T96. Neuromonitoring adolescent idiopathic scoliosis with pulse train technique

Abstract

The placement of thoracic pedicle screw is technically difficult, and the medial mal position can result in a feared risk for the spinal cord. The effectiveness of detecting mal-positioning of pedicle screws varies widely in the literature, specially at thoracic levels, but in 2014, Calancie et al. described a technique with high accuracy in preventing that in adult patients. We prospectively studied the usefulness of the technique in adolescents with idiopathic scoliosis. We studied 51 adolescent patients (41 female), mean age of 14.8 y (11–18 y) diagnosed with idiopathic scoliosis, submitted to scoliosis correction with the free-hand technique for thoracic pedicle screw placement under total intravenous anesthesia (TIVA) and intraoperative neuromonitoring (IONM) with our standard protocol updated with the multi-train stimulation technique (MTST). MTST parameters consists of a repetitive train of electric pulses applied in the track and in the implanted pedicle screw. The warning criteria was a muscle MEP recorded in lower limb muscles with stimulus below 12 mA for the track and 25 mA for the screw. Intraoperative fluoroscopy was used to check screw position if the warning criteria was achieved and after the placement of all screws if there were no alarms. The final decision of the surgeon to a given alarm considered anatomical features, the threshold and the fluoroscopy images. We studied 210 tracks/screws from T2 to L1 in the first 13 cases and 390 from T2 to T10 in the remaining 38. Track/screw threshold was below the limit 49 times (8.2%), with higher incidence for T5 and T9 (10 times each), and for T7 and T8 (8 times each). Tracks/screws were redirected successfully in 33 cases and due to their persistent lower thresholds, withdrawn in 8, all of them from the apex of the scoliosis concavity. In the remaining 8 cases the surgeon decided to let the implant in the original position considering the imaged and the screw thresholds close to the limit. In 24 cases (47%) the warning criteria were not achieved neither for track or screw, and the images agreed with the IONM findings. In 3 cases the thresholds for the screws were below 4 mA and a complete loss of TcMEP in lower limb immediately followed the screw placement. In two cases the screws were removed immediately, and the patients have transitory motor deficit, but in one case the surgeon took a long time to remove the implants and the patient become paraplegic. MTST takes no more than 10 s for each screw. The technique showed a good relationship with medial mal-positioning of the track/screw. It was not time consuming and the surgeons felt confident with the warning criteria, reducing also the X-ray exposition. Very low thresholds can even suggest spinal cord lesion demanding the cross checking with TcMEP. The technique showed to be safe and trustful for younger patients (adolescents) with idiopathic scoliosis and should be considered a standard of care for thoracic pedicle screw placement.