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Under normal circumstances, the bottom of the spinal cord floats freely within the bones of the vertebral column so that growth and movement will not damage the nerves. In some children, improper development resulting in a bony protrusion, tumor, fatty deposit, or scar tissue, may pin the spinal cord somewhere in the lower back. This results in a condition called tethered spinal cord, in which the spinal cord is tugged between the lower back and the brain stem. Because the vertebral column grows more quickly than the nerves that make up the spinal cord, tethering will cause increased tension on the cord as a child grows.
Generally tethered cord is identified by a lesion, discoloration, patch of hair, deep dimple, or a fatty tumor on the lower back, which is accompanied by low-back pain, deformed legs, hips and feet, and problems walking.
Surgery is the main treatment for tethered spinal cord. It is recommended that surgery is done early on to prevent deterioration of nerve function.
For this procedure, surgeons open the lower back to expose the site where the spinal cord is pinned, and free it by removing a protrusion or fixing a developmental abnormality.
Because the surgery is difficult, neurophysiologic intraoperative monitoring(NIOM) is used to reduce the possibility of inadvertent injury to neural structures during surgery. We monitor motor system, sensory system, and sphincter function. For motor system monitoring, we monitor the lower extremities by using electromyography (EMG). With EMG abnormalities are signified when there is a response that continues as a train.
The sensory pathways can be monitored by performing tibial somatosensory evoked potentials. During the surgery, the tibial nerve is stimulated near the medial malleolus and subcortical or cortical waveforms are recorded from scalp and non-scalp electrodes. If there is a amplitude drop of 50% or more, or a 10% prolongation in latency, this considered significant and action should be taken.
Anal or urinary sphincter dysfunction is a devastating complication of cauda equina surgery. By monitoring sphincter function, the risk of this complication can be reduced. This is the easiest of all the monitoring procedures and is the most commonly performed.
The adult spinal cord ends opposite the lower border of the L1 vertebra at T12; below that, the spinal cord contains lumbosacral nerve roots from L1-S5 that descend downward into the cauda equina. When compression or degeneration of the lumbar/sacral spine occur, a procedure called Lumbosacral Surgery is necessary. Decompression and Spinal Fusion are the two most common surgical procedures performed for lower back surgery. Spine surgery instrumentation, bone graft procedures, and the bone stimulator are sometimes used along with spinal fusion surgery.
Clinical symptoms consist of numbness, back pain, weakness and sphincter dysfunction.
IOM techs monitor the modalities of SEP’s, MEP’s, free running EMG and Motor Nerve Conductions (pedicle screw stim) during lumbosacral spine surgeries. Mixed nerve SEP and MEP’s are the NIOM techniques most often used, but they are not sensitive for nerve root injuries, but have a higher predictive value. Dermal somatosensory evoked potentials (DSEPs) can detect individual nerve root injuries but are technically difficult to resolve in the OR. For these reasons, EMG has become the most often used to monitor nerve root function, because of its sensitivity for nerve root injuries.
Nerve roots can be injured during lumbosacral spine surgery due to probing, drilling, misplaced hardware, or stretching during correction of spinal deformation. There is a 10% risk of new postoperative neurologic deficit from nerve root injury during spinal fusion for degenerative lumbosacral spine disease. The most common deficit is a new foot drop (anterior tibilais muscle) from surgical injury to the L5 nerve root. Nerve root injuries are more frequent during revision and in cases where multiple levels are focused rather than actual surgery. Clinical studies have detected postoperative symptoms from irritation or injury to the adjacent nerve roots by misplaced screws in 5% to 10% cases.
The purpose of NIOM with EMG during these lumbosacral spine surgeries is to detect early and potentially reversible surgical nerve root irritation, alerts the surgeon, and thus prevent more significant injury and postoperative deficit. Monitoring must be paused during Electro Cautery, drilling etc., because of the artifact they produce.
During Free-running EMG monitoring should be inactive under normal conditions; Blunt mechanical nerve root irritation activates the motor nerve fibers, is transmitted down the nerve and across the neuromuscular junction, and evokes recordable motor unit potentials in the monitored muscle. A minor nerve root manipulation evokes a short burst of motor unit potentials. More severe nerve root injury evokes prolonged trains of high-frequency motor evoked potentials (neurotonic discharge).Myokymic discharges are rarely seen.
Anesthesia has little to no effect on EMG or motor responses. Voluntary motor unit potentials can be seen under conditions of light anesthesia. Needle electrode movement can be easily mistaken for EMG activity. EMG cannot be monitored in the presence blockades. In addition, SEP may be required with EMG and higher doses of inhalation anesthetic may be required without neuromuscular blockade. The surgeon should be notified right away whenever neuromuscular blockade detected; they will then likely order reversal if approaching a critical phase of the procedure.
Drop of amplitude of >50% of baseline, and Increase of latency >10% of baseline signifies abnormality in IOM monitoring. The IOM technologist needs to evaluate the mechanical and non-mechanical causes as well.