Intraoperative Neurophysiological Monitoring for Minimally Invasive 1- and 2-Level Transforaminal Lumbar Interbody Fusion: Does It Improve Patient Outcome?

DISCUSSION

IOM is a valuable technique for assessing the nervous system while a patient is under general anesthesia, especially during spinal or cranial neurosurgical cases, and replaces the neurologic examination while the patient is asleep. IOM allows for the assessment of many neural structures, including the neuromuscular junction, peripheral nerve, spinal cord, brainstem, and cortex during surgery. The most commonly employed techniques during spinal procedures are (1) MEPs, (2) upper and lower SSEPs, (3) pedicle screw stimulation, and (4) spontaneous EMGs. A number of other techniques have been used over the years, including direct spinal cord stimulation and reflex monitoring. We used SSEPs, pedicle screw stimulation, and EMGs in all our monitored spine cases.

In the IOM group, monitoring did not detect the malposition of pedicle screws that necessitated revision in 5.48% of patients. In fact, the incidence of screw malpositioning was similar in both surgical groups, suggesting that the use of IOM made no difference to the incidence of pedicle screw malpositioning.

Although direct hospital cost was similar in both groups, total cost was significantly higher in the IOM group because of the added cost of using IOM. The surgery time was also significantly longer in the IOM group and probably accounts for added costs that we did not take into account in this study.

Because the analysis of our patient database showed that the use of IOM did not make a difference in the incidence of pedicle screw malpositioning and because of the comparative cost analysis for both groups of patients, we believe that the use of IOM for MIS-TLIF provides no added benefit.

Given the current climate of cost-effective medicine, developing an evidence-based algorithm for determining which spine surgeries should be monitored is important. Many surgeons want to monitor every spine case even if the chance is tiny that a patient could benefit. They argue that some patients will have improved outcomes if the IOM picks up impending damage to the nervous system. In our series, the use of IOM did not pick up medial bridge of the pedicles and nerve root irritation in 5.48% of the patients. Consequently, we think the added cost of IOM is not justified in our series. The significance of nerve root irritation or medial bridge would allow the surgeon to intraoperatively reposition the screw, but that was not noted in our experience.

Several other arguments can be made for IOM and perhaps more objective ways can be devised to analyze its cost effectiveness. Stecker¹⁵ provided a good mathematical analysis (2012) of the cost of responding to an IOM warning during surgery and other associated costs. He stated that each time the monitoring team issues an IOM warning, a number of events occur. The surgical and anesthesia teams double check what they are doing. They will commonly attempt to increase the blood pressure, administer medications, or change the level of anesthesia. Thus, operating room time and medications have a cost: C_o(W), the operative cost of responding to a warning. The cost can be expressed as C_o(W) × P(W), where P(W) is the probability that a warning will be issued. In addition, the cost of providing the monitoring for each case, C_m, is not dependent on the rate of issuing a warning. Steckler proposed that the total cost of monitoring is C_m + C_o(W) × P(W) – C(I) × P(I/W)P(W) × PC where C(I) is the cost of an injured patient, P(I/W) is the probability of injury given a warning, and PC is the probability that a true warning can be acted on to prevent injury. He concluded that if the resulting number is positive, monitoring is not cost effective. If the resulting number is negative, then monitoring is cost effective. Hence, injuries to the spinal cord resulting in permanent disability that translate into millions of dollars in medical costs make the use of IOM cost effective. For surgical procedures in which the cost of an injury is less, monitoring becomes cost effective by this calculation only when the risk of injury is high.

In our series, 5.13%-5.48% of patients had medialized screws that needed repositioning. Our incidence is comparable to the published incidence of pedicle screw malposition of 5%-10%, and only 0.5%-2% of these patients have significant nerve root irritation injuries.^16,17 Therefore, the risk of permanent nerve injury from pedicle screw malposition is not high, making the cost effectiveness of using IOM during lumbar spine surgery further questionable. We definitely did not see any benefit of the monitoring in our series. Further analysis of cost-effectiveness is required to make conclusive remarks.

The limitations of this study are as expected for a retrospective study. In addition, we did not provide any long-term data on the sequelae of pedicle screw malpositions to the patients. Our results cannot be generalized to more complex spine cases for which IOM is almost mandatory, such as microsurgical resection of intramedullary tumor or complex deformity surgeries. We plan to perform further analyses of our data in terms of costs and long-term functional outcomes.