Article Title: Encephalocele.
Author: Mohammad Al Jamal.
Editor: Aseel Rabadi, Sadeen Eid.
Reviewer: Ethar Hazaimeh.

Keywords: Cerebrospinal fluid, Congenital brain abnormality, Neural tube defect, Occipital encephalocele.

Overview

Encephalocele is a type of congenital neural tube defect (NTD) in which a sac containing the brain, meninges, and cerebrospinal fluid (CSF) forms outside the skull due to a defect in the bone. Trauma, tumors, or iatrogenic injury can all cause acquired encephaloceles. ⁽¹⁾ Depending on the intracranial content protruding through the skull opening, encephalocele defects are divided into four types: meningoencephaloceles (herniations of cerebrospinal fluid (CSF), brain tissue, and meninges); meningoceles (herniations of the meninges and CSF), glioceles (glial-lined cyst containing CSF) and atretic cephaloceles (comprising of the dura, fibrous tissue and degenerated brain tissue). ⁽²⁾ Occipital encephalocele is the most common form of this congenital disorder and is manifested as a midline swelling of different sizes over the occipital bone. ⁽³⁾

Repairing an encephalocele is not an emergency, and only individuals with skin ulcers or CSF leaks require immediate surgery to avoid complications such as meningitis. Surgery can be performed as early as two months of age but is often postponed for months to several years due to newborns’ limited total blood volume. The surgical goals are to close the skull defect with a watertight Dural closure and to restore the bone deficit. ⁽⁴⁾

Etiology

The cause of encephalocele is not completely understood, and is probably multifactorial, including both genetic and environmental factors. Individuals with a family history of NTDs (spina bifida, anencephaly) have a higher risk to develop encephalocele. The presence of the gene associated with certain disorders does not assure the development of the anomaly. However, the presence of some factors (e.g. environmental) might trigger the disease. ⁽⁵⁾ Researchers have emphasized that non-specific environmental factors (such as hyperthermia, aflatoxins, genetic background, mother’s lack of nutrition, or other environmental factors) are involved in the development of encephalocele. ⁽⁶⁾ Maternal levels of folate and its relevance for encephalocele is still ‘not clear’. ⁽⁷⁾ However, The use of folic acid (400 micrograms of folic acid daily) in the diet of pregnant women might lower the risk of some NTDs. ⁽⁴⁾

Pathophysiology

Encephaloceles are considered to be a type of NTDs series caused by neurulation failure, such as the failure of neural tube closure. ^(8,9) This theory, however, does not explain the existence of well-formed neural structures within the prolapsed sac because brain tissue is formed after the neural tube is closed. As a result, some authors propose that an encephalocele is a post-neurulation event. ^(10)(11)

According to these post-neurulation ideas, failure of mesodermal migration into the midline results in brain matter herniation. The deformed, stretched appearance of the neuroparenchyma next to the calvarial defect is best explained by this notion. Many cases, particularly occipital kinds, showed the stretched and distorted appearance of the brain next to the cranial vault defect, supporting the post-neurulation idea. Skull base encephaloceles can be caused by a primary osseous defect, a failure of bone induction due to a defective neural tube closure, a lack of fusion of ossification centers, or a persistent cricopharyngeal canal. ^(12,13)

Clinical presentation and complications

Encephalocele is frequently evident and detectable at birth as a deformity in the midline of the skull with a big projecting mass. There will be no apparent mass if it is located in the nasopharynx, but the infant may have breathing issues due to airway obstruction. ⁽¹⁴⁾

The size and location of the encephalocele influence whether the newborn develops properly or has any malformations or growth difficulties. When a frontal encephalocele extends into one of the eye orbits, proptosis might happen. Occipital encephalocele, on the other hand, can result in blindness and cognitive disability.

Still, the most common encephalocele complications are irregular CSF circulation and meningitis. Other complications to consider are: ⁽¹⁴⁾

• CSF leak
• Seizures
• Hydrocephalus
• Endoscopic recurrence
• Developmental delay
• Developmental impairment 

Workup and Evaluation

The cornerstone imaging demonstrating a fluid-filled sac through a skull defect is prenatal ultrasonography, which is commonly conducted between the 9th and 11th weeks of gestation. ^{(15, 16)} By 13 weeks, it will be clear whether the abnormality is a meningocele (without brain matter herniation) or an encephalocele. ⁽¹⁷⁾

Prenatal magnetic resonance imaging (MRI) will have easily revealed the problem, but the fetus must be sedated in order to be imaged so it is not commonly used. Postnatal brain MRI is preferred, showing the defect, its contents, brain tissue, and any accompanying anomalies. To assess the skull defect, bone anomalies, and hydrocephalus, a computed tomographic (CT) scan of the head is performed. ⁽¹⁸⁾ Angiography can be used if the defect is close to the Dural sinuses. MRI with magnetic resonance angiography (MRA) is a beneficial method to visualize the contents of the sac and its relationship to venous sinuses. ⁽¹⁹⁾

CT scan in fetal life, particularly in the first two trimesters, is not recommended because of the side effects of radiation. ⁽²⁰⁾ The significant value of postnatal CT scan comes from its excellent ability in showing the details of bony defects. ⁽²¹⁾ Yet, compared to MRI, CT scan is less effective in depicting soft tissue components in encephaloceles. ⁽²⁰⁾

Management and Treatment

Treating these defects necessitates the collaboration of a multidisciplinary team of neurosurgeons, neuro anesthetists, pediatricians, maxillofacial surgeons, and plastic surgeons. These anomalies should be treated in the first few months of life if possible because identifying the intracranial connection would be technically easier, and the Dural defect would be completely fixed. The goal of the procedure is to close the internal defect with a watertight Dural closure (intradural repair is recommended over extradural repair), close the skull defect, resect the sac, and restore the exterior bony deformity (titanium mesh or Osteopore bone scaffold filler). ⁽²²⁾ Figure (2).

The reconstruction goals are:

(a) To close open skin defects preventing infection and desiccation of viable brain tissue.

(b) To remove or invaginate nonfunctional extracranial cerebral tissue.

(c) To close the dura and do a craniofacial reconstruction with focusing on accurate skeletal reconstruction. ⁽²³⁾

Early surgical correction is necessary for children with frontoethmoidal encephalocele in order to treat and avoid facial abnormalities, binocular vision impairment, swelling that worsens due to secondary herniation of intracranial contents, and the possibility of infections of the central nervous system. The treatment of accompanying brain malformations (such as hydrocephalus) should come first, followed by a one-stage reconstructive operation. ^{(22, 24-26)}

Frontoethmoidal encephaloceles appear to have a favorable prognosis than occipital or parietal meningoencephaloceles, with an overall mortality rate of 7%-20% and a fair developmental outcome. ^{(24, 27)} The occurrence of concomitant hydrocephalus or other congenital brain abnormalities largely determines the prognosis. ⁽²⁸⁾

 

Prognosis

Many factors influence an encephalocele’s prognosis, including its location, size, the amount of brain tissue and Dural sinuses inside the sac, and the presence of hydrocephalus. ^(30,31) Patients with frontoethmoidal encephaloceles have a better prognosis than those with occipital or parietal encephaloceles. The occurrence of additional congenital brain abnormalities markedly influences the prognosis. A research paper assessing the clinical predictors found that seizures, microcephaly, and brain tissue in the sac were all associated with poor prognosis. Whereas hydrocephalus and the existence of other intracranial abnormalities were found to be significant predictors of developmental delay. ⁽³²⁾ Hydrocephalus exists in 34% of all patients prior to surgical correction of the defect and develops in 4% of them after the defect is closed. ⁽³³⁾

A long-term study of children with encephaloceles revealed that 48% developed normally, 11% had mild impairment, 16% had a substantial impairment, and 25% had severe impairment. ⁽³²⁾ Due to seizures and hydrocephalus, occipital encephaloceles have a poorer prognosis than frontal encephaloceles. Approximately half of the occipital encephalocele patients are unable to live independently in society. ⁽³⁴⁾

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