Comparison of two techniques to measure the motor nerve conduction velocity distribution

Motor fiber function in spinal muscular atrophy-analysis of conduction velocity distribution

Objectives

The motor neuron survival protein, which is deficient in spinal muscular atrophy (SMA), performs numerous cellular functions. Currently, SMA is believed to be a multi-organ disease, including lesion of various structures of the central and peripheral nervous systems. Motor nerve damage, especially in milder SMA types, is controversial. This prompted the conduct of the electrophysiological studies in adults with SMA types 2 and 3 presented in this paper.

Methods

The study group consisted of 44 adult patients with SMA types 2 and 3. All patients underwent neurological examination with Hammersmith Functional Motor Scale-Expanded (HFMSE) assessment. Standard electrophysiological studies in the ulnar nerve and conduction velocity distribution (CVD) tests were performed in all patients and controls.

Results

A prolongation of the distal latency and lowering of the motor potential amplitude with no changes in CVD were found in the whole patient group. There were no dependencies on the number of gene copies. Patients with low HFSME value had slower standard conduction velocity, CVD in upper and median quartiles, and narrower CVD spread; in milder SMA, CVD spread was greater than in controls.

Interpretation

The significant reduction in motor response amplitude in SMA seems to be primarily related to motor neuron loss and directly proportional to its severity. The coexisting rearrangement in the peripheral nerve structure is present in SMA, and this could be partially caused by a coexisting demyelinating process. Nerve remodeling mainly affects large fibers and occurs in more severe SMA types with significant disability.

The Future of Neurotoxicology: A Neuroelectrophysiological Viewpoint

Neuroelectrophysiology is an old science, dating to the 18th century when electrical activity in nerves was discovered. Such discoveries have led to a variety of neurophysiological techniques, ranging from basic neuroscience to clinical applications. These clinical applications allow assessment of complex neurological functions such as (but not limited to) sensory perception (vision, hearing, somatosensory function), and muscle function. The ability to use similar techniques in both humans and animal models increases the ability to perform mechanistic research to investigate neurological problems. Good animal to human homology of many neurophysiological systems facilitates interpretation of data to provide cause-effect linkages to epidemiological findings. Mechanistic cellular research to screen for toxicity often includes gaps between cellular and whole animal/person neurophysiological changes, preventing understanding of the complete function of the nervous system. Building Adverse Outcome Pathways (AOPs) will allow us to begin to identify brain regions, timelines, neurotransmitters, etc. that may be Key Events (KE) in the Adverse Outcomes (AO). This requires an integrated strategy, from in vitro to in vivo (and hypothesis generation, testing, revision). Scientists need to determine intermediate levels of nervous system organization that are related to an AO and work both upstream and downstream using mechanistic approaches. Possibly more than any other organ, the brain will require networks of pathways/AOPs to allow sufficient predictive accuracy. Advancements in neurobiological techniques should be incorporated into these AOP-base neurotoxicological assessments, including interactions between many regions of the brain simultaneously. Coupled with advancements in optogenetic manipulation, complex functions of the nervous system (such as acquisition, attention, sensory perception, etc.) can be examined in real time. The integration of neurophysiological changes with changes in gene/protein expression can begin to provide the mechanistic underpinnings for biological changes. Establishment of linkages between changes in cellular physiology and those at the level of the AO will allow construction of biological pathways (AOPs) and allow development of higher throughput assays to test for changes to critical physiological circuits. To allow mechanistic/predictive toxicology of the nervous system to be protective of human populations, neuroelectrophysiology has a critical role in our future.

SOMATOSENSORY CONDUCTION VELOCITY DISTRIBUTION OF MEDIAN NERVE MIDDLE PALMAR DIGITAL COMPONENT

Collision technique is one of the methods used to obtain the relative number of fibers in a nerve bundle. In 25 normal subjects, the right median nerve has been concurrently stimulated at proximal (elbow) and distal (wrist) locations, and the resultant compound action potentials (CAP) were recorded at the middle finger via ring electrodes. The delay between the two stimuli (Inter Stimulus Interval; ISI), beginning from 7 ms, has been decreased in 0.1 ms steps, until the CAPs, elicited by proximal stimulation, totally disappeared. The obtained data have been transferred to computer medium for further analysis. In this procedure, areas under proximal CAPs have been obtained for each ISI value. Using these areas, the relative numbers of fibers (%) belonging to the middle proper palmar digital (MPPD) component of sensory median nerve have been derived. The mean conduction velocities in MPPD component of sensory median nerve ranged from 40 m/s to 68 m/s. In the histogram, a large amount of heaping of the relative number of fibers has been observed in 48-59 m/s conduction velocity interval with the ratio of 64%, although there has been a 21% group having 43-47 ms conduction velocity. These results can be a guide to future studies concerning basic and clinical nerve conduction studies.

DEEP PERONEAL MOTOR NERVE CONDUCTION VELOCITY DISTRIBUTION AND CORRELATION BETWEEN NERVE CONDUCTION GROUPS AND THE NUMBER OF INNERVATED MUSCLE FIBERS

In this study, the distribution of peroneal-nerve conduction velocity was studied in 17 normal subjects, using the collision method. Paired supramaximal stimuli with predetermined interstimulus intervals (ISI) were applied at distal and proximal points of peroneal nerve and the resultant compound muscle action potentials (CMAPs) were recorded. The change in CMAP amplitudes and areas with ISI were deduced, and the relative number of fibers corresponding to each conduction velocity group (CVG) were computed. Conduction velocities of the peroneal motor nerve innervating the Extensor Digitorum Brevis (EDB) muscle were found to be in the range of 28-52 m/s and CVG innervating the greatest number appears to be in 40-48 m/s range, which consists of 70% of all fibers. These results show that, compared with the median motor nerve, deep peroneal motor nerve that innervates the EDB muscle consist of slow fibers.

The recruitment order of electrically activated motor neurons investigated with a novel collision technique

OBJECTIVE

The development of a novel collision technique for assessment of the activation order of electrically activated nerve fibers, which is an important question in functional electrical therapy or for interpretation of results of motor unit number estimates.

METHODS

Compound muscle action potentials were recorded with the belly-tendon configuration from the abductor digiti minimi. A novel modified Hopf's collision technique was applied on ten healthy male subjects to determine the distributions of conduction velocities (DCV) of all ulnar nerve fibers and of the fibers activated by electrical stimuli eliciting 20%, 50%, and 80% of the maximal muscle response.

RESULTS

The maximum nerve conduction velocity was (means+/-SE) 64.1+/-0.85m/s. The median conduction velocity of estimated DCV was 58.9+/-0.97m/s (stimulus at 20%), 58.0+/-0.98m/s (50%), 57.2+/-0.91m/s (80%), and 56.5+/-0.84m/s (whole nerve) (all different between each other, P<0.001).

CONCLUSIONS

The proposed collision technique allows the assessment of nerve conduction velocity distributions at maximal and sub-maximal stimulation levels and provided evidence for an inverse activation order of nerve fibers with electrical stimulation.

SIGNIFICANCE

The excessive fatigue seen with nerve electrical stimulation can be explained by a preferential activation of large diameter nerve fibers. The motor units first activated with electrical stimulation are likely not representative of the motor unit pool in the muscle, which poses limitations in the reliability of some of the proposed methods for motor unit counting.

Nerve conduction studies in relation to residual fatigue in Guillain-Barré syndrome

Many Guillain-Barré syndrome (GBS) and Chronic Inflammatory Demyelinating Polyneuropathy (CIDP) patients recover well, but suffer from excessive fatigue, which may persist for years and reduce the quality of life considerably. In order to determine whether residual subclinical peripheral nerve dysfunction is a possible underlying mechanism of fatigue, we performed standardized nerve conduction (NC) studies in 16 fatigued patients, mean 6.5 years after diagnosis. Thirteen were relatively well recovered from GBS and 3 had stable CIDP. In contrast to CIDP, most NC values in GBS patients were remarkably restored and within normal values. No correlations were found between the electrophysiological findings and the fatigue scores,muscle strength, or functional scores. This study demonstrates that fatigue in GBS is not explained by residual nerve dysfunction, using conventional NC measurements.

Motor unit conduction velocity distribution estimation from evoked motor responses

Action potentials travel along the muscle fibers with a specific conduction velocity that depends on their structural and functional properties. Only the estimation of muscle conduction velocity distribution (MCVD) may be able to depict this propagation heterogeneity. Based on the method proposed by Cummins et al. (Electroenceph Clin Neurophysiol, 46:647-658, 1979) to estimate nerve conduction velocity distribution (NCVD), the present paper proposes a method that modifies the Cummins' approach to make it suitable for MCVD estimation from electrically evoked motor responses. The MCVD estimation algorithm was first assessed by means of simulated signals in order to control all signal features during the optimization process. Simulations showed that estimated distributions were very close to the true ones when taking into account the specificities of the muscle action potential, due to its generation and extinction (MSE divided by 5 on distribution standard deviation). This method was then applied to real signals. Elicited motor responses were recorded on the biceps brachii of healthy subjects either during repeated maximal stimulations at 20 Hz or during increasing intensity stimulations at 1 Hz. MCVD estimates were used to analyze fatigue and motor unit recruitment processes, respectively.

Conduction velocity distribution in neurologically well-recovered but fatigued Guillain–Barré syndrome patients

Many patients with Guillain-Barré syndrome (GBS) and chronic inflammatory demyelinating polyneuropathy (CIDP) suffer from excessive fatigue. To assess whether this fatigue might be related to changes in slow-conducting nerve fibers, we determined the conduction velocity distribution (CVD) in the median nerve. Thirteen fatigued but neurologically well-recovered GBS patients, 2 fatigued and stable CIDP patients, and 19 healthy controls participated in this study. Conventional maximal nerve conduction velocities (NCVs) did not show differences between GBS patients and healthy controls. However, in both GBS and CIDP patients the CVD was altered, showing significant narrowing of the velocity distribution with loss of the fastest- and slowest-conducting fibers. These changes were most pronounced in the subgroup of patients with the lowest fatigue scores. We therefore conclude that the observed CVD changes in patients are not likely to contribute to persisting complaints of fatigue after GBS.

Correlation of motor nerve conduction velocity and number of innervated muscle fibers

Distal and proximal motor (M) responses were recorded from the "Abductor Pollicis Brevis" (APB) muscle by using the collision method; median motor nerve was stimulated at distal (elbow) and proximal (wrist) regions in a concurrent manner. The delay between two stimuli (ISI: Inter-stimulus Interval), beginning at 9 ms, was decreased by 0.1 ms steps, until the proximal potential completely disappeared. Areas of M responses recorded for each ISI were calculated. Because the area difference between two subsequent ISIs is proportional to the number of muscle fibers innervated by the conduction velocity group at that interval, the relative numbers of muscle fibers for each velocity group were calculated. The results show that the motor nerve conduction velocities belonging to the innervating APB muscle vary between 38 m/s and 57 m/s; the conduction velocity of the group innervating the greatest number of muscle nerves was found to be 55-57 m/s, which comprised 10% of all fibers.

Electrophysiologic measures of diabetic neuropathy: mechanism and meaning

Whole nerve electrophysiologic procedures afford a battery of measures that can provide a noninvasive and objective index of the onset and progression of diabetic polyneuropathy (DPN). Advances in physiologic procedures, digital hardware, and mathematical models have allowed assessment of activity in slower conducting fibers, as well as measures that reflect changes in refractory periods and threshold excitability. These expanded options can augment standard measures of maximal conduction velocity and compound amplitude and greatly enhance the sensitivity of whole nerve measure to both structural (e.g. demyelination) and "nonstructural" (e.g. redistribution of ion channels) deficits associated with DPN. The mechanisms underlying the physiologic events in DPN are multifactorial and their sequence in complex, with different mechanisms contributing to change at overlapping, but distinct points in the progression. Factors influencing early change in velocity may differ from those contributing to chronic deficits and these mechanisms may also differ in their response to various putative therapies. This review attempts to summarize the pattern of whole nerve electrophysiologic change associated with DPN, outlines the strengths and limitations of the various measures that are feasible, and discusses the specific impact of know pathophysiologic mechanisms on these end points.

Maximal and minimal motor conduction velocity in amyotrophic lateral sclerosis and X-linked bulbospinal muscular atrophy measured by Harayama's collision method

Measurement of the maximal (Vmax) and minimal (Vmin) motor nerve conduction velocities was performed in amyotrophic lateral sclerosis (ALS), bulbospinal muscular atrophy (BSMA), and control subjects. The collision method as described initially by Harayama and coworkers was used. This allowed for the correction of the velocity recovery effect (VRE) in Hopf's original method. The purpose of this study is to clarify the controversial results regarding the Vmin and the difference between Vmax and Vmin (Vmax-Vmin) in ALS and to compare these results with BSMA, and clarify the usefulness of Harayama's method. In ALS, a reduction of Vmax and Vmin, and an increase of Vmax-Vmin were found in both median and posterior tibial nerve. In BSMA, a reduction of Vmin and an increase of Vmax-Vmin in the median nerve were noted. Some patients whose results of conventional nerve conduction study were entirely within normal range showed abnormal results in Vmin and/or Vmax-Vmin. These results suggest that the correction of VRE is essential to determine a Vmin, and motor fibers with abnormally slow conduction velocities were present in ALS and BSMA. Harayama's collision method is useful to detect abnormalities of motor fibers with submaximal conduction velocities.

Decrease in nerve temperature: a model for increased temporal dispersion

A decrease in nerve temperature causes a proportional decrease in conduction velocity which, in percentage terms, is equal for all nerve fibers. The absolute decrease in conduction velocity is larger for faster conducting nerve fibers. This results in a compression and a shift to lower values of the conduction velocity distribution and an increase in temporal dispersion. The purpose of this study was to determine if these effects could be detected by a combination of two collision techniques designed to obtain the motor conduction velocity distribution and refractory period distribution. In 12 healthy volunteers we measured the conduction velocity distribution in the median nerve at nerve temperatures of 25 and 40 degrees C. The results showed that our method could detect the predicted changes in conduction velocity distribution and temporal dispersion. We conclude that temperature change is a model that can be used to study temporal dispersion. This may be a first step towards distinguishing between the effects of conduction block and (abnormal) temporal dispersion in demyelinated nerve fibers.

Interaction of nociceptive and non-nociceptive cutaneous afferents from foot sole in common reflex pathways to tibialis anterior motoneurones in humans

In six healthy subjects, the reflex responses of the tibialis anterior muscle (TA) to stimulation of the cutaneous afferents arising from plantar foot, were studied at rest and during different levels of steady voluntary contraction of the TA. At rest, the threshold of the response and the threshold of subjective pain sensation coincided. The mean latency of this TA nociceptive response was 84.7 ms. Steady voluntary contractions of the TA, which was increased progressively from 3% to 15% of the maximum voluntary contraction, produced a significant and parallel reduction in the threshold and latency of the response: at 15%, the mean latency was about 26 ms shorter than at rest and its threshold was about half (i.e. below the pain threshold). The conduction velocity of the afferents responsible for TA response at rest was within the range of A-delta pain afferents (mean 27.4 m/s), whereas during voluntary contraction it was within the A-beta fibre range (mean 45.1 m/s). This suggests that descending command makes the discharge of low-threshold, fast-conducting fibres sufficient for reflex activation of TA motoneurones (MNs). Central delay (about 4 ms) and MN recruitment order (according to the size principle) were found to be the same for both nociceptive and non-nociceptive TA reflex responses. Finally, experiments of spatial summation revealed an interaction between nociceptive and non-nociceptive inputs at a premotoneuronal level. It is therefore proposed that nociceptive and non-nociceptive cutaneous afferents arising from the foot sole use the same short-latency spinal pathway to contact TA MNs and that their relative contribution to its segmental activation is contingent upon descending command.