Clinical neurophysiology in the prognosis and monitoring of traumatic spinal cord injury

Neurocritical care and neuromonitoring considerations in acute pediatric spinal cord injury

Management of pediatric spinal cord injury (SCI) is an essential skill for all pediatric neurocritical care physicians. In this review, we focus on the evaluation and management of pediatric SCI, highlight a novel framework for the monitoring of such patients in the intensive care unit (ICU), and introduce advancements in critical care techniques in monitoring and management. The initial evaluation and characterization of SCI is crucial for improving outcomes as well as prognostication. While physical examination and imaging are the main stays of the work-up, we propose the use of somatosensory evoked potentials (SSEPs) and transcranial magnetic stimulation (TMS) for challenging clinical scenarios. SSEPs allow for functional evaluation of the dorsal columns consisting of tracts associated with hand function, ambulation, and bladder function. Meanwhile, TMS has the potential for informing prognostication as well as response to rehabilitation. Spine stabilization, and in some cases surgical decompression, along with respiratory and hemodynamic management are essential. Emerging research suggests that targeted spinal cerebral perfusion pressure may provide potential benefits. This review aims to increase the pediatric neurocritical care physician's comfort with SCI while providing a novel algorithm for monitoring spinal cord function in the ICU.

Motor Pathways Reorganization following Surgical Decompression for Degenerative Cervical Myelopathy: A Combined Navigated Transcranial Magnetic Stimulation and Clinical Outcome Study

(1) Background: Degenerative cervical myelopathy is one of the main causes of disability in the elderly. The treatment of choice in patients with clear symptomatology and radiological correlation is surgical decompression. The application of navigated transcranial magnetic stimulation (nTMS) techniques has the potential to provide additional insights into the cortical and corticospinal behavior of the myelopathic cord and to better characterize the possible extent of clinical recovery. The objective of our study was to use nTMS to evaluate the effect of surgical decompression on neurophysiological properties at the cortical and corticospinal level and to better characterize the extent of possible clinical recovery. (2) Methods: We conducted a longitudinal study in which we assessed and compared nTMS neurophysiological indexes and clinical parameters (modified Japanese Orthopedic Association score and nine-hole pegboard test) before surgery, at 6 months, and at 12 months' follow-up in a population of 15 patients. (3) Results: We found a significant reduction in resting motor threshold (RMT; average 7%), cortical silent period (CSP; average 15%), and motor area (average 25%) at both 6 months and 12 months. A statistically significant linear correlation emerged between recruitment curve (RC) values obtained at follow-up appointments and at baseline (r = 0.95 at 6 months, r = 0.98 at 12 months). A concomitant improvement in the mJOA score and in the nine-hole pegboard task was observed after surgery. (4) Conclusions: Our results suggest that surgical decompression of the myelopathic spinal cord improves the neurophysiological balance at the cortical and corticospinal level, resulting in clinically significant recovery. Such findings contribute to the existing evidence characterizing the brain and the spinal cord as a dynamic system capable of functional and reversible plasticity and provide useful clinical insights to be used for patient counseling.

Disease-specific interventions using cell therapies for spinal cord disease/injury

Traumatic spinal cord injury (SCI) may occur across the lifespan and is of global relevance. Damage of the spinal cord results in para- or tetraplegia and is associated with neuropathic pain, spasticity, respiratory, and autonomic dysfunction (i.e., control of bladder-bowel function). While the acute surgical treatment aims at stabilizing the spine and decompressing the damaged spinal cord, SCI patients require neurorehabilitation to restore neural function and to compensate for any impairments including motor disability, pain treatment, and bladder/bowel management. However, the spinal cord has a limited capacity to regenerate and much of the disability may persist, depending on the initial lesion severity and level of injury. For this reason, and the lack of effective drug treatments, there is an emerging interest and urgent need in promoting axonal regeneration and remyelination after SCI through cell- and stem-cell based therapies. This review briefly summarizes the state-of the art management of acute SCI and its neurorehabilitation to critically appraise phase I/II trials from the last two decades that have investigated cell-based therapies (i.e., Schwann cells, macrophages, and olfactory ensheathing cells) and stem cell-based therapies (i.e., neural stem cells, mesenchymal, and hematopoietic stem cells). Recently, two large multicenter trials provided evidence for the safety and feasibility of neural stem cell transplantation into the injured cord, whilst two monocenter trials also showed this to be the case for the transplantation of Schwann cells into the posttraumatic cord cavity. These are milestone studies that will facilitate further interventional trials. However, the clinical adoption of such approaches remains unproven, as there is only limited encouraging data, often in single patients, and no proven trial evidence to support regulatory approval.

Surgical Efficacy and Prognostic Factors for Acute Traumatic Central Cord Syndrome Without Fracture and Dislocation

This study was undertaken to evaluate the effectiveness of surgical treatment of acute traumatic central cord syndrome (ATCCS) without fracture and dislocation and explore surgical timing and factors influencing postoperative recovery of spinal cord function. We retrospectively collected the general and clinical data of 112 patients with ATCCS (American Spinal Injury Association impairment scale grade C or D) without fracture and dislocation who underwent surgical treatment in our hospital from January 2013 to August 2019. We used statistical methods to evaluate the safety of the operation and explore the timing of surgery and the factors influencing postoperative recovery of spinal cord function. The mean age of the 112 patients was 60.64±12.91 years. The Japanese Orthopaedic Association score and the American Spinal Injury Association motor score (AMS) of the 112 patients were significantly higher at final follow-up than at admission. No significant difference in recovery of spinal cord function was seen between the early operation group (≤4 days) and the late operation group (>4 days). Comparison of patients with a good prognosis vs a poor prognosis showed that age, intrahand muscle strength at admission, maximum spinal cord compression, maximum canal compromise, length of high-intensity signal in the spinal cord on sagittal T2-weighted magnetic resonance imaging, AMS, and American Spinal Injury Association injury grade D/C at admission had a significant effect on recovery of spinal cord function. Surgical treatment of ATCCS without fracture and dislocation is safe and effective. Age, admission AMS and American Spinal Injury Association impairment scale score, intrinsic hand muscle strength, maximum canal compromise, maximum spinal cord compression, and length of high-intensity signal in the spinal cord can be used to predict postoperative recovery of spinal cord function. [Orthopedics. 2022;45(6):325-332.].

Clinical electrodiagnostic evaluation for nerve transfer surgery in spinal cord injury: a new indication and clinical pearls

In this review, we highlight the important role of the clinical electrodiagnostic (EDX) evaluation after cervical spinal cord injury (SCI). Our discussion focuses on the need for timely, frequent, and accurate EDX evaluations in the context of nerve transfer surgery to restore critical upper limb functions, including elbow extension, hand opening, and hand closing. The EDX evaluation is crucial to define the extent of lower motor neuron lesions and determine candidacy for surgery. We also discuss the important role of the postoperative EDX evaluation in determining prognosis and supporting rehabilitation. We propose a practical framework for EDX evaluation in this clinical setting.

Quantitative electrophysiological assessments as predictive markers of lower limb motor recovery after spinal cord injury: a pilot study with an adaptive trial design

Study design

Observational, cohort study.

Objectives

(1) Determine the feasibility and relevance of assessing corticospinal, sensory, and spinal pathways early after traumatic spinal cord injury (SCI) in a rehabilitation setting. (2) Validate whether electrophysiological and magnetic resonance imaging (MRI) measures taken early after SCI could identify preserved neural pathways, which could then guide therapy.

Setting

Intensive functional rehabilitation hospital (IFR).

Methods

Five individuals with traumatic SCI and eight controls were recruited. The lower extremity motor score (LEMS), electrical perceptual threshold (EPT) at the S2 dermatome, soleus (SOL) H-reflex, and motor evoked potentials (MEPs) in the tibialis anterior (TA) muscle were assessed during the stay in IFR and in the chronic stage (>6 months post-SCI). Control participants were only assessed once. Feasibility criteria included the absence of adverse events, adequate experimental session duration, and complete dataset gathering. The relationship between electrophysiological data collected in IFR and LEMS in the chronic phase was studied. The admission MRI was used to calculate the maximal spinal cord compression (MSCC).

Results

No adverse events occurred, but a complete dataset could not be collected for all subjects due to set-up configuration limitations and time constraints. EPT measured at IFR correlated with LEMS in the chronic phases (r = −0.67), whereas SOL H/M ratio, H latency, MEPs and MSCC did not.

Conclusions

Adjustments are necessary to implement electrophysiological assessments in an IFR setting. Combining MRI and electrophysiological measures may lead to better assessment of neuronal deficits early after SCI.

Neurophysiological Changes in the First Year After Cell Transplantation in Sub-acute Complete Paraplegia

Neurophysiological testing can provide quantitative information about motor, sensory, and autonomic system connectivity following spinal cord injury (SCI). The clinical examination may be insufficiently sensitive and specific to reveal evolving changes in neural circuits after severe injury. Neurophysiologic data may provide otherwise imperceptible circuit information that has rarely been acquired in biologics clinical trials in SCI. We reported a Phase 1 study of autologous purified Schwann cell suspension transplantation into the injury epicenter of participants with complete subacute thoracic SCI, observing no clinical improvements. Here, we report longitudinal electrophysiological assessments conducted during the trial. Six participants underwent neurophysiology screening pre-transplantation with three post-transplantation neurophysiological assessments, focused on the thoracoabdominal region and lower limbs, including MEPs, SSEPs, voluntarily triggered EMG, and changes in GSR. We found several notable signals not detectable by clinical exam. In all six participants, thoracoabdominal motor connectivity was detected below the clinically assigned neurological level defined by sensory preservation. Additionally, small voluntary activations of leg and foot muscles or positive lower extremity MEPs were detected in all participants. Voluntary EMG was most sensitive to detect leg motor function. The recorded MEP amplitudes and latencies indicated a more caudal thoracic level above which amplitude recovery over time was observed. In contrast, further below, amplitudes showed less improvement, and latencies were increased. Intercostal spasms observed with EMG may also indicate this thoracic “motor level.” Galvanic skin testing revealed autonomic dysfunction in the hands above the injury levels. As an open-label study, we can establish no clear link between these observations and cell transplantation. This neurophysiological characterization may be of value to detect therapeutic effects in future controlled studies.

Spinal cord stimulation and rehabilitation in an individual with chronic complete L1 paraplegia due to a conus medullaris injury: motor and functional outcomes at 18 months

INTRODUCTION

Epidural electrical stimulation of the conus medullaris has helped facilitate native motor recovery in individuals with complete cervicothoracic spinal cord injuries (SCI). A theorized mechanism of clinical improvement includes supporting central pattern generators intrinsic to the conus medullaris. Because spinal cord stimulators (SCS) are approved for the treatment of neuropathic pain, we were able to test this experimental therapy in a subject with complete L1 paraplegia and neuropathic genital pain due to a traumatic conus injury.

CASE PRESENTATION

An otherwise healthy 48-year-old male with chronic complete L1 paraplegia with no zones of partial preservation (ZPP) and intractable neuropathic genital pain presented to our group seeking nonmedical pain relief and any possible help with functional restoration. After extensive evaluation, discussion, and consent, we proceeded with SCS implantation at the conus and an intensive outpatient physical therapy regimen consistent with the recent SCI rehabilitation literature.

DISCUSSION

Intraoperatively, no electromyography (EMG) could be elicited with epidural conus stimulation. At 18 months after implantation, his motor ZPPs had advanced from L1 to L5 on the left and from L1 to L3 on the right. Qualitative increases in lower extremity resting state EMG amplitudes were noted, although there was no consistent evidence of voluntary EMG or rhythmic locomotive leg movements. Three validated functional and quality of life (QoL) surveys demonstrated substantial improvements. The modest motor response compared to the literature suggests likely critical differences in the anatomy of such a low injury. However, the change in ZPPs and QoL suggest potential for neuroplasticity even in this patient population.

Lower extremity outcome measures: considerations for clinical trials in spinal cord injury

STUDY DESIGN

This is a focused review article.

OBJECTIVES

To identify important concepts in lower extremity (LE) assessment with a focus on locomotor outcomes and provide guidance on how existing outcome measurement tools may be best used to assess experimental therapies in spinal cord injury (SCI). The emphasis lies on LE outcomes in individuals with complete and incomplete SCI in Phase II-III trials.

METHODS

This review includes a summary of topics discussed during a workshop focusing on LE function in SCI, conceptual discussion of corresponding outcome measures and additional focused literature review.

RESULTS

There are a number of sensitive, accurate, and responsive outcome tools measuring both quantitative and qualitative aspects of LE function. However, in trials with individuals with very acute injuries, a baseline assessment of the primary (or secondary) LE outcome measure is often not feasible.

CONCLUSION

There is no single outcome measure to assess all individuals with SCI that can be used to monitor changes in LE function regardless of severity and level of injury. Surrogate markers have to be used to assess LE function in individuals with severe SCI. However, it is generally agreed that a direct measurement of the performance for an appropriate functional activity supersedes any surrogate marker. LE assessments have to be refined so they can be used across all time points after SCI, regardless of the level or severity of spinal injury.

SPONSORS

Craig H. Neilsen Foundation, Spinal Cord Outcomes Partnership Endeavor.

[Pain in patients with paraplegia]

Chronic pain is one of the most reported health problems in patients suffering from spinal cord injuries and is described by the patients as one of the most burdensome sequelae of paraplegia. Various types of pain, such as nociceptive, neuropathic and other types of pain can occur. In addition, multiple pathophysiological mechanisms based on the biopsychosocial pain model play a role in the origins of the pain. These aspects necessitate a multimodal pain management approach in this patient group. This article presents an overview of the occurrence, importance and pathophysiology of chronic pain following spinal cord injury as well as diagnostic and therapeutic approaches.

Wogonoside alleviates inflammation induced by traumatic spinal cord injury by suppressing NF-κB and NLRP3 inflammasome activation

Wogonoside possesses anti-oxidative, anti-inflammatory, anti-allergy and anti-tumor properties. The aim of the present study was to evaluate whether wogonoside alleviates spinal cord injury (SCI)-induced inflammation via nuclear factor (NF)-κB and nucleotide-binding oligomerization domain-like receptor family pyrin domain containing 3 (NLRP3) inflammasome activation. Sprague-Dawley rats were positioned in the jaws of a calibrated aneurysm clip with a closing pressure of 55 g. The jaws were placed on the dorsal and ventral surfaces of the spinal cord and left in place for 1 min. SCI rats were treated with 12, 25 and 50 mg/kg wogonoside. Following this, the locomotor function was assessed using the Basso Beattie Bresnahan scale. The water content of the spinal cord was measured, tumor necrosis factor-α (TNF-α), interleukin (IL)-1β and IL-6 levels were assessed and western blot analysis was performed to evaluate the expressions of NF-κB and NLRP3. Wogonoside was demonstrated to significantly ameliorate the SCI-induced reduction in Basso Beattie Bresnahan score (P<0.01) and significantly reduce the water content of the spinal cord in rats with SCI-induced inflammation (P<0.01). Results also indicated that treatment with wogonoside significantly reduced the levels of IL-1β, TNF-α and IL-6 in rats with SCI-induced inflammation (P<0.01), potentially via the phosphorylation of NF-κB inhibitor α. Furthermore, treatment with wogonoside inhibited the expressions of toll-like receptor 4, NLRP3 and caspase-1 protein in SCI model rats (P<0.01). In conclusion, the results of the present study suggest that wogonoside alleviates SCI-induced inflammation by suppressing NF-κB and NLRP3 inflammasome activation.

Afferent input and sensory function after human spinal cord injury

Spinal cord injury (SCI) often disrupts the integrity of afferent (sensory) axons projecting through the spinal cord dorsal columns to the brain. Examinations of ascending sensory tracts, therefore, are critical for monitoring the extent of SCI and recovery processes. In this review, we discuss the most common electrophysiological techniques used to assess transmission of afferent inputs to the primary motor cortex (i.e., afferent input-induced facilitation and inhibition) and the somatosensory cortex [i.e., somatosensory evoked potentials (SSEPs), dermatomal SSEPs, and electrical perceptual thresholds] following human SCI. We discuss how afferent input modulates corticospinal excitability by involving cortical and spinal mechanisms depending on the timing of the effects, which need to be considered separately for upper and lower limb muscles. We argue that the time of arrival of afferent input onto the sensory and motor cortex is critical to consider in plasticity-induced protocols in humans with SCI. We also discuss how current sensory exams have been used to detect differences between control and SCI participants but might be less optimal to characterize the level and severity of injury. There is a need to conduct some of these electrophysiological examinations during functionally relevant behaviors to understand the contribution of impaired afferent inputs to the control, or lack of control, of movement. Thus the effects of transmission of afferent inputs to the brain need to be considered on multiple functions following human SCI.

Spike-timing-dependent plasticity in lower-limb motoneurons after human spinal cord injury

Recovery of lower-limb function after spinal cord injury (SCI) likely depends on transmission in the corticospinal pathway. Here, we examined whether paired corticospinal-motoneuronal stimulation (PCMS) changes transmission at spinal synapses of lower-limb motoneurons in humans with chronic incomplete SCI and aged-matched controls. We used 200 pairs of stimuli where corticospinal volleys evoked by transcranial magnetic stimulation (TMS) over the leg representation of the motor cortex were timed to arrive at corticospinal-motoneuronal synapses of the tibialis anterior (TA) muscle 2 ms before antidromic potentials evoked in motoneurons by electrical stimulation of the common peroneal nerve (PCMS+) or when antidromic potentials arrived 15 or 28 ms before corticospinal volleys (PCMS-) on separate days. Motor evoked potentials (MEPs) elicited by TMS and electrical stimulation were measured in the TA muscle before and after each stimulation protocol. After PCMS+, the size of MEPs elicited by TMS and electrical stimulation increased for up to 30 min in control and SCI participants. Notably, this was accompanied by increases in TA electromyographic activity and ankle dorsiflexion force in both groups, suggesting that this plasticity has functional implications. After PCMS-, MEPs elicited by TMS and electrical stimulation were suppressed if afferent input from the common peroneal nerve reduced TA MEP size during paired stimulation in both groups. In conclusion, PCMS elicits spike-timing-dependent changes at spinal synapses of lower-limb motoneurons in humans and has potential to improve lower-limb motor output following SCI.NEW & NOTEWORTHY Approaches that aim to enhance corticospinal transmission to lower-limb muscles following spinal cord injury (SCI) are needed. We demonstrate that paired corticomotoneuronal stimulation (PCMS) can enhance plasticity at spinal synapses of lower-limb motoneurons in humans with and without SCI. We propose that PCMS has potential for improving motor output in leg muscles in individuals with damage to the corticospinal tract.

Usefulness of laser-evoked potentials and quantitative sensory testing in the diagnosis of neuropathic spinal cord injury pain: a multiple case study

STUDY DESIGN

A retrospective study.

OBJECTIVES

The aim of this study was to investigate the contribution of laser-evoked potentials (LEPs) and quantitative sensory testing (QST) to the diagnosis of neuropathic pain in patients with spinal cord injury (SCI) and inconclusive magnetic resonance imaging (MRI) findings.

SETTING

A multidisciplinary pain center.

METHODS

QST (DFNS protocol) and Tm-YAG-laser stimulation of the skin were applied within the pain site corresponding with dermatomes of altered sensation. Available MRI scans were reviewed.

RESULTS

Thirteen individuals (50±16 years) with SCI were examined. In four cases with no detectable neural lesion on MRI, all QST but three LEP were abnormal. In four patients with poorly defined spinal lesion on MRI, all QST but three LEP only were abnormal. In four cases where pain was not matching adequately with MRI lesions, all patients had abnormal LEP and QST. In one patient showing a spinal cord atrophy, LEP was normal but QST was abnormal. Findings supported the diagnoses at-level (n=5) and below-level (n=8) SCI pain. Spinothalamic tract function assessed by LEP was normal in three cases, but QST was abnormal in all cases.

CONCLUSIONS

As QST is a psychophysical examination depending on patient cooperation, we suggest that the combination of QST and LEP might be a valuable diagnostic tool to detect lesions of the somatosensory system in a subgroup of patients with neuropathic spinal cord injury pain and inconclusive MRI findings.

Characterization of Motor and Somatosensory Evoked Potentials in the Yucatan Micropig Using Transcranial and Epidural Stimulation

Yucatan micropigs have brain and spinal cord dimensions similar to humans and are useful for certain spinal cord injury (SCI) translational studies. Micropigs are readily trained in behavioral tasks, allowing consistent testing of locomotor loss and recovery. However, there has been little description of their motor and sensory pathway neurophysiology. We established methods to assess motor and sensory cortical evoked potentials in the anesthetized, uninjured state. We also evaluated epidurally evoked motor and sensory stimuli from the T6 and T9 levels, spanning the intended contusion injury epicenter. Response detection frequency, mean latency and amplitude values, and variability of evoked potentials were determined. Somatosensory evoked potentials were reliable and best detected during stimulation of peripheral nerve and epidural stimulation by referencing the lateral cortex to midline Fz. The most reliable hindlimb motor evoked potential (MEP) occurred in tibialis anterior. We found MEPs in forelimb muscles in response to thoracic epidural stimulation likely generated from propriospinal pathways. Cranially stimulated MEPs were easier to evoke in the upper limbs than in the hindlimbs. Autopsy studies revealed substantial variations in cortical morphology between animals. This electrophysiological study establishes that neurophysiological measures can be reliably obtained in micropigs in a time frame compatible with other experimental procedures, such as SCI and transplantation. It underscores the need to better understand the motor control pathways, including the corticospinal tract, to determine which therapeutics are suitable for testing in the pig model.

Spinal cord injury: how can we improve the classification and quantification of its severity and prognosis?

The preservation of functional neural tissue after spinal cord injury (SCI) is the basis for spontaneous neurological recovery. Some injured patients in the acute phase have more potential for recovery than others. This fact is problematic for the construction of clinical trials because enrollment of subjects with variable recovery potential makes it difficult to detect effects, requires large sample sizes, and risks Type II errors. In addition, the current methods to assess injury and recovery are non-quantitative and not sensitive. It is likely that therapeutic combinations will be necessary to cause substantially improved function after SCI, thus we need highly sensitive techniques to evaluate changes in motor, sensory, autonomic and other functions. We review several emerging neurophysiological techniques with high sensitivity. Quantitative methods to evaluate residual tissue sparing after severe acute SCI have not entered widespread clinical use. This reduces the ability to correlate structural preservation with clinical outcome following SCI resulting in enrollment of subjects with varying patterns of tissue preservation and injury into clinical trials. We propose that the inclusion of additional measures of injury severity, pattern, and individual genetic characteristics may enable stratification in clinical trials to make the testing of therapeutic interventions more effective and efficient. New imaging techniques to assess tract injury and demyelination and methods to quantify tissue injury, inflammatory markers, and neuroglial biochemical changes may improve the evaluation of injury severity, and the correlation with neurological outcome, and measure the effects of treatment more robustly than is currently possible. The ability to test such a multimodality approach will require a high degree of collaboration between clinical and research centers and government research support. When the most informative of these assessments is determined, it may be possible to identify patients with substantial recovery potential, improve selection criteria and conduct more efficient clinical trials.

A Rehabilomics framework for personalized and translational rehabilitation research and care for individuals with disabilities: Perspectives and considerations for spinal cord injury

Despite many people having similar clinical presentation, demographic factors, and clinical care, outcome can differ for those sustaining significant injury such as spinal cord injury (SCI) and traumatic brain injury (TBI). In addition to traditional demographic, social, and clinical factors, variability also may be attributable to innate (including genetic, transcriptomic proteomic, epigenetic) biological variation that individuals bring to recovery and their unique response to their care and environment. Technologies collectively called "-omics" enable simultaneous measurement of an enormous number of biomolecules that can capture many potential biological contributors to heterogeneity of injury/disease course and outcome. Due to the nature of injury and complex disease, and its associations with impairment, disability, and recovery, rehabilitation does not lend itself to a singular "protocolized" plan of therapy. Yet, by nature and by necessity, rehabilitation medicine operates as a functional model of "Personalized Care". Thus, the challenge for successful programs of translational rehabilitation care and research is to identify viable approaches to examine broad populations, with varied impairments and functional limitations, and to identify effective treatment responses that incorporate personalized protocols to optimize functional recovery. The Rehabilomics framework is a translational model that provides an "-omics" overlay to the scientific study of rehabilitation processes and multidimensional outcomes. Rehabilomics research provides novel opportunities to evaluate the neurobiology of complex injury or chronic disease and can be used to examine methods and treatments for person-centered care among populations with disabilities. Exemplars for application in SCI and other neurorehabilitation populations are discussed.

Effect of isoflurane on somatosensory evoked potentials in a rat model

Somatosensory evoked potentials (SEPs) are widely used in the clinic as well as research to study the functional integrity of the different parts of sensory pathways. However, most general anesthetics, such as isoflurane, are known to suppress SEPs, which might affect the interpretation of the signals. In animal studies, the usage of anesthetics during SEP measurements is inevitable due to which detailed effect of these drugs on the recordings should be known. In this paper, the effect of isoflurane on SEPs was studied in a rat model. Both time and frequency properties of the cortical recordings generated by stimulating the tibial nerve of rat's hindlimb were investigated at three different isoflurane levels. While the anesthetic agent is shown to generally suppress the amplitude of the SEP, the effect was found to be nonlinear influencing more substantially the latter part of waveform. This finding will potentially help us in future work aiming at separating the effects of anesthetics on SEP from those due to injury in the ascending neural pathways.