Review ArticleDiagnostic imaging of posterior fossa anomalies in the fetus and neonate: Part 1, normal anatomy and classification of anomalies☆
Introduction
The cerebellum is one of the earliest structures to develop; however, its development is protracted and continues into the first few postnatal months [1], [2], [3]. An awareness of its evolving structure enables more accurate recognition of a broad spectrum of abnormalities. With advances in both fetal ultrasound and magnetic resonance imaging, abnormalities of the cerebellum and posterior fossa are being increasingly diagnosed prenatally, allowing providers to anticipate management issues at the time of delivery and during early infancy and also to help the parents in comprehension of the prognosis.
Multiple posterior fossa malformations may occur either in isolation or in association with supratentorial anomalies [2], [4], [5]. In recent years, there have been changes in the terminology used and the description and mechanisms of the different entities, and therefore literature about this topic can be somewhat confusing [6], [7]. It is essential to determine the type of posterior fossa malformation as accurately as possible, as it reflects the postnatal outcome and may guide efforts at detection of associated anomalies, both within and outside the CNS. Detection of a severe hindbrain malformation, for instance, merits offering amniocentesis for genetic evaluation, as well as a detailed ultrasound and/or fetal MRI evaluation to rule out CNS and non-CNS anomalies.
This review illustrates the normal appearance of the cerebellum and posterior fossa and discusses the imaging features of various anomalies of the cerebellum and posterior fossa with prenatal and postnatal correlation.
Section snippets
Prenatal sonographic features
Sonographic evaluation of the posterior fossa is a routine part of antenatal imaging. The biometry of the cerebellum, vermis and the cisterna magna is evaluated. During prenatal imaging with ultrasound, visualization in the axial plane is required to evaluate the cerebellum. The axial plane for imaging the cerebellum is inferior to the plane used for acquiring the biparietal diameter. On the axial plane image, the cerebellar hemispheres appear hypoechoic with intermittent echogenic stripes,
Discussion of published classifications of posterior fossa malformations
Posterior fossa malformations have been classified as either embryological [37], [38], [39], etiological [40], or morphological/anatomical [41], [42], [43]. The embryological classification, although extensive, is inherently incomplete, as many conditions are only partially understood embryologically. The classification based on the etiology divides conditions into genetic and acquired, which remains rather generic. Furthermore, many conditions have overlapping etiologies.
The morphological
Anatomic approach to evaluation of posterior fossa on pre- and postnatal diagnostic imaging
In the next part of this review, we will describe an anatomy-based approach to posterior fossa malformations. Following an elaboration of an anatomic approach to diagnosis by imaging, specific abnormalities will be discussed with illustrative images. The following imaging features are to be considered to arrive at the right diagnosis (summarized in Table 1):
- 1.
Abnormal retrocerebellar fluid space
- 2.
Abnormal posterior fossa size
- 3.
Abnormal cerebellar size
- 4.
Abnormal cerebellar morphology
The first and most
Terminology of cerebellar developmental anomalies
A brief note regarding the terminology is merited to improve classification of the malformations appropriately. The term “agenesis” refers to anatomical absence of a structure, which is either complete or partial. “Hypoplasia” indicates an anatomically intact structure that is small in volume. Often, the terms agenesis and hypoplasia are used interchangeably. For instance, the term “vermian hypoplasia” is often used in the literature, although the entity might be more correctly called “vermian
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
Acknowledgments
None.
References (53)
- et al.
Defining the nature of the cerebral abnormalities in the premature infant: a qualitative magnetic resonance imaging study
J Pediatr
(2003) - et al.
Ultrasound detection of posterior fossa abnormalities in full-term neonates
Early Hum Dev
(2012) - et al.
Human malformations of the midbrain and hindbrain: review and proposed classification scheme
Mol Genet Metab
(2003) Malformations of the posterior fossa: current perspectives
Semin Pediatr Neurol
(2002)Pontocerebellar hypoplasias. An overview of a group of inherited neurodegenerative disorders with fetal onset
Brain Dev
(1993)- et al.
Normal and abnormal development of the human nervous system
(1975) - et al.
The fetal cerebellum: development and common malformations
J Child Neurol
(2011) - et al.
Late gestation cerebellar growth is rapid and impeded by premature birth
Pediatrics
(2005) - et al.
Sonographic detection of inferior vermian agenesis in Dandy-Walker malformations: prognostic implications
Radiology
(1994) - et al.
Correlation of prenatal ultrasound diagnosis and pathologic findings in fetal brain abnormalities
Ultrasound Obstet Gynecol
(2000)
Posterior fossa anomalies diagnosed with fetal MRI: associated anomalies and neurodevelopmental outcomes
Prenat Diagn
Inferior vermian hypoplasia–preconception, misconception
Ultrasound Obstet Gynecol
Dandy-Walker variant: prenatal sonographic features and clinical outcome
Radiology
The fetal cerebellar vermis: anatomy and biometric assessment using volume contrast imaging in the C-plane (VCI-C)
Ultrasound Obstet Gynecol
Posterior fossa and vermian morphometry in the characterization of fetal cerebellar abnormalities: a prospective three-dimensional ultrasound study
Ultrasound Obstet Gynecol
Closure of the cerebellar vermis: evaluation with second trimester US
Radiology
Fetal cerebellum: US appearance with advancing gestational age
Radiology
Fetal transverse cerebellar diameter measurements in normal and reduced fetal growth
Ultrasound Obstet Gynecol
Cerebral biometry in fetal magnetic resonance imaging: new reference data
Ultrasound Obstet Gynecol
Sonographic developmental milestones of the fetal cerebral cortex: a longitudinal study
Ultrasound Obstet Gynecol
The development of the fetal vermis: an in-utero sonographic evaluation
Ultrasound Obstet Gynecol
. IRPA guidelines on protection against non-ionizing radiation: the collected publications of the IRPA Non-Ionizing Radiation Committee
A pathological approach to anomalies of the posterior fossa
Birth Defects Res A Clin Mol Teratol
A magnetic resonance template for normal cerebellar development in the human fetus
Neurosurgery
How accurately does current fetal imaging identify posterior fossa anomalies?
AJR Am J Roentgenol
MRI of the fetal posterior fossa
Pediatr Radiol
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Congenital Malformations of Cerebellum
2022, Clinics in PerinatologyCitation Excerpt :One classic categorization is to divide malformations into cystic or noncystic. Additional anatomic-based classifications exist to further differentiate the cystic lesions, which may be described in terms of abnormal retrocerebellar fluid space and abnormal posterior fossa size.7 Noncystic malformations are a more diverse group of conditions that share abnormalities related to varying degrees of cerebellar agenesis or dysgenesis.9
The Role of Fetal MRI for Suspected Anomalies of the Posterior Fossa
2021, Pediatric NeurologyCitation Excerpt :Without sagittal midline views on prenatal US, it is difficult to completely image the cerebellar vermis.16 US scans may be more likely to falsely diagnose vermis hypoplasia, vermis dysplasia, or cystic malformations of the PF, if midline sagittal views are not obtained or if US occurs too early in gestation.16-19 Vermian hypoplasia and cerebellar hypoplasia may be isolated or due to specific etiologies, including Joubert syndrome and related disorders, rhombencephalosynapsis, tubulinopathies, congenital infections and/or injuries, and a myriad of genetic/metabolic syndromes.15,20
A rare case of fetal ectopic brain tissue in the posterior fossa
2020, European Journal of Obstetrics and Gynecology and Reproductive BiologySonography of the Fetal Central Nervous System
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2018, Obstetric Imaging: Fetal Diagnosis and Care: Second EditionUltrasound and CT of the posterior fossa in neonates
2018, Handbook of Clinical NeurologyCitation Excerpt :The above-described neuroimaging findings are best appreciated on MRI and the value of posterior fossa HUS is more limited in diagnosis of Joubert syndrome (Fig. 12.9). Rhombencephalosynapsis is a rare, sporadic malformation of the cerebellum (Bosemani et al., 2015; Chapman et al., 2015; Poretti et al., 2016). Key neuroimaging findings can be detected with posterior fossa HUS: small cerebellum with agenesis of the vermis, continuity of the cerebellar hemispheric white matter across midline with fused dentate nuclei (Poretti et al., 2016).
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