Sensitivity and specificity of somatosensory and neurogenic-motor evoked potentials in animals and humans

Electrodiagnostic testing in dogs with disorders of the spinal cord or cauda equina

Electrodiagnostic (EDX) testing is uncommonly utilized in dogs other than for investigation of disorders of the neuromuscular system. In dogs with diseases affecting the spinal cord or cauda equina, EDX testing can provide functional data complementary to imaging information that together can guide therapeutic and management approaches. Additionally, in some clinical scenarios, EDX testing prior to advanced imaging is integral to identifying if there is spinal cord or cauda equina involvement and can aid in determining the appropriate diagnostic path. This review will outline EDX testing methods that have been reported in dogs relating to the diagnosis, monitoring or prognosis of various conditions affecting the spinal cord and cauda equina. The various tests will be briefly outlined regarding how they are performed and what information is provided. The main focus will be on clinical applications including highlighting situations where EDX testing is useful for differentiating between neurologic and non-neurologic presentations. Additional ways these EDX techniques could be incorporated in the management of diseases of the spinal cord and cauda equina in dogs will be presented.

Descending Neurogenic Evoked Potentials Monitoring Is an Effective Alternative in Spinal Deformity Surgery Under Inhaled Anesthesia

BACKGROUND

To evaluate the reliability of descending neurogenic evoked potentials (DNEP) monitoring in spinal deformity surgery under inhaled anesthesia.

METHODS

A total of 180 consecutive patients who underwent spinal deformity surgery in our scoliosis center from July 2014 to August 2016 were reviewed. Intraoperative monitoring including somatosensory evoked potentials (SEP), motor evoked potentials (MEP), and DNEP was conducted routinely throughout operation. Patients were divided into 2 groups according to anesthesia methods: group A (n = 72, inhaled anesthesia, SEP/DNEP) and group B (n = 108, total intravenous anesthesia, SEP/MEP/DNEP). Intraoperative monitoring data were collected and analyzed.

RESULTS

Positive alerts were observed in 26 patients (14.5%), of whom 18 (10%) were confirmed as true-positive events in the study population. No false-negative events were recorded. In group A, the sensitivity and specificity of SEP and DNEP were 100% and 93.8% and 100% and 98.5%, respectively. For group B, the sensitivity and specificity of SEP/MEP and DNEP were 100% and 95.9% and 100% and 98%, respectively.

CONCLUSIONS

DNEP monitoring seemed to be effective for the detection and prevention of iatrogenic neurologic deficits during spinal deformity surgery. This study indicates that DNEP was an effective alternative in spinal deformity surgery under inhaled anesthesia.

A Retrospective Study of Surgical Correction for Spinal Deformity with and without Osteotomy to Compare Outcome Using Intraoperative Neurophysiological Monitoring with Evoked Potentials

BACKGROUND This study aimed to evaluate the effects of different combined evoked potentials monitoring modes for non-osteotomy and osteotomy surgery of spinal deformity, and to select individualized modes for various surgeries. MATERIAL AND METHODS We retrospectively reviewed a total of 188 consecutive cases undergoing spinal deformity correction. All patients were classified into 2 cohorts: non-osteotomy (Group A) and osteotomy (Group B). According to intraoperative evoked potential monitoring mode, Group A was divided into 2 sub-groups: A1 [spinal somatosensory evoked potential (SSEP)/motor evoked potential (MEP), n=67)] and A2 [SSEP/MEP/descending neurogenic evoked potential (DNEP), n=52]. Group B was classified as B1 (SSEP/MEP, n=27) and B2 (SSEP/MEP/DNEP, n=42). The demographics, surgical parameters, and evoked potential events of different combined monitoring modes were analyzed within each group. RESULTS The baselines of SSEP/MEP/DNEP in all cases were elicited successfully. Three cases with evoked potential (EP) events (2 with MEP changes and 1 with SSEP/MEP change) were noted in Group A1 and 1 with SSEP change in Group A2, with no neurological complications. Thirteen cases in Group B1 were positive for MEP intraoperatively, including 16 EP events (13 with MEP change and 3 with both SSEP+MEP changes), with no neural complications. In Group B2, 15 cases had 21 EP events, including 12 with MEP change and 2 with SSEP+MEP changes, with no complications. Postoperative neurological complications were observed in 5 of the 7 cases with SS4EP/DNEP changes. CONCLUSIONS Intraoperative simultaneous SSEP/MEP can effectively reflect neurological function in non-osteotomy spinal surgery patients. Simultaneous SSEP/MEP/DNEP can effectively avoid the unnecessary interference by false-positive results of MEP during osteotomy.

Bilateral erector spinae plane block for scoliosis surgery: Case series

Spinal correction surgery is a very invasive surgical procedure and results in severe postoperative pain. We report six cases in which Bilateral Erector Spinae Plane Block was performed for scoliosis surgery. Our aim was to provide an effective perioperative pain management and to achieve intraoperative hemodynamic stability with no interference on neuromonitoring. The technical challenges are also highlighted. An ultrasound guided scout scan is necessarry to identify the bony prominences and determine the possible multiple injection points. Erector Spinae Plane Block in scoliosis surgery is an easier and safer technique compared to epidural anesthesia and can use instrumented complex spinal surgery. This block seems to have a role in perioperative pain pathway complementing the multimodal analgesic regimen and not have interference with evocated potentials in adults. However the diffusion mechanism of the this block is not well known hence it should be awake regarding local anesthetic toxicity.

Engineering approaches to understanding mechanisms of spinal column injury leading to spinal cord injury

BACKGROUND

The mechanical interactions occurring between the spinal column and spinal cord during an injury event are complex and variable, and likely have implications for the clinical presentation and prognosis of the individual.

METHODS

The engineering approaches that have been developed to better understand spinal column and cord interactions during an injury event are discussed. These include injury models utilising human and animal cadaveric specimens, in vivo anaesthetised animals, finite element models, inanimate physical systems and combinations thereof.

FINDINGS

The paper describes the development of these modelling approaches, discusses the advantages and disadvantages of the various models, and the major outcomes that have had implications for spinal cord injury research and clinical practice.

INTERPRETATION

The contribution of these four engineering approaches to understanding the interaction between the biomechanics and biology of spinal cord injury is substantial; they have improved our understanding of the factors contributing to the spinal column disruption, the degree of spinal cord deformation or motion, and the resultant neurological deficit and imaging features. Models of the injury event are challenging to produce, but technological advances are likely to improve these models and, consequently, our understanding of the mechanical context in which the biological injury occurs.

Somatosensory and transcranial motor evoked potential monitoring in a porcine model for experimental procedures

Evoked potential monitoring has evolved as an essential tool not only for elaborate neurological diagnostics, but also for general clinical practice. Moreover, it is increasingly used to guide surgical procedures and prognosticate neurological outcome in the critical care unit, e.g. after cardiac arrest. Experimental animal models aim to simulate a human-like scenario to deduct relevant clinical information for patient treatment and to test novel therapeutic opportunities. Porcine models are particularly ideal due to a comparable cardiovascular system and size. However, certain anatomic disparities have to be taken into consideration when evoked potential monitoring is used in animal models. We describe a non-invasive and reproducible set-up useful for different modalities in porcine models. We further illustrate hints to overcome multi-faceted problems commonly occurring while using this sophisticated technique. Our descriptions can be used to answer a plethora of experimental questions, and help to further facilitate experimental therapeutic innovation.

Biomechanical Simulation of Stresses and Strains Exerted on the Spinal Cord and Nerves During Scoliosis Correction Maneuvers

STUDY DESIGN

Biomechanical analysis of the spinal cord and nerves during scoliosis correction maneuvers through numerical simulations.

OBJECTIVE

To assess the biomechanical effects of scoliosis correction maneuvers and stresses generated on the spinal nervous structures.

BACKGROUND DATA

Important forces are applied during scoliosis correction surgery, which could potentially lead to neurologic complications due to stresses exerted on the nervous structures. The biomechanical impact of the different types of stresses applied on the nervous structures during correction maneuvers is not well understood.

METHODS

Three correction techniques were simulated using a hybrid computer modeling approach, personalized to a right thoracic adolescent idiopathic scoliotic case (Cobb angle: 63°): (1) Harrington-type distraction; (2) segmental translation technique; and a (3) segmental rotation-based procedure. A multibody model was used to simulate the kinematics of the instrumentation maneuvers; a second comprehensive finite element model was used to analyze the local stresses and strains on the spinal cord and nerves. Average values of the internal medullar pressure (IMP), shear stresses, nerve compression, and strain were computed over three regions and compared between techniques.

RESULTS

Harrington distraction maneuver generated high stresses and strains over the thoracolumbar region. In the main thoracic region, the segmental translation maneuver technique induced 15% more shear stress, 25% more strain, and 62% lower nerve compression than Harrington distraction maneuver. The segmental rotation-based procedure induced 25% lower shear stresses and 18% more strain, respectively, at the apical level, as well as 72%, 57%, and 7% lower IMP, nerve compression, and strain in the upper thoracic region, compared with Harrington distraction maneuver.

CONCLUSION

This study quantified the relative stress induced on the spinal cord and spinal nerves for different correction maneuvers using a novel hybrid patient-specific model. Of the three maneuvers studied, the Harrington distraction maneuver induced the most important stresses over the thoracolumbar region.

Efficacy of Intraoperative Neurophysiologic Monitoring for Pediatric Cervical Spine Surgery

STUDY DESIGN

Clinical case series.

OBJECTIVE

To investigate the efficacy of intraoperative neuromonitoring in pediatric cervical spine surgery.

SUMMARY OF BACKGROUND DATA

Intraoperative neuromonitoring (IONM) consisting of somatosensory-evoked potentials (SSEP) and transcranial motor-evoked potentials (tcMEP) has been shown to effectively prevent permaneny neurologic injury in deformity surgery. The role of IONM during pediatric cervical spine surgery is not well documented. Advances in cervical spine instrumentation have expanded the surgical options in pediatric populations. The goal of this study is to report the ability of IONM to detect neurologic injury during pediatric cervical spine instrumentation.

METHODS

A single institution database was queried for pediatric-aged patients who underwent cervical spine instrumentation and fusion between 2011 and 2014. Age, diagnosis, surgical indication, number of instrumented levels, and a complete IONM were extracted. Sensitivity and specificity for the detection of neurologic deficits were calculated with exact 95% confidence intervals. Positive and negative predictive values were calculated with estimated 95% confidence intervals.

RESULTS

Sixty-seven patients who underwent cervical spine instrumentation were identified with a mean age of 11.6 years (range 1-18). Diagnoses included instability (27), congenital (11), kyphosis (8), fracture (7), tumor (7), arthritis (4), and basilar invagination (3). Mean number of vertebral levels fused was 4 (range 2-7). All patients underwent cervical instrumentation with SSEP and tcMEP monitoring. A significant change in tcMEP monitoring was observed in 7 subjects (10%). There were no corresponding SSEP changes in these patients. The sensitivity of combined IONM was 75% [95% CI = 24.9, 98.7] and the specificity was 98.5% [92.7, 99.9].

CONCLUSION

tcMEP is a more sensitive indicator to spinal cord injury than SSEP, which is consistent with previous studies. IONM changes in 10% of a patient population are significant enough to warrant intraoperative determination if true SCI has occurred or is underway and intervene accordingly.

LEVEL OF EVIDENCE

4.

Neurophysiological assessment of spinal cord injuries in dogs using somatosensory and motor evoked potentials

Somatosensory evoked potentials (SSEPs) and motor evoked potentials (MEPs) are non-invasive neurophysiological tests that reflect the functional integrity of sensory and motor pathways. Despite their extensive use and description in human medicine, reports in veterinary medicine are scarce. SSEPs are obtained via peripheral stimulation of sensory or mixed nerves; stimulation induces spinal and cortical responses, which are recorded when sensory pathways integrity is preserved. MEPs can be obtained via transcranial electrical or magnetic stimulation; in this case, thoracic and pelvic limb muscle responses are captured if motor pathways are preserved. This review describes principles, methodology and clinical applicability of SSEPs and MEPs in companion animal medicine. Potential interferences of anesthesia with SSEP and MEP recording are also discussed.

ELECTROMYOGRAPHY AND INSTRUMENTATION IN PATIENTS WITH IDIOPATHIC SCOLIOSIS

ABSTRACT Objective: The objective of this study is to relate the use of intraoperative electromyography with surgical time, proper placement of screws, type of curve and time spent per screw in idiopathic scoliosis correction surgery in a group of surgeons from Belo Horizonte. This study used the database of protocol evaluation of patients operated in the service, and separately analyzed the results of motor and somatosensory potentials. Methods: Retrospective study of 80 patients undergoing surgery for correction of idiopathic scoliosis between December 2008 and January 2015. A single group of Belo Horizonte spine surgeons performed the intraoperative electromyographic (EMG) monitoring. EMG was performed with stimulation of pedicle screws in patients undergoing instrumentation with pedicle screws as fixation elements. Results: The sample consisted of 85% females (mean age 17 years) and 37.5% of cases had classification type 1AN of Lenke. Of the total surgical cases, 60% had EMG changes. Of the total cases analyzed, 66.3% were true positives for the result. Conclusion: Intraoperative monitoring with EMG is a very important tool for the surgical treatment of patients with scoliosis undergoing instrumentation with pedicle screws. It enables to check if the screw is located on the correct path, helping to decrease the error rate and providing corrections to the surgical approach through a change of strategies. Moreover, it contributes to decrease the time to screw positioning and the total surgical time.

Validity and utility of monopolar spinal cord stimulation in pediatric scoliosis surgery

PURPOSE

To assess the validity and utility of monopolar stimulation (between a peridural needle and a large adhesive anode placed in the sternal area) for intraoperative monitoring in scoliosis surgery.

METHODS

This procedure was assessed during 41 operations involving either arthrodesis with posterior instrumentation or a Vertical Expandable Prosthetic Titanium Rib (VEPTR). Responses evoked by monopolar stimulation were compared with those evoked by bipolar stimulation between two epidural needle electrodes. Potentials evoked by monopolar stimulation in the upper limbs were compared with those evoked in the lower limbs during the same stimulation procedure.

RESULTS

Monopolar stimulation yielded equivalent and, if anything, more stable responses in the lower limbs. Recording in the upper limbs was satisfactory and allowed a decrease in responses to be detected in two patients. Acceptable thresholds for changes in amplitude relative to baseline were 40 % for upper limbs and 30 % for lower limbs.

CONCLUSIONS

Monopolar stimulation can be used to monitor the spinal cord during surgery for scoliosis correction. This procedure is more convenient for the surgeon and allows for the combined recording of responses in all four limbs, which can be useful in the case of surgical techniques such as those involving a VEPTR.

Monopolar-probe monitoring during spinal surgery with expandable prosthetic ribs

BACKGROUND

Intraoperative monitoring (IOM) has been proven to decrease the risk of neurological injury during scoliosis surgery. The vertical expandable prosthetic titanium rib (VEPTR) is a device that allows spinal growth. However, injuries to the spinal cord and brachial plexus have been reported after VEPTR implantation in 2 and 5% of patients, respectively. Simultaneous monitoring of these two structures requires the use of multiple time-consuming and complex methods that are ill-suited to the requirements of paediatric surgery, particularly when repeated VEPTR lengthening procedures are needed. We developed a monopolar stimulation method derived from Owen's monitoring technique. This method is easy to implement, requires only widely available equipment, and allows concomitant monitoring of the spinal cord and brachial plexus. The primary objective of this study was to assess the reliability of our technique for brachial plexus monitoring by comparing the stability of neurogenic mixed evoked potentials (NMEPs) at the upper and lower limbs.

HYPOTHESIS

We hypothesised that the coefficients of variation (CVs) of NMEPs were the same at the upper and lower limbs.

MATERIAL AND METHODS

Twelve VEPTR procedures performed in 6 patients between 1st January 2012 and 1st September 2014 were monitored using a monopolar stimulating probe. NMEPs were recorded simultaneously at the upper and lower limbs, at intervals of 150 s. The recording sites were the elbow over the ulnar nerve and the popliteal fossa near the sciatic nerve. Wilcoxon's test for paired data was used to compare CVs of the upper and lower limb NMEPs on the same side.

RESULTS

Mean CV of NMEP amplitude at the lower limbs was 16.34% on the right and 16.67% on the left; corresponding values for the upper limbs were 18.30 and 19.75%, respectively. Mean CVs of NMEP latencies at the lower limbs were 1.31% on the right and 1.19% on the left; corresponding values for the upper limbs were 1.96 and 1.73%. The risk of type I error for a significant difference between the upper and lower limbs was 0.5843 on the right and 0.7312 on the left for NMEP amplitudes and 0.7618 on the right and 0.4987 on the left for NMEP latencies.

CONCLUSION

Using an epidural active electrode and a sternal return electrode allows simultaneous stimulation of the cervical spinal cord and brachial plexus roots. The NMEPs thus obtained are as stable (reliable) at the upper limbs as at the lower limbs. This easy-to-implement method allows simultaneous monitoring of the upper and lower limbs. It seems well suited to VEPTR procedures.

LEVEL OF EVIDENCE

IV, retrospective single-centre non-randomised study.

Rare true-positive isolated SSEP loss with preservation of MEPs response during scoliosis correction

STUDY DESIGN

Case report.

OBJECTIVE

To report a case of a true-positive isolated somatosensory evoked potential (SSEP) loss with preservation of motor evoked potential (MEP) response during scoliosis correction.

SUMMARY OF BACKGROUND DATA

Combined intraoperative monitoring uses SSEPs and MEPs to decrease the probability of observing false-negative events. In combination, SSEPs and MEPs have become a standard of care for spinal deformity surgery. However, literature review reveals several cases of false-negative response with combined SSEPs and MEPs, raising the contention that intraoperative monitoring does not reliably identify all isolated selective spinal cord dysfunction.

METHODS

A 15-year-old female patient with a 65° right thoracic adolescent idiopathic scoliosis underwent correction and posterior spinal fusion with segmental pedicle screw instrumentation. After capture and derotation of the left concave rod, left-sided irreversible SSEP loss occurred whereas MEPs remained unchanged. After excluding systemic factors, anesthetic causes, or technical fault, deformity correction was released and instrumentation removed. No cortical breach was reported during pedicle screw removal.

RESULTS

Postoperatively, no clinical sensory or motor deficit was present; computed tomography demonstrated a burst left pedicle at T10 with the medial pedicle wall fragment in direct contact with the dorsal spinal cord. Magnetic resonance imaging excluded cord edema or other evidence of injury. Three days after surgery, intraoperative monitoring showed delayed latencies and amplitudes of the left SSEP. An uneventful reinsertion of instrumentation and correction excluding the left T10 pedicle screw was performed 7 days after the initial surgery.

CONCLUSION

This case report provides evidence of selective posterior spinal cord dysfunction with sparing of the anterior columns immediately after a correction maneuver and emphasizes the importance of simultaneous SSEP and MEP monitoring. To the authors' knowledge, there is no previous report of a true-positive isolated SSEP loss with preservation of MEP response during scoliosis correction.

LEVEL OF EVIDENCE

N/A.

A realidade do uso da monitorização neurofisiológica intraoperatória entre os cirurgiões de coluna brasileiros

OBJETIVO: Avaliar o uso da monitorização neurofisiológica intraoperatória (MNIO) por cirurgiões de coluna brasileiros. MÉTODO: A coleta de dados foi realizada através de um questionário aplicado em 307 cirurgiões de coluna brasileiros, durante o 11º Congresso de Cirurgia Espinhal e XIII Congresso da Sociedade Brasileira de Coluna. RESULTADOS: Dos cirurgiões entrevistados, 42% são neurocirurgiões e 58% ortopedistas. A maioria (72,3%) relatou que já fez uso do MNIO, entretanto apenas 29,6% utilizam este procedimento rotineiramente. Destes 39% são ortopedistas. Entre os neurocirurgiões, a maior parte (84%) relatou não utilizar MNIO como rotina. Nos casos de deformidade, 85,7% dos profissionais disseram usar rotineiramente a MNIO. Do total, 68,1% responderam que não tinham fácil acesso à MNIO, sendo que 10% deles atuam na região Centro-oeste do país e 11%, na região Nordeste. Dos que relataram facilidade de acesso ao procedimento, 77% atuam na região Sudeste. A média de idade dos participantes foi 41,9 anos, com mediana de 39,0, desvio padrão de 11,3 e intervalo de confiança de 1,3. Para o tempo de formação médica, a média foi 17,8 anos, com mediana de 14,0 e intervalo de confiança 1,2. Em relação ao tempo de prática em cirurgia de coluna encontrou-se que 56,3% têm até 10 anos de prática. CONCLUSÃO: A maioria dos cirurgiões de coluna já fez uso da MNIO, contudo poucos utilizam-na como rotina. A região Sudeste é onde se encontra maior facilidade de acesso à MNIO, ao contrário das regiões Centro-oeste e Nordeste.

Intraoperative monitoring in pediatric orthopedic spinal surgery: three hundred consecutive monitoring cases of which 10% of patients were younger than 4 years of age

STUDY DESIGN

Analysis of a prospective series of 300 consecutive cases undergoing intraoperative monitoring in pediatric orthopedic spinal surgery, of which 10% were children younger than 4 years.

OBJECTIVE

Determine feasibility and performance of intraoperative monitoring in children younger than 4 years. Analyze distinct physiopathologic mechanisms of relevant alerts.

SUMMARY OF BACKGROUND DATA

There are few studies in the literature concerning the intraoperative monitoring of children younger than 4 years. During childhood, the development of sensori-motor pathways is dominated by two coexisting phenomena, which have opposite effects: maturation decreasing latencies and height increasing them.

METHODS

We used intraoperative somatosensory-evoked potentials and neurogenic mixed evoked potentials with a flexible bipolar epidural electrode. Uniform total intravenous anesthesia was used.

RESULTS

Values of sensitivity and specificity of the monitoring showed slight differences between patients younger than 4 years versus older patients. There was no false-negative outcome. Various tendencies were highlighted. There were more true positive alerts for secondary etiologies than for idiopathic ones, for revision spinal surgeries than for index ones, and for boys than for girls. There were no more true positive alerts for children younger than 4 years than for older patients whereas the proportion of hemivertebrae was obviously greater for the younger group. Relevant monitoring alerts were more frequent in case of kyphoscoliosis. This is highlighted in case reports.

CONCLUSION

In some cases of kyphoscoliosis, during a posterior-based vertebral column resection, monitoring changes were corrected by positioning a rod that allowed correction of the position of the spine in the sagittal plane. Intraoperative spinal cord monitoring can be performed in children younger than 4 years and allows real-time assessment of spinal functional integrity.

Intraoperative electrophysiological monitoring in spine surgery

STUDY DESIGN

Review of the literature with analysis of pooled data.

OBJECTIVE

To assess common intraoperative neuromonitoring (IOM) changes that occur during the course of spinal surgery, potential causes of change, and determine appropriate responses. Further, there will be discussion of appropriate application of IOM, and medical legal aspects. The structured literature review will answer the following questions: What are the various IOM methods currently available for spinal surgery? What are the sensitivities and specificities of each modality for neural element injury? How are the changes in each modality best interpreted? What is the appropriate response to indicated changes? Recommendations will be made as to the interpretation and appropriate response to IOM changes.

SUMMARY OF BACKGROUND DATA

Total number of abstracts identified and reviewed was 187. Full review was performed on 18 articles.

METHODS

The MEDLINE database was queried using the search terms IOM, spinal surgery, SSEP, wake-up test, MEP, spontaneous and triggered electromyography alone and in various combinations. Abstracts were identified and reviewed. Individual case reports were excluded. Detailed information and data from appropriate articles were assessed and compiled.

RESULTS

Ability to achieve IOM baseline data varied from 70% to 98% for somatosensory-evoked potentials (SSEP) and 66% to 100% for motor-evoked potentials (MEP) in absence of neural axis abnormality. Multimodality intraoperative neuromonitoring (MIOM) provided false negatives in 0% to 0.79% of cases, whereas isolated SSEP monitoring alone provided false negative in 0.063% to 2.7% of cases. MIOM provided false positive warning in 0.6% to 1.38% of cases.

CONCLUSION

As spine surgery, and patient comorbidity, becomes increasingly complex, IOM permits more aggressive deformity correction and tumor resection. Combination of SSEP and MEP monitoring provides assessment of entire spinal cord functionality in real time. Spontaneous and triggered electromyography add assessment of nerve roots. The wake-up test can continue to serve as a supplement when needed. MIOM may prove useful in preservation of neurologic function where an alteration of approach is possible. IOM is a valuable tool for optimization of outcome in complex spinal surgery.

Validity and reliability of intraoperative monitoring in pediatric spinal deformity surgery: a 23-year experience of 3436 surgical cases

STUDY DESIGN

This was a 23-year retrospective study of 3436 consecutive pediatric orthopedic spinal surgery patients between 1995 and 2008.

OBJECTIVE

To demonstrate the effectiveness of multimodality electrophysiologic monitoring in reducing the incidence of iatrogenic neurologic deficit in a pediatric spinal surgery population.

SUMMARY OF BACKGROUND DATA

The elective nature of many pediatric spinal surgery procedures continues to drive the need for minimizing risk to each individual patient. Electrophysiologic monitoring has been proposed as an effective means of decreasing permanent neurologic injury in this population.

METHODS

A total of 3436 consecutive monitored pediatric spinal procedures at a single institution between January 1985 and September 2008 were reviewed. Monitoring included somatosensory-evoked potentials, descending neurogenic-evoked potentials, transcranial electric motor-evoked potentials, and various nerve root monitoring techniques. Patients were divided into 10 diagnostic categories. True-positive and false-negative monitoring outcomes were analyzed for each category. Neurologic deficits were classified as transient or permanent.

RESULTS

Seven of 10 diagnostic groups demonstrated true positive findings resulting in surgical intervention. Seventy-four (2.2%) potential neurologic deficits were identified in 3436 pediatric surgical cases. Seven patients (0.2%) had false-negative monitoring outcomes. These patients awoke with neurologic deficits undetected by neuromonitoring. Intervention reduced permanent neurologic deficits to 6 (0.17%) patients. Monitoring data were able to detect permanent neurologic status in 99.6% of this population. The ratio of intraoperative events to total monitored cases was 1 event every 42 surgical cases and 1 permanent neurologic deficit every 573 cases.

CONCLUSION

The combined use of somatosensory-evoked potentials, transcranial electric motor-evoked potentials, descending neurogenic-evoked potentials, and electromyography monitoring allowed accurate detection of permanent neurologic status in 99.6% of 3436 patients and reduced the total number of permanent neurologic injuries to 6.

Monitoração intraoperatória com teste de estimulação eletromiográfica dos instrumentais de pacientes submetidos à correção cirúrgica de escoliose idiopática

OBJETIVO: apresentar a metodologia empregada na monitoração neurofisiológica de pacientes submetidos a tratamento para correção de escoliose idiopática com instrumentação cirúrgica, utilizando parafusos pediculares torácicos e lombares. MÉTODOS: foram estudados, retrospectivamente, 32 prontuários de pacientes operados no Serviço de Cirurgia da Coluna Vertebral do Instituto da Coluna,Jundiaí, São Paulo, entre os anos de 2004 e 2008. A idade variou de 11 a 18 anos, e a maioria era do sexo feminino (93,75%). RESULTADOS: verificou-se que em seis pacientes houve relação entre a incidência de positividade no teste com estimulação eletromiográfica dos instrumentais e mau posicionamento dos parafusos pediculares, sem evidências de complicações após correção dos mesmos. CONCLUSÃO: os resultados confirmam a eficácia da monitoração intraoperatória.

Pediatric spinal cord injury in infant piglets: description of a new large animal model and review of the literature

OBJECTIVE

To develop a new, clinically relevant large animal model of pediatric spinal cord injury (SCI) and compare the clinical and experimental features of pediatric SCI.

METHODS

Infant piglets (3-5 weeks old) underwent contusive SCI by controlled cortical impactor at T7. Severe complete SCI was induced in 6 piglets, defined as SCI with no spontaneous return of sensorimotor function. Eight piglets received incomplete SCI, which was followed by partial recovery. Somatosensory evoked potentials, magnetic resonance imaging, neurobehavioral function, and histopathology were measured during a 28-day survival period.

RESULTS

Mean SCI volume (defined as volume of necrotic tissue) was larger after complete compared with incomplete SCI (387 +/- 29 vs 77 +/- 38 mm3, respectively, P < 0.001). No functional recovery occurred after complete SCI. After incomplete SCI, piglets initially had an absence of lower extremity sensorimotor function, urinary and stool retention, and little to no rectal tone. Sensory responses recovered first (1-2 days after injury), followed by spontaneous voiding, lower extremity motor responses, regular bowel movements, and repetitive flexion-extension of the lower extremities when crawling. No piglet recovered spontaneous walking, although 4 of 8 animals with incomplete injuries were able to bear weight by 28 days. In vivo magnetic resonance imaging was performed safely, yielded high-resolution images of tissue injury, and correlated closely with injury volume seen on histopathology, which included intramedullary hemorrhage, cellular inflammation, necrosis, and apoptosis.

CONCLUSION

Piglets performed well as a reproducible model of traumatic pediatric SCI in a large animal with chronic survival and utilizing multiple outcome measures, including evoked potentials, magnetic resonance imaging, functional outcome scores, and histopathology.

Multimodal intraoperative neuromonitoring in corrective surgery for adolescent idiopathic scoliosis: Evaluation of 354 consecutive cases

BACKGROUND

Multimodal intraoperative neuromonitoring is recommended during corrective spinal surgery, and has been widely used in surgery for spinal deformity with successful outcomes. Despite successful outcomes of corrective surgery due to increased safety of the patients with the usage of spinal cord monitoring in many large spine centers, this modality has not yet achieved widespread popularity. We report the analysis of prospectively collected intraoperative neurophysiological monitoring data of 354 consecutive patients undergoing corrective surgery for adolescent idiopathic scoliosis (AIS) to establish the efficacy of multimodal neuromonitoring and to evaluate comparative sensitivity and specificity.

MATERIALS AND METHODS

The study group consisted of 354 (female = 309; male = 45) patients undergoing spinal deformity corrective surgery between 2004 and 2008. Patients were monitored using electrophysiological methods including somatosensory-evoked potentials and motor-evoked potentials simultaneously.

RESULTS

Mean age of patients was 13.6 years (+/-2.3 years). The operative procedures involved were instrumented fusion of the thoracic/lumbar/both curves, Baseline somatosensory-evoked potentials (SSEP) and neurogenic motor-evoked potentials (NMEP) were recorded successfully in all cases. Thirteen cases expressed significant alert to prompt reversal of intervention. All these 13 cases with significant alert had detectable NMEP alerts, whereas significant SSEP alert was detected in 8 cases. Two patients awoke with new neurological deficit (0.56%) and had significant intraoperative SSEP + NMEP alerts. There were no false positives with SSEP (high specificity) but 5 patients with false negatives with SSEP (38%) reduced its sensitivity. There was no false negative with NMEP but 2 of 13 cases were false positive with NMEP (15%). The specificity of SSEP (100%) is higher than NMEP (96%); however, the sensitivity of NMEP (100%) is far better than SSEP (51%). Due to these results, the overall sensitivity, specificity and positive predictive value of combined multimodality neuromonitoring in this adult deformity series was 100, 98.5 and 85%, respectively.

CONCLUSION

Neurogenic motor-evoked potential (NMEP) monitoring appears to be superior to conventional SSEP monitoring for identifying evolving spinal cord injury. Used in conjunction, the sensitivity and specificity of combined neuromonitoring may reach up to 100%. Multimodality monitoring with SSEP + NMEP should be the standard of care.

Questionnaire Study of Neuromonitoring Availability and Usage for Spine Surgery

STUDY DESIGN

Questionnaire study presented to practicing spine surgeons.

OBJECTIVE

To evaluate surgeon preference and availability of selected electrophysiologic neuromonitoring for different spine surgeries.

SUMMARY OF BACKGROUND DATA

Maximizing the safety of spinal procedures and limiting potential iatrogenic neurologic injury has made intraoperative neuromonitoring an attractive option.

METHODS

We distributed a questionnaire to 180 orthopedic spine surgeons and neurosurgeons at a clinically oriented spine meeting asking surgeon preference and availability of various types of intraoperative neuromonitoring modalities for different types of surgical procedures. Demographic data were also gathered.

RESULTS

Somatosensory evoked potentials (SSEPs) were the most available neuromonitoring modality, followed by electromyographies and motor-evoked potentials. In both anterior and posterior cervical surgery, SSEPs were the most preferred modality. MEPs were frequently preferred in myelopathic cervical cases. Almost 70% preferred some neuromonitoring for anterior thoracic/thoracolumbar cases and 55% for posterior thoracic/thoracolumbar cases. Surgeon satisfaction was related to the number of available neuromonitoring modalities. No significant differences were found between orthopedist and neurosurgeon preferences. Fellowship-trained surgeons were more likely to use neuromonitoring for specific indications.

CONCLUSIONS

SSEPs remains the most widely available and preferred type of neuromonitoring for spine surgeons. The type of case and neurologic status of patient (eg, presence of myelopathy) affects these choices. Surgeons were more satisfied with greater neuromonitoring availability, and were more likely to use neuromonitoring if they had a fellowship background.

Spinal cord monitoring in scoliosis surgery using an epidural electrode. Results of a prospective, consecutive series of 191 cases

STUDY DESIGN

Retrospective analysis of a prospectively accrued series of 191 consecutive patients who underwent intraoperative neurophysiologic monitoring during scoliosis corrective surgery.

OBJECTIVES

To compare the monitoring outcome of idiopathic and neuromuscular scoliosis. To demonstrate the usefulness of the epidural electrode. To report sensitivity and specificity of the monitoring method employed at a single institution.

SUMMARY OF BACKGROUND DATA

Reports in the literature emphasized the difficulty to obtain data in neuromuscular patients. Multimodality spinal cord monitoring has been recommended. Despite their still debated composition, neurogenic motor-evoked potentials have proven their validity in clinical practice.

METHODS

Somatosensory and neurogenic evoked potentials were attempted in all patients presenting for scoliosis correction between 1999 and 2005. Study patients were divided into 3 groups: group 1, idiopathic; group 2, neuromuscular; and group 3, miscellaneous origins.

RESULTS

The use of the epidural electrode demonstrated significant usefulness in the ability of monitoring otherwise nonmonitored patients, especially in group 2. Inability to obtain any evoked potentials occurred in 4 cases (2.1%). Five cases were found to be true positives. An adapted and rapid intervention permitted to avoid new postoperative deficit in all cases. There was no instance of false-negative data. The overall method sensitivity was 100%, and specificity was 52.69%.

CONCLUSIONS

The use of a single epidural electrode allowing somatosensory evoked potentials recording and spinal cord stimulation alternately is a safe and valid method of intraoperative monitoring.

Intraoperative Motor Evoked Potential Monitoring: Overview and Update

Amidst controversy about methodology and safety, intraoperative neurophysiology has entered a new era of increasingly routine transcranial and direct electrical brain stimulation for motor evoked potential (MEP) monitoring. Based on literature review and illustrative clinical experience, this tutorial aims to present a balanced overview for experienced practitioners, surgeons and anesthesiologists as well as those new to the field. It details the physiologic basis, indications and methodology of current MEP monitoring techniques, evaluates their safety, explores interpretive controversies and outlines some applications and results, including aortic aneurysm, intramedullary spinal cord tumor, spinal deformity, posterior fossa tumor, intracranial aneurysm and peri-rolandic brain surgeries. The many advances in motor system assessment achieved in the last two decades undoubtedly improve monitoring efficacy without unduly compromising safety. Future studies and experience will likely clarify existing controversies and bring further advances.

[Complications of surgical treatment of spinal deformities: a prospective multicentric study of 3311 patients]

PURPOSE OF THE STUDY

The incidence of complications secondary to surgical treatment of spinal deformations remains imprecise. The purpose of this prospective multicentric observational study was to assess the incidence of intra- and postoperative complications secondary to this type of surgery to detail the observed complications and to identify favoring factors.

MATERIAL AND METHODS

For this study, we included 3311 patients who underwent surgery during a 12-month period for spinal deformation, defined as idiopathic or secondary scoliosis or kyphosis, irrespective of the localization, severity, or type of surgery performed. Four main categories of complications were studied: general, infectious, neurological, and mechanical. Pre- and intraoperative variables recorded were: epidemiological and morphological data, history of surgery for the same spinal deformation, comorbid conditions, type of deformation treated (nature, anatomic localization, severity, reducibility), type of surgery performed (approach, duration of the operation), operative blood loss, extent and localization of the fusion, associated neurological release, vertebral osteotomy or not, type of graft used. Two types of analysis were performed. The first was a descriptive analysis to detail the overall incidence of complications and the incidence of each of the four main categories. The second was a multivariate analysis designed to determine factors significantly associated with complication occurrence.

RESULTS

Mean age of the cohort was 27 +/- 18 years; 6.8% of the patients had had a prior operation for the spinal deformation. The deformation was scoliosis in 90% (mean angle 56 +/- 20 degrees) and kyphosis in 10% (mean angle 47 +/- 23 degrees). An isolated posterior approach was used for 72.5% of patients, an isolated anterior approach for 6.4%, and a combined anteroposterior approach for 21.1%. Seven hundred four patients (21.3%) had one or more complications (850 complications) during or shortly after their operation. The incidences of general, infectious, mechanical and neurological complications were: 5.7%, 4.7%, 11.5%, and 1.8% respectively. Globally, considering all types of complications, the following factors were found to be significantly associated with complication occurrence: patient age, ASA score, extent of the fusion, presence of vertebral osteotomy, inclusion of the sacrum in the arthrodesis, and initial angle of the treated deformation. For patients with scoliosis, the following factors were significantly associated with a secondary central neurological disorder: initial angle of the deformation, use of vertebral osteotomy, type of curvature with greater risk for thoracic curvatures and double thoracic and lumbar curvatures.

CONCLUSION

This work enabled us to determine the overall rate of complications after surgical treatment of spinal deformations. Certain risk factors related with complication occurrence were identified, but the heterogeneous nature of the population and the methodology used to identify these factors only allowed detection of trends. A future study by etiological group or focusing on specific complications should allow a more precise analysis of these risk factors. This overall rate of complications should be used to better inform patients and their family about the risks of this type of surgery.

Guidelines for the performance of fusion procedures for degenerative disease of the lumbar spine. Part 15: electrophysiological monitoring and lumbar fusion

Based on the medical evidence provided by the literature reviewed, there does not appear to be support for the hypothesis that any form of intraoperative monitoring improves patient outcomes following lumbar decompression or fusion procedures for degenerative spinal disease. Evidence does indicate that a normal evoked EMG response is predictive for intrapedicular screw placement (high NPV for breakout). The presence of an abnormal EMG response does not, however, exclude intrapedicular screw placement (low PPV). The majority of clinically apparent postoperative nerve injuries are associated with intraoperative changes in SSEP and/or DSEP monitoring. For this reason, changes in DSEP/SSEP monitoring appear to be sensitive to nerve root injury. There is a high-false positive rate, however, and changes in DSEP and SSEP recordings are frequently not related to nerve injury. A normal study has been shown to correlate with the lack of a significant postoperative nerve injury. There is no substantial evidence to indicate that the use of intraoperative monitoring of any kind provides useful information to the surgeon in terms of assessing the adequacy of nerve root decompression at the time of surgery.

The Initial Use of Free-running Electromyography to Detect Early Motor Tract Injury during Resection of Intramedullary Spinal Cord Lesions

OBJECTIVE:

The resection of intramedullary spinal cord lesions (ISCLs) can be complicated by neurological deficits. Neuromonitoring has been used to reduce intraoperative risk. We have used somatosensory evoked potentials (SEPs) and muscle-derived transcranial electrical motor evoked potentials (myogenic TCE-MEPs) to monitor ISCL removal. We report our retrospective experience with the addition of free-running electromyography (EMG).

METHODS:

Thirteen patients underwent 14 monitored ISCL excisions. Anesthesia was maintained with minimal inhalant to reduce motoneuron suppression and enhance the myogenic TCE-MEPs. Free-running EMG was examined in the four limbs for evidence of abnormal bursts, prolonged tonic discharge, or sudden electrical silence. Warning of an electromyographic abnormality or myogenic TCE-MEP loss prompted interventions, including blood pressure elevation, a pause in surgery, a wake-up test, or termination of surgery. Pre- and postoperative neurological examinations determined the incidence of new deficits.

RESULTS:

The combined use of free-running EMG and myogenic TCE-MEPs detected all eight patients with a new motor deficit after surgery; there was one false-positive report. In three of the eight true-positive cases, an electromyographic abnormality immediately anticipated loss of the myogenic TCE-MEPs. Two patients with abnormal EMGs but unchanged myogenic TCE-MEPs experienced mild postoperative worsening of motor deficits; myogenic TCE-MEPs alone would have generated false-negative reports in these cases.

CONCLUSION:

During resection of ISCLs, free-running EMG can supplement motor tract monitoring by TCE-MEPs. Segmental and suprasegmental elicitation of neurotonic discharges can be observed in four-limb EMG. Abnormal electromyographic bursts, tonic discharge, or abrupt electromyographic silence may anticipate myogenic TCE-MEP loss and predict a postoperative motor deficit.

Intravenous anaesthesia and repetitive transcranial magnetic stimulation monitoring in spinal column surgery

BACKGROUND

Transcranial magnetic stimulation with motor evoked potential monitoring is a non-invasive method for monitoring motor tracts during surgery. However, anaesthetic agents such as propofol and volatile agents reduce responses to single transcranial magnetic stimulation. We assessed an intravenous technique for anaesthesia to allow motor evoked potentials (MEPs) to be monitored using repetitive transcranial magnetic stimulation (rTMS).

METHODS

We applied three-pulse rTMS (TriStim) in 11 patients undergoing spinal column surgery after spinal column injury and recorded the latency and peak-to-peak amplitude of MEPs. Anaesthesia was maintained with propofol and remifentanil.

RESULTS

MEPs were monitored successfully intraoperatively in all patients.

CONCLUSIONS

It is possible to monitor intraoperative MEP using rTMS during anaesthesia with propofol and remifentanil.

A model of experimental spinal cord trauma based on computer-controlled intervertebral distraction: characterization of graded injury

STUDY DESIGN

: A new model of experimental spinal cord injury is detailed based on the application of tensile (distraction) force to the vertebral column of the rat.

OBJECTIVES

: To develop an experimental model of graded spinal cord injury by application of tensile forces to the vertebral column.

SUMMARY OF BACKGROUND DATA

: Distraction is frequently an integral component of human spinal cord injury, but the acute application of tensile forces to the spinal cord has not been modeled rigorously.

METHODS

: A computer-controlled, motorized outrigger device was used to apply a longitudinal stretching force to sublaminar hooks oriented proximally at T9 and distally at T11. Distraction force was applied using a program that varied the length, speed, and duration of its distraction. A modified 14-point Tarlov score was used to establish the presence of hindlimb dysfunction. This score was correlated with acute changes in somatosensory-evoked potential amplitude, the comprehensive open-field test of locomotor function at 4 weeks, and postmortem measurements of serotonin content and metabolism in spinal cord rostral and distal to the site of injury.

RESULTS

: Of distraction parameters, only length of distraction correlated significantly with each outcome measure. For outcome measures, open-field test inventory and distal/proximal ratio of the spinal content of serotonin were correlated most closely with final Tarlov scores. Acute somatosensory-evoked potential amplitudes proved to be an excellent index of the acute injury but were poor measures of long-term outcome.

CONCLUSIONS

: Distraction-induced spinal cord injury was uniformly mild in rats with intact facet capsular ligaments, regardless of distraction parameters. Cutting the facet joint ligaments consistently generated outcome measures associated with mild, moderate, and severe spinal cord injury at 3-, 5-, and 7-mm distraction lengths, respectively.

Intraoperative Wake-Up Test and Postoperative Emergence in Patients Undergoing Spinal Surgery: A Comparison of Intravenous and Inhaled Anesthetic Techniques Using Short-Acting Anesthetics

Surgical procedures on the vertebral column may result in spinal cord damage, leading to neurological deficits that demand immediate therapeutical intervention. We designed this study to determine which anesthetic regimen allows a rapid wake-up test during and after surgery to detect neurological deficits. Fifty-four patients were randomly allocated to the following groups: group PR (propofol/remifentanil): target-controlled infusion with propofol (plasma concentration, 2-4 microg/mL) and remifentanil 0.2-0.5 microg . kg(-1) . min(-1); group PS (propofol/sufentanil): propofol (2-4 microg/mL) and repetitive boluses of 0.1-0.2 microg/kg of sufentanil adjusted to patients requirements; and group DR (desflurane/remifentanil): desflurane/air 3.0-4.0 vol% combined with remifentanil 0.2-0.5 microg . kg(-1) . min(-1). Group PS required significantly longer times for the onset of breathing (8.9 +/- 1.6 min), elevation of the head (17.0 +/- 3.8 min), and motion of the feet (17.0 +/- 7.4 min) than group PR (6.9 +/- 2.6 min, 9.3 +/- 2.2 min, and 9.4 +/- 2.4 min, respectively) or group DR (5.4 +/- 0.8 min, 6.1 +/- 1.0 min, and 6.2 +/- 1.0 min, respectively). The anesthetic regimen with desflurane and remifentanil allowed faster awakening during and after surgery that permitted immediate neurological examination after spinal surgery compared with propofol/remifentanil.

Influence of propofol concentrations on multipulse transcranial motor evoked potentials

BACKGROUND

Motor evoked potentials can be affected by propofol anaesthesia. We studied how increasing target concentrations of propofol altered transcranial motor evoked potentials (tcMEP) during scoliosis surgery.

METHODS

Fifteen patients undergoing surgery for scoliosis were anaesthetized with remifentanil and propofol without nitrous oxide or neuromuscular blocking agents (BIS<60). tcMEP were elicited by transcranial electric multipulse stimulation of the motor cortex and recording of compound action potentials from the anterior tibialis muscle. tcMEP were obtained before surgery with propofol target values set from 4 to 8 mg litre(-1), and then during surgery. Arterial propofol concentrations were measured for each tcMEP recording.

RESULTS

Before surgery, increasing propofol reduced tcMEP amplitude in a dose-dependent manner, with no effect on latency. During surgery, at equivalent propofol concentrations, tcMEP were not statistically different from those obtained before surgery. In all except one patient, tcMEP signals were present during the entire procedure. In this patient the loss of tcMEP was unfortunately related to an anterior spinal cord lesion, which was confirmed by a wake-up test.

CONCLUSION

We found that, although propofol had a dose-dependent effect on tcMEP amplitude, anaesthesia could be maintained with remifentanil and propofol to allow recording and interpretation of tcMEP signals.

Somatosensory evoked potential monitoring of lumbar pedicle screw placement for in situ posterior spinal fusion

BACKGROUND CONTEXT

Somatosensory evoked potentials (SSEP) are commonly used to monitor the spinal cord and nerve roots during operative procedures that put those structures at risk. The utility of SSEPs to evaluate cauda equina and nerve root function during posterior spinal arthrodesis with pedicular fixation for degenerative lumbar disease has been reported anecdotally and remains controversial.

PURPOSE

An institution-wide review of the ability of SSEP readings to monitor nerve function during posterior lumbar spinal arthrodeses with transpedicular fixation for degenerative lumbar spinal disorders was undertaken.

STUDY DESIGN/SETTING

A retrospective review was undertaken. Patient history, preoperative physical examination, intraoperative anesthesia, SSEP records and the postoperative course were reviewed.

METHODS

A total of 186 consecutive arthrodeses as described above were reviewed. Patients who had anterior procedures, spondyloreduction or scoliosis correction were excluded from the study. There were 76 male and 110 female patients. Five fellowship-trained spine surgeons placed a total of 888 pedicle screws. Sixty-five percent of the patients had a principal preoperative diagnosis of spinal stenosis with degenerative spondylolisthesis. Other common diagnoses were isthmic spondylolisthesis and degenerative scoliosis. Ninety-three percent of the cases involved decompressive laminectomy. Eight percent had posterior interbody fusions. All pedicle screws were placed without the assistance of fluoroscopy or stereotactic computer-assisted guidance. Screw position was evaluated intraoperatively with standard posteroanterior and lateral radiographs. Anesthetic agents compatible with SSEP monitoring were used in all patients. SSEP baseline readings were obtained in all patients in the operating room soon after induction of general anesthesia. An acute and sustained loss of 50% of the SSEP amplitude and/or increase by 10% of latency from baseline was considered to be pathologic.

RESULTS

None of the 186 patients had significant SSEP changes. There were, however, 5 patients with postoperative radiculopathies distinct from their preoperative presentations. Early postoperative plain radiographs and computed assisted tomography (CAT) scans revealed malpositioned pedicle screws. Consequently, eight pedicle screws were either revised or removed. All patients had partial or full recovery of their new deficits after revision surgery.

CONCLUSION

We conclude that the use of SSEPs in evaluating pedicle screw placement during lumbar arthrodesis is limited. In this setting, if monitoring is required, alternative methods with greater sensitivity and efficacy should be explored.

Transcranial magnetic stimulation: review of the technique, basic principles and applications

Transcranial magnetic stimulation is rapidly developing as a powerful, non-invasive tool for studying the descending motor tracts in humans. The applications of the test in animals are for the moment restricted to small animals. However, this non-invasive, sensitive and painless technique appears promising as a test of motor tract function in horses where the neurological examination is mainly restricted to clinical evaluation and some ancillary tests, such as radiography, cerebrospinal fluid analysis and electromyography. In this review, we want to discuss the history, basic principles, technique and applications of transcranial magnetic stimulation in humans and small animals and indicate the possibilities for its use in horses. Since the great portion of this review is based on human studies, it is worthwhile to mention that the reports being described are from humans unless otherwise specified.

Monitoring scoliosis surgery with combined multiple pulse transcranial electric motor and cortical somatosensory-evoked potentials from the lower and upper extremities

STUDY DESIGN

A retrospective case review was performed.

OBJECTIVE

To assess the value, rapidity, and safety of combined multiple-pulse transcranial electric stimulation motor-evoked potential and somatosensory-evoked potential monitoring during scoliosis surgery.

SUMMARY OF BACKGROUND DATA

Leg somatosensory-evoked potentials can miss motor deficits, and a 50% amplitude warning criterion can produce false alarms.

METHODS

For this study, 33 scoliosis surgeries in neurologically normal patients under propofol/fentanyl anesthesia omitting neuromuscular blockade were monitored with four-extremity multiple-pulse transcranial electric stimulation muscle motor-evoked potentials and cortical somatosensory-evoked potentials. Instead of amplitude criteria, parallel (same-direction) change was used to identify systemic alteration and nonparallel (one- or two-limb) deterioration to identify focal neurologic compromise. Clinical observation and intraoperative electroencephalography were used to assess adverse effects.

RESULTS

Instantaneous motor-evoked potentials and rapidly reproducible cortical somatosensory-evoked potentials provided comprehensive feedback every 0.8 to 6.7 minutes (median, 2.4 minutes) without adverse effects. Parallel (systemic) changes without alarm or deficit included motor-evoked potential fading or temporary loss and leg somatosensory-evoked potential amplitudes below 50% of initial, maximum, or median intraoperative values in 10% to 37% of the cases. Three nonparallel changes occurred: 1) abrupt bilateral leg somatosensory-evoked potential 20% to 30% reduction without motor-evoked potential change during instrumentation resolving spontaneously over 30 minutes, with transient postoperative sensory symptoms; 2) right-arm somatosensory-evoked potential and motor-evoked potential reduction during hyperabduction restored after repositioning, without deficit; 3) abrupt bilateral leg motor-evoked potential loss preceding 30% to 60% somatosensory-evoked potential reduction during derotation rapidly restored after instrumentation release, without deficit.

CONCLUSIONS

In neurologically normal patients, the combined methods are safe and rapid, and could improve the sensitivity and specificity of scoliosis monitoring. Arm controls facilitate differentiation between systemic alterations and focal neurologic compromise.

Safety of intraoperative transcranial electrical stimulation motor evoked potential monitoring

This article reviews intraoperative transcranial electrical stimulation (TES) motor evoked potential (MEP) monitoring safety based on comparison with other clinical and experimental brain stimulation methods and clinical experience in more than 15000 cases. Comparative analysis indicates that brain damage and kindling are highly unlikely. There have been remarkably few adverse events. Pulse train TES-induced or coincidental seizures (n = 5) are rare, probably because of very brief (<0.03 second) stimuli, anesthesia, and the general absence of predisposing cerebral conditions. Soft bite blocks may prevent tongue or lip laceration (n = 29) or mandibular fracture (n = 1). Rare cardiac arrhythmia (n = 5) and intraoperative awareness (n = 1) may be coincidental. Minor scalp burns (n = 2) are rare. Although possible, no spinal epidural recording electrode complications or injuries resulting from TES-induced movement were found. There have been no recognized adverse neuropsychological effects, headaches, or endocrine disturbances. Comprehensive relative contraindications include epilepsy, cortical lesions, convexity skull defects, raised intracranial pressure, cardiac disease, proconvulsant medications or anesthetics, intracranial electrodes, vascular clips or shunts, and cardiac pacemakers or other implanted biomedical devices. Otherwise unexplained intraoperative seizures and possibly arrhythmias are indications to abort TES. With appropriate precautions in expert hands, the well-established benefits of TES MEP monitoring decidedly outweigh the associated risks.

Combined spinal cord monitoring using neurogenic mixed evoked potentials and collision techniques

STUDY DESIGN

Neurogenic mixed evoked potentials are used routinely to monitor the spinal cord during spine surgery. This study investigates the differential sensory-motor contribution by using collision techniques.

OBJECTIVE

To demonstrate that neurogenic mixed evoked potentials do contain a motor component.

SUMMARY OF BACKGROUND DATA

Spinal cord monitoring is now routinely used during spine deformity surgery. Neurogenic mixed evoked potentials (i.e., potentials recorded from lower limb nerves after spinal cord stimulation) represent a reliable and sensitive technique. However, their specificity (sensory and motor spinal pathways) remains debated.

METHODS

Neurogenic mixed evoked potentials and collisions were performed in 24 consecutive patients during scoliosis surgery. Neurogenic mixed evoked potentials were elicited by a high thoracic spinal test stimulation and recorded from the tibial nerve at the ankle. A peripheral conditioning stimulation was delivered at the popliteal fossa 15 ms before spinal stimulation, inducing an ascending volley. The antidromic ascending motor component stops at the anterior horn cell level, whereas the orthodromic sensory component reaches the dorsal columns. The 15-ms interstimulus interval between peripheral conditioning and spinal test stimulation makes the collision with descending volleys occur in the spinal cord. The descending sensory volley is blocked, whereas the descending motor volley is unaffected.

RESULTS

Reproducible evoked potentials were recorded from the tibial nerve in all the patients studied when the conditioning stimulation was performed. These conditioned neurogenic mixed evoked potentials consist of a small and polyphasic wave whose amplitude represents approximately 26% that of the wave of unconditioned neurogenic mixed evoked potentials. It is likely that they correspond to motor spinal pathway activation.

CONCLUSION

Both standard and conditioned neurogenic mixed evoked potentials are proposed to provide combined sensory and motor spinal pathway monitoring.

Discrepancy between decreases in the amplitude of compound muscle action potential and loss of motor function caused by ischemic and compressive insults to the spinal cord

We examined the relationship between decreases in the amplitude of the compound muscle action potential (CMAP), caused by ischemic and compressive insults to the spinal cord, and postoperative motor deficits. Results were compared with those for other evoked potentials commonly used for multimodal monitoring of the spinal cord. CMAP was more sensitive than the other evoked potentials employed to ischemic and compressive insults to the spinal cord, although the disappearance of CMAP did not always result in a residual motor deficit. A decrease of more than 50% in the amplitude of the motor-evoked potential (MEP) from the spinal cord correlated well with the postoperative motor deficit. CMAP is a sensitive tool for the early detection of spinal cord impairment caused by ischemic or compressive insults to the spinal cord. The time after the disappearance of the CMAP amplitude was important for predicting postoperative motor deficit, but it is also necessary to employ CMAP concomitantly with other conductive potentials in spinal cord monitoring.

The benefit of neurogenic mixed evoked potentials for intraoperative spinal cord monitoring during correction of severe scoliosis: a case study

STUDY DESIGN

A case report is presented.

OBJECTIVE

To present a case in which surgical correction of a severe scoliotic curve caused unilateral loss of neurogenic mixed evoked potential data despite unchanged somatosensory data.

SUMMARY OF BACKGROUND DATA

Surgical correction of large scoliotic curves presents a risk to the function of the spinal cord. Multimodality intraoperative neurophysiologic monitoring of the spinal cord is recommended during such procedures.

METHODS

A 13-year-old girl with severe double major scoliosis underwent a staged operative procedure for correction of her spine deformity. Intraoperative neurophysiologic monitoring using somatosensory-evoked potentials and neurogenic mixed evoked potentials was performed for each stage.

RESULTS

During the final stage (a T4-L5 posterior instrumentation and fusion) left neurogenic mixed evoked potential data were lost approximately 45 minutes after placement of the left-side, correcting rod. The surgeon was warned of the data change. Set bolts were loosened at all fixation points, and the data quickly returned to within normal limits of baseline. Somatosensory data never approached warning criteria at any point during surgery. The patient awakened with no neurologic deficit.

CONCLUSIONS

Neurophysiologic monitoring using both somatosensory-evoked potentials and neurogenic mixed evoked potentials is recommended when surgery is performed to correct spine deformity. The Stagnara wake-up test, somatosensory-evoked potentials, and neurogenic mixed evoked potentials are important components of spinal cord monitoring during surgery, and should be used together for optimal protection of neurologic function.

Factors related to false- versus true-positive neuromonitoring changes in adolescent idiopathic scoliosis surgery

STUDY DESIGN

A retrospective study of 134 adolescent patients who underwent surgical correction of idiopathic scoliosis between June 1992 and August 1998 was conducted.

OBJECTIVE

To examine factors related to changes in somatosensory-evoked potentials with or without neurogenic motor-evoked potentials.

SUMMARY OF BACKGROUND DATA

Studies document and demonstrate threshold criteria for changes in neuromonitoring that predict changes in spinal cord function. Rates of false-negative occurrences are low, yet higher rates of false-positive findings may result.

METHODS

All the patients had somatosensory monitoring, and 71 patients had both somatosensory-evoked potential and neurogenic motor-evoked potential monitoring. Gender, age, curve types, duration of surgery, type and amount of instrumentation, and amount of correction were examined for their effects on monitoring. Estimated blood volume loss as well as high and low mean arterial pressure and its variance were assessed at the start, middle, and conclusion of the procedure.

RESULTS

According to the findings, 122 patients (91%) had no monitoring changes and no postoperative neurologic deficit. Six patients (4.5%) had false-positive readings. Six patients had a postoperative motor or sensory deficit, all of which resolved within 18 months. False-positive readings were associated with greater variability in mean arterial pressure. No consistent predictions could be made about the incidence of cord injury if neuromonitoring changes returned to baseline before the end of surgery.

CONCLUSIONS

Questions remain about the predictive accuracy of somatosensory-evoked and neurogenic motor-evoked potentials. According to the findings in this study, in which there were no false-negative readings and a modest false-positive rate, continued use of these methods is recommended. Higher false-positive rates were seen in patients with greater lability in mean arterial pressure. A wake-up test is recommended for all cases in which threshold monitoring changes occur because cases of spinal cord injury may exist even when monitored variables return to baseline.

The use of magnetic motor evoked potentials in horses with cervical spinal cord disease

The aim of this study was to investigate the use of magnetic motor evoked potentials as an ancillary diagnostic test in horses with cervical cord lesions. Transcranial magnetic stimulation was performed in 12 ataxic horses and the results of the evoked responses were compared to those found in normal horses. The latency and peak-to-peak amplitude of the potentials in the 12 ataxic horses were significantly different from those measured in normal horses. The configuration of the abnormal potentials was also polyphasic. Normalisation of the evoked potentials occurred in none of the horses, presented after a period of clinical improvement. These findings demonstrate that the technique is also able to detect lesions in horses with subtle clinical signs of incoordination. Magnetic transcranial stimulation is a valuable ancillary test to assess the integrity of the motor tracts. The technique is painless and safe and shows good sensitivity to detect lesions along the descending motor pathways.