Determination of TOF Test Threshold for Obtaining Reliable Pedicle Screw Stimulation Test During Lumbar Spine Surgery

During lumbar spine fusion surgery intraoperative neurophysiological monitoring is in routine
use for prevention of possible nerve injuries during placement of screw into pedicle. Pedicle
screw stimulation test is performed to assess if screw placement is encroaching on the nerve
roots. Relative distance between the pedicle screw and the neighbouring root can be estimated
by the intensity of the current required to activate the root and appropriate muscle. A
properly placed screw can be distinguished from those perforating the pedicle wall by its
higher minimum level of electrical current needed to elicit a muscle response. The minimum
level is deemed as threshold. This test is based on compound muscle action potential (CMAP)
and use of neuromuscular blocking agents (NMBA) should be avoided because of possible cause
of false negative results of screw stimulation test. Neuromuscular blocking agents, which are
in routine use during anesthesia, will have effect on muscle action potential. When NMBA are
used current stimulus will depolarize the same number of axons and the associated muscle
response will be present but of lower amplitude because some percentage of motor fibers will
be blocked by activity of NMBA. At this point, stronger stimulus is needed to recruit
additional axons/muscle fibers , and hence, the measured threshold is elevated.

Train of four (TOF) test is method used to determine level of neuromuscular blockade, by
stimulation of peripheral nerve and following induced muscle contractions. Interpretation of
muscle contractions may be by subjective (visual) or objective (quantitative) method.
Quantitative TOF test may be used prior screw stimulation test by calculating T4/T1 ratio and
obtaining quantitative value which shows level of neuromuscular blockade. Residual
neuromuscular blockade may be present before screw stimulation test, and effect on accuracy
of this test in this situation was not clearly investigated in recent studies.

The purpose of this study would be to determine changes of screw stimulation thresholds under
different range of neuromuscular blockade.Therefore, acceptable neuromuscular blockade
threshold (determined by TOF test) acceptable for obtaining reliable screw stimulation test
should be determined. Screw stimulation test may be performed when neuromuscular blockade is
absent and compared to repeated screw stimulation test when neuromuscular blockade is
present. Different levels of neuromuscular blockade may provide different results on screw
stimulation test, so if difference is statistically significant, induced neuromuscular
blockade level may be set as threshold value, acceptable for obtaining reliable testing
results.