Brain encephalocele is a rare congenital cranial anomaly where a developmental fusion defect in the fetal skull allows meninges, cerebrospinal fluid, and neural parenchyma to protrude outward. Requiring advanced prenatal fetal neuroimaging, early surgical repair, and dural reconstruction, management aims to preserve functional brain tissue and control secondary hydrocephalus.
What Is Encephalocele?
Brain encephalocele is a rare congenital cranial malformation in which a developmental fusion defect of the fetal skull bones allows brain tissue and/or its protective membranes to protrude outward through a bone defect. It is classified under neural tube defects (NTDs) and represents one of the severe forms of congenital central nervous system anomalies. It is evaluated alongside other pediatric dysraphic conditions and structural malformations like Arnold–Chiari Malformation or diastematomyelia.
Neural tube closure normally occurs around the 3rd to 4th week of embryonic development. When this critical fusion process fails, an opening remains within the cranial bones, creating a pathway for herniation and leading to the formation of an encephalocele.
Types of Encephalocele
The content of the herniated sac varies; therefore, encephaloceles are divided into distinct anatomical subtypes:
Meningocele
A sac containing exclusively cerebrospinal fluid (CSF) and meningeal membranes protruding through the bony skull defect. No neural parenchyma or brain tissue is present within the herniated sac.
Meningoencephalocele
Both meninges and functional brain tissue (parenchyma) extend into the herniated sac. This represents the most frequent clinical subtype of cranial encephalocele.
Encephalomyelocele
A complex, severe developmental malformation where brain tissue, meninges, and elements of the upper spinal cord herniate together through an extensive craniocervical or occipitocervical defect.
Most cases present at birth with a visible, fluid-filled or skin-covered sac-like mass on the head and are frequently associated with additional structural central nervous system anomalies.
Embryology and Pathophysiology
Failure of neural tube closure during early embryogenesis disrupts primary cranial bone mesoderm induction and suture fusion. The resulting skull defect allows hydrostatic cerebrospinal fluid pressure to push underlying meninges and neural tissues outward, forming a progressive herniated sac.
The herniated sac may contain a variable combination of:
- Cerebrospinal fluid (CSF)
- Meninges (dura, arachnoid, and pia mater)
- Dysplastic or viable brain parenchyma
- In selective complex cases, major cerebral vascular structures
The severity of long-term neurodevelopmental outcomes correlates strongly with the volume of herniated functional brain tissue and associated intracranial malformations.
Anatomical Locations
Encephaloceles are classified geographically based on the anatomical site of the cranial bone defect:
Calvarial Locations
- Occipital Region: Most common anatomical location in Western populations (~75%), located between the foramen magnum and lambda.
- Parietal Region: Situated along the sagittal suture line between the anterior and posterior fontanelles.
- Bregmatic Area: Positioned at the junction of the coronal and sagittal sutures.
Basal & Anterior Locations
- Frontonasal Region: Most frequent in Southeast Asian populations, protruding at the junction of the frontal and nasal bones (ethmoidal or naso-orbital).
- Sphenoidal / Basal Region: Protrudes through the skull base into the nasopharynx, ethmoid sinus, or orbit, often presenting without external facial deformity.
Etiology and Risk Factors
There is no single definitive cause of encephalocele. A multifactorial etiology involving genetic, environmental, and maternal metabolic factors is recognized:
Genetic & Chromosomal Associations
- Trisomy 13 (Patau syndrome) and Trisomy 18 (Edwards syndrome)
- Meckel–Gruber syndrome and Walker–Warburg syndrome
- Knobloch syndrome and Roberts syndrome
- Frontonasal dysplasia and Dissegmental dysplasia
Environmental & Maternal Factors
- Maternal folate (folic acid) deficiency during pre-conception and early organogenesis
- Pre-gestational or gestational maternal diabetes
- Teratogenic drug exposures (e.g., valproate or specific antiepileptics)
- Maternal obesity, radiation exposure, or amniotic band sequence
Clinical Features and Symptoms
The clinical presentation varies according to lesion size, anatomical location, sac contents, and coexisting brain malformations:
- Local Signs: Externally visible cranial mass, thinned or hyperpigmented overlying skin, potential active cerebrospinal fluid (CSF) leakage, and secondary craniofacial deformities.
- Neurological Manifestations: Global developmental delay, intellectual impairment, motor spasticity, visual or auditory deficits, and focal seizures.
- Associated Hydrocephalus: Ventricular enlargement due to CSF pathway obstruction at the aqueduct or skull base.
Prenatal Diagnosis
Modern obstetric screening allows high rates of intrauterine detection during routine prenatal care:
Prenatal Ultrasonography
Identifies bony cranial defects, demonstrates herniated sac morphology, and detects associated cranial anomalies during mid-trimester anatomical ultrasound scans.
Fetal MRI
Provides superior soft-tissue contrast to differentiate sac contents (meninges vs. brain parenchyma), map cerebral vasculature, and guide postnatal neurosurgical planning.
Treatment & Long-Term Management
Microsurgical repair is the primary treatment for encephalocele. Goals of surgical intervention include carefully excising or repositioning the herniated sac contents, preserving viable non-herniated brain tissue, achieving watertight dural closure to prevent CSF leaks and meningitis, and reconstructing overlying cranial osseous defects.
Surgery is typically scheduled during early infancy under pediatric neurosurgical care. Long-term management involves monitoring for secondary hydrocephalus (frequently requiring ventriculoperitoneal shunting or endoscopic third ventriculostomy), formal neurodevelopmental assessments, and multidisciplinary pediatric rehabilitation.
Frequently Asked Questions
What is an encephalocele?
An encephalocele is a rare congenital neural tube defect where a gap in the skull allows fluid, protective membranes (meninges), or brain tissue to protrude outward in a sac.
What is the difference between a meningocele and a meningoencephalocele?
A meningocele sac contains only cerebrospinal fluid and protective membranes without brain tissue. A meningoencephalocele contains both meninges and actual brain tissue (parenchyma).
How is an encephalocele diagnosed before birth?
Encephaloceles are typically diagnosed during routine second-trimester prenatal ultrasounds and confirmed using detailed fetal MRI to evaluate brain tissue involvement.
What causes an encephalocele?
It is caused by incomplete closure of the neural tube during the 3rd to 4th week of embryonic development, influenced by genetic factors and maternal nutrition, particularly folic acid deficiency.
How is an encephalocele treated?
Treatment involves pediatric neurosurgery during early infancy to repair the dural opening, preserve brain tissue, prevent fluid leaks, and reconstruct the skull defect.
Can hydrocephalus occur with an encephalocele?
Yes. Hydrocephalus frequently develops in infants with encephaloceles due to blocked cerebrospinal fluid pathways, often requiring a ventriculoperitoneal (VP) shunt.