Electrophysiologic evaluation of lumbosacral single nerve roots using compound muscle action potentials

Transcutaneous electrical stimulation applied to the vertebral column produces compound muscle action potentials (CMAPs) from the leg muscles. Using this method, we evaluated the efferent pathways of the lumbosacral nerve roots. The subjects were 26 healthy volunteers and 31 patients with lumbar disc herniation (LDH). CMAP recordings were obtained from the bilateral vastus medialis, tibialis anterior, extensor digitorum brevis, and abductor hallucis muscles using low-output-impedance stimulation. In normal subjects, the CMAP latency increased linearly with the distance between the stimulating electrode and the recording electrode, with little difference in latency between the left and the right sides in each subject. The CMAP amplitude was significantly lower in the patients with LDH, and the latency was also prolonged when the stimulating electrode was placed above the lesion. This technique may thus be a useful noninvasive method for assessing lumbosacral nerve root function in patients with LDH.     

Neurophysiological tests of respiratory function by compound muscle action potentials from the diaphragm. Detection of lesions in the higher cervical cord

We recorded compound muscle action potentials (CMAPs) from the diaphragm in 15 normal volunteers, nine patients with lesions of the lower cervical cord (C5 to C8), one completely quadriplegic patient (C6) and seven patients with lesions at a higher cervical level (C1 to C4). Transcranial magnetic stimulation and electrical stimulation of the phrenic nerve were carried out. When the centre of the coil was placed on the interauricular line at a point 3 cm lateral to the vertex on the scalp, the CMAPs from the diaphragm had the largest amplitude and the shortest latency. There was no difference in the mean latency of the CMAPs recorded by transcranial magnetic stimulation in the normal volunteers and in the patients with lesions of the lower cervical cord. In the quadriplegic patient, the latency of the CMAPs was not delayed, but was prolonged in the patients with lesions at a higher level. Those evoked by electrical stimulation of the phrenic nerve were not prolonged in the patients with higher lesions. Our findings suggest that the prolongation of the latency by transcranial magnetic stimulation reflects dysfunction of the higher cervical cord. The combination of transcranial magnetic stimulation and electrical stimulation of the phrenic nerve can detect the precise level of the lesion in the motor tract to the diaphragm.     

Subdural air limits the elicitation of compound muscle action potentials by high-frequency transcranial electrical stimulation

High-frequency transcranial electrical stimulation was performed in 8 patients undergoing surgery in the sitting position. Following the opening of the dura of the posterior fossa changes in compound muscle action potentials were observed. These changes were not attributable to surgical manoeuvres at the brain stem or spinal cord, or to anaesthetic changes. In all these cases intraoperative fluoroscopy of the skull revealed a subdural air collection underneath the stimulation electrodes. Such a subdural air collection, not infrequent in patients operated on in the sitting position, limits the application of high-frequency transcranial electrical stimulation as a monitoring technique. It remains unclear if this effect is due to the increasing distance between scalp and cortex and the insulating effect of subdural air, or due to displacement of the motor cortex. The practical importance of this report is derived from the increasing application of intraoperative motor pathway monitoring.     

Post-tetanic transcranial motor evoked potentials augment the amplitude of compound muscle action potentials recorded from innervated and non-innervated muscles

Background Context

Transcranial electrical stimulation used to produce motor evoked potentials (TES-MEPs) and subsequent compound muscle action potential (CMAP) recording is widely used to monitor motor function during surgery when there is risk of damaging the spinal cord. Nonetheless, some muscles do not produce CMAP amplitudes sufficient for intraoperative monitoring.

Diagnostic utility of waveform analysis of compound muscle action potentials for carpal tunnel syndrome

PURPOSE: To determine the diagnostic utility of waveform analysis of compound muscle action potentials (CMAP) for carpal tunnel syndrome (CTS). METHODS: A total of 131 hands in 71 patients diagnosed with CTS (grouped according to severity) and 80 hands in 44 normal subjects were evaluated using nerve conduction test through the carpal tunnel combined with waveform analysis of CMAP. RESULTS: Compared to normal subjects, the sensory nerve conduction velocity and mean frequency of the CMAP waveform were significantly reduced in patients with CTS. Compared with distal motor latency and sensory nerve conduction velocity, the mean frequency of the CMAP decreased significantly with increasing clinical severity. CONCLUSION: This study suggests that waveform analysis of CMAP is of diagnostic value in CTS, and is also of value in objective evaluation of postoperative recovery of carpal median nerve dysfunction.     

Effect of spinal cord ischemia on compound muscle action potentials and spinal evoked potentials following spinal cord stimulation in the dog

We recorded the compound muscle action potentials (CMAP) from the soleus muscle and spinal evoked potentials (SpEP) from the caudal spinal cord after stimulation of the rostral spinal cord via epidural electrodes in 18 dogs. We examined the changes of CMAP and SpEP after the cord was made ischemic by clamping the aorta at different levels. Clamping the abdominal aorta below the artery of Adamkiewicz (five dogs) did not change either CMAP or SpEP significantly. When the thoracic aorta was clamped above the radicular and Adamkiewicz arteries (10 dogs), CMAP disappeared entirely within 5-10 min of occlusion, but SpEP showed only minimal latency prolongation limited to the late peaks in seven of 10 dogs. In the remaining three dogs, both CMAP and SpEP disappeared within 30 min of occlusion but with the more rapid deterioration of CMAP than of SpEP. Progressive ligation of lumbar arteries (three dogs) from caudal to rostral levels did not change CMAP until ligation of Adamkiewicz artery. Then CMAP disappeared, but SpEP remained unchanged. These findings suggest that CMAP and SpEP are mediated through different pathways and that CMAP reflect anterior cord function. The described electrophysiologic technique would be useful to monitor spinal cord motor function during surgery of the spine, spinal cord, or thoracoabdominal aorta.     

A CT-based study investigating the relationship between pedicle screw placement and stimulation threshold of compound muscle action potentials measured by intraoperative neurophysiological monitoring

Purpose

Neurophysiological monitoring aims to improve the safety of pedicle screw placement, but few quantitative studies assess specificity and sensitivity. In this study, screw placement within the pedicle is measured (post-op CT scan, horizontal and vertical distance from the screw edge to the surface of the pedicle) and correlated with intraoperative neurophysiological stimulation thresholds.

Methods

A single surgeon placed 68 thoracic and 136 lumbar screws in 30 consecutive patients during instrumented fusion under EMG control. The female to male ratio was 1.6 and the average age was 61.3 years (SD 17.7). Radiological measurements, blinded to stimulation threshold, were done on reformatted CT reconstructions using OsiriX software. A standard deviation of the screw position of 2.8 mm was determined from pilot measurements, and a 1 mm of screw—pedicle edge distance was considered as a difference of interest (standardised difference of 0.35) leading to a power of the study of 75 % (significance level 0.05).

Results

Correct placement and stimulation thresholds above 10 mA were found in 71 % of screws. Twenty-two percent of screws caused cortical breach, 80 % of these had stimulation thresholds above 10 mA (sensitivity 20 %, specificity 90 %). True prediction of correct position of the screw was more frequent for lumbar than for thoracic screws.

Conclusion

A screw stimulation threshold of >10 mA does not indicate correct pedicle screw placement. A hypothesised gradual decrease of screw stimulation thresholds was not observed as screw placement approaches the nerve root. Aside from a robust threshold of 2 mA indicating direct contact with nervous tissue, a secondary threshold appears to depend on patients’ pathology and surgical conditions.     

The effect of anesthetic agents on descending spinal cord evoked potential and the compound muscle action potentials elicited by stimulation at the cerebral motor cortex and the spinal cord]

Recently, intraoperative monitoring of the motor tract by descending spinal cord motor evoked potentials (MEP) and compound muscle action potentials (CMAP) has been applied in clinical testing. Since several reports have mentioned the vulnerability of these potentials to anesthetic agents, experimental studies were carried out on the relationship between these potentials and anesthesia using 41 adult cats. The effects of anesthesia on changes in amplitude of the direct wave (D wave) and indirect wave (I wave) of the MEP and CMAPs were investigated. These potentials were generated by stimulation of the spinal cord and the motor cortex, respectively. Enflurane (2%), halothane (1%) and isoflurane (1.5%) with pure oxygen decreased the amplitude of the I wave to less than 50% of the control level. The CMAP after stimulation of the spinal cord was degraded to less than 30%, and the CMAP after cortical stimulation vanished completely. Only the D wave was stable against inhalational anesthetic agents. Sixty-seven percent nitrous oxide with the above concentrations of these inhalational anesthetic agents decreased the amplitude of the I wave to less than 30% and the CMAP evoked by spinal cord stimulation vanished. The effect of modified NLA (diazepam and pentazocine) on these potentials was weaker than that of the inhalational anesthetic agents.     

Stepping-like movements in humans with complete spinal cord injury induced by epidural stimulation of the lumbar cord: electromyographic study of compound muscle action potentials

STUDY DESIGN: It has been previously demonstrated that sustained nonpatterned electric stimulation of the posterior lumbar spinal cord from the epidural space can induce stepping-like movements in subjects with chronic, complete spinal cord injury. In the present paper, we explore physiologically related components of electromyographic (EMG) recordings during the induced stepping-like activity. OBJECTIVES: To examine mechanisms underlying the stepping-like movements activated by electrical epidural stimulation of posterior lumbar cord structures. MATERIALS AND METHODS: The study is based on the assessment of epidural stimulation to control spasticity by simultaneous recordings of the electromyographic activity of quadriceps, hamstrings, tibialis anterior, and triceps surae. We examined induced muscle responses to stimulation frequencies of 2.2-50 Hz in 10 subjects classified as having a motor complete spinal cord injury (ASIA A and B). We evaluated stimulus-triggered time windows 50 ms in length from the original EMG traces. Stimulus-evoked compound muscle action potentials (CMAPs) were analyzed with reference to latency, amplitude, and shape. RESULTS: Epidural stimulation of the posterior lumbosacral cord recruited lower limb muscles in a segmental-selective way, which was characteristic for posterior root stimulation. A 2.2 Hz stimulation elicited stimulus-coupled CMAPs of short latency which were approximately half that of phasic stretch reflex latencies for the respective muscle groups. EMG amplitudes were stimulus-strength dependent. Stimulation at 5-15 and 25-50 Hz elicited sustained tonic and rhythmic activity, respectively, and initiated lower limb extension or stepping-like movements representing different levels of muscle synergies. All EMG responses, even during burst-style phases were composed of separate stimulus-triggered CMAPs with characteristic amplitude modulations. During burst-style phases, a significant increase of CMAP latencies by about 10 ms was observed. CONCLUSION: The muscle activity evoked by epidural lumbar cord stimulation as described in the present study was initiated within the posterior roots. These posterior roots muscle reflex responses (PRMRRs) to 2.2 Hz stimulation were routed through monosynaptic pathways. Sustained stimulation at 5-50 Hz engaged central spinal PRMRR components. We propose that repeated volleys delivered to the lumbar cord via the posterior roots can effectively modify the central state of spinal circuits by temporarily combining them into functional units generating integrated motor behavior of sustained extension and rhythmic flexion/extension movements. This study opens the possibility for developing neuroprostheses for activation of inherent spinal networks involved in generating functional synergistic movements using a single electrode implanted in a localized and stable region.     

An experimental study of postoperative monitoring for innervated free muscle graft by the compound muscle action potential in rabbits

This experiment establishes the principles of using the compound muscle action potential (CMAP) as a possible postoperative monitor for free muscle grafts. Twenty rabbits were divided into two groups of ten each to investigate the effects of ischemia on CMAP of the muscles. Rectus femoris model was used and contralateral muscle was used as control. In all muscles total normothermic ischemia of 1.5 hours to mimic the time needed for transfer and inset of the flap was followed by occlusion of the artery in one group and vein in another group after 3 hours. During this ischemia of 1 hour, the CMAP amplitudes decreased and the latencies were prolonged. Latency prolongation was detected within 10 minutes of total, arterial, or venous ischemia. During the revascularization, both amplitude and latency improved, but not to the original values at the start. The results show that CMAP monitoring can provide easily detectable, objective indication of vascular compromise to a muscle graft within as early as 10 minutes of total, arterial, and venous ischemia. Changes in latency are more constant and predictable compared with amplitude changes. This method can provide continuous monitoring and can be used in buried muscle grafts.     

The effect of septic shock on skeletal muscle action potentials in the primate.

Techniques developed for the in vivo study of cellular physiology have been applied to septic shock in primates. Measurements of skeletal muscle transmembrane resting and action potentials were correlated with an analysis of fluid and electrolyte changes in the intracellular and extracellular compartments of skeletal muscle. The data obtained indicated a marked depletion of muscle extracellular water and an increase in intracellular sodium chloride and water content during shock. The significant decrease of resting membrane potential was associated with a decrease in amplitude of the action potential and prolongation of both the repolarization and depolarization time. In addition, there was a decrease of muscle intracellular potassium concentration during shock. This study demonstrates that the alterations in cellular membranes in hemorrhagic shock and septic shock are similar.     

Innervation of the trapezius muscle by the intra-operative measurement of motor action potentials

Although the surgical anatomy of the spinal accessory nerve and the cervical plexus has been extensively described, the exact motor innervation of the trapezius has been controversial. Attempts to resolve this question have involved anatomic or electrophysiologic studies in human embryos and animals. Extrapolation of the results to adult humans may not be correct. Accurate identification of muscle innervation is obtainable by intra-operative measurement of motor action potentials produced by direct stimulation of the accessory nerve and the cervical plexus. The study involved 14 patients undergoing supraomohyoid or modified neck dissections. Under direct vision, stimulating electrodes were placed on the identified nerves and motor action potentials, and latencies were recorded by surface electrodes placed over the three portions of the trapezius. In 13 patients, when the accessory nerve was stimulated, motor action potentials were obtained in 13 of 13 in the first portion, 11 of 13 in the second portion, and 10 of 13 in the third portion of the trapezius. In the last patient, the accessory nerve ended in the sternocleidomastoid muscle, and innervation of the trapezius was via C3 as demonstrated by motor action potentials. Responses when the roots of the cervical plexus were stimulated varied. Three patterns were seen: In the first group (seven patients), motor action potentials were distinct from those recorded when the accessory nerve was stimulated. Additionally, latencies were different from those of the accessory nerve. The second group (four patients) had motor action potentials that were similar to those obtained from stimulation of the accessory nerve, although their corresponding latencies were different. In two patients, no motor action potentials were recorded when the cervical plexus was stimulated. The results suggest that motor innervation of the trapezius is variable. The accessory nerve, when present, provides the most important input to the trapezius. Motor innervation from the cervical plexus is unpredictable, although it appears to be present in the majority of patient studies.     

An electrophysiological study of the smooth muscle of the human colon.

Electrical recordings were made in vitro from preparations of human colonic smooth muscle from surgically resected specimens. The behaviour of the taenia consisted of regular spike action potentials based on a slow wave rhythm (22 +/- 5 c.p.m.), with tetanic contractions of the muscle. The actions of cholinergic drugs were studied and experiments performed to investigate the mechanism of the action potentials. The circular muscle produced clusters of spikes with solitary contractions. The differences between the two muscle layers may be of relevance to understanding the colonic electromyogram as recorded in vivo.