Encephalocele refers to the herniation of cranial contents through a defect in the skull. This term includes both encephaloceles, which contain meninges and brain, as well as meningoceles, which consist of meninges and cerebrospinal fluid. Empty meningoceles are not as common as encephaloceles. Encephaloceles are subdivided according to their location: occipital and frontal. This distinction is important, as each of the conditions has a different prognosis based on their location. In occipital encephaloceles, the defect lies between the lambda and foramen magnum (Simpson et al., 1984). These encephaloceles are considered part of the spectrum of neural tube defects. Sincipital or frontal encephaloceles are situated between the bregma and anterior margin of the ethmoid bone. The majority of these encephaloceles extend into the root of the nose, and they are not considered a form of neural tube defect, but rather are likely secondary to an environmental exposure. Occipital encephaloceles are considerably more common than frontal encephaloceles in populations of European descent, whereas frontal encephaloceles are much more common in populations from Southeast Asia.

The cause of encephalocele is unknown, but occipital encephaloceles are thought to be caused by failure of closure of the rostral neural pore. The majority of encephaloceles occur in the midline; however, they may also result from disruption of fetal-skull formation, such as in the amnion rupture sequence (Chervenak et al., 1984). Encephaloceles occur between 25 and 50 days of gestation for anterior defects and up to 60 days for posterior defects (Brown and Sheridan-Pereira, 1992).

In a recent review from New Zealand, encephaloceles were seen in one in 13,000 births (Monteith et al., 2005). In an earlier U.S. study extending more than 17 years, Wiswell et al. (1990) documented 112 cases of encephalocele in 763,364 livebirths, or 0.15 per 1000 livebirths. Of these infants with encephaloceles, 40% had associated congenital anomalies. Encephalocele is more often associated with extra-CNS malformations than anencephaly or spina bifida (Kalien et al., 1998). In an Australian study, the incidence of encephalocele was shown to be 0.08 per 1000 total births (David and Proudman, 1989). In Australians of European descent, the occipital location for the encephalocele is more common, whereas in patients of Southeast Asian extraction, the frontal location is more common (Richards, 1992). The United Kingdom, with its generally higher rate of all neural tube defects, has a higher incidence of encephalocele—between 0.3 and 0.6 per 1000 livebirths (Fleming et al., 1991).

Encephaloceles are associated with maternal rubella, diabetes, genetic syndromes (such as Meckel–Gruber), and amniotic bands. They have been produced experimentally with X-irradiation and hypervitaminosis A. In the Western world, 75% of encephaloceles are located in the occipital region (Chervenak et al., 1984). The frontoethmoidal encephalocele is more prevalent in Malaysia, Thailand, Indonesia, Burma, and parts of Russia. In Burma, a high prevalence of encephalocele has been noted among rural and peasant rice farmers (David and Proudman, 1989). There is no relationship between encephalocele and parental age.

The sonographic appearance of encephalocele is diverse, depending on its contents. Encephaloceles can appear as a purely cystic mass, a solid mass with a gyral pattern continuous with the cranium, or a combined cystic and solid mass (Graham et al., 1982; Winter et al., 1993). A calvarial defect is usually identified. In most European studies, the encephalocele is present in the occipital region 75% of the time, in the frontal/sincipital region 15% of the time, and at the vertex 5% of the time. An encephalocele that is observed in an asymmetric or atypical location may be related to the amniotic-band syndrome or limb–body wall complex (Graham et al., 1982; Winter et al., 1993). Encephaloceles have been noted to have a unique “cyst within a cyst” (Figures 12-1A and 12-1B) or “target sign” appearance when the encephalocele is viewed from the en face position (Goldstein et al., 1991; Adetiloye et al., 1993).

To diagnose an encephalocele sonographically, the following considerations should be taken into account:

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  The mass should be seen attached to the fetal head or move with the fetal head.

  
  
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  A bony defect should be revealed.

  
  
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  Intracranial anatomic abnormalities should be detected, such as hydrocephalus.

  
  
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  The spine should be examined to exclude associated spina bifida.

  
  
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  The fetal kidneys should be examined, because of a high incidence of association with renal cystic disease (Meckel–Gruber syndrome).

In 7% to 15% of cases, neural tube defects are shown to be present in association with the encephalocele (Fleming et al., 1991). In addition, microcephaly is seen in 20% of cases. Other associated central nervous system (CNS) anomalies that may occur with encephalocele include agenesis of the corpus callosum, orofacial clefting, craniosynostosis, Dandy–Walker malformation, Arnold–Chiari malformation, hemifacial microsomia, Klippel–Feil anomaly, iniencephaly, and myelomeningocele (Cohen and Lemire, 1982).

In one study, Budorick et al. (1995) reviewed 26 cases of prenatally diagnosed encephalocele. Seventy-one percent of these cases were in the occipital location. Sixty-five percent of cases had associated major congenital anomalies. CNS features that were observed and helped with the diagnosis of encephalocele included visible skull defect (seen in 96% of cases), ventriculomegaly (23%), microcephaly (50%), beaked tectal plate (38%), and flattened occiput (38%).

In another study, Goldstein et al. (1991) reviewed the prenatal sonograms of encephaloceles in 15 fetuses. Of these, 13 were in the occipital location, 1 in the ethmoidal, and 1 frontoparietal. These authors could not accurately distinguish prenatally between meningocele and meningoencephalocele. They also noted an inward depression of the frontal bones (the lemon sign) in 33% of fetuses with encephalocele. Of the nine cases in which prenatal chromosome studies were performed, four (44%) were abnormal, including trisomy 13, trisomy 18, mosaic trisomy 20, and an unbalanced chromosome translocation. The outcome for the remaining 11 patients included 5 terminations of pregnancy, 2 stillbirths, and 4 neonatal deaths. Similarly, Wininger and Donnenfeld (1994) reviewed 15 cases of prenatally diagnosed encephalocele.

These authors also demonstrated a high incidence of associated major anomalies (60% of cases). The mean gestational age at detection of these cases was 12 to 14 weeks. These authors identified three multifactorial disorders, two cases of chromosomal abnormalities, and two autosomal recessive syndromes.

Important considerations in the sonographic differential diagnosis of encephalocele include location of the extracranial mass (midline or lateral), contents of the mass (cystic or solid), and whether or not the anomaly is associated with an underlying cranial bony defect (Sherer et al., 1993). Encephaloceles are typically midline cystic structures that overlie or project through a defect in the calvarium. The diagnosis is helped by the identification of herniated brain tissue and associated hydrocephalus.

A major consideration in the differential diagnosis is cystic hygroma (Table 12-1). An occipital meningocele that contains only cerebrospinal fluid can be confused with a cystic hygroma. With cystic hygromas, however, the margins of the mass blend into the skin line, and a bony defect in the cranial vault should not be visible. Cystic hygromas frequently contain septations and there may be fewer CNS anomalies associated with cystic hygromas (see Chapters 31 and 32). In addition, other findings of hydrops, such as pleural effusions and ascites, are often found with cystic hygroma (Pearce et al., 1985; Goldstein et al., 1991). Encephaloceles can also be confused with cervical teratomas, which are solid or heterogeneous but have no demonstrable brain tissue. Hemangioma is an additional soft tissue mass that can be confused with encephalocele; they are heterogeneously echogenic, with no identifiable skull defects. They form an obtuse angle with the adjacent skull, as opposed to encephaloceles, which typically form an acute angle with the adjacent skull (Bronshtein et al., 1992; Winter et al., 1993). Sherer et al. (1993) described the case of a 27-week-old fetus with a right retroauricular mass thought to overlay a bony defect of the skull. This was initially diagnosed as an encephalocele, but it was later shown to be a subcutaneous hemangioma. Other considerations in the differential diagnosis include scalp edema (Winter et al., 1993), a clump of fetal hair (Noriega et al., 2001), epidermal scalp cyst (Shahabi and Busine, 1998), and branchial cleft cysts.