Low torcular Herophili position and large brainstem-tentorium angle in fetuses with open spinal dysraphism at 11-13 weeks' gestation.

OBJECTIVE: To evaluate whether in fetuses with open spina bifida (OSB) the tentorium can be seen to be displaced downwards and vertically oriented by the time of the 11-13-week scan and whether this is reflected in an alteration of the brainstem-tentorium (BST) angle. METHODS: The study population was recruited between 2015 and 2020 from three fetal medicine referral centers and comprised a control group and a study group of pregnancies with OSB. The control group was recruited prospectively and included singleton pregnancies with a normal sonographic examination after first-trimester combined screening for chromosomal abnormalities and normal outcome. The study group was selected retrospectively and included all cases with OSB between 2015 and 2020. All cases underwent detailed ultrasound assessment at 11 + 0 to 13 + 6 weeks' gestation. The position of the torcular Herophili (TH) was identified in the midsagittal view of the fetal brain with the use of color Doppler and was considered as a proxy for the insertion of the tentorium on the fetal skull. The BST angle was calculated in the same view and was compared between the two groups. RESULTS: Sixty normal fetuses were included in the control group and 22 fetuses with OSB in the study group. In both groups, the BST angle was found to be independent of gestational age or crown-rump length (P = 0.8815, R2 = 0.0003861 in the controls, and P = 0.2665, R2 = 0.00978 in the OSB group). The mean BST angle was 48.7 ± 7.8° in controls and 88.1 ± 1.18°, i.e. close to 90°, in fetuses with OSB. Comparison of BST-angle measurements between the control group and cases with OSB showed a statistically significant difference (P = 0.0153). In all fetuses with OSB, the downward displacement of the TH and tentorium was clearly visible at the 11-13-week scan. CONCLUSIONS: In fetuses with OSB, the BST angle is significantly larger than in normal controls, with the tentorium being almost perpendicular to the brainstem. This sign confirms the inferior displacement of the tentorium cerebelli with respect to its normal insertion on the occipital clivus as early as the first trimester of pregnancy and is useful in the diagnosis of Chiari-II malformation at this early stage. In fetuses with OSB, the low position of the tentorium and TH is clearly visible, even subjectively, at the 11-13-week scan. © 2021 International Society of Ultrasound in Obstetrics and Gynecology.

Position of the choroid plexus of the fourth ventricle in first- and second-trimester fetuses: a novel approach to early diagnosis of cystic posterior fossa anomalies.

OBJECTIVE: To describe the sonographic appearance and position of the choroid plexus of the fourth ventricle (4V-CP) between 12 and 21 weeks' gestation in normal fetuses and in fetuses with Dandy-Walker malformation (DWM) or Blake's pouch cyst (BPC). METHODS: The study population comprised 90 prospectively recruited normal singleton pregnancies and 41 pregnancies identified retrospectively from our institutional database that had a suspected posterior fossa anomaly at 12-13 weeks' gestation based on the ultrasound finding of abnormal hindbrain spaces. In all cases the final diagnosis was confirmed by prenatal and/or postnatal magnetic resonance imaging or postmortem examination. All pregnancies underwent a detailed ultrasound assessment, including a dedicated examination of the posterior fossa, at 12-13 weeks, 15-16 weeks and 20-21 weeks of gestation. Two-dimensional ultrasound images of the midsagittal and coronal views of the brain through the posterior fontanelle and three-dimensional volume datasets were obtained. Multiplanar orthogonal image correlation with volume contrast imaging was used as the reference visualization mode. Two independent operators, blinded to the fetal outcome, were asked to classify the 4V-CP as visible or not visible in both normal and abnormal cases, and to assess if the 4V-CP was positioned inside or outside the cyst in fetuses with DWM and BPC. RESULTS: Of the 41 fetuses with apparently isolated cystic posterior fossa anomaly in the first trimester, eight were diagnosed with DWM, 29 were diagnosed with BPC and four were found to be normal in the second trimester. The position of the 4V-CP differed between DWM, BPC and normal cases in the first- and second-trimester ultrasound examinations. In particular, in normal fetuses, no cyst was present and, in the midsagittal and coronal planes of the posterior fossa, the 4V-CP appeared as an echogenic oval-shaped structure located inside the 4V apparently attached to the cerebellar vermis. In fetuses with DWM, the 4V-CP was not visible in the midsagittal view because it was displaced inferolaterally by the cyst. In contrast, in the coronal view of the posterior brain, the 4V-CP was visualized in all cases with DWM at 12-13 weeks, with a moderate decrease in the visualization rate at 15-16 weeks (87.5%) and at 20-21 weeks (75%). In the coronal view, the 4V-CP was classified as being outside the cyst in all DWM cases at 12-13 weeks and in 87.5% and 75% of cases at 15-16 and 20-21 weeks, respectively. In fetuses with BPC, the 4V-CP was visualized in all cases in both the midsagittal and coronal views at 12-13 weeks and in 100% and 96.6% of cases, respectively, at 15-16 weeks. In the coronal view, the 4V-CP was classified as being inside the cyst in 28 (96.6%), 27 (93.1%) and 25 (86.2%) cases at 12-13, 15-16 and 20-21 weeks, respectively. The medial segment of the 4V-CP was visualized near the inferior part of the vermis. CONCLUSIONS: Our study shows that longitudinal ultrasound assessment of the 4V-CP and its temporal changes from 12 to 21 weeks is feasible. The 4V-CP is located inside the cyst, just below the vermis, in BPC and outside the cyst, inferolaterally displaced and distant from the vermian margin, in DWM, consistent with the pathogenesis of the two conditions. The position of the 4V-CP is a useful sonographic marker that can help differentiate between DWM and BPC as early as in the first trimester of pregnancy. © 2021 International Society of Ultrasound in Obstetrics and Gynecology.

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.

Three-dimensional volumetric study with VOCAL in normal and abnormal posterior fossa fetuses.

Purpose: The aim of this study was to compare volumetric parameters in the abnormal and normal posterior fossa using the Virtual Organ Computer-aided AnaLysis (VOCAL™) technique to determine whether fetuses with an abnormal posterior fossa have different volumes.Methods: A prospective study was conducted on 17 fetuses with an abnormal posterior fossa including, Dandy Walker malformation (DWM) (n = 6), vermian hypoplasia (VH) (n = 3), mega cisterna magna (MCM) (n = 8), and 99 healthy control fetuses from 20 to 34 weeks' gestation. Measurement of the fetal cisterna magna and cerebellar volume was performed in the standard transcerebellar plane through the VOCAL™ method. To establish the correlation of volumes with gestational age, polynomial regression analysis was performed. For comparison between groups, univariate ANCOVA was performed using gestational age as a covariate. The reliability was analyzed by the intraclass correlation coefficient (ICC).Results: Cerebellar volume and cisterna magna volume were correlated with gestational age. Posterior fossa volume was significantly larger in DWM (p < .0001) and MCM (p < .0001) in comparison to the control group. In VH group, cisterna magna volume does not seem to expand (p = .298). Cerebellar volume does not seem to change in subgroups when the influence of gestational age is discarded (p = .09). The ratio of cerebellar volume to the cisterna magna volume decreases significantly in abnormal fetuses (p < .0001). Good intraobserver and interobserver reliabilities were found for both cerebellum and cisterna magna measurements.Conclusions: Volume analysis may have a role in discrimination of different posterior fossa pathologies.

A comprehensive review of the clivus: anatomy, embryology, variants, pathology, and surgical approaches.

INTRODUCTION: The clivus is a bony structure formed by the fusion of the basioccipital and basispheniod bone at the sphenooccipital synchondrosis. This downward sloping structure from the dorsum sellae to the foramen magnum is derived from mesoderm and ectoderm properties. METHODS: This comprehensive review of the clivus will discuss its basic anatomy, embryology, pathological findings, and surgical implications. The clivus is an endochondral bone, formed under two processes; first, a cartilaginous base is developed, and it is secondly reabsorbed and replaced with bone. Knowledge of its embryological structure and growth of development will clarify the pathogenesis of anatomical variants and pathological findings of the clivus. CONCLUSIONS: Understanding the anatomy including proximity to anatomical structures, adjacent neurovasculature properties, and anatomical variants will aid neurosurgeons in their surgical management when treating pathological findings around the clivus.

Reference Ranges for Transvaginal Examined Fossa Posterior Structures in Fetuses from 45 to 84 mm Crown-Rump Length.

OBJECTIVE: The study aimed to describe reference values for structures of the posterior fossa in fetuses with a crown-rump length (CRL) between 45 and 84 mm. MATERIALS AND METHODS: This was a prospective, cross-sectional study including 216 normal appearing fetuses. In transvaginal acquired 3-dimensional volume blocks, the longest diameter of the vermis (VE), posterior membranous area (PMA), medulla-oblongata-pons angle (MOPA), diameters of the medulla oblongata (MO) and pons (PO), and the area of Blake's pouch (BP) were measured. Polynomial or linear regression analysis were performed to calculate the mean, 5th and 95th centile according to CRL. In 20 fetuses, intra- and interobserver repeatability were calculated. RESULTS: There is a curvilinear correlation between CRL and PO (PO [mean] = 1.3893 + 0.004356 × CRL + 0.000002610 × CRL3; SD = 1.6818 - 0.03765 × CRL + 0.000003831 × CRL3; R2 = 0.489); CRL and MO (MO [mean] = 1.5959-0.001905 × CRL + 0.000003362*CRL3; SD = -0.1417 + 0.005404 × CRL + 0.0000004988 × CRL3; R2 = 0.525); CRL and VE (VE [mean] = -0.3640 + 0.04302 × CRL+ 0.000001486 × CRL3; SD = 0.5854 - 0.004812 × CRL + 0.0000005896 × CRL3; R2 = 0.643); CRL and PMA (PMA [mean] = 0.6901 + 0.04307 × CRL - 0.0000008459 × CRL3; SD = -0.4232 + 0.02026 × CRL - 0.000001320 × CRL3; R2 = 0.272); CRL and BP (mm2; BP [mean] -12.2067 + 0.3334 × CRL - 0.00001262 × CRL3; SD = -1.6431 + 0.06380 × CRL+ 0.0000003257 × CRL3; R2 = 0.289). The relation between CRL and MOPA (°) is best described by a linear regression (MOPA [mean] = 79.6332 + 0.6122 × CRL; SD = 4.8453 + 0.07333 × CRL; R2 = 0.318). CONCLUSION: We provide reference values for anatomical structures of the posterior fossa of fetuses between 45 and 84 mm CRL. The established reference values might ease the diagnosis of fetal malformations in early pregnancy.

Post Mortem Diagnosis of Blake's Pouch Cyst: A Presentation of Distended Cyst at Necropsy.

BACKGROUND: Blake's pouch cyst forms from non-permeabilization of Blake's pouch. It is difficult to visualize at necropsy as the cyst ruptures easily into the 4th ventricle during dissection. CASE REPORT: Based upon prenatal imaging, delicate dissection allowed post-mortem confirmation of the Blake's pouch cyst. CONCLUSIONS: This highlights the importance of utilizing premortem imaging to help guide the postmortem dissection and documentation of a posterior fossa cyst.

Embryology of the craniocervical junction and posterior cranial fossa, part II: Embryogenesis of the hindbrain.

Although pathology of the hindbrain and its derivatives can have life altering effects on a patient, a comprehensive review on its embryology is difficult to find in the peer-reviewed medical literature. Therefore, this review article, using standard search engines, seemed timely. The embryology of the hindbrain is complex and relies on a unique timing of various neurovascular and bony elements. Derailment of these developmental processes can lead to a wide range of malformations such as the Chiari malformations. Therefore, a good working knowledge of this embryology as outlined in this review of the hindbrain is important for those treating patients with involvement of this region of the central nervous system. Clin. Anat. 31:488-500, 2018. © 2018 Wiley Periodicals, Inc.

Embryology of the craniocervical junction and posterior cranial fossa, part I: Development of the upper vertebrae and skull.

Although the embryology of the posterior cranial fossa can have life altering effects on a patient, a comprehensive review on this topic is difficult to find in the peer-reviewed medical literature. Therefore, this review article, using standard search engines, seemed timely. The embryology of the posterior cranial fossa is complex and relies on a unique timing of various neurovascular and bony elements. Derailment of these developmental processes can lead to a wide range of malformations such as the Chiari malformations. Therefore, a good working knowledge of this embryology as outlined in this review of its bony architecture is important for those treating patients with involvement of this region of the cranium. Clin. Anat. 31:466-487, 2018. © 2018 Wiley Periodicals, Inc.

Valoración del cerebelo fetal mediante resonancia magnética / Evaluation of the fetal cerebellum by magnetic resonance imaging

La valoración del cerebelo fetal debe realizarse siempre por protocolo en las ecografías realizadas durante la gestación. Diferentes motivos como limitaciones técnicas o hallazgos ecográficos poco concluyentes han convertido la sospecha de patología del cerebelo fetal en una de las indicaciones más frecuentes de resonancia magnética (RM) prenatal. Aunque la ecografía fetal es la técnica de imagen de elección para su valoración, la RM muestra con mayor detalle la anatomía de la fosa posterior y las anomalías del desarrollo del cerebelo fetal, lo que permite un diagnóstico prenatal más preciso. Describimos la anatomía normal del cerebelo fetal mediante RM, así como las diferentes patologías que afectan a su normal desarrollo, y revisamos la terminología más apropiada para definirla, su diagnóstico diferencial y el papel de la RM en la evaluación prenatal de la fosa posterior (AU)

Obstetric protocols dictate that the fetal cerebellum should always be assessed during sonograms during pregnancy. For various reasons, including technical limitations or inconclusive sonographic findings, suspicion of cerebellar abnormalities is one of the most common indications for prenatal magnetic resonance imaging (MRI). Although sonography is the imaging technique of choice to assess the cerebellum, MRI shows the anatomy of the posterior fossa and abnormalities in the development of the fetal cerebellum in greater detail and thus enables a more accurate prenatal diagnosis. We describe and illustrate the normal anatomy of the fetal cerebellum on MRI as well as the different diseases that can affect its development. Moreover, we review the most appropriate terminology to define developmental abnormalities, their differential diagnoses, and the role of MRI in the prenatal evaluation of the posterior fossa (AU)

Clival canal and clival foramen development in the fetal and infant basioccipital.

PURPOSE: There is a paucity of research regarding both the development and the prevalence of the clival canal or clival foramen in fetuses. Reports that have examined child and adult populations have posited ideas for the development of the canal and foramen; however, they have done so in the absence of anatomical data from the fetal population. Therefore, the present study was performed to elucidate the development of the clival canal and foramen through the assessment of perinatal basioccipitals. METHODS: This study analyzed 104 basioccipital bones, 60 from fetuses and 44 from newborns and infants. Dorsal surfaces of basioccipitals were assessed for the presence of anatomical variation with particular attention to the presence of clival canals and foramina. Among cases in which the presence of a clival canal or clival foramen was suspected, cannulation was performed for verification. RESULTS: Of the 104 basioccipitals analyzed, 1 (0.96%) had a clival foramen. Clival canals were identified in seven basioccipitals (7:104; 6.73%), four of which were from fetuses. Trends in anatomical variations among basioccipitals were also identified and categorized. These categories were then evaluated relative to age in order to elucidate ontogeny. A model is presented to explain the development of the clival foramen, the clival canal, and the basioccipital, in general. CONCLUSIONS: The presence of a clival canal or foramen should be considered even among individuals of fetal age. The findings of this osteological study suggest that the clival canal and foramen develop around vascular structures and, therefore, signify vascular connections among nearby venous plexuses.

Enlarged posterior fossa on prenatal imaging: differential diagnosis, associated anomalies and postnatal outcome.

INTRODUCTION: The primary aim of this study was to ascertain the prevalence of the individual conditions and of associated anomalies in fetuses with the prenatal diagnosis of enlarged posterior fossa (PF) and to explore the diagnostic accuracy of ultrasound in these anomalies. The secondary aim was to evaluate the postnatal outcome of children affected by PF anomalies. MATERIAL AND METHODS: All fetuses with enlarged PF detected by prenatal sonography at a referral center from 2001 to 2015 were analyzed retrospectively. Some were also studied by fetal magnetic resonance imaging (MRI) or volume ultrasound examinations. Fetal sonographic and MRI were compared using following classification: Dandy-Walker malformation (DWM); megacisterna magna (MCM); Blake's pouch cyst; isolated vermian hypoplasia; vermian agenesis; PF arachnoid cyst (AC); and cerebellar hypoplasia (CH). RESULTS: The ultrasound diagnoses of the 69 fetuses were as follows: MCM (n = 29; of these isolated n = 15), DWM (n = 28, isolated n = 4), vermian hypoplasia (n = 5, isolated n = 4), Blake's pouch cyst (n = 4, isolated n = 1), CH (n = 2; none isolated) and AC in the PF (n = 1, isolated). Thirteen of the 41 karyotyped fetuses were aneuploid, including seven with DWM. Associated malformations were found in 37/69 cases. There were 39 live births, including 11 with confirmed DWM, six of whom show a normal development. Twelve infants with isolated MCM show normal development. There were eight false-positive prenatal diagnoses (or resolution until birth) of "enlarged PF": three with Blake's pouch cyst, two with MCM and one with vermian hypoplasia. CONCLUSIONS: An enlarged PF requires specific diagnoses for the best possible counseling. The term "Dandy-Walker variant" should not be used anymore. Isolated MCM and Blake's pouch cyst can either resolve or be normal variants, but may also indicate the presence of a more severe anomaly or associated malformations.

Sonoembryology of the fetal posterior fossa at 11 + 3 to 13 + 6 gestational weeks on three-dimensional transvaginal ultrasound.

OBJECTIVE: To describe the sonographic appearance and temporal changes of the structures of the posterior cranial fossa in fetuses at a crown-rump length (CRL) between 45 and 84 mm in transvaginal acquired three-dimensional volume blocks. METHODS: This was a prospective, cross-sectional, observational study including 80 fetuses, whose mothers attended Kepler University Hospital Linz or the Ambulatorium für Fetalmedizin Feldkirch for first-trimester sonography. Three-dimensional volume blocks were acquired in a standardized way and after processing the sonographic characteristics of the brainstem, cerebellar vermis, choroid plexus, anterior membranous area (AMA) and Blake's metapore were described. Measurements of the length of the cerebellar vermis, the length of the AMA and the medulla-oblongata-pons angle (MOPA) were performed. In 20 fetuses the intra- and interobserver repeatability was calculated. RESULTS: The sonomorphologic characteristics of posterior fossa structures as cerebellar vermis, AMA, Blake's metapore, choroid plexus, pons and medulla oblongata were described. There is a significant correlation between CRL and vermis length, CRL and MOPA and CRL and AMA. CONCLUSIONS: Transvaginal three-dimensional sonography allows a detailed depiction of the structures of the posterior fossa and their temporal course in early pregnancy. © 2016 John Wiley & Sons, Ltd.

Congenital basis of posterior fossa anomalies.

The classification of posterior fossa congenital anomalies has been a controversial topic. Advances in genetics and imaging have allowed a better understanding of the embryologic development of these abnormalities. A new classification schema correlates the embryologic, morphologic, and genetic bases of these anomalies in order to better distinguish and describe them. Although they provide a better understanding of the clinical aspects and genetics of these disorders, it is crucial for the radiologist to be able to diagnose the congenital posterior fossa anomalies based on their morphology, since neuroimaging is usually the initial step when these disorders are suspected. We divide the most common posterior fossa congenital anomalies into two groups: 1) hindbrain malformations, including diseases with cerebellar or vermian agenesis, aplasia or hypoplasia and cystic posterior fossa anomalies; and 2) cranial vault malformations. In addition, we will review the embryologic development of the posterior fossa and, from the perspective of embryonic development, will describe the imaging appearance of congenital posterior fossa anomalies. Knowledge of the developmental bases of these malformations facilitates detection of the morphological changes identified on imaging, allowing accurate differentiation and diagnosis of congenital posterior fossa anomalies.

Congenital abnormalities of the posterior fossa.

The frequency and importance of the evaluation of the posterior fossa have increased significantly over the past 20 years owing to advances in neuroimaging. Nowadays, conventional and advanced neuroimaging techniques allow detailed evaluation of the complex anatomic structures within the posterior fossa. A wide spectrum of congenital abnormalities has been demonstrated, including malformations (anomalies due to an alteration of the primary developmental program caused by a genetic defect) and disruptions (anomalies due to the breakdown of a structure that had a normal developmental potential). Familiarity with the spectrum of congenital posterior fossa anomalies and their well-defined diagnostic criteria is crucial for optimal therapy, an accurate prognosis, and correct genetic counseling. The authors discuss the spectrum of posterior fossa malformations and disruptions, with emphasis on neuroimaging findings (including diagnostic criteria), neurologic presentation, systemic involvement, prognosis, and risk of recurrence.

Transsphenoidal meningocele: an anatomical study using human fetuses including report of a case.

An asymptomatic transsphenoidal meningoencephalocele was discovered incidentally by fiber laryngoscopic examination in a 62-year-old man suffering from hoarseness due to dysplasia of the vocal cord epithelium. To provide a better understanding of the pathogenesis of this anomaly, we performed histologic observations of paraffin-embedded specimens of 42 human fetal heads at 12-16 weeks of gestation. At these stages, ossification had started in the clivus but the sphenoid sinus was not developed. In contrast to the very low incidence of the intra- or trans-sphenoidal remnant of Rathke's pouch after birth, we found (1) the typical mid-line cleft of the sphenoid body in two specimens (2/42 or 4.8 %) and (2) a duct-like, sellar inferior protrusion ending in the sphenoid body in 12 specimens (12/42 or 28.6 %). The cyst-like structure in the protrusion (two specimens) seemed to be composed of obstructed veins. The intra- and trans-sphenoidal anomalies were observed more frequently in specimens without ossification of the sphenoid body than in those with ossification. However, irrespective of ossification, a cyst-like remnant of the most upper part of Rathke's pouch was always seen between the anterior and posterior lobes of the developing pituitary gland. In addition, the bursa pharyngea was seen in four specimens and we confirmed that the notochord was attached to the bursa in each case. The consistent remnant of the intrasellar Rathke's pouch appeared to explain the high incidence of Rathke's cleft cyst in adults. The relatively high incidence of intrasphenoidal anomalies in fetuses (14/42) suggested that the intra- or trans-sphenoidal remnant of Rathke's pouch was physiologically closed by ossification of the sphenoid body.

Posterior brain in fetuses with trisomy 18, trisomy 13 and triploidy at 11 to 13 weeks' gestation.

OBJECTIVE: To measure changes in the posterior fossa of first-trimester fetuses with trisomy 18, trisomy 13 and triploidy. METHODS: Brain stem (BS) diameter and BS to occipital bone (BSOB) diameter were measured in images of the midsagittal view of the face at 11(+0) to 13(+6) weeks from 45 trisomy 18, 21 trisomy 13 and 15 triploid fetuses and compared with values in 162 euploid fetuses. RESULTS: In euploid fetuses BS and BSOB diameters increased significantly with crown-rum length and the BS to BSOB ratio decreased. In all three aneuploidies BSOB diameter was significantly higher than in euploid fetuses. In trisomy 18 and trisomy 13, the BS diameter and BS to BSOB ratio were decreased. The BS to BSOB ratio was below the 5(th) percentile in 16 (35.6%), 17 (81.0%) and 5 (33.3%) of trisomy 18, trisomy 13 and triploidy, respectively. In 7 (8.6%) of the aneuploid fetuses there was open spina bifida and in all these cases the BS to BSOB ratio was above the 95(th) percentile. CONCLUSIONS: At 11 to 13 weeks' gestation many fetuses with trisomy 18, trisomy 13 and triploidy have measurable abnormalities in the posterior brain.

Brainstem-vermis and brainstem-tentorium angles allow accurate categorization of fetal upward rotation of cerebellar vermis.

OBJECTIVE: To evaluate the role of the brainstem-vermis (BV) and brainstem-tentorium (BT) angles in the differential diagnosis of upward rotation of the fetal cerebellar vermis. METHODS: The BV and BT angles were measured retrospectively on median sonographic views of the brain in 31 fetuses at 19-28 weeks' gestation with upward rotation of the cerebellar vermis due to Blake's pouch cyst (n = 12), Dandy-Walker malformation (n = 12) and cerebellar vermian hypoplasia (n = 7). Eighty normal fetuses at 20-24 weeks were included as controls. RESULTS: In the control group, BV and BT angles were 9.1 ± 3.5° (range, 4-17°) and 29.3 ± 5.8° (range, 21-44°), respectively. The BV angle was significantly increased in each of the three subgroups of anomalies: Blake's pouch cyst (23 ± 2.8°; range, 19-26°), vermian hypoplasia (34.9 ± 5.4°; range, 24-40°) and Dandy-Walker malformation (63.5 ± 17.6°; range, 45-112°), the angle increasing with increasing severity of the condition. The BT angle had a similar pattern but there was overlap among the different groups. CONCLUSION: The BV angle and, to a lesser degree, the BT angle are simple and reproducible measurements that provide valuable additional information for the categorization of upward rotation of the fetal cerebellar vermis. From mid gestation, a BV angle > 45° is strongly suggestive of a Dandy-Walker malformation, while a measurement < 30° favors the diagnosis of a Blake's pouch cyst.