‘Choroid bar’: easy‐to‐seek marker of normal posterior fossa at 12–14 weeks’ gestation

Objectives

Our objectives were: (1) to assess the visualization rate of the choroid bar in a consecutive series of 306 first trimester scans; (2) to verify – in this cohort of fetuses ‐ the normalcy of the posterior fossa later in pregnancy; (3) to confirm the non‐visualization of the choroid bar in a retrospective series of fetuses with posterior fossa malformations.

Methods

The study include a prospective and a retrospective series. The former includes 306 fetuses undergoing routine obstetric ultrasound at our Unit both in the first and the second trimester over the last 6 months, the latter includes 12 cases of posterior fossa malformations. In the prospective study, the choroid bar – defined as a visually continuous, homogeneously hyperechoic thick structure bridging the cisterna magna from side to side – was sought at the end of the 1^st trimester nuchal translucency scan. In the retrospective study, previously acquired three‐dimensional volume datasets were processed in order to assess whether the choroid bar could be visualized in case of open spinal dysraphisms and vermian cystic anomalies. In the prospective study patients, the confirmation of a normal posterior fossa was based on the sonographic aspect of this anatomic region at the time of the 2^nd trimester anomaly scan at 19‐21 gestational weeks, while in the retrospective study it was based on necropsy results, when available, or further direct imaging of the defect later in pregnancy.

Results

In the prospective study, the choroid bar could be visualized in all 306 fetuses: on transabdominal ultrasound in 287 (93.8%) cases, on transvaginal in 19 (6.2%). The choroid bar was displayed with a ventral/dorsal approach in 67 (21.9%) cases, with a lateral approach in 56 (18.3%) cases and with both in 183 (59.8%) cases. All 306 cases were confirmed to have a sonographically normal posterior fossa at 19‐21 gestational weeks. On the contrary, in the retrospective study, it was never possible to reproduce the choroid bar.

Conclusions

We have described a new sign – the choroid bar – consistent with a normal posterior fossa at 12‐14 gestational weeks. The choroid bar represents an option to screen for major abnormalities of the posterior fossa, since it allows to suspect both open spinal dysraphisms and posterior fossa cystic malformations being at the same time very easy to visualize, for it can be displayed with all lines of insonation.

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