Anencephaly and encephaloceleAuthorsTadanori Tomita, MDHideki Ogiwara, MD, PhDSection EditorsMarc C Patterson, MD, FRACPLeonard E Weisman, MDDeputy EditorAlison G Hoppin, MD

Disclosures

Last literature review version 19.2: May 2011 |This topic last updated: June 17, 2011 (More)

INTRODUCTION — Myelomeningocele, anencephaly, and encephalocele are the most

common NTDs. Anencephaly is an open NTD as the affected region of the cranial neural tube

is exposed to the body surface. It is a severe defect, and is not compatible with survival.

Encephalocele is a herniation of the brain and/or meninges through a defect in the skull

(cranium bifidum) that is “closed” or covered with skin.

The clinical features, diagnosis, and management of these conditions are discussed here.

Myelomeningocele is discussed in separate topic reviews. (See "Pathophysiology and clinical

manifestations of myelomeningocele (spina bifida)" and "Overview of the management of

myelomeningocele (spina bifida)".)

EMBRYOLOGY OF THE NEURAL TUBE — The central nervous system (CNS) 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 [1]. Neural tube

defects (NTDs) result from failure of the neural tube closure between 25 and 28 days after

conception. The embryology of the neural tube is discussed in a separate topic review. (See

"Pathogenesis and types of occult spinal dysraphism", section on 'Normal cord development

in humans'.)

ANENCEPHALY — Anencephaly is a severe defect of development of the neuraxis, in which

the developing forebrain and variable amounts of the brainstem are exposed in utero and

fail to develop or are destroyed [2,3]. It results from failure of the rostral neuropore to close

[4].

Incidence — According to data from birth certificates reported to the National Center for

Health Statistics, the prevalence of anencephaly in the United States in 2001 was 9.40 per

100,000 live births [5]. During 1999 to 2004, 2116 cases of anencephaly were reported in

the United States [6]. Hispanic infants had the highest prevalence. The malformation is more

common in girls than boys, in whites than blacks, and in mothers at the younger and older

extremes of age [7]. The rate of anencephaly in live births underestimates the actual rate of

occurrence because of unknown numbers of spontaneous abortion or pregnancy termination

of affected fetuses.

The birth prevalence of anencephaly decreased 20 percent between the 1999 to 2000

period and the 2003 to 2004 survey periods [6]. This decline is thought to be due to

mandatory fortification of foods with folic acid, which began in January 1998.

There is considerable variation in the rates of anencephaly and other NTDs worldwide, with

particularly high prevalence in Ireland and the British Islands as compared with continental

Europe [8]. Rates in Asia and Africa tend to be lower, although there are pockets of higher

prevalence in the northern provinces of China and in India, which are not easily explained by

geographic or ethnic gradients [9].

Inheritance — The risk of recurrence for NTDs (spina bifida or anencephaly) is

approximately 2 to 4 percent when there is one affected sibling [10-14]. With two affected

siblings, the risk is approximately 10 percent [15]. The risk of recurrence appears to be

higher in countries such as Ireland where the prevalence of NTDs is high [16].

Prenatal diagnosis — Prenatal diagnosis is accomplished by maternal screening of serum

alpha-fetoprotein (AFP) levels and ultrasonography. Prenatal detection usually is followed by

termination of the pregnancy.

The sonographic diagnosis of anencephaly is based upon the absence of brain and calvarium

superior to the orbits on coronal views of the fetal head. The sonographic diagnosis of this

condition is highly accurate and should not be missed on any routine second- or third-

trimester ultrasound examination. Another sonographic feature is polyhydramnios, which

develops in up to 50 percent of the cases during the second and third trimester due to

decreased fetal swallowing. (See "Ultrasound diagnosis of neural tube defects", section on

'Anencephaly'.)

Clinical features — In the most common type, the forebrain and variable amounts of upper

brainstem are involved [7]. Exposure in utero results in destruction of neural tissue, which

appears as a hemorrhagic, fibrotic, nonfunctioning mass (picture 1). Major portions of the

CNS are absent or malformed. The hypothalamus is typically missing. The cerebellum,

brainstem, optic nerves, and spinal cord may be malformed. Underdevelopment or absence

of the pituitary leading to adrenal hypoplasia is always present [17].

Large portions of the cranium are absent in this disorder. The frontal, parietal, and portions

of the occipital bones are most often affected. The absent cranial vault results in the

characteristic appearance of bulging eyes and absent neck. The defect in the skull

sometimes extends through the level of the foramen magnum and involves the cervical

spine. This abnormality is known as holoacrania [7].

Associated malformations — Other congenital malformations frequently accompany

anencephaly. In a registry of 456 affected stillborn and liveborn infants in British Columbia,

additional malformations, including cleft lip and/or palate or omphalocele, occurred in 12.7

percent [18]. Craniofacial and ocular anomalies often occur. Cardiac, pulmonary, renal, and

skeletal malformations may be associated. Aganglionosis is a frequent finding [19].

Neurologic function — Neonates with anencephaly typically have brainstem function, with

spontaneous breathing and often with suck, root, and gag responses. However, they are

permanently unconscious. Without intensive care, the majority of them die within two days

of birth, and none survive longer than two weeks [7].

Management — There are no neurosurgical management options. Since anencephalic

infants can survive for no more than a few weeks, and in most developed countries where

abortion is legal these pregnancies are interrupted earlier, prevention is the most important

in its management.

Because of their poor prognosis, anencephalic infants have been considered as potential

organ donors for transplantation. To assess the feasibility of this approach, one group

modified the medical care of 12 live-born anencephalic infants for one week to determine

whether organ viability could be maintained and whether the criteria of total brain death

could be met. Among six anencephalic infants who received intensive care during the first

week after birth, organ function was maintained, but five were not candidates for organ

donation because they had persistent brainstem activity. Among six other infants who did

not receive intensive care during the first week of life, most organs were damaged,

precluding organ donation. These findings suggest that anencephalic infants are not good

candidates for organ donation because they do not generally meet criteria for brain death

until their clinical condition has declined to the point where the solid organs are damaged

[20].

Prevention — Several lines of evidence suggest a link between folic acid deficiency and the

development of NTDs including anencephaly. Risk factors for NTDs include dietary deficiency

of folic acid, administration of folic acid antagonists such as trimethoprim, carbamazepine,

phenytoin and phenobarbital, and genetic polymorphisms in genes encoding folate-

dependent enzymes. Moreover, randomized trials have consistently shown that folic acid

supplementation reduces the incidence of NTDs. (See "Folic acid for prevention of neural

tube defects", section on 'Pathogenesis'.)

Other contributors to NTD risk that are probably not related to folate metabolism include

maternal diabetes mellitus with poor glycemic control during the first trimester,

hyperthermia, and some genetic syndromes. (See "Folic acid for prevention of neural tube

defects", section on 'Treatment failures' and "Pregnancy risks in women with type 1 and

type 2 diabetes mellitus", section on 'Congenital malformations'.)

Periconceptional folic acid supplementation is recommended for all women who are

pregnant or who may become pregnant. Higher doses of folic acid supplements are usually

recommended for women who are taking anticonvulsant drugs or who have had a previous

pregnancy affected by a NTD. (See "Folic acid for prevention of neural tube defects", section

on 'Folic acid supplementation'.)

ENCEPHALOCELE — 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 (CSF). The mechanism causing 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 skull or meninges may occur later [21]. Nasofrontal lesions are thought to

result from defective separation of neural and surface ectoderm at the site of final closure of

the rostral neuropore [22].

Classification — Classification of encephalocele is based on the cause and anatomical

location of the skull defect.

Primary encephaloceles are congenital and present at birth. Primary encephaloceles

are divided into three major types:

Sincipital (fronto-ethmoidal)

Basal (trans-sphenoidal, spheno-ethmoidal, trans-ethmoidal, and spheno-orbital)

(picture 2)

Occipital (picture 3A-B)

Secondary encephaloceles are acquired and commonly due to trauma or a

postsurgical defect [23-26].

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 [27]. These cases included

91 liveborn infants, 18 stillbirths, and six 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 about 85 percent of encephaloceles take this form [28]. 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 [29]. Most of the affected individuals in this family were

developmentally normal and had normal neurologic exams 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 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; they may

present as a nasal or epipharyngeal mass, difficulty breathing, recurrent upper

tract infections, nasal discharges, recurrent meningitis, or CSF leaks [28].

An occipital encephalocele usually is obvious at the time of birth and 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. Occipital encephalocele also may be

associated with hind-brain anomaly (Chiari III malformation) in which herniating

occipital/cerebellar tissues distort the posterior fossa structures. (See "Chiari

malformations".)

The herniated 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 [30,31]. 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 [32].

Associated anomalies — Infants with encephalocele frequently have other malformations

that may be part of recognized syndromes [27,33,34]. In a series from Atlanta, at least one

other major structural congenital defect was present in 17 of 83 (20 percent) of affected

liveborn infants with encephalocele [27]. This is a smaller proportion than in other series,

due to exclusion of infants with chromosomal abnormalities, other major CNS 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 [7]. (See "Renal cystic diseases in children", section on 'Renal cysts

in malformative syndromes' 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) [7,35].

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.

Neuroimaging should be performed to evaluate the intracranial components of the

malformation and identify any associated brain or vascular anomalies. CT scans are effective

for detection of the extent of cranial defects. MR scans are 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

AFP levels and ultrasonography. With the latter, encephaloceles appear as a defect in the

calvarium containing a cystic or solid mass with a gyral pattern that is contiguous with the

brain [36].

Prenatal ultrasonography detects approximately 80 percent of encephaloceles. Fetal

magnetic resonance imaging (MRI) has higher sensitivity for NTDs, and can be used in cases

where ultrasound results are not optimal or the diagnosis is uncertain, or to evaluate high-

risk cases. (See "Prenatal screening and diagnosis of neural tube defects", section on

'Prenatal diagnosis'.)

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 [37]. In patients

with basal encephaloceles or 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 [28]:

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,

and optic pathway may be present in the encephalocele sac.

Occipital encephaloceles — Surgical management of patients with occipital

encephalocele depends on 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 [38]. 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, then herniated brain is transpositioned into the new intracranial

space as the ventricle contracts [39].

To preserve herniated occipital and cerebellar parenchyma, the tentorium is incised,

creating infratentorial space, and the cerebral cortex retracted into this space [40].

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 [28].

The prognosis for patients with occipital encephalocele depends on 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 [41]. 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 (mental retardation). In a

separate series of 30 patients with occipital encephalocele, the mortality rate was 29

percent [42]. 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 [35]. 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 about 20 percent of patients with congenital encephalocele

[35]. 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 [43]. In

a literature review of 18 cases of encephaloceles with seizure, six (33 percent) were extra-

temporal and 12 (67 percent) were temporal [44]. 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.

SUMMARY AND RECOMMENDATIONS

Anencephaly

Anencephaly is a severe defect of development of the neuraxis, in which the

developing forebrain and variable amounts of the brainstem are exposed in utero

and destroyed or fail to develop (picture 1). Neonates with anencephaly typically

have brainstem function, but are permanently unconscious; none survive longer

than two weeks. (See 'Neurologic function' above.)

Prenatal diagnosis of anencephaly is accomplished by maternal screening of serum

alpha-fetoprotein (AFP) levels and ultrasonography. Prenatal detection usually is

followed by termination of the pregnancy. (See 'Prenatal diagnosis' above.)

Risk factors for NTDs include dietary deficiency of folic acid, administration of folic

acid antagonists such as trimethoprim, and several types of antiepileptic drugs,

and genetic polymorphisms in genes encoding folate-dependent enzymes. Folic

acid supplementation given from before the time of conception reduces the

incidence of NTDs, and is recommended for all women who are pregnant or who

may become pregnant. (See 'Prevention' above.)

Encephalocele

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, about 80 percent of encephaloceles are detected prenatally.

(See 'Encephalocele' 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 varies 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 (picture 2) 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 encephaloceles usually are obvious at the time of birth and many are

diagnosed prenatally by ultrasonography (picture 3A). 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 encephaloceles depends on the type of neural tissue that is protruding

beyond the skull. (See 'Occipital encephaloceles' above.)

ACKNOWLEDGMENT — The editorial staff at UpToDate, Inc. would like to acknowledge Dr.

Marvin Fishman and Dr. Grace Villarreal, who contributed to an earlier version of this topic

review.

Use of UpToDate is subject to the Subscription and License Agreement.

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TOPIC OUTLINE

INTRODUCTION EMBRYOLOGY OF THE NEURAL TUBE ANENCEPHALY Incidence Inheritance Prenatal diagnosis Clinical features Associated malformations Neurologic function Management Prevention ENCEPHALOCELE Classification Incidence Inheritance Clinical features Associated anomalies Diagnosis - Prenatal diagnosis Management - Sincipital encephaloceles - Basal encephaloceles - Occipital encephaloceles Outcome SUMMARY AND RECOMMENDATIONS Anencephaly Encephalocele ACKNOWLEDGMENT REFERENCES

GRAPHICS

FIGURES Cephalocaudad neurulation PICTURES Anencephaly Nasal encephalocele Occipital encephalocele MRI Chiari III malformation

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