The evolution of operative technology has allowed correction of complex spinal deformities. Neurological deficits following spinal instrumentation is a devastating complication and the risk is especially high in those with complex sagittal and coronal plane deformities. Prior to intraoperative evoked potential monitoring, spinal cord function was tested using the Stagnara Wake up test, typically performed after instrumentation once the desired correction has been achieved. This test is limited as it does not reflect the timeframe in which the problem occurred and it may be dangerous to some patients. Intraoperative neuromonitoring allows timely feedback of the effect of instrumentation and curve correction on the spinal cord. Pedicle screws that are malpositioned can result in poor fixation or neuronal injury. Evoked EMG monitoring can aid in accurate placement. A positive EMG response can alert the surgeon to a potential pedicle breech and allow them to reassess the placement of their hardware intraoperatively. The stimulation threshold is affected by the amount of surrounding bone acting as an insulator to electrical conduction and is variable in different regions of the spine. In the non-deformed, lumbar spine stimulation thresholds have been established. Such guidelines have not been well-developed for the thoracic spine, or for severely scoliotic spines. Thus our primary objective was to compare the stimulation threshold of the apical pedicle on the concave side to the stimulation threshold of the pedicles at the upper and lower instrumented levels.

Intraoperative EMG stimulation thresholds were done at 192 apical pedicles on the concave side of the deformity and then compared to those thresholds found at 169 terminal level pedicles. Only pedicles for which a stimulation threshold was found were reported and excluded those where a breech was suspected. The lowest stimulation required for an EMG response was documented to a maximum stimulation of 20 mA.

The mean threshold at the apex was 16.62 milliamps (mA) compared to 18.25mA at the terminal levels. This was compared with the t-test and showed a statistically significant difference (p<0.05).

In this study we report only the thresholds for the concave side, the pedicle that is most likely to be reduced in size. The threshold for stimulation is reduced compared to those seen at the highest and lowest instrumented level. Most of the apexes are located in the mid-thoracic spine with the highest instrumented levels being in the high thoracic spine and the lowest levels being in the lumbar spine. This study provides preliminary evidence that the apical, concave pedicle has a lower threshold than the end pedicles and one cannot rely on established thresholds from different areas of the spine. The surgeon should be cognisant of these differences when instrumenting at the apical level. Ongoing work is examining the convex apex threshold as well as the relationship between the effect of age and a diagnosis other than adolescent idiopathic scoliosis.

To examine the effect of lateral spine curvature on somatosensory evoked potentials (SSEP) in patients with adolescent idiopathic scoliosis (AIS) compared to normal controls. We hypothesise that patients with AIS will show increased latency in their SSEPs when bending into their curve suggesting that their spinal cord is more sensitive to this increased lateral curvature.

Patients were recruited from the paediatric scoliosis clinic in a single centre. Inclusion criteria were: diagnosis of AIS, age 10–18 years, major thoracic curve measuring greater than 10 degrees on Cobb measurement, and undergoing nonoperative management. Exclusion criteria were: any detectable neurologic deficit, and previous surgery on the brain or spine. SSEP recordings were obtained via stimulation of the posterior tibial nerve with surface electrode and measurement of the cortical response over the scalp. All recordings were performed three times: with the patient in neutral standing and maximum right and left side bending.

SSEP recordings show that when AIS subjects bend into their curve, latency slows by an average of 0.5ms. However there was a bimodal distribution with most subjects showing minimal change (3ms). This subset was statistically different from both a control group, and the larger AIS group.

There appears to be a subset of patients with AIS who have subclinical spinal cord dysfunction demonstrated by abnormal SSEPs. This may place these patients at slightly higher risk of neurologic injury at the time of surgery.