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.

Non-visualization of choroid plexus of fourth ventricle as first-trimester predictor of posterior fossa anomalies and chromosomal defects.

OBJECTIVE: To assess non-visualization of the choroid plexus of the fourth ventricle (CP-4V) as a simple, qualitative and reproducible first-trimester ultrasound feature of the posterior fossa for the prediction of central nervous system (CNS) anomalies and chromosomal defects. METHODS: First-trimester three-dimensional ultrasound datasets of the fetal brain were obtained prospectively from 65 consecutive normal singletons and retrospectively from 27 fetuses identified as having an abnormal posterior fossa on first-trimester ultrasound examination, and randomly combined to form the final study group. The stored ultrasound volumes were analyzed offline by two accredited sonologists, who were not aware of the final diagnoses. The CP-4V was assessed by multiplanar navigation and classified as visible or non-visible in its normal position depending on whether or not the echogenic structure that separates the fourth ventricle from the cisterna magna was identified in both midsagittal and axial planes. Correlation with subsequent second-trimester ultrasound, fetal magnetic resonance imaging, or postmortem or postnatal findings was performed to determine the predictive value of the first-trimester findings. RESULTS: Among the 92 ultrasound datasets analyzed, 73 (79%) were acquired transabdominally and 19 (21%) transvaginally. The CP-4V was classified as visible in 64 cases and non-visible in 28 cases, with agreement between the two observers in both sagittal and axial planes in all but one case. Twelve of the 28 (43%) fetuses with non-visible CP-4V were subsequently diagnosed as having a CNS malformation (open spina bifida (n = 6), Dandy-Walker malformation (n = 2), Blake's pouch cyst (n = 2), cephalocele (n = 1) and megacisterna magna (n = 1)). In addition, 20 of these 28 (71%) fetuses had aneuploidy (trisomy 18 (n = 10), triploidy (n = 5), trisomy 13 (n = 3), Turner syndrome (n = 1) or trisomy 21 (n = 1)). There was only one false-positive case, in which the CP-4V was classified as absent in a normal fetus. CONCLUSIONS: Non-visualization of the CP-4V in the first trimester appears to be a strong marker of posterior fossa anomalies and chromosomal defects. Qualitative evaluation of this anatomic structure is simple, feasible and reproducible, and its routine assessment during the first-trimester scan may facilitate the early detection of CNS anomalies and associated fetal aneuploidy. Copyright © 2017 ISUOG. Published by John Wiley & Sons Ltd.

Screening for fetal spina bifida at the 11-13-week scan using three anatomical features of the posterior brain.

OBJECTIVE: To evaluate the contribution of examination of specific anatomical features of the fetal posterior brain on mid-sagittal first-trimester ultrasound examination to the early detection of open spina bifida. METHODS: Four independent observers reviewed a series of 260 mid-sagittal first-trimester ultrasound images from 52 cases of open spina bifida and 208 normal fetuses. The following analysis was performed by each reviewer for each image: Herman score calculation, intracranial translucency score (CFEF-IT) calculation and determination of presence or absence of three anatomical criteria: intracranial translucency (IT), caudal displacement of the brainstem and cisterna magna. The sensitivity and the false-positive rate for spina bifida detection were calculated for each of the latter three criteria. A secondary analysis was performed on the subset of images achieving a Herman score ≥ 7. RESULTS: The highest detection rate for spina bifida was achieved by non-visualization of the cisterna magna, with associated sensitivity of 50-73% and 39-76%, respectively, for all images and for the subset of images achieving a Herman score ≥ 7. Posterior shift of the brainstem achieved the highest detection rate (86%), but for a single reviewer only. The level of variation in performance between observers was also greatest for this sign. Absence of IT was associated with a lower detection rate for all observers. Overall, an abnormal posterior brain presenting at least one of these three criteria was associated with a detection rate ranging from 50 to 90%. CONCLUSION: In the detection of spina bifida, non-visualization of the cisterna magna achieved the best screening performance. Both non-visualization of the IT and posterior shift of the brainstem were associated with acceptable but lower detection rates. A prospective evaluation of changes in the posterior brain is needed to allow assessment of the most pertinent criteria for first-trimester screening for spina bifida.

Cisterna magna width at 11-13 weeks in the detection of posterior fossa anomalies.

OBJECTIVE: To construct reference ranges for cisterna magna (CM) width at 11-13 weeks' gestation in healthy fetuses and determine whether open spina bifida and posterior fossa anomalies could be diagnosed in the first trimester. METHODS: This was a retrospective study. CM width reference ranges were constructed based on the measurements obtained from 80 healthy fetuses with normal postnatal outcome undergoing routine first-trimester ultrasound at 11-13 weeks, using the Lambda-Mu-Sigma method. CM was measured in the fetal mid-sagittal view, as routinely used for nuchal translucency assessment. In addition, first-trimester ultrasound images in 11 fetuses with open spina bifida or posterior fossa anomalies, most of which were diagnosed later in pregnancy, were retrospectively reviewed, and CM measurements were compared against reference ranges. RESULTS: CM width was noted to increase with gestational age in normal fetuses. The anomalies in the 11 fetuses we reviewed were: open spina bifida (n = 5), megacisterna magna (n = 3), Blake's pouch cyst (n = 2) and posterior fossa arachnoid cyst (n = 1). All fetuses with open spina bifida had a CM width below the 5(th) percentile. In the two fetuses with Blake's pouch cyst and in two with megacisterna magna, CM width was above the 95(th) percentile. In one of the fetuses with a megacisterna magna and the one with an arachnoid cyst, CM width was within normal range. CONCLUSION: We have constructed reference ranges for CM width at 11-13 weeks using the mid-sagittal view. It appears that first-trimester CM width can be used as a marker for the early detection of open spina bifida. However, our findings need to be confirmed in prospective large series.

Three-dimensional sonography of the posterior fossa in fetuses with open spina bifida at 11-13 weeks' gestation.

OBJECTIVES: To investigate the posterior fossa of normal fetuses and fetuses with open spina bifida in stored three-dimensional (3D) volumes and to describe signs that might allow early detection of this defect. METHODS: A prospective study of 3D volumes of the fetal brain obtained from 10 normal fetuses and three fetuses with open spina bifida was undertaken. Measurements of the anteroposterior diameters of the cisterna magna and fourth ventricle were taken in the tilted axial view. In the mid-sagittal plane the brainstem (BS) diameter and the brainstem-occipital bone (BSOB) distance were measured. The BS/BSOB ratio was calculated. All measurements were expressed as Z-scores. Structural analysis of the differences in the posterior fossa between normal fetuses and fetuses with open spina bifida was undertaken. RESULTS: In normal fetuses all measurements were within ±2.5 Z-scores. In three fetuses with open spina bifida the BS Z-scores were 2.7, 2.8 and 2.8; the BSOB scores were -3.4, -2.8 and -2.9; the cisterna magna scores were -5.6, -3.7 and -4.2; and the BS/BSOB ratio scores were 4.1, 9.7 and 8.9. In normal fetuses the cisterna magna was posterior to the fourth ventricle and extended along its entire length. In fetuses with open spina bifida the cisterna magna was partially or completely obliterated. CONCLUSIONS: Assessment of the cranial posterior fossa is feasible at 11-13 weeks' gestation. There are distinct signs in fetuses with open spina bifida which can be evaluated by ultrasonography.

Appearance of the fetal posterior fossa at 11 + 3 to 13 + 6 gestational weeks on transabdominal ultrasound examination.

OBJECTIVES: To describe the sonographic appearance of the structures of the posterior cranial fossa in fetuses at 11 + 3 to 13 + 6 weeks of pregnancy and to determine whether abnormal findings of the brain and spine can be detected by sonography at this time. METHODS: This was a prospective study including 692 fetuses whose mothers attended Innsbruck Medical University Hospital for first-trimester sonography. In 3% (n = 21) of cases, measurement was prevented by fetal position. Of the remaining 671 cases, in 604 there was either a normal anomaly scan at 20 weeks or delivery of a healthy child and in these cases the transcerebellar diameter (TCD) and the anteroposterior diameter of the cisterna magna (CM), measured at 11 + 3 to 13 + 6 weeks, were analyzed. In 502 fetuses, the anteroposterior diameter of the fourth ventricle (4V) was also measured. In 25 fetuses, intra- and interobserver repeatability was calculated. RESULTS: We observed a linear correlation between crown-rump length (CRL) and CM (CM = 0.0536 × CRL - 1.4701; R2 = 0.688), TCD (TCD = 0.1482 × CRL - 1.2083; R2 = 0.701) and 4V (4V = 0.0181 × CRL + 0.9186; R2 = 0.118). In three patients with posterior fossa cysts, measurements significantly exceeded the reference values. One fetus with spina bifida had an obliterated CM and the posterior border of the 4V could not be visualized. CONCLUSIONS: Transabdominal sonographic assessment of the posterior fossa is feasible in the first trimester. Measurements of the 4V, the CM and the TCD performed at this time are reliable. The established reference values assist in detecting fetal anomalies. However, findings must be interpreted carefully, as some supposed malformations might be merely delayed development of brain structures.

Rotation of the vermis as a cause of enlarged cisterna magna on prenatal imaging.

OBJECTIVES: Dandy-Walker complex is a continuum of developmental anomalies of the posterior fossa which includes vermian rotation. However, vermian rotation alone may be benign. The aim of this study was to describe our experience with sagittal-plane prenatal ultrasound in the diagnosis of rotation of the vermis in cases of suspected enlarged cisterna magna on routine antenatal imaging, and to describe the follow-up of these patients. METHODS: Seven women, who were referred to our ultrasound unit for evaluation of an enlarged fetal cisterna magna and suspected agenesis of the vermis on axial-plane imaging, underwent further multiplanar studies of the posterior fossa and measurements of the vermis. RESULTS: The mean maternal age was 27 (range, 20-33) years and the mean gestational age at diagnosis was 19.5 (range, 18-31) weeks. The standard axial plane image showed a 'direct communication' between the cisterna magna and the fourth ventricle. In the mid-sagittal plane, the vermis was clearly delineated, with posterosuperior rotation. Vermis size was within normal limits for gestational age in all cases. Findings were confirmed by prenatal magnetic resonance imaging (MRI) in two cases and postnatal MRI and/or sonography in five. During a mean follow-up of 4.5 (range, 1-7.5) years, all children developed normally, with no neurological complications. CONCLUSION: The finding of an enlarged cisterna magna on standard- (axial-)plane ultrasound should be evaluated further in the sagittal plane to determine whether the cause is rotation of a normal vermis. This may spare patients unnecessary tests, anxiety and, in some cases, pregnancy termination.

Linear echoes in the fetal cisterna magna.

Linear echoes are seen in the fetal cisterna magna, (CM) on obstetrical sonography. These echoes often are paired, joining as they descend toward the base of the posterior fossa. Histologic correlation suggests that these echoes are most consistent with dural folds, which likely represent the inferior attachment of the falx cerebelli. A prospective series of 322 prenatal studies was performed in which the sonographer was asked to look for the linear echoes in the cisterna magna and image them. Linear echoes were identified in 84% of all fetuses studied. Identification of these echoes was dependent on CM size, in that they were seen less commonly when the CM was less than 3 mm in diameter. However, their identification was not dependent on gestational age. In addition, 18 fetuses with Dandy Walker cyst or Dandy Walker variant were evaluated and in 16 linear echoes were not seen. We conclude that recognition of normal anatomy within the fetal brain, specifically the fetal CM, is helpful for identifying abnormalities in the size of the CM, whether large or small.

Fetal cisterna magna septa: a normal anatomic finding.

A linear hyperechoic structure in the cisterna magna, previously described as the straight sinus, was investigated during routine obstetric ultrasound (US) examination. One hundred three fetal US examinations were performed to determine the prevalence and appearance of this finding. Also, US water-bath examination of 18 fixed postmortem specimens and fetuses was performed, and results were correlated with those of gross and microscopic examination of the fetal brain. The structure was identified in 95 (92%) prenatal examinations between 15 and 38 menstrual weeks. It was located posteriorly between the cerebellar hemispheres in the cisterna magna. In those 95 cases in which it was identified, it appeared as a single echogenic line in 31 cases (33%), as two echogenic lines in 47 (49%), or as three or more lines in 17 (18%). When two echogenic lines were identified, they appeared "cystlike" in 17 (18%) of the 95 examinations. Correlation of the gross and histologic findings demonstrated these lines to represent normal subarachnoid septa rather than the straight sinus.