Clinical Imaging xxx (2014) xxx–xxx

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Review Article

Diagnostic imaging of posterior fossa anomalies in the fetus and neonate: part 2, posterior fossa disorders☆ Teresa Chapman a,b,⁎, Sowmya Mahalingam b, Gisele E. Ishak a,b, Jason N. Nixon a,b, Joseph Siebert c, Manjiri K. Dighe b a b c

Department of Radiology, Seattle Children's Hospital, MA.7.220, 4800 Sand Point Way NE, Seattle, WA, 98105 Department of Radiology, University of Washington Medical Center, Box 357115, 1959 NE Pacific Street, Seattle, WA 98195-7117 Department of Pathology, Seattle Children's Hospital, PC.8.720, 4800 Sand Point Way NE, Seattle, WA, 98105

a r t i c l e

i n f o

Article history: Received 1 August 2014 Received in revised form 16 October 2014 Accepted 20 October 2014 Available online xxxx Keywords: Posterior fossa Congenital brain malformation Prenatal imaging Cerebellum

a b s t r a c t This second portion of a two-part review illustrates examples of posterior fossa disorders detectable on prenatal ultrasound and MRI, with postnatal or pathology correlation where available. These disorders are discussed in the context of an anatomic classification scheme described in Part 1 of this posterior fossa anomaly review. Assessment of the size and formation of the cerebellar hemispheres and vermis is critical. Diagnoses discussed here include arachnoid cyst, Blake's pouch cyst, Dandy–Walker malformation, vermian agenesis, Joubert syndrome, rhombencephalosynapsis, Chiari II malformation, ischemia, and tumors. © 2014 Elsevier Inc. All rights reserved.

1. Introduction As discussed in the Part 1 of this two-part review, recognizing a posterior fossa anomaly or acquired disorder is an important role of prenatal imaging. Defining the developmental implications of these disorders then becomes a challenge for the clinical team caring for the pregnant patient and her family. In recent years, changes in the terminology of the different entities affecting the posterior fossa of the fetal brain require that the reader frames an understanding of these disorders in the context of updated information. In the first part of this review, we describe a diagnostic approach with imaging studies that includes identification of posterior fossa size, presence of abnormal retrocerebellar fluid, and determination of cerebellar size and morphology. This second part of the review illustrates the imaging features of various anomalies of the cerebellum and posterior fossa with prenatal and postnatal correlation.

will give rise to the cerebellar hemispheres and vermis. The more inferiorly situated posterior membranous area represents the site of the future foramen of Magendie. Nonfenestration of the foramen will cause normal, transient dilatation of the primitive developing fourth ventricle in an inferior and posterior direction, and the appearance of this ballooning vesicle has been termed a Blake's pouch cyst (BPC) (discussed in more detail below, Section 3) [1]. If the cyst undergoes subsequent delayed fenestration, a mega cistern magna can result [2]. The cyst can also persist into postnatal life, termed a persistent Blake's pouch cyst. If the developmental abnormality is more extensive and involves the plica choroidea and anterior membranous area as well, there will be varying degrees of agenesis of the fourth ventricular choroid, vermis, and cerebellar hemispheres [3]. When additionally associated with an enlarged posterior fossa, the classic Dandy–Walker malformation (DWM) is present. 3. Megacisterna magna

2. Brief overview of fourth ventricle embryology The fourth ventricular roof is separated in early development into the anterior and posterior membranous areas, divided by the plica choroidea, or primitive choroid plexus. The anterior membranous area ☆ Funding: None. ⁎ Corresponding author. Department of Radiology, MA.7.200, Seattle Children's Hospital, 4800 Sand Point Way NE, Seattle, WA 98105. Tel.: + 1-206-987-1577; fax: + 1-206-987-2341. E-mail address: [email protected] (T. Chapman).

Megacisterna magna (MCM) refers to benign enlargement of the subarachnoid spaces of the posterior fossa, with a normally-developed cerebellum. There is characteristic absence of a persistent retrocerebellar cyst and free communication between a normally formed fourth ventricle and the subarachnoid space through a patent median foramen [3]. It is considered to be a normal variant. Prognosis is uniformly excellent in the absence of additional anomalies [3]. Prenatal ultrasound will demonstrate a cisterna magna that measures greater than 1 cm in maximal anteroposterior dimension on a standard oblique axial image through the posterior fossa (Fig. 1a & b).

http://dx.doi.org/10.1016/j.clinimag.2014.10.012 0899-7071/© 2014 Elsevier Inc. All rights reserved.

Please cite this article as: Chapman T, et al, Diagnostic imaging of posterior fossa anomalies in the fetus and neonate: part 2, posterior fossa disorders, Clin Imaging (2014), http://dx.doi.org/10.1016/j.clinimag.2014.10.012

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[5]. Differential considerations on ultrasound include a persistent BPC, vermian dysplasia, and a DWM. Fetal magnetic resonance imaging (MRI) of MCM will demonstrate enlargement of the posterior fossa subarachnoid space, similar to that seen on ultrasound (Fig. 1c). Mass effect, as seen with a posterior fossa arachnoid cyst, is absent. A midline sagittal posterior fossa image will characteristically demonstrate a normal contour of the fourth ventricle and cerebellar vermis. In contrast to the malformations detailed in this review, the vermis will not be abnormally elevated away from the brainstem (the tegmento-vermian angle will approach zero), and the vermis will be normal in size [5]. Postnatal ultrasound and MR will demonstrate findings similar to prenatal imaging, with prominence of the cisterna magna. There can be mild associated enlargement of the posterior fossa with subtle scalloping of the occipital calvarium, but marked bony remodeling or mass effect on normal posterior fossa structures suggests an alternate diagnosis. Cerebellar morphology and growth remain normal. 4. Blake's pouch cyst

Fig. 1. Mega cisterna magna incidentally detected on prenatal ultrasound. (a) Axial sonographic image from a 36-week prenatal ultrasound demonstrates enlargement of the cisterna magna (2.3 cm marked by calipers) with normal cerebellar contours. Sagittal reformat (b) from a three-dimensional US shows a fully formed vermis and large cisterna (arrow). (c) Midline sagittal T2-weighted image from a prenatal MR at 38-weeks gestational age confirms enlargement of the cisterna magna with normally formed cerebellum and normal tegmento-vermian angle (angle measurement shown in c). Postnatal images confirmed mega cisterna magna in this fetus (not shown here).

As described above (Section 1.2), BPC refers to the abnormal persistence of a normal developmental membrane past the 10th gestational week. Lack of normal fenestration of the median foramen results in impaired cerebrospinal fluid (CSF) egress from the ventricular system into the subarachnoid space and the nonfenestrated membrane balloons out into a retrocerebellar cyst (Fig. 2a & b). By definition, the vermis and cerebellar hemispheres are normally formed [2,6]. Depending on the patency of the foramina of Luschka, persistent BPC can therefore present with hydrocephalus of varying severity. Presence of hydrocephalus is the major determinant of prognosis, as this is often an otherwise benign and asymptomatic anomaly. On prenatal ultrasound, persistent BPC can present with apparent enlargement of the posterior subarachnoid space, mimicking MCM on the standard posterior fossa view. However, an adequate midline sagittal image with either ultrasound or MR can provide characteristic features. These include (a) normal vermian size and morphology; (b) counterclockwise rotation of the vermis with enlargement of the fourth ventricle and increased tegmento-vermian angle; and (c) visualization of the cyst roof at the inferior aspect of the cerebellar vermis. Difficulty in distinguishing inferior vermian compression from partial agenesis can make prenatal distinction between inferior vermian dysplasia with retrocerebellar cyst and persistent BPC difficult or impossible [3]. Use of three-dimensional sonographic technique may allow for more accurate assessment of vermian morphology on reconstructed midline sagittal views and can accentuate subtle differences between the cyst contents and the surrounding subarachnoid CSF in the posterior fossa [2,7,8]. Late fenestration has been documented in up to 60% of cases, and postnatal imaging may demonstrate only a prominent cisterna magna, or may be entirely normal (Fig. 2c) [2]. If present postnatally, BPC will demonstrate identical imaging findings to prenatal imaging. Postnatal distinction of cerebellar vermian dysplasia and BPC is facilitated by high-resolution mid-sagittal imaging demonstrating normal vermian lobulation in the latter [3]. Post contrast T1-weighted MR imaging can also be helpful by delineating the location of the enhancing fourth ventricular choroid plexus: it is deficient in vermian agenesis, but is displaced into the roof of a BPC. The choroid plexus will be normally located in the case of mega cisterna magna and posterior fossa arachnoid cyst [8]. Use of contrast is only appropriate with postnatal imaging, as the risk of fetal neurotoxicity from gadolinium exposure in utero warrants avoiding this contrast agent during fetal MRI. 5. Dandy–Walker malformation

The cisterna magna is measured from the posterior margin of the echogenic cerebellar vermis to the inner table of the occipital calvarium [4]. Cerebellar biometry and morphology will be normal, and no evidence of mass effect on the underlying cerebellum should be present

The DWM is believed to result from failure of normal development of the membranous roof of the primitive fourth ventricle [1,3,9]. The DWM consists of partial agenesis or absence of the cerebellar vermis

Please cite this article as: Chapman T, et al, Diagnostic imaging of posterior fossa anomalies in the fetus and neonate: part 2, posterior fossa disorders, Clin Imaging (2014), http://dx.doi.org/10.1016/j.clinimag.2014.10.012

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abnormalities, coarctation of aorta and cardiac defects, and eye defects). Reported non-CNS anomalies include cardiac defects (most common), cleft lip and cleft palate, and neural tube defects [10]. On imaging in the midsagittal plane, a rotated vermis, an enlarged posterior fossa, and a posterior fossa cyst communicating with the fourth ventricle through the defect provided by the agenetic vermis can be seen (Fig. 3). Hydrocephalus is usually present, although not a criterion for diagnosis of DWM. 6. Arachnoid cyst Considered developmental variants rather than true malformations, posterior fossa arachnoid cysts are important mimics of the DWM on prenatal ultrasound when located in the cisterna magna (Fig. 4). Histologically, they represent abnormal fluid collections that develop within the leaves of the arachnoid membrane [1]. There is no direct communication with the surrounding subarachnoid space. Beyond mimicking other posterior fossa abnormalities, their clinical significance lies in their ability to exert mass effect on surrounding structures. Prenatal ultrasound will demonstrate an anechoic fluid collection with mass effect on the surrounding structures (Fig. 4a & b). Fetal MR will demonstrate a similar collection with fluid signal intensity. Large arachnoid cysts can enlarge the posterior fossa and mimic DWMs, but the cerebellum is compressed and not dysplastic [5]. Postnatal imaging features are similar to those seen prenatally (Fig. 4c). 7. Joubert syndrome

Fig. 2. Persistent BPC. Schematic illustration demonstrates cystic distention of a developmental membrane that normally fenestrates and dissolves in fetal development (BP=Blake's pouch; 4th=fourth ventricle;*=extra-axial CSF). Midline sagittal T2-weighted image from a 20-week fetal MR (a) demonstrates prominence of the CSF space in the posterior fossa with enlargement of the fourth ventricle and widening of the tegmento-vermian angle (arrow). Differentiation of an elevated and compressed vermis from vermian agenesis can be difficult on prenatal imaging. Midline sagittal constructive interference in steady state sequence from a postnatal MR at 2 years of age (b) demonstrates a retrocerebellar cyst (arrow) with normal lobulation of the vermis.

with upward rotation of the vermis, variable dysplasia, or agenesis of the cerebellar hemispheres, cystic dilatation of the fourth ventricle, and enlargement of the posterior fossa with upward displacement of the tentorium and torcula [1,3,9]. It can occur as an isolated abnormality or associated with other syndromes such as Walker–Warburg, Meckel Gruber, Coffin–Siris, Frazier cryptophthalmos, Aicardi syndrome, and chromosome syndromes such as trisomy 9, 13, and 18. In the classic DWM, there are multiple associated central nervous system (CNS) abnormalities such as hydrocephalus (70–90%), occipital encephalocele (up to 16%), polymicrogyria or heterotopias (5–10%), partial or complete agenesis of the corpus callosum (30%), and the PHACE syndrome (posterior fossa malformation, facial hemangiomas, and arterial

In 1969, Marie Joubert et al., first described Joubert Syndrome (JS) in four siblings with cerebellar vermis agenesis, ataxia, episodic tachypnea, oculomotor apraxia, and intellectual disability [11]. With the advent of cross-sectional imaging, a pathognomonic midbrain–hindbrain malformation, the molar tooth sign (MTS), was described first in JS and subsequently in several other conditions [12–14]. Recently, the term Joubert syndrome and related disorders (JSRD) has been adopted to describe all disorders presenting with MTS on brain imaging [15]. JSRD are clinically heterogeneous conditions characterized by core features of hypotonia, ataxia, intellectual disability and variable involvement of the retina, kidney, liver, and other tissues/organs in subsets of patients. JSRD is predominantly autosomal recessive with rare instances of X-linked inheritance. Over 20 genes so far have been described making JSRD part of an expanding group of conditions called ciliopathies [16,17]. Prenatal ultrasound (US) diagnosis of Joubert's syndrome is difficult due to the lack of specific signs, and the only recognizable finding may be vermian dysplasia. Prenatal US needs to be complemented with MRI in order to make the diagnosis [18]. The essential features of the MTS on MRI are seen on axial images and include thick, elongated, and horizontal superior cerebellar peduncles (SCPs) with a narrow pontomesencephalic junction (isthmus) and a deep interpeduncular fossa [12]. The findings are thought to be due to absence of decussation of the SCP at the pontomesencephalic junction [19]. MTS can be identified as early as 22 weeks gestation age [20]. On postnatal MRI in JS, the hindbrain malformation is characterized by variable dysplasia or agenesis of the vermis, a deep interpeduncular groove, and thickened SCPs, creating a molar tooth appearance on axial images (Fig. 5a). On sagittal images, the roof of the fourth ventricle is oriented more horizontally, and the fastigium is positioned more superiorly than usual (Fig. 5b). The fourth ventricle opens widely immediately distal to the aqueduct (rather than gradually tapering), and the foramen of Magendie is often enlarged. These findings reflect the horizontal SCP and cerebellar vermian anomalies. The vermis varies from mildly hypoplastic to absent. Dysplasia of the anterior vermis is common with typical midline clefting seen best on coronal images [21]. Diagnosis of a JSRD has implications for diagnostic work-up to identify retinal, kidney, and liver complications, as well as for genetic testing and recurrence risk [14]. Recognition of the MTS is essential for the early

Please cite this article as: Chapman T, et al, Diagnostic imaging of posterior fossa anomalies in the fetus and neonate: part 2, posterior fossa disorders, Clin Imaging (2014), http://dx.doi.org/10.1016/j.clinimag.2014.10.012

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Fig. 3. DWM on prenatal imaging. Transverse ultrasound image (a) through the posterior fossa shows a small cerebellum with a key-hole deformity (white arrow), concerning for a posterior fossa anomaly. Note asymmetric nuchal thickening along the occipital region (double arrows). Sagittal (b) image from a fetal MR study confirms the large posterior fossa (arrowhead) and absent vermis. Axial image (c) through the posterior fossa shows dysplastic cerebellar hemispheres (black arrow) and demonstrates an occipital meningocele (white arrow). Axial T2 image from a postnatal MR (d) demonstrates a large posterior fossa cyst, directly communicating with the fourth ventricle. The cerebellar vermis is severely agenetic (not seen on these images), and the cerebellar hemispheres are hypoplastic/agenetic and asymmetrical. There is a bony defect in the occipital bone (arrow) with a large occipital meningocele.

identification of these patients to allow early treatment of medical complications and allow for informed reproductive decision-making. 8. Rhombencephalosynapsis Rhombencephalosynapsis (RES) is a rare developmental anomaly of the posterior fossa in which there is complete or partial absence of the vermis, with varying degrees of midline fusion of the cerebellar hemispheres and dentate nuclei. It rarely occurs as an isolated anomaly and can be seen in association with a number of intra- and extracranial abnormalities [22]. It frequently occurs with other midline fusion anomalies including mesencephalosynapsis (collicular fusion and aqueductal stenosis or atresia), diencephalosynapsis (thalamic fusion with third ventricular atresia), and holoprosencephaly. It is also described in patients with the vertebral anomalies, anal atresia, cardiovascular anomalies, tracheo-esophageal fistula, renal anomalies, and limb defects association [22]. The most common congenital syndrome associated with RES is Gomez–Lopez–Hernandez syndrome, characterized by RES, scalp alopecia, and abnormal head shape [23]. Neurodevelopmental outcome is predicted both by degree of cerebellar fusion and presence of associated anomalies [24]. Early prenatal imaging findings of RES are subtle and easily overlooked, often demonstrating only cerebellar hypoplasia with

associated mild ventriculomegaly. Prospective sonographic identification of cerebellar hemispheric fusion is often missed but can be suggested by lack of vermian clefting on axial or coronal views of the cerebellum (Fig. 6a) [25]. Third-trimester ultrasound and fetal MR are more sensitive for the diagnosis of this entity and can demonstrate hemispheric fusion and absent or agenetic vermis [26]. Lack of the primary vermian fissure and rounding of the normal fastigial point can be helpful findings on the midline sagittal view [5]. In practice, the diagnosis of RES is often not made until postnatal imaging is performed. On MR, visualization of the cerebellar hemispheric fusion is easily accomplished (Fig. 6a). Axial images will demonstrate a small, diamond- or keyhole-shaped fourth ventricle with posterior fusion of the deep cerebellar white matter and dentate nuclei [27]. A complete evaluation of associated supra- and infratentorial abnormalities is required for prognosis. 9. Chiari malformation Chiari II malformation (also called the Arnold–Chiari malformation, first described in the late 1800s by Chiari then in the early 1900s by two scientists in Arnold's laboratory) is a complex consequence of myelomeningocele, a defect arising from failure of normal closure of the neural tube [28]. The abnormality at the level of the spine results

Please cite this article as: Chapman T, et al, Diagnostic imaging of posterior fossa anomalies in the fetus and neonate: part 2, posterior fossa disorders, Clin Imaging (2014), http://dx.doi.org/10.1016/j.clinimag.2014.10.012

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Fig. 4. Cisterna magna arachnoid cyst incidentally discovered on routine prenatal imaging. Axial sonographic image (a) through the posterior fossa from an ultrasound at 31 weeks demonstrates a large posterior fossa cystic structure (large arrow) with anteriorly compressed cerebellum (small arrows). Sagittal sonographic image (b) from a cranial ultrasound on Day 1 of life demonstrates cystic mass in the posterior fossa. Note the compression on the cerebellum (Cb). Sagittal T2-weighted image (c) from a postnatal MR on Day 2 of life shows the markedly enlarged posterior fossa cyst and mass effect upon the fully formed cerebellar vermis (arrow).

in abnormal brain development. Various animal models and experience with fetal surgery support a multifactorial pathogenesis, including neuronal damage due to spinal cord uncovering and hydrodynamic alterations secondary to a chronic CSF leak influencing the neural axis development [29–31]. The primary manifestation of a Chiari II malformation is a small posterior fossa and low insertion of the tentorium cerebelli, with resultant crowding of the parenchyma (Fig. 7a) [32]. The cerebellar tissue develops abnormally, and both dysplasia and hypoplasia may be appreciable by imaging. The cerebellar vermis herniates superiorly above the tentorium, and the vermis and cerebellar tonsils may herniate inferiorly below the level of the foramen magnum. The mass effect also manifests with inferior displacement of the lower brainstem, a small and inferiorly displaced fourth ventricle, aqueductal stenosis, enlargement of the foramen magnum, and distorted stretching of the mesencephalic tectum (“tectal beaking”). Additional findings in the supratentorial brain often include dysplasia and hypoplasia of the corpus callosum, hydrocephalus, gyral malformations and interdigitation across the falx, and

Fig. 5. JS. Axial T2-weighted image of a 7-month-old male (a) shows fourth ventricle pointed anteriorly (arrow), with thickened horizontal SCPs (double arrows) giving a molar tooth configuration of the midbrain. Sagittal T1-weighted image (b) demonstrates agenetic and dysplastic vermis (black arrow) with a large fourth ventricle which is upwardly convex (double black arrow). In situ view (c) of posterior fossa contents in a fetus following termination of pregnancy at approximately 24 weeks shows abnormal upward angulation of the tentorium cerebelli (arrows) and cerebellar hemispheres (C), deficient vermis (arrowhead), and enlarged cisterna magna.

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Fig. 7. MR and CT findings of Chiari II malformation in a neonate with myelomeningocele. Sagittal T1-weighted image of the brain (a) through the midline shows a small posterior fossa and pointed morphology of the herniating inferior cerebellar tonsil (white arrow), cervical cord syrinx (arrowhead), and corpus callosal dysgenesis (black arrow). Calvarial three-dimensional surface-shaded reconstructions from a CT (b) show clearly the lacunar skull seen in Chiari II malformation, with numerous calvarial pits and thinning.

Fig. 6. RES. Axial T2-weighted image (a) from a postnatal MR of a newborn with prenatally identified small cerebellum and ventriculomegaly (not shown) demonstrates absent vermis and fusion of folia and interfoliate sulci across the midline (arrow), confirming the diagnosis of RES. Ex-situ (b) view of posterior cerebellum in a different male fetus following termination of pregnancy at approximately 21 weeks shows fusion of right and left hemispheres (RES). The fetus also had aqueductal stenosis and ventriculomegaly.

enlargement of the massa intermedia [28,32,33]. The calvarium may also be affected in a Chiari II malformation, with a mesenchymal dysplasia of the calvarial plates, leading to the “luckenschadel” or lacunar skull (Fig. 7b) [34]. Computed tomography (CT) three-dimensional surface reformats of the skull nicely depict the numerous pits and thinning of the inner and outer tables caused by abnormal collagen development and ossification. This abnormality normalizes within the first year of life. Prenatal assessment with fetal ultrasound will document the myelomeningocele, which occurs most often in the lumbar region. The shape of the fetal calvarium is altered, with concave scalloping of the frontal bones, termed the lemon sign (Fig. 8a). The lemon sign is typically diagnosed at 16–22 weeks, and the finding may disappear after 25 weeks gestational age [35,36]. It is important to be aware that the lemon sign is not specific to myelomeningocele and may also be seen in the context of encephalocele and even in congenital brain anomalies not involving the neural tube [37]. Another fetal ultrasound finding in Chiari II is the effaced cisterna magna and the compressed fetal cerebellum, which has an unusual elongated morphology, referred to as the “banana sign” (Fig. 8a) [38,39]. An additional finding in the prenatal sonographic diagnosis of Chiari II after 25 weeks gestational age (GA) is a triangular shape of the posterior horn of the lateral ventricle in coronal plane imaging

(Fig. 8b) [39]. Ventriculomegaly is easily appreciated by ultrasound. Fetal MR imaging will show additional key prognostic factors, including degree of herniation, brainstem dysmorphology, corpus callosal dysgenesis, and cerebral cortical or white matter abnormalities [40]. Fetal surgical repair of myelomeningocele has seen many advances and promising results in the past decade, with reversal of the hindbrain herniation following surgery [31]. Research in this area is ongoing to determine the long-term effects of this type of prenatal intervention. Postnatally, myelomeningocele closure, tethered cord release, and ventriculoperitoneal shunting are typically performed. Posterior fossa decompression may also be performed in severe cases [41].

10. Early ischemic and/or hemorrhagic abnormalities Posterior fossa ischemic insult with or without hemorrhage is an often-overlooked entity. Although not as common as the germinal matrix hemorrhage, it is more prevalent than previously thought [42]. Advances in sonographic resolution and the recent availability of fetal MRI have increased the recognition of this condition. Ultrasound reveals increased echogenicity with ill-defined margins in the cerebellar hemispheres (Fig. 9). In the infant, the best sonographic approach is through the mastoid fontanelle [43]. Small cerebellar bleeds may be seen in premature infants and are thought to originate in the posterior fossa germinal matrix (at the external granular layer of the cerebellum), with similar pathophysiology to the supratentorial germinal matrix hemorrhage. Other etiologies of cerebellar ischemia/hemorrhage include intrauterine infection, coagulopathies, venous sinus thrombosis, and neonatal alloimmune thrombocytopenia [44].

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Fig. 9. Posterior fossa hemorrhage in a preterm infant. Sagittal (a) and coronal (b) ultrasound images from a 15-day-old neonate born at 25 weeks gestational age show ventricular enlargement, intraventricular clot, and echogenic areas (arrow) in the posterior fossa and midbrain region distinct from the echogenic vermis, consistent with hemorrhage.

Fig. 8. Fetal diagnosis of Chiari II malformation by ultrasound. Axial view of the head (a) in a 19-week gestational age fetus with low lumbosacral myelomeningocele (not shown) demonstrates intracranial and calvarial findings of this malformation. Frontal bone inward scalloping, the “lemon sign,” is designated by arrowheads, and effacement of the cisterna magna with a small, compressed cerebellum (calipers and arrows) is called the banana sign. A third prenatal finding is the triangular morphology of the lateral ventricle atrium, as shown in a different 18-week gestational age fetus (b, ventricular margins marked by arrowheads), a finding that persists later in gestation than the lemon and banana signs. (c) Ex situ lateral view of the brain of a 21-week female fetus with Chiari Type II malformation. Note the inferior herniation of cerebellar tonsillar tissue along the upper cervical spinal cord (arrow). The cerebellum exhibits subarachnoid hemorrhage as well. The fetus also had ventriculomegaly and a lumbar myelomeningocele.

11. Posterior fossa masses Fetal brain tumors are rare, accounting only for 10% of all fetal tumors. Of these, the majority are supratentorial masses [45–47]. Reviews show that they are generally diagnosed at a gestational age of 28 weeks (ranging from 18 to 36 weeks). The most common histopathology is the teratoma (a germ cell tumor which is also the most common tumor type in the extracranial fetus as well), followed by astrocytoma, primitive neuroectodermal tumor (PNET), and craniopharyngioma. Of these tissue types, the teratoma and PNET (cerebellar medulloblastoma) are nearly the only tumor types that arise in the posterior fossa in the fetus. Fetal brain tumors universally have a terrible prognosis, with the exception of lipomas and choroid plexus papillomas [46,48]. On imaging, teratomas and primitive tumors tend to be heterogeneous, with both cystic components and hemorrhage. MR imaging has the advantage of more clearly delineating the full extent of the tumor than ultrasound.

Brain tumors in neonates are uncommon, especially compared to older children. The peak age of pediatric brain tumors is between ages 5 and 10 [48]. As with the fetal population, the majority of neonatal tumors in the posterior fossa are teratomas (approximately one third of cases). Astrocytoma is the second most common neonatal tumor, although the vast majority of these in the neonate are supratentorial. PNET accounts for approximately 10–13% of neonatal brain tumors [48]. This tumor type is a highly aggressive malignant tumor that arises from the neural crest. An additional posterior fossa tumor that is seen in infancy is the atypical teratoid/rhabdoid tumor (ATRT), which carries a worse prognosis than the medulloblastoma and is invariably fatal. This tumor primarily originates from the cerebellum, but may also originate from the brainstem or cerebral hemispheres. Brain invasion and early spread through the CSF lead to a short survival, ranging from 6 to 11 months [49]. Posterior fossa tumors demonstrating high cellularity tend to reflect this on diffusion-weighted imaging, with restricted diffusion (Fig. 10) [50]. Staging for these tumors includes MR imaging of the brain and the total spine with contrast, to evaluate for spread of disease through the entire neural axis. 12. Prognosis of posterior fossa malformations Prognostic information should be provided regarding fetal survival, cognitive outcome, motor development, associated anomalies that can be expected in association with the detected anomaly, and the possible risk of recurrence in future pregnancies. The prognosis depends largely on the type of condition and associated anomalies detected antenatally. Clinical outcome appears to be strongly related to other intracranial anomalies (degree of hydrocephalus, cortical malformations, and corpus callosal dysgenesis) and extracranial and chromosomal anomalies [50,51]. Hence, a detailed complete anatomic scan is very critical upon detection of posterior fossa anomaly. In addition, the severity of cerebellar hypoplasia, agenesis, or dysgenesis has been shown to be associated with poor intellectual outcome [52–54]. Co-occurrence of a brainstem

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13. Conclusion Our understanding of posterior fossa and cerebellar malformations is growing, and recent literature supports adopting a nomenclature based on anatomy and morphometry of abnormalities. The fundamental diagnostic approach includes determining whether the posterior fossa is abnormal in size, to recognize the presence of a retrocerebellar cyst, and to assess the cerebellar dimensions to determine if the parenchyma has developed normally. It is imperative that an accurate diagnosis be established since counseling and future management depends largely on the type of condition and associated anomalies detected antenatally. Furthermore, a unified approach to prenatal and postnatal posterior fossa anomalies will enrich the ability of outcomes studies to appropriately provide more prognostic information for infants born with these abnormalities. Acknowledgments None. References

Fig. 10. Posterior fossa fetal ATRT. Third-trimester fetal ultrasound (36 weeks gestational age) through the posterior fossa in axial plane with color Doppler (a) shows a heterogeneous mass with focal areas of echogenicity (white arrows) that could signify either calcification or intratumoral hemorrhage. Massive obstructive hydrocephalus is also appreciable (v=ventricle). Further evaluation with fetal MR imaging was performed. Sagittal T2-weighted single-shot fast spin echo (b) correlates with the ultrasound, and shows T1- and T2-hyperintensities within the posterior fossa tumor, consistent with hemorrhage (arrow). Postnatal brain MR imaging was performed for complete staging work-up. Sagittal T1-weighted image (c) mirrors the findings on fetal MR and demonstrates the complete obliteration of normal posterior fossa anatomy by the large solid mass (arrows), as well as marked hydrocephalus (v=ventricle). Complete spine imaging (not shown) did not reveal any CSF spread of disease. The infant expired on Day 2 of life.

abnormality associated with a cerebellar anomaly carries a greater risk for severely abnormal neurodevelopmental outcome or neonatal death [55]. Although isolated mega cisterna magna and BPC without hydrocephalus have normal developmental outcome, the prognosis of patients with isolated inferior vermian agenesis is variable [54–56], which makes counseling for these often difficult-to-distinguish entities challenging. Importantly, isolated mega cisterna magna, uncomplicated BPC, and most cases of isolated inferior vermian agenesis have normal developmental outcome [54–56].

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Please cite this article as: Chapman T, et al, Diagnostic imaging of posterior fossa anomalies in the fetus and neonate: part 2, posterior fossa disorders, Clin Imaging (2014), http://dx.doi.org/10.1016/j.clinimag.2014.10.012