Fetal Advanced Neurosonography in the First Trimester of Pregnancy Open Access

The detection of central nervous system (CNS) malformations by ultrasound during pregnancy is mostly during the second-trimester ultrasound examination [1]. However, some of these malformations can be detected or suspected as early as the first trimester. Both, national [2] and international [3] guidelines specify a minimum set of requirements for anatomical assessment of the CNS between 11 and 14 weeks of pregnancy. These requirements include evaluating ossification, contour, and shape of the skull in the axial plane; confirming the presence of two distinct cerebral hemispheres separated by the interhemispheric falx; and verifying that the choroid plexuses occupy the majority of the lateral ventricles (the “butterfly sign”).

Due to the improvements in high-resolution ultrasound technology, the CNS can now be examined in greater detail. Thus, the latest version of the Fetal Neurosonography guidelines from the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) describes the systematic multiplanar assessment of the CNS [4].

In 2009, Chaoui et al. [5] introduced intracranial translucency as an early indicator of open neural tube defects (NTDs) in 2009. Since then, numerous studies have described additional markers that can predict both NTDs [6‒8] and other severe CNS pathologies, particularly those affecting the posterior fossa [9‒11]. However, none of the previously mentioned guidelines currently advises the routine use of these markers.

In this study, we propose an advanced protocol for CNS evaluation in the first trimester of pregnancy, incorporating multiplanar assessment along with various predictive markers of CNS abnormalities reported in the literature. The objectives of this study were to determine whether this proposed protocol is feasible in a tertiary center as part of routine screening ultrasounds and to evaluate the reproducibility of the proposed parameters and markers.

Ethical approval for this study was granted by the Institutional Review Board (IRB) of the Hospital Universitari Dexeus, Barcelona, Spain (FSD-NEU-2023-12). This is a prospective observational study was conducted from July to November 2023 at the Hospital Universitari Dexeus in Barcelona, a tertiary-level center.

Inclusion criteria were singleton pregnancies with fetal CRL from 60 to 82 mm (12+3–13+6 weeks), normal basic ultrasound examination and the signed informed consent. The exclusion criterion was the suspicion of fetal malformations at the time of inclusion. The maternal body mass index (BMI) was recorded at the time of the examination, as well as the total time spent (basic study plus advanced study) in the exploration.

The study was performed by two ultrasound specialists (M.A.R. and M.E.) with more than 10 years of experience in fetal neurosonography. Both the basic and advanced ultrasound assessments were conducted using the Samsung Hera W10 (Samsung Electronics Iberia SAU HME Health and Medical Equipment, Seoul, South Korea) and Voluson E8/E10 (GE Healthcare, Chicago, IL, USA). All patients underwent both transvaginal and transabdominal ultrasounds with 3–5 MHz and 10 MHz probes.

The examination was divided into a basic examination (considered part of conventional screening) and an advanced evaluation (performed only when the basic examination was feasible and normal). Details of the systematic evaluation for both the basic and advanced assessments are provided below.

The basic examination was performed using transventricular and transthalamic sections. The following structures were evaluated subjectively (Fig. 1):

• Cranial ossification: degree of ossification and the absence of bone defects

• Interhemispheric line: complete integrity

• Choroid plexuses: presence and filling of almost all LVs (“Butterfly sign” morphology)

• III ventricle and thalami: their presence was evaluated in pregnancies of more than 13 weeks.

In all patients, both transvaginal and transabdominal ultrasound examinations were performed. The advanced evaluation was carried out only if the basic examination was feasible and showed normal findings.

The advanced examination included a multiplanar assessment of the CNS (axial and sagittal planes), along with both qualitative and quantitative evaluations of specific predictive markers of CNS pathology in all patients.

• Axial sections:

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    Transventricular: insula, LVs, and choroid plexuses (Fig. 2)

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    Transthalamic: Sylvian aqueduct and tectum (Fig. 3)

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    Transcerebellar: cerebellar hemispheres (Fig. 4a)

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    Posterior fossa: IV ventricle (IVV), choroid plexus of the IV ventricle (PC), cisterna magna (CM) (Fig. 4b).

• Sagittal sections:

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    Identify 4 lines and 3 spaces (Fig. 5)

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    Diencephalon, brain stem (BS), IVV, PC, CM, occipital bone (Fig. 5).

• Axial sections:

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    Transventricular axial section (Fig. 2):

    • Choroid plexus length (PL) (mm)

    • Lateral ventricle length (VL) (mm)

    • PL/VL ratio [12]

    • Insula (measured perpendicular to the interhemispheric line, from inside to inside, between the cranial bone and the maximum indentation of the insula) (mm)

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    Transthalamic axial section (Fig. 3):

    • Measurement from the aqueduct to the occipital bone (Ac-Oc) [8] (mm)

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    Transcerebellar axial section: (axial oblique) (Fig. 4a).

    • Transcerebellar diameter (TCD) distance between the ends of the cerebellar hemispheres (mm).

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    Axial section of posterior fossa (parallel and inferior to the transcerebellar) (Fig. 4b):

    • IVV: from BS posterior border to PC [9] (mm)

    • CM: from posterior border of PC to internal border of occipital bone [9] (mm).

• Sagittal sections:

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    BS: from the posterior border of the sphenoid to the anterior border of the IVV (IT) [7] (mm) (Fig. 6)

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    IT: from the posterior edge of the BS to the PC [6, 7] (mm) (Fig. 7)

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    CM: from posterior border of PC to internal border of occipital bone [10] (mm) (Fig. 8)

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    BSOB: is the space between the BS and the occipital bone. We must measure it from the anterior edge of the IT to the anterior edge of the occipital bone It would be the IT+CM [7] complex (mm) (Fig. 9)

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    BS/BSOB ratio [7].

For each variable, we documented whether it could be evaluated and which approach (transabdominal or transvaginal) provided the best assessment. Both qualitative and quantitative analyses were performed in real time during the examination (ONLINE) by the sonographer conducting the ultrasound (M.A.R. or E.M.). Images were archived and anonymized, as were all patient data. An offline analysis of the quantitative parameters was then performed on the stored images at least 1 week later by both the first and second examiners to assess concordance. Study data were entered into an electronic form: Research Electronic Data Capture (REDCap).

Normal quantitative variables were reported as means and standard deviations, while non-normal variables were expressed as medians and interquartile ranges. Frequency tables were used for nominal variables.

Intra- and interobserver agreement for quantitative parameters was assessed using the intraclass correlation coefficient (ICC) for continuous variables. ICC values were interpreted as follows: <0.40 (poor), 0.40–0.59 (sufficient), 0.60–0.74 (good), and >0.75 (excellent). Bland-Altman plots were used to quantify the differences between two measurements and to determine the 95% confidence range within which these differences are expected to lie. To calculate the ICC in an absolute agreement model, we used a mixed-effects model for intraobserver agreement and a two-way random-effects model for interobserver agreement.

We also analyzed the median time required for extended scans and compared these data with those from a random cohort of 240 conventional screening cases of normal fetuses at comparable gestational ages, performed by the same operators. For this comparison, we used the Mann-Whitney test. All statistical analyses were conducted using IBM SPSS Statistics software.

A total of 98 fetuses with CRL of 60–68 mm were analyzed. Two were excluded due to pathological findings (1 case of hydrops with a congenital heart disease and one case of omphalocele). Of the remaining 96, 92 (95.8%) were finally included in the study, as the basic examination could not be adequately performed in 4 cases. In the included group (n = 92), the mean maternal BMI was 23.4 (range 17–34) and the mean fetal CRL was 68 mm (range 64–68). Among those not included (n = 4), the mean BMI was 29 (range 27–32) and the mean fetal CRL was 70 mm (range 68–81).

The full extended study was completed in 86 of the 92 fetuses (93.48%). No statistically significant differences in BMI or CRL were found between the group that underwent the complete extended examination and the group in which only the basic examination was performed (Table 1).

For the qualitative assessment (Table 2), in the axial plane, the insula and lateral ventricles were evaluated in 100% of cases and in >95% of the cases, the remaining structures were visualized. In sagittal planes, all structures were visualized in 100% of cases. Regarding the quantitative assessment (Table 3), in axial sections PL, VL, and insula were measured in 100% of cases and in >95% of the cases the remaining structures were measured. In the sagittal sections, all structures were measured in 100% of cases.

The best approach for evaluating each structure was as follows:

• Basic examination: transvaginal in 52% of cases, transabdominal in 12%, and both approaches in 36%.

• Advanced examination: axial views were assessed transvaginally in 84% of cases and transabdominally in 16%, while sagittal views were evaluated transabdominally in 93% of cases and transvaginally in 7%.

Intra- and interobserver concordance for the quantitative parameters was evaluated in 60 cases using the intraclass correlation coefficient (ICC) (Table 4) and Bland-Altman plots (Fig. 10). Overall, a good intra- and interobserver correlation was found. The lowest ICC values (both intra- and interobserver) were observed for the PL/VL and BS/BSOB ratios, although the Bland-Altman plots showed good distribution. Consequently, all analyzed parameters can be considered reproducible.

Finally, we compared the mean time required for the extended ultrasound protocol with data from a control group of 241 conventional screening examinations of fetuses without pathology, at similar gestational ages, performed by the same operators. The mean time in the study group was 13 min, whereas in the control group it was 10 min (p < 0.05).

Current ultrasound guidelines specify the minimum structures to be evaluated in the first trimester of pregnancy [2, 3]. According to these recommendations, all cases of acrania, alobar holoprosencephaly, and encephalocele should be diagnosed [13]. Consequently, most abnormalities are diagnosed only in the second or even third trimester. Thus, Syngelaki et al. [14] reported that at 11–13 weeks, they diagnosed all cases of acrania, alobar holoprosencephaly, and encephalocele, 59% (35/59) of cases of spina bifida and 13% (2/15) of cerebellar or vermis hypoplasia. The rest of the CNS abnormalities were diagnosed in the second or third trimester.

With advances in ultrasound resolution, it is now possible to identify subtle fetal anatomical details at an earlier gestational stage. Several studies have examined the correlation between high-resolution ultrasound imaging and anatomical findings [12, 15]. Through the protocolized assessment of different structures of the fetal head, added to the conventional screening examination or just in selected cases, conditions that are typically diagnosed late (such as ventriculomegaly and open NTDs) can be suspected in the first trimester. In the first trimester of pregnancy, ventriculomegaly is not defined by an enlarged atrium but rather by a reduced ratio between the choroid plexus and the lateral ventricle. Manengold et al. [12] assessed the ratio of the diameter, length, and area of these structures in a transventricular section, proposing ventriculomegaly if this ratio (choroid plexus/lateral ventricle) was below the 5th percentile, with reported sensitivities of 82.4%, 94.1%, and 94.1%, respectively. Another example can be found in the open DTN markers. In 2009, Chaoui et al. [5] introduced the absence of the IVV on the sagittal section of the fetal head (routinely used to measure nuchal translucency) as a marker of OBS. They termed this the “intracranial translucency” (IT) sign. In this sagittal section, four lines and three spaces (Fig. 5) are typically visible; their absence (fewer lines and/or spaces) should prompt suspicion of a potential abnormality. Subsequent studies focused on identifying Arnold Chiari II signs associated with OBS in the first trimester, such as a decreased CM and BSOB, or an increased BS and BS/BSOB ratio, which would prompt a direct search for the defect [7, 16]. Other publications have described how to assess these posterior fossa collapse markers in axial views (decreased CM, decreased Ac-Oc distance) [8‒10]. Using these markers, early posterior fossa anomalies (e.g., Dandy-Walker malformation, Blake’s pouch cyst) may be suspected if there is an increased IT and CM, or even fusion of these two spaces, together with a decreased BS/BSOB ratio [11, 17]. Additionally, the absence of the IVV plexus (PC) is considered a common marker for different CNS anomalies [18].

In this study, we aimed to standardize an advanced early neurosonographic examination. Our proposed protocol includes subjective assessment in both axial and sagittal planes of certain CNS structures in addition to the basic examination, such as aqueduct, tectum, cerebellum, PC, CM, BS, IVV, along with quantitative evaluation of specific markers previously identified as predictors of pathology in the literature. In 86 out of 92 scans (93.48%), all proposed structures in the extended protocol could be assessed and measured. Each structure was visualized and measured in over 95% of cases, reaching 100% in the axial trasventricular and in the sagittal sections. Notably, our study we selected fetuses with a CRL >60 mm who had a feasible and normal basic examination. The most challenging measurements involved identifying the aqueduct and tectum and determining the Ac-Oc distance, although this was still achieved in 95.65% (88/92) of cases (88/92) (Tables 2, 3).

In the extended study, the optimal approach for axial views was transvaginal (84%), whereas for sagittal views it was transabdominal (93%). This suggests that both approaches are advisable for an advanced neurological examination in the first trimester. Although the difference in mean examination time between the study group and the control group was statistically significant (3 extra minutes; p < 0.05), we do not consider this an excessive amount of time. Therefore, integrating certain markers – particularly simple qualitative indicators such as the 4l-3e sign or an axial transventricular assessment – into routine screenings could improve abnormality detection without significantly extending the procedure time. In cases where pathology is suspected or risk factors are present, an extended neurosonographic examination is advised.

To perform an early neurosonographic evaluation, it is essential to have reliable and updated reference curves and a strict and well-defined measurement protocol. Some parameters analyzed in this study lack established normal ranges for these gestational weeks (e.g., insula measurements). In other cases, existing reference data were derived from transabdominal scans, even though many of these structures are better visualized transvaginally as is the case of the TCD and all axial measurements [9]. Based on these observations, we propose developing reference curves for all quantitative parameters in accordance with the protocol described in the Methods section.

In conclusion, advanced CNS assessment in the first trimester is feasible by incorporating multiplanar ultrasound evaluation and predictive markers of CNS abnormalities. This approach demonstrates a good intra- and interobserver concordance and does not excessively increase overall ultrasound examination time.

This study was conducted as a final project for the Fetal Neurology Postgraduate Program organized by Fetal i+D Education Barcelona^® with the auspices of the Càtedra d’Investigacio en Obstetrícia i Ginecologia de la Universitat Autònoma de Barcelona.

Ethical approval for this study was granted by the Institutional Review Board (IRB) of the Hospital Universitari Dexeus, Barcelona, Spain (FSD-NEU-2023-12). Written informed consent was given by all participants.

The authors declare no conflicts of interest.

This study was not supported by any sponsor or funder.

M.A.R. and M.I. developed the idea of the study, substantially contributed to the design, review and interpretation of the results. I.R. carried out a lot of work by performing all the statistical analysis. M.E.: significantly contributed by collecting data. M.A.R., P.P., and L.P. wrote a major part of the manuscript and G.A. reviewed all the documents for submission. All authors have read and approved the manuscript.

All data generated or analyzed during this study are included in this article. Further inquiries can be directed to the corresponding author.