An Audit of the Posterior Fossa Characterization in Open Spina Bifida Based on Tertiary Center Experience

Spina bifida aperta (SBA), often referred to as myelomeningocele (MMC) or myeloschisis, is a devastating yet non-lethal congenital malformation with complex physical and neurodevelopmental sequelae. Its prevalence is approximately 4.9 per 10,000 live births in Europe and 3.17 in the United States. SBA results in motor and sensory deficits, their extension being defined by the upper level of the anatomical defect. These range, as the level increases, from bladder, bowel and sexual dysfunction, to involvement of the lower and even upper extremities, and secondary orthopaedic disabilities. Despite improvements in prevention, diagnosis, and postnatal management, SBA remains a major source of morbidity and mortality throughout the world. Children with SBA almost invariably have an associated Chiari II hindbrain malformation (CM II) and ventriculomegaly. The Chiari II malformation is characterized by posterior fossa (PF) and brain stem abnormalities, with downward displacement and compression of the cerebellum and brain stem.

Geerdink demonstrated that morphometric measures reliably quantify the morphological distortions of Chiari II malformation on postnatal MR images. These included downward herniation of the cerebellum, displacement of pons, medulla and fourth ventricle, medullary kinking, abnormal fourth ventricle, tentorium and mesencephalic tectum. The mamillopontine distance (MPD) and the cerebellar width were the most sensitive and specific determinant of Chiari II. Some foetuses with SBA have ventriculomegaly and its degree is believed to be predictive for the need of postnatal shunting.

In 2011, the Management of Myelomeningocele Study (MOMS) demonstrated the benefit of in utero repair of MMC as the need for ventricular shunting at 12 months was reduced and motor outcome at 30 months improved. Fetuses with the suspicion of SBA should be assessed comprehensively to counsel parents about the expected outcome and whether they would be candidates for fetal surgery. In this assessment, fetal MR has a crucial role to characterize the extent of brain abnormalities, the level of the lesion, and to rule out additional anomalies in fetuses with SBA. For fetal surgery eligibility, the presence of CM II on MR is a necessary finding. Many measurements have been proposed to describe the typical PF changes in fetuses with SBA, yet the reproducibility of these has rarely been studied. These include the transverse cerebellar diameter (TCD), the transverse diameter of the posterior fossa (TDPF), the midsagittal PF area, the level of cerebellar herniation and the clivus-supraocciput angle (CSA). These parameters have also been shown to change after in utero repair of SBA in small series and at different time points after fetal surgery, yet no study has consistently reported early postoperative assessment in utero.

The aims of this study were threefold: (1) to assess the reproducibility of measurements of the brain stem and PF that have been suggested to be representative on postnatal 8,9 and prenatal MR imaging (2) apply those parameters that were shown to be reproducible, to discriminate fetuses with SBA from gestational age matched fetuses with a normal PF; (3) and document early changes in these measurements one week after fetal surgery.