Posterior fossa malformations: main features and limits in prenatal diagnosis

Fetal Brain Development: Regulating Processes and Related Malformations

This paper describes the contemporary state of knowledge regarding processes that regulate normal development of the embryonic–fetal central nervous system (CNS). The processes are described according to the developmental timetable: dorsal induction, ventral induction, neurogenesis, neuronal migration, post-migration neuronal development, and cortical organization. We review the current literature on CNS malformations associated with these regulating processes. We specifically address neural tube defects, holoprosencephaly, malformations of cortical development (including microcephaly, megalencephaly, lissencephaly, cobblestone malformations, gray matter heterotopia, and polymicrogyria), disorders of the corpus callosum, and posterior fossa malformations. Fetal ventriculomegaly, which frequently accompanies these disorders, is also reviewed. Each malformation is described with reference to the etiology, genetic causes, prenatal sonographic imaging, associated anomalies, differential diagnosis, complimentary diagnostic studies, clinical interventions, neurodevelopmental outcome, and life quality.

Indications for magnetic resonance imaging of the fetal central nervous system: recommendations from the European Society of Paediatric Radiology Fetal Task Force

Fetal central nervous system MRI is a well-established method to complement a high-quality fetal ultrasound and to clarify sonographically detected abnormalities in complex pregnancies. However, there is still worldwide heterogeneity and confusion regarding the indications of fetal central nervous system MRI, which has roots in differences among countries regarding the performance of ultrasound examinations and legislation on pregnancy termination. The purpose of this article is to clarify the indications for fetal central nervous system MRI by focusing on the ultrasound findings that guide further investigation with MRI and highlight the strengths and the weaknesses of each modality on imaging the fetal central nervous system.

Fetal MRI assessment of posterior fossa anomalies: A review

Prenatal ultrasound (US) is the first prenatal imaging tool for screening and evaluation of posterior fossa malformations since it is noninvasive, widely available, and safe for both mother and child. Fetal MRI is a widely used secondary technique to confirm, correct, or complement questionable US findings and plays an essential role in evaluating fetuses with suspected US findings and /or positive family history. The main sequences of fetal MRI consist of T2-weighted (T2w) ultrafast, single-shot sequences. Axial, coronal, and sagittal images are typically acquired allowing for a detailed evaluation of the posterior fossa contents. Also, various complimentary sequences, such as T1w, T2*w gradient sequences, or advanced techniques, including diffusion-weighted imaging, diffusion tensor imaging, and magnetic resonance spectroscopy, may provide additional information based on the studied malformation. Inclusion of these techniques should be done with careful risk-benefit analysis. The use of fetal MRI also aims to evaluate for associated anomalies. In addition, prenatal diagnosis of posterior fossa malformations is still a challenge but advances in knowledge in human developmental anatomy, genetic, and imaging recognition patterns have enabled us to shed some light on prognostic information that will help with the counseling of families. Finally, high-resolution late third trimester fetal MRI offers a safe alternative to early postnatal MR imaging, basically taking advantage of the uterine environment as a kind of "maternal incubator." Our goal is to discuss the spectrum of prenatal posterior fossa pathologies that can be studied by fetal MRI and their key neuroimaging features.

Biometric assessments of the posterior fossa by fetal MRI: A systematic review

BACKGROUND

Posterior fossa abnormalities (PFAs) are commonly identified within routine screening and are a frequent indication for fetal magnetic resonance imaging (MRI). Although biometric measurements of the posterior fossa (PF) are established on fetal ultrasound and MRI, qualitative visual assessments are predominantly used to differentiate PFAs.

OBJECTIVES

This systematic review aimed to assess 2-dimensional (2D) biometric measurements currently in use for assessing the PF on fetal MRI to delineate different PFAs.

METHODS

The protocol was registered (PROSPERO ID CRD42019142162). Eligible studies included T2-weighted MRI PF measurements in fetuses with and without PFAs, including measurements of the PF, or other brain areas relevant to PFAs.

RESULTS

59 studies were included - 6859 fetuses had 62 2D PF and related measurements. These included linear, area and angular measurements, representing measures of PF size, cerebellum/vermis, brainstem, and supratentorial measurements. 11 measurements were used in 10 or more studies and at least 1200 fetuses. These dimensions were used to characterise normal for gestational age, diagnose a range of pathologies, and predict outcome.

CONCLUSION

A selection of validated 2D biometric measurements of the PF on fetal MRI may be useful for identification of PFA in different clinical settings. Consistent use of these measures, both clinically and for research, is recommended.

Prenatal visualisation of the torcular herophili by means of a Doppler technology highly sensitive for low-velocity flow in the expert assessment of the posterior fossa: a prospective study

OBJECTIVE

To evaluate the usefulness of a Doppler technology highly sensitive for low-velocity flow in the antenatal imaging of the torcular herophili (TH) in the second trimester of pregnancy.

DESIGN

Prospective study.

SETTING

Referral Fetal Medicine Unit.

POPULATION

Non-consecutive series of singleton pregnancies submitted to antenatal neurosonogram between 20 and 28 weeks of gestation.

METHODS

A midsagittal section of the fetal brain was obtained by insonating through the anterior fontanelle, then the MV-Flow™ and LumiFlow™ presets were selected to visualise the TH as the posterior confluence of the superior sagittal sinus and the straight sinus.

MAIN OUTCOME MEASURES

Evaluation of the anatomic relationship of the TH with the 'transpalatal line' joining the upper bony palate to the fetal skull.

RESULTS

A total of 99 pregnant women were recruited, including one fetus with open spina bifida, one with Dandy-Walker malformation (DWM) and two with Blake's pouch cysts. In normal fetuses, the TH appeared to lie on or just below the 'transpalatal line'. In the cases of Blake's pouch cyst, the position of the TH appeared normal if compared with controls, whereas in DWM a supra-elevated position of the TH in respect of the transpalatal line was demonstrated. Finally, in the fetus with Chiari II malformation the TH was identified below the 'transpalatal plane'.

CONCLUSIONS

Prenatal ultrasound visualisation of the TH by means of newly developed Doppler technologies characterised by high sensitivity for low-velocity flow is feasible and allows the indirect evaluation of the insertion of cerebellar tentorium in the second trimester.

TWEETABLE ABSTRACT

Prenatal imaging of the torcular herophili using a Doppler technology highly sensitive for low-velocity flow.

Quantitative diagnostic advantages of three-dimensional ultrasound volume imaging for fetal posterior fossa anomalies: Preliminary establishment of a prediction model

OBJECTIVES

To quantitatively assess prenatal diagnostic performance of three-dimensional ultrasound (3D-US) for posterior fossa anomalies (PFA) and establish a preliminarily 3D-US prediction model.

METHODS

Sixty singleton fetuses suspected of PFA by 2D-US presented their detailed 3D-US evaluation. The surface area of vermis (SAV), brainstem-vermis, and brainstem-tentorium angles were measured by 3D-US. The good prognosis was defined as normal neurodevelopmental outcome. MRI and autopsy were the diagnostic reference standard.

RESULTS

There was a significant difference between 2D-US (60.0%, 36/60) and 3D-US (94.8%, 55/58) for the diagnostic accuracy (P < .01). Prenatal 3D-US prediction model was established with observed/expected SAV as the main predictor (area under the curve [AUC]: 0.901; 95% CI, 0.810-0.992, P < .001). When it was more than 107.5%, the prognosis seemed to be good (sensitivity: 96.4%, specificity: 26.7%), which led to consideration of mega cisterna magna, Blake pouch cyst, or small arachnoid cyst. The prognosis appeared to be poor when it was less than 73% (sensitivity: 71.4%, specificity: 100%), and the diagnosis tended to be a Dandy-Walker malformation, vermian hypoplasia, and cerebellar hypoplasia. Brainstem-vermis and brainstem-tentorium angles were the secondary indicators (AUC: 0.689 vs 0.761; 95% CI, 0.541-0.836 vs 0.624-0.897, P = .014 vs.001).

CONCLUSIONS

It seems that the exact types of PFA can be effectively diagnosed by quantitative indicators of 3D-US.

The cerebellar "tilted telephone receiver sign" enables prenatal diagnosis of PHACES syndrome

OBJECTIVES

To describe a unique posterior fossa neuroimaging characteristic of prenatal PHACES syndrome (PS): unilateral cerebellar hypoplasia (UCH) and ipsilateral posterior fossa (PF) cyst communicating with an asymmetrically distended 4th ventricle.

METHODS

The registries of seven prenatal diagnosis centers were searched for cases with PF findings and a postnatal diagnosis of PS. All records were evaluated for ultrasound and MRI findings and the postnatal outcome. PS was diagnosed after birth according to the consensus statement on diagnostic criteria for PS from 2009. The imaging findings of the PS fetuses were compared to a group of consecutive cases with fetal UCH, whose postnatal diagnosis was not PS.

RESULTS

The PS group included 10 fetuses. All were referred due to UCH accompanied by an ipsilateral retrocerebellar cyst. All pregnancies resulted in livebirths, all newborns had a large segmental facial hemangioma. In all PS fetuses the affected cerebellar hemisphere was upwardly displaced by an ipsilateral PF cyst communicating with an asymmetrically distended 4th ventricle. An upwardly rotated and deviated vermis merged with the contralateral cerebellar peduncles forming an elongated oblique connection between the cerebellar hemispheres, resulting in a unique cerebellar shape, "a tilted telephone receiver sign" (TTRS), on the coronal plane through the upper vermis.The non-PS group included 11 fetuses with UCH: clastic cerebellar lesions (8) and a unilateral PF arachnoid cyst (3). The TTRS was not depicted in any of them (p < 0.0005).

CONCLUSIONS

The cerebellar TTRS is a specific fetal imaging feature of PHACES syndrome enabling its prenatal diagnosis.

Prenatal imaging of posterior fossa disorders. A review

With advances in fetal imaging, prenatal diagnosis of posterior fossa anomalies has been greatly improved. Based on the anatomical approach proposed by Guibaud and Desportes in 2006, the main anomalies depicted according to the algorithm includes: (1) increased "fluid-filled" space of the posterior fossa, (2) abnormal biometry of the cerebellum, and (3) abnormal cerebellar anatomy. In this review, the spectrum of PF anomalies is covered in an attempt to update this approach in the light of both our experience, more than a decade since this algorithm was published, and the latest data in the literature.

Establishment of normative values for the fetal posterior fossa by magnetic resonance imaging

OBJECTIVE

Suspected Dandy-Walker continuum anomalies constitute a significant percentage of prenatal cases evaluated by magnetic resonance imaging (MRI). To unify the description of posterior fossa malformations, we sought to establish objective measurements for the posterior fossa in normal fetuses between 18 and 37 weeks gestation.

METHODS

T2-weighted images of normal fetal brains in sagittal projection were obtained from fetal magnetic resonance (MR) studies of normal brains performed from 2009 to 2017.121 fetal brains were included in the analysis. Three radiologists reviewed images and recorded the following for each case: superior posterior fossa angle (SPFA), posterior fossa perimeter, and tegmento-vermian angle (TVA).

RESULTS

For each feature, the mean of the measurements, the percentage of absolute difference of the reader measurement compared with mean measurement, and the interclass correlation (ICC) were calculated. Values are reported as mean ± standard deviation. Perimeter increases linearly with age, whereas the SPFA and the TVA are independent of gestational age. For all included cases, the SPFA averaged 100.9° ± 8° and the TVA averaged 2.5° ± 2.3°.

CONCLUSION

The superior posterior fossa angle, a novel measurement, and the posterior fossa perimeter can be used for establishing the expected size of the posterior fossa in second- and third-trimester fetuses by MRI.

Fetal cerebellar disorders

The embryologic development of the cerebellum extends over a long time period, thus making it vulnerable to a broad spectrum of malformations and disruptions. Knowledge of the main steps of fetal posterior fossa development; the normal imaging patterns at different stages of embryogenesis; the large spectrum of cerebellar malformations; and their clinical presentations enables diagnosis and precise counseling of parents. Sonography is the most important imaging method for the screening of cerebellar malformations since it is noninvasive, widely available, and safe for both mother and child. The ultrasonographic approach for the evaluation of the fetal posterior fossa is based on the classic transabdominal visualization of axial planes with addition when indicated of a more comprehensive, multiplanar transvaginal or transfundal approach, including coronal and sagittal imaging planes. Fetal magnetic resonance imaging (MRI) has become an adjunct to prenatal ultrasound since the 1980s. Good-quality images have been obtained thanks to the implementation of fast and ultrafast MRI sequences. Fetal MRI has higher-contrast resolution than prenatal sonography and may contribute to the differentiation of normal from abnormal tissue. Both prenatal neurosonography and fetal MRI enable accurate prenatal diagnosis of most posterior fossa anomalies.

Ultrasound and CT of the posterior fossa in neonates

Ultrasound, CT and MRI may all be used in the evaluation of the posterior fossa in neonates depending on the clinical scenario. Ultrasonography is particularly valuable for the evaluation of the neonatal brain because of the lack of ionizing radiation and the ability to perform exams at the bedside and, importantly, advancements in ultrasound technology now allow for diagnostic-quality imaging. While CT is still the initial imaging modality of choice in most neurologic emergencies, in the neonate, ultrasound is the first line in nontraumatic emergencies. The goal of this chapter is to discuss the ultrasound technique for evaluation of the cerebellum, to describe the normal sonographic and CT appearance of posterior fossa and to provide the classical findings of the most common cerebellar abnormalities. While ultrasound is able to accurately diagnosis a majority of cerebellar abnormalities in neonates, subsequent MR imaging remains essential to confirm findings and to detect associated abnormalities.

Systematic review and meta-analysis of isolated posterior fossa malformations on prenatal imaging (part 2): neurodevelopmental outcome

OBJECTIVES

Diagnosis of isolated posterior fossa anomalies in children is biased by the fact that only those that are symptomatic are brought to the attention of the appropriate clinical personnel, and the reported rate is often affected by the adoption of different nomenclature, diagnostic criteria, outcome measures, duration of follow-up and neurodevelopmental tools. The aim of this systematic review was to explore the neurodevelopmental outcome of fetuses with a prenatal diagnosis of isolated posterior fossa anomalies.

METHODS

MEDLINE and EMBASE were searched electronically, utilizing combinations of the relevant medical subject heading terms for 'posterior fossa' and 'outcome'. Studies assessing the neurodevelopmental outcome in children with a prenatal diagnosis of isolated posterior fossa malformations were considered eligible. The posterior fossa anomalies analyzed included Dandy-Walker malformation (DWM), mega cisterna magna (MCM), Blake's pouch cyst (BPC) and vermian hypoplasia (VH). Two authors reviewed all abstracts independently. Quality assessment of the included studies was performed using the Newcastle-Ottawa Scale for cohort studies. Meta-analyses of proportions were used to combine data, and between-study heterogeneity was explored using the I(2) statistic.

RESULTS

A total of 1640 articles were identified; 95 were assessed for eligibility and a total of 16 studies were included in the systematic review. The overall rate of abnormal neurodevelopmental outcome in children with a prenatal diagnosis of DWM was 58.2% (95% CI, 21.8-90.0%) and varied from 0-100%. In those with a prenatal diagnosis of MCM, the rate of abnormal neurodevelopmental outcome was 13.8% (95% CI, 7.3-21.9%), with a range of 0-50%. There was no significant association between BPC and the occurrence of abnormal neurodevelopmental delay, with a rate of 4.7% (95% CI, 0.7-12.1%) and range of 0-5%. Although affected by the very small number of studies, there was a non-significant occurrence of abnormal neurodevelopmental delay in children with a prenatal diagnosis of VH, with a rate of 30.7% (95% CI, 0.6-79.1%) and range of 0-100%.

CONCLUSIONS

Fetuses diagnosed with isolated DWM are at high risk of abnormal neurodevelopmental outcome, while isolated MCM or BPC have a generally favorable outcome. The risk of abnormal developmental delay in cases with isolated VH needs to be further assessed. In view of the wide heterogeneity in study design, time of follow-up, neurodevelopmental tests used and the very small number of included cases, further future large prospective studies with standardized and objective protocols for diagnosis and follow-up are needed in order to ascertain the rate of abnormal neurodevelopmental outcome in children with isolated posterior fossa anomalies. Copyright © 2015 ISUOG. Published by John Wiley & Sons Ltd.

Systematic review and meta-analysis of isolated posterior fossa malformations on prenatal ultrasound imaging (part 1): nomenclature, diagnostic accuracy and associated anomalies

OBJECTIVE

To explore the outcome in fetuses with prenatal diagnosis of posterior fossa anomalies apparently isolated on ultrasound imaging.

METHODS

MEDLINE and EMBASE were searched electronically utilizing combinations of relevant medical subject headings for 'posterior fossa' and 'outcome'. The posterior fossa anomalies analyzed were Dandy-Walker malformation (DWM), mega cisterna magna (MCM), Blake's pouch cyst (BPC) and vermian hypoplasia (VH). The outcomes observed were rate of chromosomal abnormalities, additional anomalies detected at prenatal magnetic resonance imaging (MRI), additional anomalies detected at postnatal imaging and concordance between prenatal and postnatal diagnoses. Only isolated cases of posterior fossa anomalies - defined as having no cerebral or extracerebral additional anomalies detected on ultrasound examination - were included in the analysis. Quality assessment of the included studies was performed using the Newcastle-Ottawa Scale for cohort studies. We used meta-analyses of proportions to combine data and fixed- or random-effects models according to the heterogeneity of the results.

RESULTS

Twenty-two studies including 531 fetuses with posterior fossa anomalies were included in this systematic review. The prevalence of chromosomal abnormalities in fetuses with isolated DWM was 16.3% (95% CI, 8.7-25.7%). The prevalence of additional central nervous system (CNS) abnormalities that were missed at ultrasound examination and detected only at prenatal MRI was 13.7% (95% CI, 0.2-42.6%), and the prevalence of additional CNS anomalies that were missed at prenatal imaging and detected only after birth was 18.2% (95% CI, 6.2-34.6%). Prenatal diagnosis was not confirmed after birth in 28.2% (95% CI, 8.5-53.9%) of cases. MCM was not significantly associated with additional anomalies detected at prenatal MRI or detected after birth. Prenatal diagnosis was not confirmed postnatally in 7.1% (95% CI, 2.3-14.5%) of cases. The rate of chromosomal anomalies in fetuses with isolated BPC was 5.2% (95% CI, 0.9-12.7%) and there was no associated CNS anomaly detected at prenatal MRI or only after birth. Prenatal diagnosis of BPC was not confirmed after birth in 9.8% (95% CI, 2.9-20.1%) of cases. The rate of chromosomal anomalies in fetuses with isolated VH was 6.5% (95% CI, 0.8-17.1%) and there were no additional anomalies detected at prenatal MRI (0% (95% CI, 0.0-45.9%)). The proportions of cerebral anomalies detected only after birth was 14.2% (95% CI, 2.9-31.9%). Prenatal diagnosis was not confirmed after birth in 32.4% (95% CI, 18.3-48.4%) of cases.

CONCLUSIONS

DWM apparently isolated on ultrasound imaging is a condition with a high risk for chromosomal and associated structural anomalies. Isolated MCM and BPC have a low risk for aneuploidy or associated structural anomalies. The small number of cases with isolated VH prevents robust conclusions regarding their management from being drawn. Copyright © 2015 ISUOG. Published by John Wiley & Sons Ltd.

Consensus paper: radiological biomarkers of cerebellar diseases

Hereditary and sporadic cerebellar ataxias represent a vast and still growing group of diseases whose diagnosis and differentiation cannot only rely on clinical evaluation. Brain imaging including magnetic resonance (MR) and nuclear medicine techniques allows for characterization of structural and functional abnormalities underlying symptomatic ataxias. These methods thus constitute a potential source of radiological biomarkers, which could be used to identify these diseases and differentiate subgroups of them, and to assess their severity and their evolution. Such biomarkers mainly comprise qualitative and quantitative data obtained from MR including proton spectroscopy, diffusion imaging, tractography, voxel-based morphometry, functional imaging during task execution or in a resting state, and from SPETC and PET with several radiotracers. In the current article, we aim to illustrate briefly some applications of these neuroimaging tools to evaluation of cerebellar disorders such as inherited cerebellar ataxia, fetal developmental malformations, and immune-mediated cerebellar diseases and of neurodegenerative or early-developing diseases, such as dementia and autism in which cerebellar involvement is an emerging feature. Although these radiological biomarkers appear promising and helpful to better understand ataxia-related anatomical and physiological impairments, to date, very few of them have turned out to be specific for a given ataxia with atrophy of the cerebellar system being the main and the most usual alteration being observed. Consequently, much remains to be done to establish sensitivity, specificity, and reproducibility of available MR and nuclear medicine features as diagnostic, progression and surrogate biomarkers in clinical routine.

Normal fetal posterior fossa in MR imaging: new biometric data and possible clinical significance

BACKGROUND AND PURPOSE

Posterior fossa malformations are a common finding in prenatal diagnosis. The objectives of this study are to re-evaluate existing normal MR imaging biometric data of the fetal posterior fossa, suggest and evaluate new parameters, and demonstrate the possible clinical applications of these data.

MATERIALS AND METHODS

This was a retrospective review of 215 fetal MR imaging examinations with normal findings and 5 examinations of fetuses with a suspected pathologic posterior fossa. Six previously reported parameters and 8 new parameters were measured. Three new parameter ratios were calculated. Interobserver agreement was calculated by using the intraclass correlation coefficient.

RESULTS

For measuring each structure, 151-211 MR imaging examinations were selected, resulting in a normal biometry curve according to gestational age for each parameter. Analysis of the ratio parameters showed that vermian lobe ratio and cerebellar hemisphere ratio remain constant with gestational age and that the vermis-to-cisterna magna ratio varies with gestational age. Measurements of the 5 pathologic fetuses are presented on the normal curves. Interobserver agreement was excellent, with the intraclass correlation coefficients of most parameters above 0.9 and only 2 parameters below 0.8.

CONCLUSIONS

The biometry curves derived from new and existing biometric data and presented in this study may expand and deepen the biometry we use today, while keeping it simple and repeatable. By applying these extensive biometric data on suspected abnormal cases, diagnoses may be confirmed, better classified, or completely altered.

Assessment of fetal midbrain and hindbrain in mid-sagittal cranial plane by three-dimensional multiplanar sonography. Part 1: comparison of new and established nomograms

OBJECTIVE

To construct nomograms for fetal midbrain (MB) and hindbrain (HB) dimensions, assessed in the mid-sagittal cranial plane by three-dimensional multiplanar sonographic reconstruction (3D-MPR).

METHODS

This was a prospective cross-sectional study of 334 healthy fetuses in low-risk singleton pregnancies between 16 and 35 gestational weeks. All sonographic volumes were obtained by sagittal acquisition. The following MB and HB parameters were evaluated in the mid-sagittal cranial plane using 3D-MPR: MB parameters tectal length (TL) and anteroposterior midbrain diameter (APMD), and HB parameters anteroposterior pons diameter (APPD), superoinferior vermian diameter (SIVD), anteroposterior vermian diameter (APVD) and anteroposterior diameter of the fourth ventricle (APDFV). The measurements were presented as growth charts according to gestational age.

RESULTS

MB and HB biometry were best assessed between 19 and 29 weeks. During this period, adequate visualization was achieved for successful measurement of TL in 90.9% of cases, APMD in 86.6%, APPD in 73.7%, SIVD in 74.2%, APVD in 71% and APDFV in 71%. There was a linear growth pattern, with Pearson correlation coefficients of 0.79 for TL, 0.88 for APMD, 0.91 for APPD, 0.95 for SIVD, 0.88 for APVD and 0.88 for APDFV (P < 0.0001 for each). The mean intra- and interobserver variations for the MB measurements and vermian diameters ranged between 4.3% and 9%. APPD and APDFV showed highest mean variations: 9.0% and 19.4% (intraobserver) and 11.6% and 17.7% (interobserver), respectively.

CONCLUSION

We present new nomograms for assessment of the fetal MB and HB using 3D-MPR in the mid-sagittal cranial plane. To our knowledge, these are the first proposed nomograms for fetal MB dimensions.

Diagnostic imaging of posterior fossa anomalies in the fetus and neonate: part 2, Posterior fossa disorders

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.

Prenatal unilateral cerebellar hypoplasia in a series of 26 cases: significance and implications for prenatal diagnosis

OBJECTIVE

To define imaging patterns of unilateral cerebellar hypoplasia (UCH), discuss possible pathophysiological mechanisms and underline the etiology and prognosis associated with these lesions.

METHODS

In this retrospective study we reviewed the charts of 26 fetuses diagnosed between 2003 and 2011 with UCH, defined by asymmetrical cerebellar hemispheres with or without decreased transverse cerebellar diameter. The review included analysis of the anatomy of the cerebellar hemispheres, including foliation, borders and parenchymal echogenicity, and of the severity of the hypoplasia. Data from clinical and biological work-up and follow-up were obtained.

RESULTS

Our series could be divided into two groups according to whether imaging features changed progressively or remained constant during follow-up. In Group 1 (n = 8), the progression of imaging features, echogenic cerebellar changes and/or hyposignal in T2*-weighted MR images were highly suggestive of ischemic/hemorrhagic insult. In Group 2 (n = 18), imaging features remained constant during follow-up; UCH was associated with abnormal foliation in three proven cases of clastic lesions, a cystic lesion was noted in three cases of PHACE (posterior fossa anomalies, hemangioma, arterial anomalies, cardiac abnormalities/aortic coarctation, eye abnormalities) syndrome and, in the remaining cases, UCH remained unchanged, with no imaging pattern typical of hemorrhage. In 24 cases the infant was liveborn and follow-up was continued in 23, for a mean period of 3 years. Among these, neurological complications were identified in seven (in one of seven (at a mean of 46 months) in Group 1 and in six of 16 (at a mean of 35 months) in Group 2). The surface loss of cerebellar hemisphere was > 50% in 19/24 fetuses and the vermis was clearly normal in appearance in 19/24. Predisposing factors for fetal vascular insult were identified in eight cases: these included maternal alcohol addiction, diabetes mellitus, congenital cytomegalovirus infection and pathological placenta with thrombotic vasculopathy and infarctions.

CONCLUSION

UCH is defined as a focal lesion of the cerebellum that may be secondary to hemorrhage and/or ischemic insult, suggesting a clastic origin, particularly when imaging follow-up reveals changes over time. UCH may also be a clue for the prenatal diagnosis of PHACE syndrome. The amount of surface loss of cerebellar hemisphere does not correlate with poor prognosis. UCH with normal vermis is often associated with normal outcome.

Multidimensional analysis of fetal posterior fossa in health and disease

Fetal magnetic resonance imaging (MRI) is now routinely used to further investigate cerebellar malformations detected with ultrasound. However, the lack of 2D and 3D biometrics in the current literature hinders the detailed characterisation and classification of cerebellar anomalies. The main objectives of this fetal neuroimaging study were to provide normal posterior fossa growth trajectories during the second and third trimesters of pregnancy via semi-automatic segmentation of reconstructed fetal brain MR images and to assess common cerebellar malformations in comparison with the reference data. Using a 1.5-T MRI scanner, 143 MR images were obtained from 79 normal control and 53 fetuses with posterior fossa abnormalities that were grouped according to the severity of diagnosis on visual MRI inspections. All quantifications were performed on volumetric datasets, and supplemental outcome information was collected from the surviving infants. Normal growth trajectories of total brain, cerebellar, vermis, pons and fourth ventricle volumes showed significant correlations with 2D measurements and increased in second-order polynomial trends across gestation (Pearson r, p < 0.05). Comparison of normal controls to five abnormal cerebellum subgroups depicted significant alterations in volumes that could not be detected exclusively with 2D analysis (MANCOVA, p < 0.05). There were 15 terminations of pregnancy, 8 neonatal deaths, and a spectrum of genetic and neurodevelopmental outcomes in the assessed 24 children with cerebellar abnormalities. The given posterior fossa biometrics enhance the delineation of normal and abnormal cerebellar phenotypes on fetal MRI and confirm the advantages of utilizing advanced neuroimaging tools in clinical fetal research.

Main congenital cerebral anomalies: how prenatal imaging aids counseling

The purpose of this article is to discuss some common cerebral lesions that may be detected during prenatal screening: corpus callosum dysgenesis, absent septum pellucidum, localized parenchymal ischemic-hemorrhagic lesions, megacisterna magna, Blake's pouch cyst, posterior fossa arachnoid cyst and Dandy-Walker malformation. For each cerebral defect, the main imaging findings are reminded, certain differential diagnoses are discussed and prenatal diagnostic accuracy is analyzed with emphasis on uncertainties encountered during analysis of ultrasound or magnetic resonance images. Detecting cerebral lesions in fetuses requires rapid counseling by neuropediatricians. Keeping in mind that the prenatal diagnostic accuracy is not 100%, the neuropediatricians have to answer the parents' questions regarding the outcome of the unborn child as well as the risk of recurrence for future pregnancies. This article is based on the authors' large experience in both prenatal imaging and neurocounseling. The frequently asked questions are set up. Answers are provided, underscoring the importance of an appropriate description of the cerebral defect, and therefore the pivotal role of prenatal imaging. However, prenatal neurocounseling remains challenging and the parents must be aware of uncertainties regarding both diagnostic accuracy and prognostic evaluation.

Fetal posterior fossa dimensions: normal and anomalous development assessed in mid-sagittal cranial plane by three-dimensional multiplanar sonography

OBJECTIVES

To construct nomograms of the dimensions of the fetal posterior fossa (PF), assessed in the mid-sagittal plane in the second and third trimesters, and to assess how measurements from fetuses with PF abnormalities deviate from our normal ranges.

METHODS

This was a prospective cross-sectional study of 378 healthy fetuses in low-risk singleton pregnancies between 15 and 35 weeks. PF size was evaluated in the mid-sagittal plane of the fetal head using three-dimensional multiplanar reconstruction (3D-MPR). The borders of the PF were defined from the clivus to the tentorium (clivotentorial distance, CTD) and from the occipital bone to the level of the upper mesencephalic edge (tecto-occipital distance, TOD), and the posterior fossa area (PFA) and perimeter (PFP) were assessed. Growth charts were produced. Thirty-nine fetuses diagnosed with PF malformations were analyzed by calculating the z-scores of PFA, PFP, TOD and CTD, relative to the developed nomograms.

RESULTS

Of the 378 healthy fetuses initially included, there were 281 with adequate visualization of the PF borders; i.e. PF mid-sagittal plane morphometry was feasible in 74.3% of cases. There was a linear relationship between each of PFA, PFP, TOD and CTD, and gestational age, with Pearson correlation coefficients of 0.97, 0.97, 0.96 and 0.95, respectively (P < 0.001 for each). Chiari II malformation (CM-II) and Dandy-Walker malformation (DWM) were associated with the greatest difference in PF size compared with normal; PFA z-scores exceeded 2.6 in all five DWM cases and were below -2.66 in all 11 CM-II cases.

CONCLUSIONS

Sonographic evaluation of fetal PF size in the mid-sagittal plane is feasible. Our constructed nomograms provide reference data that may be helpful when evaluating PF congenital malformations.

Fetal cerebral imaging - ultrasound vs. MRI: an update

The purpose of this article is to analyze the advantages and limitations of prenatal ultrasonography (US) and magnetic resonance imaging (MRI) in the evaluation of the fetal brain. These imaging modalities should not be seen as competitive but rather as complementary. There are wide variations in the world regarding screening policies, technology, skills, and legislation about termination of pregnancy, and these variations markedly impact on the way of using prenatal imaging. According to the contribution expected from each technique and to local working conditions, one should choose the most appropriate imaging modality on a case-by-case basis. The advantages and limitations of US and MRI in the setting of fetal brain imaging are displayed. Different anatomical regions (midline, ventricles, subependymal area, cerebral parenchyma, pericerebral space, posterior fossa) and pathological conditions are analyzed and illustrated in order to compare the respective contribution of each technique. An accurate prenatal diagnosis of cerebral abnormalities is of utmost importance for prenatal counseling.

The fetal cerebellum: development and common malformations

The cerebellum undergoes a protracted development, making it particularly vulnerable to a broad spectrum of developmental events. Acquired destructive and hemorrhagic insults may also occur. The main steps of cerebellar development are reviewed. The normal imaging patterns of the cerebellum in prenatal ultrasound and magnetic resonance imaging (MRI) are described with emphasis on the limitations of these modalities. Because of confusion in the literature regarding the terminology used for cerebellar malformations, some terms (agenesis, hypoplasia, dysplasia, and atrophy) are clarified. Three main pathologic settings are considered and the main diagnoses that can be suggested are described: retrocerebellar fluid enlargement with normal or abnormal biometry (Dandy-Walker malformation, Blake pouch cyst, vermian agenesis), partially or globally decreased cerebellar biometry (cerebellar hypoplasia, agenesis, rhombencephalosynapsis, ischemic and/or hemorrhagic damage), partially or globally abnormal cerebellar echogenicity (ischemic and/or hemorrhagic damage, cerebellar dysplasia, capillary telangiectasia). The appropriate timing for performing MRI is also discussed.