screening, evaluation, and pregnancy management", section on 'Prenatal screening and diagnosis'.)
With ultrasonography, encephaloceles appear as a defect in the calvarium containing a cystic or solid mass with a gyral pattern that is contiguous with the brain [21]. Prenatal ultrasonography detects approximately 80 percent of encephaloceles. Fetal MRI has higher sensitivity for neural tube defects and can be used in cases where ultrasound results are not optimal or the diagnosis is uncertain, or to evaluate high-risk cases. (See "Neural tube defects: Overview of prenatal screening, evaluation, and pregnancy management", section on 'Follow-up of screen-positive MSAFP results'.)
Differential diagnosis — Lesions which can appear similar to an encephalocele include neoplasms, congenital cysts, vascular malformations, and inflammatory lesions (table 1) [1]. The considerations vary depending upon the location of the encephalocele. In general, encephaloceles are more likely to vary with respiration or crying. Imaging usually distinguishes the encephalocele by demonstrating the skull defect and communication with the intracranial space.
MANAGEMENT — When diagnosed prenatally, vaginal delivery may be safe if the lesion is relatively small. Large encephaloceles require cesarean section.
Surgical treatment is appropriate in most cases unless the encephalocele is massive and there is severe microcephaly or other lethal anomalies. The procedure basically consists of removing the overlying sac and closing the defect including the dural defect [22]. In patients with basal encephaloceles or cerebrospinal fluid (CSF) leakage, prompt closure is important to reduce the risk of infection. Patients with hydrocephalus usually undergo ventriculo-peritoneal shunt placement prior to encephalocele repair to prevent postoperative CSF leaks.
Sincipital encephaloceles — Unless there is CSF leakage, sincipital encephaloceles can be treated electively, ideally early in infancy to avoid deleterious effect of an enlarging encephalocele on craniofacial structures. Complete surgical repair includes [12]:
●Resection of the herniated mass to be flush with the floor of the anterior cranial base
●Repair of the dural and cranium defect
●Correction of hypertelorism, when appropriate, with:
•Reconstruction of the nasal elements
•Alignment of the horizontal ocular axis
•Cannulation of obstructed nasolacrimal ducts
Basal encephaloceles — Basal encephaloceles require prompt surgical repair because of high prevalence of meningitis. Ideally, a craniofacial team usually consisting of neurosurgeons and plastic surgeons perform the repair. Reconstruction of the skull and dural defect is important to prevent meningitis. In the case of sphenoidal encephalocele, particular caution must be used during the repair because the third ventricle, hypothalamus, pituitary gland, and optic pathway may be present in the encephalocele sac. The approach route of the repair may be transcranial, transpalatal, endonasal, or a combination of these [23,24].
Occipital encephaloceles — Surgical management of patients with occipital encephalocele depends upon the type of neural tissue that is protruding beyond the skull, as assessed by inspection during surgery. If the herniating mass consists of gliotic tissue, the mass is transected flush with the skull.
If the mass contains normal brain, attempts should be made to preserve it. Several techniques have been described:
●Expansion cranioplasties have been used to accommodate large amounts of herniated neural tissues [25]. One technique of expansion cranioplasty uses tantalum mesh to create an extracranial space for herniating tissues.
●Ventricular volume reduction is a two-stage technique in which the ventricles are artificially expanded, followed by intracranial transposition of the neural tissue. In the first stage, the dural sac is closed; this increases ventricular pressure to produce ventriculomegaly. After hydrocephalus develops, a ventriculoperitoneal shunt is placed, and then herniated brain is transpositioned into the new intracranial space as the ventricle contracts [26].
●To preserve herniated occipital and cerebellar parenchyma, the tentorium is incised, creating infratentorial space, and the cerebral cortex retracted into this space [27].
Parietal encephalocele — An atretic parietal encephalocele does not have viable neural structures and can be amputated at the small cranium defect. In some cases, however, there is a fenestrated sagittal sinus, which should not be disrupted.
A large parietal encephalocele may require a cranial expansion for repair if there is substantial normal brain within the encephalocele. However, a giant encephalocele with associated microcephaly has limited cerebral function, and an amputation of the herniated brain is justified for nursing purposes.
OUTCOME — For sincipital encephaloceles, craniofacial operations can usually be performed without intraoperative mortality or major complications. Most patients with nasofrontal encephaloceles have normal or near-normal intelligence and ultimately do well following the repair [12].
The prognosis for patients with occipital encephalocele depends upon the extent of herniated neural tissue in the sac and on the presence of associated anomalies. In one series, 17 percent of all patients with occipital encephaloceles had normal mental and physical development, and 83 percent were mentally and/or physically impaired [28]. The subset with few anomalies and a small amount of neural herniation had a 53 percent chance of being physically and mentally normal, 28 percent chance of normal intelligence but physical impairment, and 19 percent chance of intellectual disability. In a separate series of 30 patients with occipital encephalocele, the mortality rate was 29 percent [29]. Factors related to prognosis include the size of the sac, the contents of the neural tissue, hydrocephalus, infection, and pathologies that accompany the condition.
One series from a single center suggests that hydrocephalus and other intracranial abnormalities predict neurodevelopmental outcome, but the location (type) of the cephalocele does not [20]. In a retrospective analysis of 85 cases of cephaloceles (68 encephaloceles and 17 meningoceles), 40 were frontal, 33 occipital, and 12 parietal. The most common associated intracranial abnormalities included hydrocephalus (27 percent), seizure disorder (20 percent), corpus callosal abnormalities (18 percent), and cerebral dysgenesis (13 percent). Cognitive development was normal in 48 percent of patients and mildly, moderately, or severely delayed in 11, 16, and 25 percent of patients, respectively. On multivariate analysis, only hydrocephalus and the presence of intracranial abnormalities were significant predictors of developmental delay.
Seizure disorders are present in approximately 20 percent of patients with congenital encephalocele [20]. Rates may vary if patients with occult lesions are included; such patients may present with seizures, and small occult encephaloceles are discovered as a secondary finding [30]. In a literature review of 18 cases of encephaloceles with seizure, 6 (33 percent) were extra-temporal and 12 (67 percent) were temporal [31]. For the purpose of seizure control, all extra-temporal encephaloceles were managed with resection of the encephalocele. Among patients with temporal lobe encephaloceles, eight (67 percent) were treated with encephalocele resection and extended lobectomy, and four (33 percent) were treated only with resection of the encephalocele. The prognosis after surgery was generally good, and 17 patients were seizure-free postoperatively. A subsequent retrospective study found that temporal encephaloceles were present on MRI in approximately 10 percent of patients with temporal lobe epilepsy; smaller temporal encephaloceles (diameter <6.25 mm) were more likely to be asymptomatic [32].
SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Congenital malformations of the central nervous system".)
SUMMARY AND RECOMMENDATIONS
●An encephalocele is a type of malformation in which there are calvarial and dural defects with extracranial herniation of leptomeninges and sometimes brain and cerebrospinal fluid (CSF). In populations where prenatal ultrasonography is routinely performed, approximately 80 percent of encephaloceles are detected prenatally. (See 'Embryology' above and 'Prenatal diagnosis' above.)
●Isolated encephaloceles (ie, those that are not associated with other congenital anomalies) generally are not familial. Those with associated anomalies may be part of specific genetic syndromes such as Meckel-Gruber syndrome, in which case the recurrence risk may be substantial. (See 'Inheritance' above and 'Associated anomalies' above.)
●Primary encephaloceles are divided into sincipital, basal, and occipital types; the management and the prognosis vary among these types. (See 'Classification' above.)
●Sincipital encephaloceles range from occult lesions that are not noticeable to those with marked craniofacial deformities (hypertelorism, telecanthus, orbital dystopia, or unilateral micro/anophthalmos). Unless there is CSF leakage, sincipital encephaloceles can be treated electively, ideally early in infancy. (See 'Sincipital encephaloceles' above.)
●Basal encephaloceles may be trans-sphenoidal, spheno-ethmoidal, trans-ethmoidal, and spheno-orbital (image 2 and image 1) and may or may not be apparent on external inspection. They may present as a nasal or epipharyngeal mass, difficulty breathing, recurrent upper tract infections, nasal discharges, recurrent meningitis, or CSF leaks. They require prompt surgical repair because of high prevalence of meningitis. (See 'Basal encephaloceles' above.)
●Occipital and parietal encephaloceles usually are obvious at the time of birth (picture 1 and picture 2 and image 3). Many are diagnosed prenatally by ultrasonography. Those of relatively large size may be associated with cranial nerve deficits, poor sucking and feeding, spasticity, blindness, seizures, or developmental delay. Surgical management of patients with occipital and parietal encephaloceles depends upon the type of neural tissue that is protruding beyond the skull. (See 'Occipital encephaloceles' above and 'Parietal encephalocele' above.)
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