To: SRS Colleagues
The attached information statement on Intraoperative Neurophysiological Monitoring of Spinal Cord Function During Spinal Deformity Surgery was developed to provide updated information to the membership. The information expresses the opinion that intraoperative neurophysiological spinal cord monitoring is not investigational and is a widely used adjunct as an integral part of surgical deformity correction procedures.
The advent of rigid spinal surgical implants has allowed surgeons to correct increasingly complex spinal deformities. However, strong corrective forces applied to spinal deformities has significant risk for neurologic deficit including loss of motor function in the lower extremities. As a consequence there exists a need for a method to warn the spine surgeon of impending neurological deficits during surgery. With reliable warning changes in the surgical technique may be implemented in an attempt to restore the normal neurophysiology of the spinal cord. Schmitt's (1981) report from the Scoliosis Research Society Morbidity and Mortality Committee cited an incidence of 0.5% spinal cord injuries during the period 1971 to 1979. MacEwen et al. (1975) survey showed an incidence of 0.72%. Wilber et al. (1986) reported a 17% incidence of neurologic complications including 13% transient sensory changes and 4% major spinal cord injuries. More recently, Diab, et.al. reviewed 1301 consecutive surgical cases for adolescent idiopathic scoliosis and reported a neural complication rate of 0.69% (Diab, et.al. 2007). The risk of complications is greatest when patients have kyphosis, congenital scoliosis, pre-existing neurological impairment, or have been in traction preoperatively. The impairment may be from direct stretching of the spinal cord, compression of the cord, trauma during fitting of the orthopedic instrumentation, or interference with blood flow (Nuwer, 1988, Drummond, et al., 2003) Any procedure that can be used to reduce the rate of post-operative complications following surgery to the spine has tremendous attraction for patients and surgeons alike.
Until the entry of Intraoperative evoked potential monitoring, the only other available method of observing spinal cord function is through the Stagnara Wake-up test (Vauzelle et al., 1973). This test has been available since the mid-1970’s, but has many limitations (Stephen, et.al., 1996; Schwartz, et al., 1997, Padberg, et.al., 1999). It is usually performed after the desired correction has been achieved; therefore, the exact moment of neurological dysfunction remains obscure. Occasionally it may be dangerous to intraoperatively wake up patients with certain primary diseases (Mostegl, et.al., 1988). Some patients may not be able to cooperate with the wake-up test because of age or mental status. In situations where the wake-up test can be performed it is an accepted method of assessing spinal cord function.
A number of neurophysiologic spinal cord monitoring techniques have been proposed (Tamaki and Kubota, 2007). Somatosensory cortical evoked potentials (SSEP) monitoring serves as an early warning system to detect deficit intraoperatively (Schwartz et al, 1997, Devlin and Schwartz., 2007) SSEP monitoring is performed to assess the functional status of peripheral nerves and sensory tracts in the spinal cord. SSEP is elicited by stimulating either a sensory or mixed peripheral nerve. The neuronal response to the electrical stimulation can be recorded from various locations along the neural pathway. SSEPs can be significantly affected by the halogenated anesthetics (Schwartz, et al, 1997; DiCindio and Schwartz, 2005). Intraoperative monitoring equipment consists of three systems: stimulus, recording, and data storage. Since the report by Nash et al.(1977) on routine somatosensory cortical evoked potential (SSEP) monitoring of scoliosis surgery, hundreds of articles on the topic of evoked potential monitoring of spinal cord function during operations of the spine have been reported in the world literature.
Results of research demonstrating the value of somatosensory evoked potential monitoring of spinal surgery has been published by several authors. A review of these procedures was provided by Nuwer (1988). For example, Dinner et al. (1986) monitored somatosensory evoked potentials and post-operative deficit in 220 patients. They reported that marked changes in SSEP responses indicated a high chance of developing a neurological deficit, and if there was no change the chance of any neurological postoperative deficit was extremely low. Bieber et al. (1988) reported data on 275 consecutive patients who were treated by posterior spinal instrumentation and fusion with intraoperative monitoring using SSEP. Intraoperatively, six patients demonstrated significant changes in evoked potentials during instrumentation of the spine. With immediate removal of the instrumentation evoked potentials returned to baseline. All patients were neurologically normal postoperatively. Brown et al. (1984) reported SSEP results in 300 patients. Three neurologic deficits were documented intraoperatively and confirmed postoperatively. There were four cases in which changes in evoked potentials led to change in the operative procedure, with no subsequent neurologic deficit. Jones et al. (1983) report a series of 138 patients in which three patients were noted to have a reduction of potentials during distraction with improvement following revision of the surgical procedure. These authors concluded that changes in cord function can be reversed when the cause is quickly remedied.
While SSEP monitoring reduces the risk of permanent neurological deficit, there are reports of false negative cases where the patient awoke with a neural deficit that was not detected by SSEP monitoring (Ginsberg, et.al., 1985; Schwartz, et.al. 2007) . The direct stimulation and recording from the spinal cord evoked potentials (SpEP) has been described as an alternative to monitoring sensory pathways within the spinal cord.
Direct spinal cord stimulation has also been described as an alternative to motor cortex stimulation (Owen, et.al., 1988; Lashinger, et.al., 1988). This method, known as neurogenic motor evoked potentials (NMEP) is now considered as “backfiring” or antidromic conduction of the large sensory afferent fibers within the posterior columns of the spinal cord based on animal and human studies (Toleikis, et.al., 2000; Su and Haghighi, 1992), but remains an adjunct to somatosensory evoked potential monitoring.
The advent of transcranial motor evoked potentials (tcMEPs) over the past decade has facilitated direct intraoperative assessment of the spinal cord motor tracts for the identification of emerging motor deficits. There is now substantial evidence demonstrating the safety and efficacy of tcMEPs monitoring during spine surgery (DiCindio, et al, 2003, Hilibrand, et al, 2006, Schwartz, et al. 2007, Langaloo, et al. MacDonald, 2003; Burke et.al., 1998; Calancie, et.al., 1998). Typically, 150 – 400 mV of electrical stimulation is required for adequate motor cortex activation. The resultant response can be recorded from spinal cord or target muscles. The tcMEPs are extremely sensitive to inhalational anesthetics. Intravenous anesthesia can provide favorable anesthesia to perform tcMEPs. Schwartz et al (2007) recently reported on the safety of transcranial stimulation even in the presence of cardiac disease, pace-makers and history of epilepsy in a large patient series. Their data showed that the single biggest complication was the potential for tongue-bite since cortical stimulation in patients without neuromuscular blockage can cause severe jaw musculature contraction.
The combined monitoring of sensory evoked potentials and motor evoked potentials during spine surgery decreases the false-negative rates of reporting.(Iwasaki, et.al. 2003, Leppanen, et.al. 2005; Hilibrand, et al. 2006, Schwartz, et.al. 2007). It has been conclusively demonstrated that intraoperative spinal cord monitoring facilitates detection of impending spinal cord deficit and facilitates early responses that are likely to preserve spinal cord function (Lyon, et.al. 2004; Schwartz, et.al., 2007).
Pedicle screw instrumentation systems for spinal arthrodesis are in widespread use. Malpositioned screws can induce loss of fixation, neuronal injury, and pain syndromes. Intraoperative evoked EMG monitoring of pedicle screws has proven to be a simple, safe, and efficacious technique in accurate placement of pedicle screws. A positive EMG response at or below a constant-current of <6-10 mA may be an indication for inspection, redirection, or removal of the instrument or implant (Glassman, et al., 1995). However, the stimulation of thoracic pedicle screws is more challenging than for lumbar screws, and from T1 – T6 the monitoring results may not be reliable. Normal free-run EMG response is predictive of the lack of nerve root injury or irritation. An abnormal EMG response during a spine procedure may or may not be associated with a clinical deficit (Leppanen, RE, 2005), while on the contrary, normal EMG responses do not insure against lateral breeches.
Monitoring services during surgery may be provided by a variety of personnel. Technologists acquire the intraoperative data and relay that information to the surgeon, anesthesiologist, neurologist, Ph.D. neurophysiologist, or other professional for interpretation. Interpretation may be made in the Operating Room or by remote consultation on a continuous or intermittent basis. In some instances the interpretation is completed from collected data following the operative procedure. Currently, there are several pathways for certification of intraoperative monitoring personnel. Technologists who are certified in neurophysiologid intraoperative monitoring (CNIM) have demonstrated successful completion of training, experience, and examination in techniques for acquiring intraoperative neurological monitoring data. Professionals other than physicians who interpret the data also have several avenues for certification. Diplomates of the American Board of Neurophysiological Monitoring (D-ABNM) have met specific educational requirements. Also, the American Academy of Audiology and the American Speech, Language and Hearing Association have scope of practice statements indicating that licensed audiologists with specialty training in intraoperative neurophysiology are among those professionals capable of rendering an interpretation of the intraoperative monitoring data. Surgeons should understand the qualifications and roles of the personnel responsible for data acquisition and interpretation of intraoperative neurophysiological monitoring data (Devlin and Schwartz, 2007).
Finally, the Scoliosis Research Society and the European Spinal Deformity Society surveyed their membership regarding the use of intraoperative monitoring of somatosensory evoked potentials in spinal surgery (Dawson et al. 1991). A retrospective analysis of 60,366 heterogeneous surgical cases from the respondent surgeon's memory found 364 cases of postoperative neurologic deficit, 263 of which were identified with SSEP in place (i.e. true-positive) and 101 that were not detected with SSEP (false-negative cases). In a second part of the survey, analysis of data obtained on 33,000 heterogeneous spine procedures found 248 false-positive, 161 true-positive, and 25 false-negative cases. The authors of the survey concluded that SSEP is a "useful adjunct to the spinal surgeons' armamentarium" and that "the wake-up test should also be considered for cases with increased risk of postoperative neurological deficits".
In conclusion, a substantial body of research has demonstrated that neurophysiologic monitoring can assist in the early detection of complications and possibly prevent post-operative morbidity in patients undergoing operations on the spine. In view of the accumulated research and clinical experience demonstrating the effectiveness of neurophysiologic monitoring, the Scoliosis Research Society concludes that the use of intraoperative spinal cord neurophysiological monitoring during operative procedures including instrumentation is not investigational. The majority of SRS members utilize neurophysiologic monitoring routinely in the correction of spinal deformity. The Scoliosis Research Society considers neurophysiological monitoring the preferred method for the early detection of an evolving or impending spinal cord deficit during surgical manipulation of the spine. The wake up test is a useful adjunct to neurophysiologic monitoring for the detection of neurologic spinal cord deficits.
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