Posterior Root-muscle Reflexes Research Articles

9. Multimodal intraoperative evoked potential monitoring (IOM) for hip surgery

Introduction The incidence of sciatic nerve palsy after hip surgery ranges from 2.7% in primary total hip arthroplasty up to 7.6% in developmental dysplasia surgery. So far, intraoperative monitoring using multimodal evoked potential for hip surgeries to assess sciatic and femoral nerves functional integrity has not been described. Material and methods Thirty patients who underwent hip surgery using IOM of sciatic and femoral nerves have been evaluated. All the patients received total intravenous anaesthesia. Multimodal IOM includes: (a) somatosensory evoked potentials (SEP) and (b) motor evoked potentials (MEP) recorded bilaterally in lower extremities limb muscles from quadriceps, tibialis anterior, gastrocnemius, peroneal and abductor hallucis bilaterally. In addition, we also have developed a new IOM technique: (c) posterior root muscle reflex (PRMR) and (d) anterior root muscle response (ARMR). To elicit PRMR and ARMR, percutaneous stimulation of cauda equine was performed by self-adhesive electrodes placed over the skin of the projection of the first and third lumbar interspinous space (anode) and over the abdomen skin of the umbilicus (cathode). A paired stimuli of identical parameters are used with an interstimulus interval of 50 ms and duration of 0.5–1 ms. The recording electrodes for PRMR and ARMR were identical as the electrodes used for recording MEPs. Results Baselines were successful in all patients. Detection of significant changes occured in eight patients (8/30, 26,6%). In six patients (6/30, 20%) changes were reversible and in two patients (2/30, 6,7%) changes were irreversible and correlated with the postoperative neurological deficit. Conclusion Multimodal IOM techniques applied to hip surgery allow identification of the momentthat may damage peripheral nerves of roots before an irreversible injury occurred and/or avoid permanent postoperative neurological deficits. PRMR and ARMR could emerge as a new intraoperative monitoring techniques to assess the functional integrity of lumbosacral plexus and nerves. These new techniques have potential to give more specific information than MEPs and SEPs.

Locomotor rhythm and pattern generating networks of the human lumbar spinal cord: an electrophysiological and computer modeling study

The concept of the neural control of human locomotion has undergone changes in the past decades. In spite of the encephalization and the erect, bipedal mode of walking, independent observations imply that unperturbed locomotor patterns can be generated by similar spinal neural circuits as in other vertebrates [1,2]. However, little is known about the organization of these rhythm and pattern generating networks in humans. It has been shown that the human lumbar spinal cord isolated from supraspinal control due to traumatic spinal cord injury (SCI) can generate rhythmic, locomotor-like activity in response to sustained epidural spinal cord stimulation of certain frequencies [1]. In the supine position, in which the subjects were tested, afferent feedback is minimized. The rhythmic activities consist of a series of stimulus time-related rhythmically modulated posterior-root muscle (PRM) reflexes, each initiated in posterior root afferents and electromyographically recorded as compound muscle action potentials (CMAPs) [3]. The relation between individual stimuli and responses, as well as their characteristics, allow for the identification of mechanisms beyond the information gained from the overall electromyographic (EMG) patterns. Here, EMG activities of quadriceps, hamstrings, tibialis anterior and triceps surae, bilaterally in response to epidural stimulation at 20 Hz-40 Hz were analyzed in 10 individuals with motor complete posttraumatic SCI. Forty segments (duration: 10 s) of rhythmical activities found in all four-muscle groups of one lower limb at the same time were identified in 7 subjects. Phases of bursting and suppressed activities were identified. Latencies of PRM reflexes were calculated. Furthermore, a computational network model of neurons with Hodgkin-Huxley-like membrane dynamics was developed to test whether hypothesized rhythm and pattern generating networks would reproduce the recorded data. A core rhythm-generating network model [4] was extended by adding conduction delays, presynaptic inhibition and dis-inhibition of parallel central pathways. In all 10-s segments, rhythmical activities of all muscle groups had the same cycle frequency. In-between muscles, rhythmic activity occurred either synchronous or alternating, within phases resembling flexion or extension. No other phase-relations were observed. PRM reflexes constituting bursts during the extension phases had monosynaptic latencies. These responses were suppressed during flexion and were replaced by delayed, oligosynaptic PRM reflexes in quadriceps, tibialis anterior and triceps surae. Computer simulation confirmed that a network model with a half-center organization, phasic modulation of the monosynaptic reflex gain, presynaptic control of primary afferents together with the selection of alternative interneuronal pathways (within the flexor half-center) reliably reproduce the electrophysiological findings. The electrophysiological data as well as the computer simulations give insight into the organization of the human spinal rhythm and pattern generating networks and reveal common control characteristics with the central pattern generators for locomotion described in animal experimental work.

Selectivity of transcutaneous stimulation of lumbar posterior roots at different spinal levels in humans

Posterior root-muscle (PRM) reflexes can be elic- ited by epidural as well as recently also transcutaneous elec- trical stimulation of the lumbar spinal cord. We tested the se- lectivity of transcutaneous spinal cord stimulation by eliciting PRM reflexes and recording the compound muscle action po- tentials of the thigh and shank. The stimulation set-up con- sisted of an electrode array with 7 levels centered at the spinal level of T12. With the variation of the stimulation level a defi- nite change in the recruitment of PRM reflexes was observed. Even though the distribution of the generated electrical field with transcutaneous electrodes is unfocused in relation to the target structures, a selective stimulation is possible.

Modification of Reflex Responses to Lumbar Posterior Root Stimulation by Motor Tasks in Healthy Subjects

Dynamic task-dependent regulation of reflexes controlled by the central nervous system plays an integral part in neurocontrol of locomotion. Such modifications of sensory-motor transmission can be studied by conditioning a test reflex with specific motor tasks. To elicit short-latency test reflexes, we applied a novel transcutaneous spinal cord stimulation technique that depolarizes large-diameter posterior root afferents. These responses, termed posterior root-muscle (PRM) reflexes, are equivalent to the monosynaptic Hoffmann (H)-reflex but can be evoked in several muscles simultaneously. We elicited PRM reflexes in quadriceps, hamstrings, tibialis anterior, and triceps surae in subjects with intact nervous system. During three different conditioning-test paradigms in a standing position, that is, volitional unilateral single- and multi-joint lower limb movements and leaning backward/forward, we recorded characteristic movement-induced modulations of PRM reflexes in the thigh and leg muscle groups. We could thus demonstrate that monosynaptic PRM reflexes in functional extensor and flexor muscles of the thigh and leg can be elicited in upright standing subjects and can be modulated during the execution of postural maneuvers. The significance is that transcutaneous posterior root stimulation allows extending H-reflex studies of a single muscle to the assessment of synaptic transmission of two-neuron reflex arcs at multiple segmental levels simultaneously.

Posterior root–muscle reflexes elicited by transcutaneous stimulation of the human lumbosacral cord

Continuous epidural stimulation of lumbar posterior root afferents can modify the activity of lumbar cord networks and motoneurons, resulting in suppression of spasticity or elicitation of locomotor-like movements in spinal cord-injured people. The aim of the present study was to demonstrate that posterior root afferents can also be depolarized by transcutaneous stimulation with moderate stimulus intensities. In healthy subjects, single stimuli applied through surface electrodes placed over the T11-T12 vertebrae with a mean intensity of 28.6 V elicited simultaneous, bilateral monosynaptic reflexes in quadriceps, hamstrings, tibialis anterior, and triceps surae by depolarization of lumbosacral posterior root fibers. The nature of these posterior root-muscle reflexes was demonstrated by the duration of the refractory period, and by modifying the responses with vibration and active and passive movements. Stimulation over the L4-L5 vertebrae selectively depolarized posterior root fibers or additionally activated anterior root fibers within the cauda equina depending on stimulus intensity. Transcutaneous posterior root stimulation with single pulses allows neurophysiological studies of state- and task-dependent modulations of monosynaptic reflexes at multiple segmental levels. Continuous transcutaneous posterior root stimulation represents a novel, non-invasive, neuromodulative approach for individuals with different neurological disorders.