Plasticity of lower limb motor axons after cervical cord injury

Does Essential Tremor Alter the Axonal Excitability Properties of Lower Motor Neurons?

PURPOSE

Automated nerve excitability testing has identified that the altered excitability of lower motor neuron (LMN) axons in central diseases is because of trans-synaptic plasticity. Essential tremor (ET) is considered a central disorder caused by an altered cerebellar circuit. This study aimed to identify alterations in the excitability of distal motor axons in subjects with ET, with the intention of clarifying whether a trans-synaptic mechanism or LMN adaptation for tremor affects the LMNs of subjects with ET.

METHODS

Twenty-one consecutive patients diagnosed with ET underwent a clinical and electrophysiological evaluation. For the enrolled cases and 45 age- and gender-matched healthy controls, automated nerve excitability testing with threshold tracking techniques (QTRACS software with TRONDF multiple-excitability protocol) was used to evaluate multiple nerve excitability indices in distal median nerve motor axons.

RESULTS

The automated protocol calculated the strength-duration time constant, parameters of threshold electrotonus and current-threshold relationship, and the recovery cycle of excitability. Comparisons of the automated nerve excitability testing parameters revealed no significant differences between the ET and control groups in any of strength-duration time constant, threshold electrotonus, current-threshold relationship, and recovery cycle, whereas the rheobase was higher in the ET group (3.4 ± 1.1 vs. 2.3 ± 1.1, mean ± standard error mean; P < 0.01).

CONCLUSIONS

With the exception of an increased rheobase in ET subjects, no significant differences were observed in LMN excitability between the ET subjects and their controls. The extent of plasticity or adaptation in LMNs may be limited to a major change in central processes that exert marked effects on the pool of LMNs.

Measurement of axonal excitability: Consensus guidelines

Measurement of axonal excitability provides an in vivo indication of the properties of the nerve membrane and of the ion channels expressed on these axons. Axonal excitability techniques have been utilised to investigate the pathophysiological mechanisms underlying neurological diseases. This document presents guidelines derived for such studies, based on a consensus of international experts, and highlights the potential difficulties when interpreting abnormalities in diseased axons. The present manuscript provides a state-of-the-art review of the findings of axonal excitability studies and their interpretation, in addition to suggesting guidelines for the optimal performance of excitability studies.

Ih contributes to increased motoneuron excitability in restless legs syndrome

KEY POINTS

Restless legs patients complain about sensory and motor symptoms leading to sleep disturbances. Symptoms include painful sensations, an urge to move and involuntary leg movements. The responsible mechanisms of restless legs syndrome are still not known, although current studies indicate an increased neuronal network excitability. Reflex studies indicate the involvement of spinal structures. Peripheral mechanisms have not been investigated so far. In the present study, we provide evidence of increased hyperpolarization-activated cyclic nucleotide-gated (HCN) channel-mediated inward rectification in motor axons. The excitability of sensory axons was not changed. We conclude that, in restless legs syndrome, an increased HCN current in motoneurons may play a pathophysiological role, such that these channels could represent a valuable target for pharmaceutical intervention.

ABSTRACT

Restless legs syndrome is a sensorimotor network disorder. So far, the responsible pathophysiological mechanisms are poorly understood. In the present study, we provide evidence that the excitability of peripheral motoneurons contributes to the pathophysiology of restless legs syndrome. In vivo excitability studies on motor and sensory axons of the median nerve were performed on patients with idiopathic restless legs syndrome (iRLS) who were not currently on treatment. The iRLS patients had greater accommodation in motor but not sensory axons to long-lasting hyperpolarization compared to age-matched healthy subjects, indicating greater inward rectification in iRLS. The most reasonable explanation is that hyperpolarization-activated cyclic nucleotide-gated (HCN) channels open at less hyperpolarized membrane potentials, a view supported by mathematical modelling. The half-activation potential for HCN channels (Bq) was the single best parameter that accounted for the difference between normal controls and iRLS data. A 6 mV depolarization of Bq reduced the discrepancy between the normal control model and the iRLS data by 92.1%. Taken together, our results suggest an increase in the excitability of motor units in iRLS that could enhance the likelihood of leg movements. The abnormal axonal properties are consistent with other findings indicating that the peripheral system is part of the network involved in iRLS.

Early intensive hand rehabilitation is not more effective than usual care plus one-to-one hand therapy in people with sub-acute spinal cord injury ('Hands On'): a randomised trial

QUESTION

What is the effect of adding an intensive task-specific hand-training program involving functional electrical stimulation to a combination of usual care plus three 15-minute sessions per week of one-to-one hand therapy in people with sub-acute tetraplegia?

DESIGN

A parallel group, randomised, controlled trial. Participants were randomly assigned (1:1) via a computer-generated concealed block randomisation procedure to either a control or experimental intervention.

PARTICIPANTS

Seventy people with C2 to T1 motor complete or incomplete tetraplegia within 6 months of injury. Participants were recruited from seven spinal units in Australia and New Zealand.

INTERVENTION

Experimental participants received intensive training for one hand. Intensive training consisted of training with an instrumented exercise workstation in conjunction with functional electrical stimulation for 1hour per day, 5 days per week for 8 weeks. Both groups received usual care and 15minutes of one-to-one hand therapy three times per week without functional electrical stimulation.

OUTCOME MEASURES

The primary outcome was the modified Action Research Arm Test reflecting arm and hand function, which was assessed at the end of the intervention, that is, 11 weeks after randomisation. Secondary outcomes were measured at 11 and 26 weeks.

RESULTS

Sixty-six (94%) participants completed the post-intervention assessment and were included in the primary intention-to-treat analysis. The mean (SD) modified Action Research Arm Test score for experimental and control participants at the post-intervention assessment was 36.5 points (SD 16.0) and 33.2 points (SD 17.5), respectively, with an adjusted mean between-group difference of 0.9 points (95% CI -4.1 to 5.9).

CONCLUSION

Adding an intensive task-specific hand-training program involving functional electrical stimulation to a combination of usual care plus three 15-minute sessions per week of one-to-one hand therapy does not improve hand function in people with sub-acute tetraplegia.

REGISTRATION

Australian and New Zealand Trial Registry ACTRN12609000695202 and ClinicalTrials.gov NCT01086930. [Harvey LA, Dunlop SA, Churilov L, Galea MP, Spinal Cord Injury Physical Activity (SCIPA) Hands On Trial Collaborators (2017) Early intensive hand rehabilitation is not more effective than usual care plus one-to-one hand therapy in people with sub-acute spinal cord injury ('Hands On'): a randomised trial. Journal of Physiotherapy 63: 197-204].

Upper Limb Recovery in Spinal Cord Injury: Involvement of Central and Peripheral Motor Pathways

BACKGROUND AND OBJECTIVE

The course of central and peripheral motor recovery after cervical spinal cord injury (SCI) may be investigated by electrophysiological measures. The goal of this study was to compare the 2 over the first year after injury in relation to motor gains.

METHODS

Compound motor action potentials (CMAPs), motor-evoked potentials (MEPs), and F-waves were recorded from the abductor digiti minimi and CMAP and F-waves from abductor hallucis muscles in 305 patients at about 15 days, 1 month, 3 months, 6 months, and 12 months following an acute traumatic SCI.

RESULTS

The MEP amplitudes and F-wave persistences were lower with more severe sensorimotor impairment. They steadily increased in most subgroups within 6 months after SCI. The amplitude of the CMAPs was low for the first 3 months in the most severely affected participants. This was also found for CMAPs from tibial nerve originating well below the cervical lesion level. Improvement in neurophysiological parameters correlated with improved upper extremity motor scores.

CONCLUSION

The results point to a systematic interrelation of corticospinal transmission, spinal motoneuron excitability, and its axon function, respectively. Electrophysiological correlates of neural excitability show distinct spatial and temporal interrelations within central and peripheral motor pathways following acute cervical SCI. A strong secondary deterioration within the peripheral motor system with incomplete or no recovery depends on anatomical distance caudal to lesion and on lesion severity. Electrophysiological assessments may increase the sensitivity of interventional studies in addition to clinical measures.

Early intensive hand rehabilitation is not more effective than usual care plus one-to-one hand therapy in people with sub-acute spinal cord injury ('Hands On'): a randomised trial

QUESTION

What is the effect of adding an intensive task-specific hand-training program involving functional electrical stimulation to a combination of usual care plus three 15-minute sessions per week of one-to-one hand therapy in people with sub-acute tetraplegia?

DESIGN

A parallel group, randomised, controlled trial. Participants were randomly assigned (1:1) via a computer-generated concealed block randomisation procedure to either a control or experimental intervention.

PARTICIPANTS

Seventy people with C2 to T1 motor complete or incomplete tetraplegia within 6 months of injury. Participants were recruited from seven spinal units in Australia and New Zealand.

INTERVENTION

Experimental participants received intensive training for one hand. Intensive training consisted of training with an instrumented exercise workstation in conjunction with functional electrical stimulation for 1 hour per day, 5 days per week for 8 weeks. Both groups received usual care and 15 minutes of one-to-one hand therapy three times per week without functional electrical stimulation.

OUTCOME MEASURES

The primary outcome was the modified Action Research Arm Test reflecting arm and hand function, which was assessed at the end of the intervention, that is, 11 weeks after randomisation. Secondary outcomes were measured at 11 and 26 weeks.

RESULTS

Sixty-six (94%) participants completed the post-intervention assessment and were included in the primary intention-to-treat analysis. The mean modified Action Research Arm Test score for experimental and control participants at the post-intervention assessment was 36.5 points (SD 16.0) and 33.2 points (SD 17.5), respectively, with an adjusted mean between-group difference of 0.9 points (95% CI -4.1 to 5.9).

CONCLUSION

Adding an intensive task-specific hand-training program involving functional electrical stimulation to a combination of usual care plus three 15-minute sessions per week of one-to-one hand therapy does not improve hand function in people with sub-acute tetraplegia.

REGISTRATION

Australian and New Zealand Trial Registry ACTRN12609000695202 and ClinicalTrials.gov NCT01086930.

In vivo impact of presynaptic calcium channel dysfunction on motor axons in episodic ataxia type 2

Ion channel dysfunction causes a range of neurological disorders by altering transmembrane ion fluxes, neuronal or muscle excitability, and neurotransmitter release. Genetic neuronal channelopathies affecting peripheral axons provide a unique opportunity to examine the impact of dysfunction of a single channel subtype in detail in vivo. Episodic ataxia type 2 is caused by mutations in CACNA1A, which encodes the pore-forming subunit of the neuronal voltage-gated calcium channel Cav2.1. In peripheral motor axons, this channel is highly expressed at the presynaptic neuromuscular junction where it contributes to action potential-evoked neurotransmitter release, but it is not expressed mid-axon or thought to contribute to action potential generation. Eight patients from five families with genetically confirmed episodic ataxia type 2 underwent neurophysiological assessment to determine whether axonal excitability was normal and, if not, whether changes could be explained by Cav2.1 dysfunction. New mutations in the CACNA1A gene were identified in two families. Nerve conduction studies were normal, but increased jitter in single-fibre EMG studies indicated unstable neuromuscular transmission in two patients. Excitability properties of median motor axons were compared with those in 30 age-matched healthy control subjects. All patients had similar excitability abnormalities, including a high electrical threshold and increased responses to hyperpolarizing (P < 0.00007) and depolarizing currents (P < 0.001) in threshold electrotonus. In the recovery cycle, refractoriness (P < 0.0002) and superexcitability (P < 0.006) were increased. Cav2.1 dysfunction in episodic ataxia type 2 thus has unexpected effects on axon excitability, which may reflect an indirect effect of abnormal calcium current fluxes during development.

Sensory axonal dysfunction in cervical radiculopathy

OBJECTIVE

The aim of this study was to evaluate changes in sensory axonal excitability in the distal nerve in patients with cervical radiculopathy.

METHODS

The patients were classified by the findings of cervical MRI into two subgroups: 22 patients with C6/7 root compression and 25 patients with cervical cord and root compression above/at C6/7. Patients were investigated using conventional nerve conduction studies (NCS) and nerve excitability testing. Sensory nerve excitability testing was undertaken with stimulation at the wrist and recording from digit II (dermatome C6/7). The results were compared with healthy controls. Both preoperative and postoperative tests were performed if the patient underwent surgery.

RESULTS

Sensory axonal excitability was significantly different in both cohorts compared with healthy controls, including prolonged strength-duration time constant, reduced S2 accommodation, increased threshold electrotonus hyperpolarisation (TEh (90-100 ms)), and increased superexcitability. The changes in these excitability indices are compatible with axonal membrane hyperpolarisation. In five patients who underwent surgery, the postoperative sensory excitability was tested after 1 week, and showed significant changes in TE (TEh (90-100 ms) and TEh slope, p<0.05) between presurgery and postsurgery.

CONCLUSIONS

The present study demonstrated distal nerve axonal hyperpolarisation in patients with cervical radiculopathy. These findings suggest that the hyperpolarised pattern might be due to Na(+)-K(+) ATPase overactivation induced by proximal ischaemia, or could reflect the remyelinating process. Distal sensory axons were hyperpolarised even though there were no changes in NCS, suggesting that nerve excitability testing may be more sensitive to clinical symptoms than NCS in patients with cervical radiculopathy.

Short-term peripheral nerve stimulation ameliorates axonal dysfunction after spinal cord injury

There is accumulating evidence that peripheral motor axons deteriorate following spinal cord injury (SCI). Secondary axonal dysfunction can exacerbate muscle atrophy, contribute to peripheral neuropathies and neuropathic pain, and lead to further functional impairment. In an attempt to ameliorate the adverse downstream effects that developed following SCI, we investigated the effects of a short-term peripheral nerve stimulation (PNS) program on motor axonal excitability in 22 SCI patients. Axonal excitability studies were undertaken in the median and common peroneal nerves (CPN) bilaterally before and after a 6-wk unilateral PNS program. PNS was delivered percutaneously over the median nerve at the wrist and CPN around the fibular head, and the compound muscle action potential (CMAP) from the abductor pollicis brevis and tibialis anterior was recorded. Stimulus intensity was above motor threshold, and pulses (450 μs) were delivered at 100 Hz with a 2-s on/off cycle for 30 min 5 days/wk. SCI patients had consistently high thresholds with a reduced CMAP consistent with axonal loss; in some patients the peripheral nerves were completely inexcitable. Nerve excitability studies revealed profound changes in membrane potential, with a "fanned-in" appearance in threshold electrotonus, consistent with membrane depolarization, and significantly reduced superexcitability during the recovery cycle. These membrane dysfunctions were ameliorated after 6 wk of PNS, which produced a significant hyperpolarizing effect. The contralateral, nonstimulated nerves remained depolarized. Short-term PNS reversed axonal dysfunction following SCI, may provide an opportunity to prevent chronic changes in axonal and muscular function, and may improve rehabilitation outcomes.

Evolution of peripheral nerve function in humans: novel insights from motor nerve excitability

While substantial alterations in myelination and axonal growth have been described during maturation, their interactions with the configuration and activity of axonal membrane ion channels to achieve impulse conduction have not been fully elucidated. The present study utilized axonal excitability techniques to compare the changes in nerve function across healthy infants, children, adolescents and adults. Multiple excitability indices (stimulus-response curve, strength-duration time constant, threshold electrotonus, current-threshold relationship and recovery cycle) combined with conventional neurophysiological measures were investigated in 57 subjects (22 males, 35 females; age range 0.46-24 years), stimulating the median motor nerve at the wrist. Maturational changes in conduction velocity were paralleled by significant alterations in multiple excitability parameters, similarly reaching steady values in adolescence. Maturation was accompanied by reductions in threshold (P < 0.005) and rheobase (P = 0.001); depolarizing and hyperpolarizing electrotonus progressively reduced (P < 0.001), or 'fanned-in'; resting current-threshold slope increased (P < 0.0001); accommodation to depolarizing currents prolonged (P < 0.0001); while greater threshold changes in refractoriness (P = 0.001) and subexcitability (P < 0.01) emerged. Taken together, the present findings suggest that passive membrane conductances and the activity of K(+) conductances decrease with formation of the axo-glial junction and myelination. In turn, these functional alterations serve to enhance the efficiency and speed of impulse conduction concurrent with the acquisition of motor skills during childhood, and provide unique insight into the evolution of postnatal human peripheral nerve function. Significantly, these findings bring the dynamics of axonal development to the clinical domain and serve to further illuminate pathophysiological mechanisms that occur during development.

An Examination of the Motor Unit Number Index (MUNIX) in muscles paralyzed by spinal cord injury

The objective of this study was to assess whether there is evidence of motor unit loss in muscles paralyzed by spinal cord injury (SCI), using a measurement called motor unit number index (MUNIX). The MUNIX technique was applied in SCI (n=12) and neurologically intact (n=12) subjects. The maximum M waves and voluntary surface electromyography (EMG) signals at different muscle contraction levels were recorded from the first dorsal interosseous (FDI) muscle in each subject. The MUNIX values were estimated using a mathematical model describing the relation between the surface EMG signal and the ideal motor unit number count derived from the M wave and surface EMG measurements. We recorded a significant decrease in both maximum M wave amplitude and in estimated MUNIX values in paralyzed FDI muscles, as compared with neurologically intact muscles. Across all subjects, the maximum M wave amplitude was 8.3 ± 4.4 mV for the paralyzed muscles and 14.4 ± 2.0 mV for the neurologically intact muscles (p<0.0001). These measurements, when combined with voluntary EMG recordings, resulted in a mean MUNIX value of 112 ± 71 for the paralyzed muscles, much lower than the mean MUNIX value of 228 ± 49 for the neurologically intact muscles (p<0.00001). A motor unit size index was also calculated, using the maximum M wave recording and the MUNIX values. We found that paralyzed muscles showed a mean motor unit size index value of 80.7 ± 17.7 ìV, significantly higher than the mean value of 64.9 ± 10.1 ìV obtained from neurologically intact muscles (p<0.001). The MUNIX method used in this study offers several practical benefits compared with the traditional motor unit number estimation technique because it is noninvasive, induces minimal discomfort due to electrical nerve stimulation, and can be performed quickly. The findings from this study help understand the complicated determinants of SCI induced muscle weakness and provide further evidence of motoneuron degeneration after a spinal injury.

Do the motor manifestations of Parkinson disease alter motor axon excitability?

BACKGROUND

Axonal excitability is altered in common medical conditions such as stroke, multiple sclerosis, and spinal cord injury. Given the motor neuron changes in the presence of rigidity and tremor in Parkinson disease, we examine whether there are also changes in motor axon excitability.

METHODS

Axonal excitability studies were performed in 15 Parkinson subjects and 12 age-matched control subjects.

RESULTS

There was no significant difference in excitability indices between Parkinson subjects and control subjects.

CONCLUSIONS

It is unlikely that the lack of change in the excitability indices reflects a balance between the effects of bradykinesia ("underactivity") and the effects of rigidity and tremor ("overactivity") on the motoneuron and its axon. It is more likely that plastic changes in motoneuron properties do not occur symmetrically with decreases and increases in activity, being more profound when activity levels are interrupted and less obvious when they are enhanced.

Peripheral neuropathy in hereditary spastic paraplegia due to spastin (SPG4) mutation--a neurophysiological study using excitability techniques

OBJECTIVE

To identify peripheral nerve abnormalities in hereditary spastic paraplegia (HSP) due to mutations in the spastin gene (spastic paraplegia 4, SPG4) using standard nerve conduction (NCS) and novel tests of axonal excitability.

METHODS

Eleven patients with known mutations in spastin were assessed with NCS for the upper and lower limbs, and axonal excitability testing on the median nerve.

RESULTS

Standard nerve conduction studies revealed a sensorimotor neuropathy in two patients. Excitability studies on median motor axons showed an isolated abnormality (increased strength-duration time constant), but those on sensory axons were normal in nine patients with normal routine nerve conduction studies.

CONCLUSIONS

Peripheral neuropathy occurs in HSP patients with SPG4 mutations, but axonal excitability studies provide limited additional evidence for subclinical peripheral nerve dysfunction, and add little further to standard nerve conduction studies.

SIGNIFICANCE

The features of HSP due to SPG4 mutations include sensorimotor polyneuropathy. The value of excitability studies is limited in individual patients.

Dysfunction of axonal membrane conductances in adolescents and young adults with spinal muscular atrophy

Spinal muscular atrophy is distinct among neurodegenerative conditions of the motor neuron, with onset in developing and maturing patients. Furthermore, the rate of degeneration appears to slow over time, at least in the milder forms. To investigate disease pathophysiology and potential adaptations, the present study utilized axonal excitability studies to provide insights into axonal biophysical properties and explored correlation with clinical severity. Multiple excitability indices (stimulus–response curve, strength–duration time constant, threshold electrotonus, current–threshold relationship and recovery cycle) were investigated in 25 genetically characterized adolescent and adult patients with spinal muscular atrophy, stimulating the median motor nerve at the wrist. Results were compared with 50 age-matched controls. The Medical Research Council sum score and Spinal Muscular Atrophy Functional Rating Scale were used to define the strength and motor functional status of patients with spinal muscular atrophy. In patients with spinal muscular atrophy, there were reductions in compound muscle action potential amplitude (P < 0.0005) associated with reduction in stimulus response slope (P < 0.0005), confirming significant axonal loss. In the patients with mild or ambulatory spinal muscular atrophy, there was reduction of peak amplitude without alteration in axonal excitability; in contrast, in the non-ambulatory or severe spinal muscular atrophy cohort prominent changes in axonal function were apparent. Specifically, there were steep changes in the early phase of hyperpolarization in threshold electrotonus (P < 0.0005) that correlated with clinical severity. Additionally, there were greater changes in depolarizing threshold electrotonus (P < 0.0005) and prolongation of the strength-duration time constant (P = 0.001). Mathematical modelling of the excitability changes obtained in patients with severe spinal muscular atrophy supported a mixed pathology comprising features of axonal degeneration and regeneration. The present study has provided novel insight into the pathophysiology of spinal muscular atrophy, with identification of functional abnormalities involving axonal K⁺ and Na⁺ conductances and alterations in passive membrane properties, the latter linked to the process of neurodegeneration.

Severe Degeneration of Peripheral Motor Axons After Spinal Cord Injury: A European Multicenter Study in 345 Patients

Objective. There are indications that perilesional and remote peripheral motor axons may degenerate after spinal cord injury (SCI). The authors investigated the magnitude and dependence on severity of SCI of this degeneration as well as whether motor axons so affected can recover. Methods. The function of the peripheral motor axons was investigated by recording compound muscle action potentials (CMAPs) in 345 patients with cervical SCI. CMAP amplitude changes in the abductor digiti minimi (ADM) and abductor hallucis (AH) muscles were investigated in 3 groups with SCIs of differing severity: patients with permanent complete SCI (ASIA Impairment Scale [AIS] A-remain), patients with initially complete SCI converting to incomplete lesion (AIS A-convert), and patients with incomplete injury (Incomplete). Results. Significant decreases in ADM and AH CMAP amplitudes were found in groups A-remain and A-convert. In group A-remain and group A-convert, the authors found a partial, although significant, recovery of ADM CMAP amplitude occurring between 5 and 12 months postinjury. In group A-remain, they found significant recovery of the AH CMAP amplitude. Conclusion . Following SCI, peripheral motor axons below the level of the lesion exhibit severe degeneration. There is partial, although significant, recovery of CMAP during the second half year following SCI. The observed motor axon dysfunction is likely a result of transsynaptic degeneration. The peripheral motor axon dysfunction observed after SCI is of sufficient magnitude that it may affect muscle power and thus contribute to impairment of recovery of functional activities in patients with SCI.

Axonal ion channels from bench to bedside: a translational neuroscience perspective

Over recent decades, the development of specialised techniques such as patch clamping and site-directed mutagenesis have established the contribution of neuronal ion channel dysfunction to the pathophysiology of common neurological conditions including epilepsy, multiple sclerosis, spinal cord injury, peripheral neuropathy, episodic ataxia, amyotrophic lateral sclerosis and neuropathic pain. Recently, these insights from in vitro studies have been translated into the clinical realm. In keeping with this progress, novel clinical axonal excitability techniques have been developed to provide information related to the activity of a variety of ion channels, energy-dependent pumps and ion exchange processes activated during impulse conduction in peripheral axons. These non-invasive techniques have been extensively applied to the study of the biophysical properties of human peripheral nerves in vivo and have provided important insights into axonal ion channel function in health and disease. This review will provide a translational perspective, focusing on an overview of the investigational method, the clinical utility in assessing the biophysical basis of ectopic symptom generation in peripheral nerve disease and a review of the major findings of excitability studies in acquired and inherited neurological disease states.