Single-fiber EMG study of the flexor carpi radialis H reflex

Advancing Intraoperative Neurophysiological Monitoring With Human Reflexes

Human reflexes are simple motor responses that are automatically elicited by various sensory inputs. These reflexes can provide valuable insights into the functioning of the nervous system, particularly the brainstem and spinal cord. Reflexes involving the brainstem, such as the blink reflex, laryngeal adductor reflex, trigeminal hypoglossal reflex, and masseter H reflex, offer immediate information about the cranial-nerve functionality and the overall state of the brainstem. Similarly, spinal reflexes such as the H reflex of the soleus muscle, posterior root muscle reflexes, and sacral reflexes provide crucial information about the functionality of the spinal cord and peripheral nerves. One of the critical benefits of reflex monitoring is that it can provide continuous feedback without disrupting the surgical process due to no movement being induced in the surgical field. These reflexes can be monitored in real time during surgical procedures to assess the integrity of the nervous system and detect potential neurological damage. It is particularly noteworthy that the reflexes provide motor and sensory information on the functional integrity of nerve fibers and nuclei. This article describes the current techniques used for monitoring various human reflexes and their clinical significance in surgery. We also address important methodological considerations and their impact on surgical safety and patient outcomes. Utilizing these methodologies has the potential to advance or even revolutionize the field of intraoperative continuous monitoring, ultimately leading to improved surgical outcomes and enhanced patient care.

Peripheral Nerve Magnetoneurography With Optically Pumped Magnetometers

Electrodiagnosis is routinely integrated into clinical neurophysiology practice for peripheral nerve disease diagnoses, such as neuropathy, demyelinating disorders, nerve entrapment/impingement, plexopathy, or radiculopathy. Measured with conventional surface electrodes, the propagation of peripheral nerve action potentials along a nerve is the result of ionic current flow which, according to Ampere’s Law, generates a small magnetic field that is also detected as an “action current” by magnetometers, such as superconducting quantum interference device (SQUID) Magnetoencephalography (MEG) systems. Optically pumped magnetometers (OPMs) are an emerging class of quantum magnetic sensors with a demonstrated sensitivity at the 1 fT/√Hz level, capable of cortical action current detection. But OPMs were ostensibly constrained to low bandwidth therefore precluding their use in peripheral nerve electrodiagnosis. With careful OPM bandwidth characterization, we hypothesized OPMs may also detect compound action current signatures consistent with both Sensory Nerve Action Potential (SNAP) and the Hoffmann Reflex (H-Reflex). In as much, our work confirms OPMs enabled with expanded bandwidth can detect the magnetic signature of both the SNAP and H-Reflex. Taken together, OPMs now show potential as an emerging electrodiagnostic tool.

Jendrassik maneuver effect on spinal and brainstem reflexes

The effect of Jendrassik Maneuver (JM) has been extensively studied on monosynaptic reflexes in numerous muscles below the level at which the maneuver was performed. Here we hypothesize that the effect of JM could be observed also on other reflexes, indicating a widespread influence of performing a motor act such as the JM. We examined polysynaptic reflexes caudal (i.e., the withdrawal reflex of the lower extremities) and rostral (i.e., the blink reflex to supraorbital nerve stimulation) to the level of JM contraction. We have assessed soleus tendon (T) reflex; withdrawal reflex in tibialis anterior and soleus muscle; blink reflex (BR), blink reflex excitability recovery curve (BR-ER) and prepulse inhibition of the blink reflex. Our results showed that (1) T-reflex amplitude increased during JM and decreased just after and 15 min after JM; (2) no change in the withdrawal reflex; (3) R2 area of BR reduced significantly just after or 15 min after JM; (4) Prepulse inhibition in BR reduced significantly during JM; (5) no change in BR-ER. Our results indicate that JM leads to generalized effects on neural excitability at both caudal and rostral levels. Furthermore, JM has a selective effect on excitability of reflex circuitries.

F wave, A wave, H reflex, and blink reflex

Late responses include F waves, A waves, H reflex, and the blink reflex. These responses help enhance routine nerve conduction studies. Despite the use of F waves in multiple clinical applications, their studies can technically challenge even the most experienced electromyographers. They vary in latency, amplitude, and configuration, whereas A waves show no change in latency or morphology. Electrical stimulation of the supraorbital branch of the trigeminal nerve on one side results in a reflexive activation of the facial nucleus causing contraction of the orbicularis oculi muscle, short latency R1 ipsilaterally, and long latency R2 bilaterally. F waves can help determine the presence of a polyneuropathy. A waves can reflect axonal damage. H reflexes provide nerve conduction measurements along the entire length of the nerve, demonstrating abnormalities in neuropathies and radiculopathies. Abnormalities in the blink reflex can suggest the presence of an acoustic neuroma or a demyelinating polyneuropathy, which can affect the cranial nerves. This reflex, which also needs appropriate technical expertise, helps to assess cranial nerves V and VII along with their connections in the pons and medulla. The blink reflex, the electrical version of the corneal reflex, represents a polysynaptic reflex.

Single-fiber F waves compared with conventional surface F waves, and their utility in detecting early diabetic neuropathy

INTRODUCTION

The single-fiber F-wave (SFF-wave) technique assesses the entire length of single motor fibers using a concentric needle. Herein we investigated the utility of this approach in the detection of early diabetes-related neuropathy, and compared it with the use of conventional surface F waves (CF waves).

METHODS

Sixteen patients with diabetes and either no neuropathy or mild neuropathy were assessed and compared with 16 age- and height-matched control participants.

RESULTS

Both CF and SFF waves were abnormal in all 5 patients who had mild neuropathy. However, SFF waves demonstrated subclinical abnormalities in 7 of 11 patients (64%) with no neuropathy, whereas only 2 of these patients (18%) had prolonged CF waves. Minimum F-wave latency was comparable between techniques, but maximum SFF-wave latency was more frequently prolonged, as these delayed motor units were better isolated, rather than buried among summated CF-wave responses.

DISCUSSION

SFF waves highlight the segmental involvement in diabetic neuropathy, and use of the SFF-wave technique detects more abnormalities than with CF waves. Muscle Nerve 58: 665-670, 2018.

Can the F-Response be Volitionally Repressed during Functional Electrical Stimulation?

Objective  The purpose of this study was to test if the F-response can be repressed volitionally. Normally, the F-response is used for clinical diagnostics but it also has an important influence on the design of a neural prosthesis involving functional electrical stimulation (FES) and the use of volitional myoelectric signal (MES) for control. Methods  Ten neurologically normal subjects were trained to reduce the level of the F-response from the anterior tibial muscle. The nerve to the anterior tibial (TA) muscle was stimulated with constant intensity and frequency (16.6 pulses per second) and the surface myoelectric signal (MES) from the muscle was digitally processed to estimate the F-level. Training was carried out by giving the subject visual feedback on a computer screen of the F-level during the stimulation with the task of keeping the level as low as possible. Each subject had five sessions consisting of 20 stimulation tests, lasting 30 sec each. The subjects acted as their own control and changes in the F-level during the stimulation tests, sessions, and trials, were analyzed. Results  There was a significant (p < 0.001) increase in the F-response level within the test period of constant stimulation, but a significant (p < 0.001) decrease from the first to the last test in the session was found. From the first to the last session of a trial, the change was found not significant. Conclusion  The level of the F-response may change locally, but there is no indication that a subject can volitionally learn to repress the response, even when given feedback information about the actual level. Therefore the F-waves in the myoelectric signal from a stimulated muscle has to be accounted for when designing devices using a stimulated muscle response for myoelectric control such as eliminating the F-interval from the recorded signal.

Spinal reflex excitability changes after cervical and lumbar spinal manipulation: a comparative study

BACKGROUND CONTEXT

Spinal manipulation (SM) is a commonly employed nonoperative treatment modality in the management of patients with neck, low back or pelvic pain. One basic physiologic response to SM is a transient decrease in motoneuron activity as assessed using the Hoffmann reflex (H-reflex) technique. Previous research from our laboratory indicates that both SM with a high-velocity, low-amplitude thrust and mobilization without thrust produced a profound but transient attenuation of motoneuronal activity of the lumbosacral spine in asymptomatic subjects. To date, effects of cervical SM procedures on the excitability cervical motoneuron pools are unknown.

PURPOSE

The objective of this research was to a gain a more complete understanding of the physiologic effects of SM procedures on motoneuron activity, by comparing the effects of regional SM on cervical and lumbar motoneuron pool excitability.

STUDY DESIGN/SETTING

Maximal H-reflex amplitudes were recorded before and after SM in both the cervical and lumbar regions of asymptomatic subjects in two successive experimental sessions.

PATIENT SAMPLE

Asymptomatic, young healthy volunteers were used in this study.

OUTCOME MEASURES

Changes in flexor carpi radialis and gastrocnemius H-reflex amplitudes before and after SM procedures.

METHODS

H-reflexes recorded form the tibial and median nerves were evaluated before and after lumbar and cervical SM, respectively.

RESULTS

Both Lumbar and cervical SM produced a transient but significant attenuation of motoneuron excitability. The attenuation of the tibial nerve H-reflex amplitude was proportionately greater than that of the median nerve, which occurred after cervical SM.

CONCLUSIONS

SM procedures lead to transient suppression of motoneuron excitability, as assessed by the H-reflex technique. Lumbar spine SM appears to lead to greater attenuation of motoneuron activity compared with that of the cervical region. Thus, these two distinct regions of the spine may possess different responsiveness levels to spinal manipulative therapy.

Clinical impact of single-fiber electromyography

The major clinical impact of single-fiber electromyography has been from its role in confirming, or excluding, the diagnosis of myasthenia gravis (MG). Jitter measurements also have a clinical role in demonstrating changes in disease severity in patients with MG and Lambert-Eaton myasthenic syndrome, in demonstrating subtle changes in motor unit architecture and physiology in patients with nerve and muscle diseases, and in demonstrating the remote effects of locally injected botulinum toxin. In addition to these clinical roles, the ability to identify the activity from single muscle fibers makes it possible to mark the discharges of single motor units. This, along with information gained by jitter and fiber-density measurements, has uniquely increased our understanding of motor unit organization and function in normal and disease states.

[Importance of neuromuscular "jitter" under stimulation in the diagnosis of myasthenia gravis]

Ninety five patients with global muscular weakness or purely extraocular weakness were included in a retrospective study. Electrical micro stimulation and single fiber electromyography were performed in all, for neuromuscular jitter evaluation in myasthenia gravis diagnosis. In our study, increased jitter was more often present (70% of generalised myasthenia gravis and 57% of ocular myasthenia gravis) than decrement after repetitive nerve stimulations (58% of generalised myasthenia gravis and 14% of ocular myasthenia gravis). Increased jitter was also found in non-myasthenic patients. With the aim of a better sensitivity and specificity of the electrophysiological diagnosis for myasthenia gravis a protocol is described.

Late facilitation of the human soleus H reflex induced by sustained isometric maneuver

Studying the effect of spinal cord reinforcement maneuvers (SCRMs) on H reflex assists in understanding aspects of motor control. Our objective was as follows: (1) to elucidate the effects of four neck positions (neck resting at neutral position (control); passive hyperflexion of the neck; hyperextension of the neck with simultaneous abdominal contraction; and sustained active neck hyperflexion); (2) to evaluate the temporal changes of soleus H reflexes repeatedly evoked after a period of sustained neck flexion. We used a prospective, intrinsically controlled trial of the effects of these SCRMs on the H reflexes and M-responses in ten healthy volunteers. Pre- and postmaneuver measures included H reflex and M-response latencies and amplitudes, H/M maximum amplitude ratio, and H threshold. The four maneuvers showed no significant effect on the H reflex or M-response measures. To investigate temporal changes in the H reflex amplitude, H reflexes were repeatedly evoked at two-minute intervals after a one-minute period of active neck flexion. The amplitude of the H reflex was enhanced (P = 0.0356; analysis of variance), and the post hoc least significant difference test was significant at four minutes postmaneuver. Peak magnitude of the H reflex occurred at four minutes after relaxation, and the response returned to pretest baseline at eight minutes. The results of this study document the time course of repeated H reflexes after SCRM, and the timing of the H reflex was found to be a contributing variable that should be considered in future study designs.

Correlates of motor unit size, recruitment threshold, and H-reflex jitter

The aim of this research was to study the neurophysiology of the anterior horn cell (AHC) using single-fiber EMG (SFEMG) study of the flexor carpi radialis (FCR) H-reflex. Twelve men and 7 women, ages 20-80 years, were studied. The mean H-jitter was 138 +/- 59 microseconds. H-jitter increased with age (while the M-jitter did not) and was greater in men than in women. There was a direct correlation between the H-jitter and H-latency which was used as an indirect measure of the AHC's size. Given that small AHCs have a higher input resistance than large ones, the H-jitter can be used as an indirect indicator of the AHC's input resistance and therefore its size. When subjects fell asleep, the H-jitter increased over tenfold the baseline value, raising the possibility of an alternative, oligosynaptic pathway. H-reflex jitter studies provide a useful clinical neurophysiological tool for the study of AHC physiology.

Single motor unit H-reflex in motor neuron disorders

The latency fluctuation of single motor unit potentials (MUPH) in the H-reflex is greater than the latency fluctuation of MUPs in the direct (MUPM) and recurrent (MUPF) responses. This has been attributed to the variability in the impulse generation at the site of nerve stimulation, and to the variation in the synaptic delay at the anterior horn cell. We studied the latency fluctuation of single motor unit H-reflex in patients with motor neuron disorders (MND) in comparison with normal subjects. The mean jitter of the H-reflex was 264.3 +/- 17.8 microseconds (mean +/- SEM) in 30 MUPH recorded from 10 patients with ALS, 302.7 +/- 25.2 microseconds in 16 MUPH from 6 patients with chronic motor neuron diseases, as compared with 137.4 +/- 7.3 microseconds in 34 MUPH recorded from 10 normal subjects. This difference, which persisted even after the correction for the latency variation of MUPM, cannot be explained on the basis of an enhanced reciprocal inhibition. Thus, the increased latency fluctuation of the single motor unit H-reflex in patients with MND may reflect changes in the motoneuron pool excitability that may be secondary to altered intrinsic electrophysiological properties of motoneurons, or to an abnormal temporal and spatial summation of synaptic inputs on motoneurons.