Nitrous oxide and isoflurane are synergistic with respect to amplitude and latency effects on sensory evoked potentials

Intraoperative somatosensory evoked potential (SEP) monitoring: an updated position statement by the American Society of Neurophysiological Monitoring

Somatosensory evoked potentials (SEPs) are used to assess the functional status of somatosensory pathways during surgical procedures and can help protect patients' neurological integrity intraoperatively. This is a position statement on intraoperative SEP monitoring from the American Society of Neurophysiological Monitoring (ASNM) and updates prior ASNM position statements on SEPs from the years 2005 and 2010. This position statement is endorsed by ASNM and serves as an educational service to the neurophysiological community on the recommended use of SEPs as a neurophysiological monitoring tool. It presents the rationale for SEP utilization and its clinical applications. It also covers the relevant anatomy, technical methodology for setup and signal acquisition, signal interpretation, anesthesia and physiological considerations, and documentation and credentialing requirements to optimize SEP monitoring to aid in protecting the nervous system during surgery.

Intraoperative Monitoring of Scoliosis Surgery in Young Patients

SUMMARY

Intraoperative neurophysiologic monitoring has added substantially to the safety of spinal deformity surgery correction since its introduction over four decades ago. Monitoring routinely includes both somatosensory evoked potentials and motor evoked potentials. Either modality alone will detect almost all instances of spinal cord injury during deformity correction. The combined use of the two modalities provides complementary information, can permit more rapidly identification of problems, and enhances safety though parallel redundancy should one modality fail. Both techniques are well established and continue to be refined. Although there is room for provider preference, proper monitoring requires attention to technical detail, understanding of the underlying physiology, and familiarity with effects of commonly used anesthetic agents.

Traumatic Optic Neuropathy: Update on Management

Traumatic optic neuropathy is one of the causes of visual loss caused by blunt or penetrating head trauma and is classified as both direct and indirect. Clinical history and examination findings usually allow for the diagnosis of traumatic optic neuropathy. There is still controversy surrounding the management of traumatic optic neuropathy; some physicians advocate observation alone, while others recommend steroid therapy, surgery, or both. In this entry, we tried to highlight traumatic optic neuropathy's main pathophysiologic mechanisms with the most available updated treatment. Recent research suggests future therapies that may be helpful in traumatic optic neuropathy cases.

Consensus document for multimodal intraoperatory neurophisiological monitoring in neurosurgical procedures. Basic fundamentals

The present work aims to establish a guide to action, agreed by anaesthesiologists and neurophysiologists alike, to perform effective intraoperative neurophysiological monitoring for procedures presenting a risk of functional neurological injury, and neurosurgical procedures. The first section discusses the main techniques currently used for intraoperative neurophysiological monitoring. The second exposes the anaesthetic and non-anaesthetic factors that are likely to affect the electrical records of the nervous system structures. This section is followed by an analysis detailing the adverse effects associated with the most common techniques and their use. Finally, the last section describes a series of guidelines to be followed upon the various intraoperative clinical events.

Gas Anesthesia Impairs Peripheral Auditory Sensitivity in Barn Owls (Tyto alba)

Auditory nerve single-unit recordings were obtained from two groups of young barn owls (age, between posthatching days 11 and 86) in terminal experiments under two different anesthetic regimes: ketamine (6-11 mg/kg) plus xylazine (∼2 mg/kg); or isoflurane (1-1.5%) in oxygen, delivered via artificial respiration. In a second series of minimally invasive experiments, auditory brainstem responses (ABRs) were recorded in the same four adult barn owls (Tyto alba; age, between 5 and 32 months) under three different anesthetic protocols: ketamine (10 mg/kg) plus xylazine (3 mg/kg), isoflurane (1-1.5%), and sevoflurane (2-3%) in carbogen. Finally, the ABR measurements on adult owls were repeated in terminal experiments including more invasive procedures such as artificial respiration and higher isoflurane dosage. The main finding was a significant deterioration of auditory sensitivity in barn owls under gas anesthesia, at the level of the auditory nerve (i.e., a very peripheral level of the auditory system). The effect was drastic in the young animals that experienced threshold elevations in auditory nerve single-unit responses of ≥20 dB. ABR thresholds assessed repeatedly in experiments on adult owls were also significantly higher under isoflurane and sevoflurane, on average by 7 and 15 dB, compared with ketamine/xylazine. This difference already occurred with minimal dosages and was reversibly enlarged with increased isoflurane concentration. Finally, there was evidence for confounding detrimental effects associated with artificial respiration over many hours, which suggested oxygen toxicity.

SSEP in Therapeutic Hypothermia Era

PURPOSE

The reliability of somatosensory evoked potentials (SSEPs) in predicting outcome in comatose survivors of cardiac arrest treated with therapeutic hypothermia (TH) has been questioned. We investigated whether the absence of cortical (N20) responses was a reliable predictor of a nonawakening in the setting of TH.

METHODS

A retrospective review was conducted in cardiac arrest survivors treated with TH admitted to a single tertiary care hospital from April, 2010 to March, 2013 who underwent SSEP testing at various time points after cardiac arrest. N20 responses were categorized as normal, present but abnormal, bilaterally absent, or inadequate for interpretation. Neurologic outcome was assessed at discharge by the Cerebral Performance Category Scale (CPC).

RESULTS

Ninety-three SSEP studies were performed in 73 patients. Fourteen patients had absent N20 responses; all had poor outcome (CPC 4-5). Eleven patients had absent N20 s during hypothermia, three of whom had follow-up SSEPs after rewarming and cortical responses remained absent. Fifty-seven patients had N20 peaks identified and had variable outcomes. Evaluation of 1 or more N20 peaks was limited or inadequate in 11.4% of SSEPs performed during the cooling because of artifact.

CONCLUSIONS

Somatosensory evoked potentials remain a reliable prognostic indicator in patients undergoing TH. The limited sample size of patients who had SSEP performed during TH and repeated after normothermia added to the effect of self-fulfilling prophecy limit the interpretation of the reliability of this testing when performed during cooling. Further prospective, multicenter, large scale studies correlating cortical responses in SSEPs during and after TH are warranted. Technical challenges are commonplace during TH and caution is advised in the interpretation of suboptimal recordings.

A randomized crossover study of the effects of lidocaine on motor- and sensory-evoked potentials during spinal surgery

BACKGROUND CONTEXT

Lidocaine has emerged as a useful adjuvant anesthetic agent for cases requiring intraoperative monitoring of motor-evoked potentials (MEPs) and somatosensory-evoked potentials (SSEPs). A previous retrospective study suggested that lidocaine could be used as a component of propofol-based intravenous anesthesia without adversely affecting MEP or SSEP monitoring, but did not address the effect of the addition of lidocaine on the MEP and SSEP signals of individual patients.

PURPOSE

The purpose of this study was to examine the intrapatient effects of the addition of lidocaine to balanced anesthesia on MEPs and SSEPs during multilevel posterior spinal fusion.

STUDY DESIGN

This is a prospective, two-treatment, two-period crossover randomized controlled trial with a blinded primary outcome assessment.

PATIENT SAMPLE

Forty patients undergoing multilevel posterior spinal fusion were studied.

OUTCOME MEASURES

The primary outcome measures were MEP voltage thresholds and SSEP amplitudes. Secondary outcome measures included isoflurane concentrations and hemodynamic parameters.

METHODS

Each participant received two anesthetic treatments (propofol 50 mcg/kg/h and propofol 25 mcg/kg/h+lidocaine 1 mg/kg/h) along with isoflurane, ketamine, and diazepam. In this manner, each patient served as his or her own control. The order of administration of the two treatments was determined randomly.

RESULTS

There were no significant within-patient differences between MEP threshold voltages or SSEP amplitudes during the two anesthetic treatments.

CONCLUSIONS

Lidocaine may be used as a component of balanced anesthesia during multilevel spinal fusions without adversely affecting the monitoring of SSEPs or MEPs in individual patients.

Effects of different etomidate doses on intraoperative somatosensory-evoked potential monitoring

BACKGROUND

Somatosensory-evoked potentials (SSEPs) are widely used for intraoperative spinal cord monitoring. Although many general anesthetics inhibit SSEPs, etomidate has been reported to boost SSEPs. This clinical study aimed to test whether etomidate doses less than 0.3 mg/kg amplify SSEP monitoring.

METHODS

Patients were divided into four groups: A, B, C, and D. Etomidate doses of 0.1, 0.2, and 0.3 mg/kg were infused into patients in groups A, B, and C, respectively, after baseline SSEPs were obtained. Group D patients were infused with saline. In the subsequent 15 min, the amplitudes and latencies of SSEPs were recorded and compared.

RESULTS

Etomidate exhibited amplification effects on SSEPs, and this effect increased with dose escalation. The amplitude changes in groups A, B, and C were significantly different (P = 0.002, P = 0.000, and P = 0.000, respectively) from that of group D. The amplitude change was largest in group C and significantly greater than those in groups A and B (P = 0.006, P = 0.000). Latency was not significantly affected (P < 0.05) by etomidate.

CONCLUSION

Small doses of etomidate that were less than 0.3 mg/kg had dose-related amplification effects on SSEP monitoring.

Neuromonitoring for scoliosis surgery

The use of intraoperative neuromonitoring (IONM) during pediatric scoliosis repair has become commonplace to reduce the risk of potentially devastating postoperative neurologic deficits. IONM techniques include somatosensory evoked potentials, motor evoked potentials, electromyography, and intraoperative wake-up tests. Special considerations for scoliosis repair in pediatric patients include preexisting neurologic deficits and young patients with immature neural pathways in whom neurophysiologic monitoring may prove difficult or unreliable.

Effects of prophylactic and therapeutic teriflunomide in transcranial magnetic stimulation-induced motor-evoked potentials in the dark agouti rat model of experimental autoimmune encephalomyelitis

Teriflunomide is a once-daily oral immunomodulatory agent recently approved in the United States for the treatment of relapsing multiple sclerosis (RMS). This study investigated neurophysiological deficits in descending spinal cord motor tracts during experimental autoimmune encephalomyelitis (EAE; a model of multiple sclerosis) and the functional effectiveness of prophylactic or therapeutic teriflunomide treatment in preventing the debilitating paralysis observed in this model. Relapsing-remitting EAE was induced in Dark Agouti rats using rat spinal cord homogenate. Animals were treated with oral teriflunomide (10 mg/kg daily) prophylactically, therapeutically, or with vehicle (control). Transcranial magnetic motor-evoked potentials were measured throughout the disease to provide quantitative assessment of the neurophysiological status of descending motor tracts. Axonal damage was quantified histologically by silver staining. Both prophylactic and therapeutic teriflunomide treatment significantly reduced maximum EAE disease scores (P < 0.0001 and P = 0.0001, respectively) compared with vehicle-treated rats. Electrophysiological recordings demonstrated that both teriflunomide treatment regimens prevented a delay in wave-form latency and a decrease in wave-form amplitude compared with that observed in vehicle-treated animals. A significant reduction in axonal loss was observed with both teriflunomide treatment regimens compared with vehicle (P < 0.0001 and P = 0.0014, respectively). The results of this study suggest that therapeutic teriflunomide can prevent the deficits observed in this animal model in descending spinal cord motor tracts. The mechanism behind reduced axonal loss and improved motor function may be primarily the reduced inflammation and consequent demyelination observed in these animals through the known effects of teriflunomide on impairing proliferation of stimulated T cells. These findings may have significant implications for patients with RMS.

Optic nerve monitoring

Orbital and anterior skull base surgery is generally performed close to the prechiasmatic visual pathway, and clear strategies for detecting and handling visual pathway damage are essential. To overcome the common problem of a missed clinical examination because of an uncooperative or unresponsive patient, flash visual evoked potentials and electroretinograms should be used. These electrophysiologic examination techniques can provide evidence of intact, pathologic, or absent conductivity of the visual pathway when clinical assessment is not feasible. Visual evoked potentials and electroretinograms are thus essential diagnostic procedures not only for primary diagnosis but also for intraoperative evaluation. A decision for or against treatment of a visual pathway injury has to be made as fast as possible due to the enormous importance of the time elapsed with such injuries; this can be achieved additionally using multislice spiral computed tomography. The first-line conservative treatment of choice for such injuries is megadose methylprednisolone therapy. Surgery is used to decompress the orbital compartment by exposure of the intracanalicular part of the optic nerve in the case of optic canal compression. Modern craniomaxillofacial surgery requires detailed consideration of the diagnosis and treatment of traumatic visual pathway damage with the ultimate goal of preserving visual acuity.

Effects of the α₂-adrenergic receptor agonist dexmedetomidine on neural, vascular and BOLD fMRI responses in the somatosensory cortex

This article describes the effects of dexmedetomidine (DEX) - the active ingredient of medetomidine, which is the latest popular sedative for functional magnetic resonance imaging (fMRI) in rodents - on multiple unit activity, local field potential (LFP), cerebral blood flow (CBF), pial vessel diameter [indicative of cerebral blood volume (CBV)], and blood oxygenation level-dependent (BOLD) fMRI. These measurements were obtained from the rat somatosensory cortex during 10 s of forepaw stimulation. We found that the continuous intravascular systemic infusion of DEX (50 μg/kg/h, doses typically used in fMRI studies) caused epileptic activities, and that supplemental isoflurane (ISO) administration of ~0.3% helped to suppress the development of epileptic activities and maintained robust neuronal and hemodynamic responses for up to 3 h. Supplemental administration of N(2)O in addition to DEX nearly abolished hemodynamic responses even if neuronal activity remained. Under DEX + ISO anesthesia, spike firing rate and the delta power of LFP increased, whereas beta and gamma power decreased, as compared with ISO-only anesthesia. DEX administration caused pial arteries and veins to constrict nearly equally, resulting in decreases in baseline CBF and CBV. Evoked LFP and CBF responses to forepaw stimulation were largest at a frequency of 8-10 Hz, and a non-linear relationship was observed. Similarly, BOLD fMRI responses measured at 9.4 T were largest at a frequency of 10 Hz. Both pial arteries and veins dilated rapidly (artery, 32.2%; vein, 5.8%), and venous diameter returned to baseline slower than arterial diameter. These results will be useful for designing, conducting and interpreting fMRI experiments under DEX sedation.