Figure 1: HASTE image demonstrates a protruding mass in the fetal ethmoid region containing herniating brain tissue. The right frontal lobe is dysplastic and filled with CSF.

Figure 2: Axial HASTE image through the fetal brain again demonstrates the dysplastic right frontal lobe replaced by CSF. Abnormal brain tissue in the right frontal region is herniating anteriorly into the right frontal ethmoidal region.

Figure 3: Axial HASTE image through the fetal brain demonstrates a soft tissue mass in the right frontal ethmoidal region containing herniated brain tissue.

Right frontal ethmoidal encephalocele.

Encephaloceles represent approximately 10-15% of all neural tube defects (NTD). An encephalocele is a developmental abnormality in which part of the CNS herniates through a cranial defect. It is caused by the failure of the surface ectoderm to separate from the neuroectoderm early in embryonic development. This results in a bony defect in the skull table, which allows herniation of the meninges (cranial meningocele) or herniation of brain tissue. Encephaloceles can be classified as containing only CSF and meninges (meningocele), neural tissue and meninges (meningoencephalocele), or neural tissue, meninges, and ventricle (hydroencephalomeningocele). Encephaloceles may be occipital (75-80%), frontoethmoidal (13-15%), parietal (10-12%), or sphenoidal. Approximately 90% of cases involve the midline. There is considerable variation in the incidence of encephaloceles, ranging from 1 in 3000 live births in Southeast Asia to 1 in 10,000 live births in North America . The location of the encephalocele also varies with geographical location, with occipital region encephaloceles more common in North America and Western Europe , frontoethmoidal encephaloceles more common in Southeast Asia , and frontal encephaloceles more common in the Far East , particularly in the China . Encephaloceles occur more commonly in females than in males.

Similar to NTDs, the etiology and pathogenesis of encephaloceles is poorly understood. However, there is correlation with the maternal use of sodium arsenate, clofibrate, and vitamin A. Other intracranial and extracranial malformations are noted in at least 60% of patients with encephalocele. Associated anomalies include the findings in genetic syndromes such as Meckel-Gruber, von Voss, Chemke, Roberts, and Knobloch syndromes. Other associated brain abnormalities that may occur in isolation or as part of genetic or non-genetic syndromes include spina bifida, agenesis of the corpus callosum, Arnold-Chiari II malformation, Dandy-Walker malformation, and brain migrational anomalies. The risk of chromosomal abnormalities is 13-44%, therefore, karyotyping should be recommended. The most common associated chromosomal anomaly is trisomy 18.

Currently, most encephaloceles are diagnosed antenatally on routine ultrasound scanning and are present at birth. Some, particularly sphenoidal encephaloceles, may become apparent later in childhood. Maternal serum alpha-fetoprotein (AFP) levels are elevated in only 3% of patients because most encephaloceles are covered by skin and hence do not leak AFP. Postnatally, infants may present with cerebrospinal fluid rhinorrhea and recurrent meningitis. Postnatal presentation also depends on the associated malformations and the size and contents of the defect.

Ultrasound ( US ) remains the mainstay of fetal imaging, however, its resolution of the fine detail of the brain and/or CNS is sometimes limited by the mother's body habitus, the surrounding amniotic fluid, and the position of the fetus. Fetal MRI provides superior detail of central nervous system (CNS) anomalies and can contribute complementary information for planned fetal or postpartum surgery. The entire cerebrospinal axis including the vertebral bony marrow space, the cord size and contour, the cerebrospinal fluid extradural interface, lipomatous tissue, and the skin covering the lesion can be better evaluated on MRI than on ultrasound scan. MRI also provides superior visualization of posterior fossa anatomy, fluid-filled and /or cerebrospinal fluid filled structures, the cranial defect, and the herniated contents. Due to the high incidence of coexisting anomalies, MRI offers the possibility of elucidating the extent of underlying pathologies of the entire neuroaxis. Additional information provided by MRI influences patient management and my play a decisive role in the consideration of interruption of pregnancy. Upon delivery diagnosis, further imaging is critical for treatment planning regardless of the size of the defect. MRI and MRA have become the standard in preoperative evaluation, determining the extent of neural tissue herniation into the defect, a factor that is generally considered one of the most important prognostic indicators. In addition, operative planning with regards to venous sinus drainage can be determined. Due to the high incidence of coexisting intracranial and extracranial anomalies, complete imaging is important because additional anomalies can affect prognosis, decrease long-term functional status, and overall operative success.

References:

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2. Khan A.N. and I. Turnbull. Encephalocele. (2002). eMedicine (04/19/04).
3. Birnbacher R, Messerschmidt AM, and AP Pollack. Diagnosis and Prevention of Neural Tube Defects. Current Opinion in Urology. 2002; 12: 461-464.
4. Blaicher W, Prayer D, and G. Bernascheck. Magnetic Resonance Imaging and Ultrasound in the Assessment of the Fetal Central Nervous System. Journal of Perinatal Medicine. 2003; 31: 459-468.