Neurophysiological events caused by IOHFT were present in 13.5% of this mixed patient group, with the majority of occurrences being observed shortly after application of IOHFT. However, events were also observed during all other stages of surgery. The study revealed that patients with a lower height were more likely to experience an IOHFT event. Additionally, there was a suggestive negative association between flexibility of the spine and IOHFT event.

IONM events related to IOHFT during deformity surgery has been reported in literature before. The number of traction events differs between studies. Some studies ([15] n = 22), [16] n = 23), [1] n = 45) did not find any traction-related events, while others reported numbers that exceeded our number, up to 48% ([11] n = 36). A different definition of event cannot explain the differences between these percentages, as criteria of Tøndevold et al. were stricter (50% MEP amplitude decrease [15]) than in our study (80% MEP amplitude decrease), with less events. Neither do differences in the interpretation of the cause of event explain the differences between studies: in 77% of our patients, the cause was unambiguously IOHFT, as it occurred prior to incision, not blurred by other parameters such as surgical actions, bloodloss, or anesthetic fade [17, 18]. Further, the included diagnosis groups cannot explain the differences: Rushton et al. ([12] n = 104) and Lewis et al. [11] included patients with AIS and had a higher number of traction events compared to Tøndevold et al. [15], who included only non-ambulatory patients. In our study, the incidence of IOHFT events was lower in the AIS (8.5%) than in NMS (21%).

There are many ways to apply traction. Some surgeons apply traction on the head and anchor the lower limbs [12, 16], while others apply weights with 20% of body weight on the head combined with 50% of body weight evenly distributed between the legs [11]. A comparison of these studies revealed no relationship between the amount of traction and the number of events.

We found that patients smaller than 162 cm had an almost four times higher risk of an IOHFT event compared to taller patients. The predicting effect of height was also found within the AIS patient group, indicating that the predictor is not confounded by pathology. Differences in height between patients might explain the discrepancies between studies. None of the studies mentioned height; however, the variance is not expected to be very high and cannot completely explain the large differences.

Tøndevold et al. included patients with very high flexibility of the spine (60%) compared to other studies [11,12,13, 16]. Lewis et al. [11] included patients with large thoracic curves with less flexibility and had a high number of traction-related MEP changes. These studies support our association between less flexibility, a larger Cobb angle, and an increased risk of IOHFT events, and (partly) explain the large variability in the number of events between studies. Whenever the patient gets older, the spine gets less flexible; therefore, Lewis et al. [11] suggested a less apical tolerance of large thoracic curves to changes in spinal cord perfusion. However, this cannot explain the asymmetric events we found, as the anterior spinal artery supplies the corticospinal tract bilaterally. Possibly, the spinal cord is more vulnerable on the concave side, explaining the more common left-sided events in patients with a right convex thoracic scoliosis, where the left side of the spinal cord lies compromised in the spinal canal. Whenever the Cobb angle is larger, the spinal cord is stretched tighter.

All IOHFT events concerning upper and lower extremities occurred pre-incision in only NMS. Events involving all extremities indicate the affection of the cervical spinal cord. Previous studies [19,20,21] more often found abnormalities of the cervical spine, such as canal stenosis, in patients with CP than in healthy subjects. Although these studies were conducted in adults, the cervical spine in younger CP patients may also be more vulnerable to IOHFT events due to cervical abnormalities. To identify IOHFT events, it is essential to set a baseline before IOHFT is applied and to regularly check MEP amplitudes while using IOHFT. IONM usually includes MEPs of the upper extremities as a control for events in the lower extremities, since the cervical spinal cord is not involved in the correction of the deformity. However, when IOHFT is applied, the MEPs of the upper extremities add value to the interpretation and prevention of neurological events caused by IOHFT.

The present study included patients with a broad range of Cobb angles. Previous studies found a more profound effect of IOHFT in more severe scoliotic curves. Erdem et al. [2] concludes that in AIS patients with Cobb angles between 70° and 90°, IOHFT is effective. Lewis et al. [11] advised to use IOHFT in thoracic AIS curves  > 80°, and thoracolumbar or lumbar curve  > 60°, or in cases with small pedicles, and Rushton et al. [12] confirms the advice to use IOHFT only in larger curves. Since the risks of IOHFT were found to be related to body length, larger Cobb angles, and stiffer curves, one might consider not to use traction in patients with a shorter stature and/or stiffer curves with a Cobb angle  < 70°. According to these results, we decided not use IOHFT in curves  < 70° in future surgeries. However, this decision depends on the surgical procedure. Nowadays, the bipolar surgical technique [22] is used more often instead of the posterior spinal fusion technique, which was used in the present study. The bipolar surgical procedure consists of only two small incisions, after which rods are inserted under the fascia. Soft tissue and facet-joints remain intact. A large part of the curve correction has to be obtained by traction. Whenever this kind of surgical procedure is used, IOHFT must be reconsidered to optimize the correction of the scoliotic curve.

In patients having a stiff curve  > 90°–100°, preoperative halo-gravity traction is indicated in order to make the curve more flexible [23]. As we found a relation between a stiffer curve and IOHFT event, the risks of IOHFT might decrease by preoperative traction when making the spine more flexible, underpinning the use of pre-operative traction in these patients.

The used definition of “IOHFT-event” presumes that all IOHFT events can be resolved after release of traction, which is never proved. A variety of mechanisms can cause a MEP amplitude decrease. To quickly reverse the event and prevent neurologic deficits, several actions were simultaneously taken. Apart from the removal of IOHFT, perfusion of the spinal cord was optimized by increasing blood pressure and hematocrit, while surgical manipulations were avoided [24]. We might have excluded patients who had an event that was only partly caused by IOHFT. To prospectively study the effect of IOHFT on an event, the recovery of MEP amplitudes after only the release of traction should be measured. When the patient is hemodynamically unstable and experiencing extensive blood loss, we consider it unethical. Further, when an event was caused by contusion of the spinal cord, a sudden decrease in MEP amplitudes was expected, which could not be reversed by release of traction. These kinds of events were excluded from our definition.

After an event, one might consider to decrease the amount of traction, although we found no significant effect of the percentage of traction compared to body weight, as long as it remained beneath 20% on the head and 35% on each leg.

Complications of IOHFT postoperatively are implied by Weiner and Silver’s study [8]. This study approached the use of traction from a historical perspective and concluded that the great forces of IOHFT could cause serious neurological complications such as paralysis. Recent studies using traction in combination with IONM, including ours, have reported no postoperative neurological complications caused by IOHFT. IOHFT can be used safely in all patients, but only when using IONM during all stages of the surgery with an adequate response on events [1, 2, 5, 11, 12, 16, 25, 26]. None of the procedures had to be aborted due to IOHFT events.