Summary of Evidence
The current systematic review examined the current literature on IOUS and spinal decompression surgery for the cervical, thoracic, and lumbar spine. Although we did not identify a universal definition, there was a common description of decompression that was qualitative. Adequate decompression was frequently characterized by the ventral aspect of the spinal cord being "free floating" within the cerebrospinal fluid. This included no contact with the anterior soft tissue elements such as the OPLL or intervertebral disks.
There was insufficient evidence in favor of any quantifiable definition. Only one study attempted to quantify the severity of compression by IOUS using measures of the diameter of the spinal cord before and after decompression. Additionally, the pulsatility of the spinal cord, previously thought to be a surrogate measure of extrinsic compression of the cord, was also an inadequate definition given not all patients demonstrated pulsations.
The clinical significance of IOUS in decompression spine surgery was less clear. There was conflicting evidence suggesting a clinical correlation with the qualitative "free floating" definition and improved clinical outcomes. We identified nine studies of 15 (three studies in cervical spine,[8,9,14] six studies in thoracic,[21–25] 0 studies in lumbar) that documented an improved clinical score with adequate free floating (+) decompression compared to inadequate decompression free floating (–). The remainder six studies of 15 found no difference in clinical outcomes (clinical scores included the Japanese Orthopedic Association Scoring System, Recovery rate by Hirabayashi method, or Nurick grading scale).[10,11,16,26,27,32]
Evidence does exist that IOUS may prove to be a beneficial tool to help surgeons identify the adequacy of their decompression intraoperatively. For compressive lesions above L1 level, in the cervical and thoracic spine, ultrasound allows us to see structures that cannot normally be visualized in the canal due to the dangers of cord retraction. Conversely, for levels below L1, the nerve roots can readily be retracted after laminotomy without injury, to allow for direct visualization of compressive structures in the canal. Nonetheless, there are studies that show IOUS provides an additional adjunct to identify residual disk material that would otherwise be missed in routine decompression. For example, Montalvo et al reported that after routine lumbar discectomy, residual disk material was found in 41% (17/41) patients using IOUS. This subsequently led to further decompression in 16 of these patients by using ultrasound to visualize the removal of additional disk fragments. In the remaining one patient, the surgeon judged that the residual disk fragment to be insignificant and to not warrant further decompression. Aoyama et al also reported cases where IOUS altered the intraoperative course. In 30 patients undergoing lumbar disc herniation surgery, IOUS detected residual herniated disc in 7% (two of 30), that was missed by the surgeon and these patients went on to receive further decompression.
There were limitations to this study. Due to the heterogeneity of the included studies such as variability in study design, patient populations, and reported outcomes, we could not pool the studies and only a qualitative analysis could be performed. Nonetheless, we identified a common qualitative definition for decompression in the cervical, thoracic and lumbar spine that will be useful as an intraoperative tool. However, future studies are needed to investigate the correlation between decompression morphology and patient outcomes to better understand the clinical value. There are still important gaps to address for the routine use of IOUS in spine surgery including a better understanding of the interobserver reliability, association with clinical outcomes, and systematic acquisition of ultrasound images.
Spine. 2022;47(2):E73-E85. © 2022 Lippincott Williams & Wilkins