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Normative data for fetal cisterna magna length measurementbetween 18 and 24 weeks of pregnancy

Edward Araujo Júnior & Wellington P. Martins &

Liliam Cristine Rolo & Claudio Rodrigues Pires &Sebastião Marques Zanforlin Filho

Received: 16 August 2013 /Accepted: 25 September 2013 /Published online: 10 October 2013# Springer-Verlag Berlin Heidelberg 2013

AbstractPurpose The aim of this study was to determine normativedata for fetal cisterna magna length (CML) measurement in aBrazilian population.Methods This was a retrospective cross-sectional study on 3,862 normal singleton pregnancies between the 18th and 24thweeks of pregnancy. Fetal CML was measured in the axialplane of the fetal head, at lateral ventricle level, including thecavum septum pellucidum, thalamus, third ventricle, andtransverse cerebellar diameter. The anteroposterior measure-ment was made between the posterior border of the cerebellarvermis and the internal face of the occipital bone. To assess thecorrelation between CML and gestational age (GA), polyno-mial equations were calculated, with adjustments using deter-mination coefficient (R2).Results The mean CML ranged from 4.29±0.93 mm at 18 to18+6 weeks to 5.58±1.23 mm at 24 to 24+6 weeks ofpregnancy. There was a good correlation between CML and

GA, best represented by a linear equation: CML=0.535+0.208*GA (R2=0.084).Conclusion We established normative data for fetal CML in thesecond trimester of pregnancy, in a large Brazilian population.

Keywords Reference range . Fetus . Cisternamagna length .

Brazilian population

Introduction

Cisterns are voids larger than the subarachnoid space, inwhich there is greater accumulation of cerebrospinal fluid thanin other regions of the central nervous system [1]. The cisternamagna is the largest of these and is located in the posteriorfossa, between the internal face of the cerebellum and thedorsal part of the medulla oblongata, and continues caudallyas the subarachnoid space of the medulla, to connect with thefourth ventricle through its medial opening [2].

Evaluation of the fetal posterior fossa is an important part ofassessing the nervous system because a variety ofmalformationsmay occur, including Dandy-Walker syndrome, mega cisternamagna, and hypoplasia/hypogenesis of the vermis [3].

Measurement of the anteroposterior length of the fetalcisterna magna, done in the second trimester of pregnancy, isan important parameter for identifying several abnormalitiesof the posterior fossa. Most authors have taken 10 mm to bethe upper limit for diagnosing abnormalities of the posteriorfossa [4–6], although some other authors have shown that thelength of the cisterna magna increases with increasing gesta-tional age [7, 8].

It is known that ethnicity has an influence on the magnitudeof the transverse diameter of the fetal cerebellum [9], but thereare no studies assessing ethnicity in relation to measurementson the length of the cisterna magna. Only two studies have

E. Araujo Júnior (*) : C. R. Pires : S. M. Zanforlin FilhoReferral Center for Teaching of Diagnostic Imaging (CETRUS),Rua Carlos Weber, 956, apto. 113 Visage, Alto da Lapa,São Paulo, São Paulo, Brazil 05303-000e-mail: araujojred@terra.com.br

E. Araujo Júnior : L. C. RoloDepartment of Obstetrics, Paulista School of Medicine,Federal University of São Paulo (EPM-UNIFESP),São Paulo, São Paulo, Brazil

W. P. MartinsDepartment of Obstetrics and Gynecology, Ribeirão Preto School ofMedicine, University of São Paulo (DGO-FMRP-USP),Ribeirão Preto, São Paulo, Brazil

W. P. MartinsUltrasonography and Retraining Medical School of Ribeirão Preto(EURP), Ribeirão Preto, São Paulo, Brazil

Childs Nerv Syst (2014) 30:9–12DOI 10.1007/s00381-013-2298-y

evaluated the behavior of measurements of the anteroposteriordiameter of the fetal cisterna magna during the second trimes-ter of gestation, in different populations [7, 10], and these werestudies with small sample sizes that may not have representedthe real importance of ethnicity in these measurements.

The aim of the present study was to determine referencevalues for measurements on the fetal cisterna magna duringthe second trimester of pregnancy, among a large sample ofthe Brazilian population.

Methods

A retrospective cross-sectional study covering the period fromJune 2006 toMay 2013 was conducted among 3,862 pregnantwomen who were referred for ultrasonography to beperformed in their second trimester, between the 18th and24th weeks. This study was approved by the Research EthicsCommittee of the Referral Center for the Teaching of Diag-nostic Imaging (CETRUS). The inclusion criteria were asfollows: single pregnancy; absence of fetal malformationsidentified on ultrasonography examination; and absence ofchronic maternal diseases such as arterial hypertension anddiabetes mellitus.

All the ultrasonography examinations were performed ab-dominally, using 3–5 MHz transducers. The length measure-ments on the cisternamagna were obtained in an axial plane ofthe fetal cephalic center, at the level of the lateral ventricles.The reference points used were the cavum septum pellucidum,third ventricle, thalamus, and transverse diameter of the cere-bellum, with an angle of 30° between the transducer and theplane of the fetal head, as recommended by Goldstein et al.[11] (Fig. 1a). Measurements of the anteroposterior diameterwere obtained along straight lines between the posterior bor-der of the cerebellar vermis and the internal face of theoccipital bone (Fig. 1b). A single measurement was made oneach fetus, and no information on postnatal outcomes wasobtained. All the evaluations were supervised by two exam-iners (CR and SMZF), with 20 years of experience of ultra-sonography for obstetrics.

The data were transferred to a spreadsheet in the Excel2007 software (Microsoft Corp., Redmond, WA, USA) andwere analyzed using the PASW software (version 18.0, SPSSInc., Chicago, IL, USA) and the GraphPad software (version5.0, GraphPad Software, San Diego, CA, USA). To describethe sample, means, standard deviations, and minimum andmaximum values were used. To construct reference ranges forthe length of the cisterna magna as a function of gestationalage, polynomial regression was performed and adjustmentswere made by means of determination coefficient (R2). The1st, 5th, 50th, 95th, and 99th percentiles for each gestationalweek (from the 18th to the 24th week) were determined inaccordance with the proposal of Altman and Chitty [12].

Results

Out of the 3,874 fetuses initially selected, 12 were excludedbecause malformations were identified through ultrasound scan.

Fig. 1 Fetal head axial plane at the level of the lateral ventricles to cisternamagna length measurement. a The reference landmarks (white arrows):cavum septum pellucidum (csp), thalamus (tha), and transverse cerebelardiameter (tcd); b the anteroposterior diameter was obtained between theposterior border of the cerebellar vermis and the internal face of the occipitalbone (white arrow)

Table 1 Means, standard deviations, and minimum and maximumvalues for measurements of fetal cisterna magna length as a function ofgestational age

GA (weeks) N CML (mm)

Mean SD Min. Max.

18 to 18+6 128 4.29 0.93 2.60 7.00

19 to 19+6 354 4.54 1.01 2.30 7.00

20 to 20+6 594 4.78 1.18 2.00 12.00

21 to 21+6 723 5.00 1.12 2.60 9.00

22 to 22+6 743 5.19 1.18 2.30 9.00

23 to 23+6 684 5.42 1.22 2.00 9.10

24 to 24+6 636 5.58 1.23 2.00 10.90

GA gestational age, CML cisterna magna length

10 Childs Nerv Syst (2014) 30:9–12

Thus, for the final statistical analysis, 3,862 fetuses wereevaluated.

The means for maternal age, gestational age, biparietal diam-eter, cranial circumference, abdominal circumference, femorallength, and estimated fetal weight were respectively: 27.40±6.46 years (range, 11.75–51.92 years); 22.02±1.69 weeks(range, 18–24.86 weeks); 53.13±5.76 mm (range, 33–69 mm);193.8±20.48 mm (range, 101.4–246.6 mm); 171.70±21.17(range, 113.1–249.8 mm); 37.62±4.98 mm (range, 18–51.4 mm); and 495.9±147.8 g (range, 191.8–979.8 g).

The mean fetal cisterna magna length (CML) was 4.29±0.93 mm (range, 2.60–7.00 mm) in the 18 to 18+6 week andincreased to 5.58±1.23 mm (range, 2–10.8 mm) in the 24 to24+6 week (Table 1). A good correlation between the fetalCML and gestational age (GA) was observed and was bestrepresented by a linear equation: CML=0.535+0.208*GA(R2=0.084) (Fig. 2). Table 2 shows the 1st, 5th, 50th, 95th,and 99th percentiles for the fetal CML measurements at eachgestational age evaluated.

Discussion

Abnormalities of the posterior fossa comprise a heterogenousgroup of findings with greatly varying prognoses. Therefore, itis essential to have precise interpretation and localization ofthese abnormalities so that counseling and follow-up for thepregnancy can be provided [13].

In this study, we determined reference values for the lengthof the anteroposterior diameter of the fetal cisterna during thesecond trimester, among a large sample of the Brazilian pop-ulation. The lower limit of 18 weeks was used because the

vermis is not yet completely formed before this gestationalage [14], which could give rise to false diagnoses of abnor-malities of the posterior fossa.

It is known that measurements of the transverse diameter ofthe cerebellum made using two-dimensional ultrasonography[9] and of cerebellar volume using three-dimensional ultraso-nography [15] vary between different ethnic groups. There-fore, since the posterior border of the vermis is one of themeasurement limits of the cisterna magna, it can be expectedthat measurements on the cisterna magna will also vary be-tween different ethnic groups, even though no information onthis has been published in the literature.

Only three studies evaluating the behavior of measurementsof the anteroposterior diameter of the fetal cisterna magna overthe course of pregnancy have been published [7, 8, 10]. Two ofthese were on a Turkish population (homogenous) [7, 8], andthe third was on a Brazilian population (heterogeneous) [10]. Inthe study conducted by Koktner et al. [7], the length of the fetalcisterna magna was evaluated among 194 pregnant women

Fig. 2 Scatter plot for fetalcisterna magna lengthmeasurement as a function ofgestational age. 50th percentile,solid line; 5th and 95thpercentiles, dashed lines

Table 2 Estimated per-centiles for measure-ments of fetal cisternamagna length as a func-tion of gestational age

GA gestational age,CML cisterna magnalength

GA CML (mm)

1st 5th 50th 95th 99th

18 1.9 2.6 4.3 5.9 6.6

19 2.0 2.7 4.5 6.2 6.9

20 2.2 2.9 4.7 6.5 7.2

21 2.3 3.0 4.9 6.8 7.5

22 2.4 3.2 5.1 7.0 7.8

23 2.5 3.3 5.3 7.3 8.1

24 2.6 3.5 5.5 7.6 8.4

Childs Nerv Syst (2014) 30:9–12 11

between their 16th and 24th weeks. A linear correlation withgestational age was observed, such that the mean fetal cisternamagna length increased from 3.59 to 5.90 mm over the gesta-tional interval from 18 to 24 weeks. In the study by Serhatliogluet al. [8], 130 pregnant women were evaluated from the 16th tothe 38th week, and statistically significant differences in cister-na magna length were observed in the second and third trimes-ters. However, no reference values for each gestational ageevaluated were presented. Comparing these with our results,we observed that our mean values were higher than those ofKoktner et al. at the gestational ages of 18, 19, and 20 weeks,similar at the ages of 21 and 22 weeks, and lower at the ages of23 and 24 weeks.

A study by de Barros et al. [10] comparedmeasurements ofthe anteroposterior diameter of the fetal cisternamagna among69 fetuses of gestational ages of 18 to 24 weeks, using two-dimensional and three-dimensional ultrasonography bymeansof the XI VOCAL method (multi-slice view). In comparingthe measurements made using two-dimensional ultrasonogra-phy, we observed that the measurements were overestimatedat all the gestational ages evaluated. However, because of thesmall sample size of the study by de Barros et al. [10], wecannot come to any definitive conclusions regarding the in-fluence of the ethic factor.

In summary, we determined reference values for measure-ments on the anteroposterior diameter of the fetal cisternamagna between the 18th and 24th weeks of gestation in aBrazilian population. Because of the large sample size, webelieve that these values may serve as a reference in cases ofsuspected abnormalities of the posterior fossa.

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