Normalization of the Middle Cerebral Artery Resistance Index: A Sign for Adequate Untethering of the Spinal Cord and Regression of Hindbrain Herniation after Fetal Spina Bifida Repair?

Introduction: In fetuses with open spina bifida (fSB), reduced middle cerebral artery resistance index (MCA-RI) has been described. In theory, compression of specific neuronal pathways due to hindbrain herniation (HH) leads to an imbalance of the autonomic nervous system, favoring sympathetic pathways with increased peripheral vasoconstriction resulting in reduced MCA-RI (similar to the brain sparing effect). While untethering the neural placode, the unavoidable surgical manipulation may lead to a temporarily increased tension of the spinal cord and even aggravate HH and compression of neuronal pathways. We assume that along with untethering the spinal cord, the downward pull disappears with a consequent ascensus of the cord, and HH regression (i.e., the hindbrain moves cephalad and out of the foramen magnum) and MCA values normalize after fSB repair. To confirm this hypothesis, the aim of this study was to compare MCA-RI values obtained before, during, and after fetal surgery. Methods: In this prospective study, 28 consecutive patients operated for fSB were included. Ultrasound examinations of the MCA Doppler were performed pre-, intra-, and postoperatively with special focus on measurements during surgery, particularly while untethering the placode. Postoperatively, the HH regression was assessed. Regression of HH is defined as any degree of hindbrain decompression, which is marked by an ascensus of the hindbrain into the posterior fossa and, consequently, by the presence of cerebrospinal fluid around the cerebellum seen on US. MCA measurements were then compared with the postoperative HH regression. Results: MCA-RI before surgery was 0.82 ± 0.02. Intraoperative MCA-RI obtained during untethering was significantly lower than the presurgical values (0.75 ± 0.11, p = 0.03). After fSB repair, the MCA-RI was significantly higher than during surgery (0.83 ± 0.08, p = 0.04). Sonographic findings showed preoperative HH in 24/28 cases (86%), where 23/24 (96%) regressed after fSB repair. Conclusion: This study provides evidence that there might be cerebral blood flow redistribution, potentially due to iatrogenic forces applied to the spinal cord during the fetal operation. Postoperative HH regression seems to be associated with normalization of cerebral hemodynamics.