Ischemic resistance of cutaneous afferents and motor axons in patients with amyotrophic lateral sclerosis

Neurophysiological and imaging biomarkers of lower motor neuron dysfunction in motor neuron diseases/amyotrophic lateral sclerosis: IFCN handbook chapter

This chapter discusses comprehensive neurophysiological biomarkers utilised in motor neuron disease (MND) and, in particular, its commonest form, amyotrophic lateral sclerosis (ALS). These encompass the conventional techniques including nerve conduction studies (NCS), needle and high-density surface electromyography (EMG) and H-reflex studies as well as novel techniques. In the last two decades, new methods of assessing the loss of motor units in a muscle have been developed, that are more convenient than earlier methods of motor unit number estimation (MUNE),and may use either electrical stimulation (e.g. MScanFit MUNE) or voluntary activation (MUNIX). Electrical impedance myography (EIM) is another novel approach for the evaluation that relies upon the application and measurement of high-frequency, low-intensity electrical current. Nerve excitability techniques (NET) also provide insights into the function of an axon and reflect the changes in resting membrane potential, ion channel dysfunction and the structural integrity of the axon and myelin sheath. Furthermore, imaging ultrasound techniques as well as magnetic resonance imaging are capable of detecting the constituents of morphological changes in the nerve and muscle. The chapter provides a critical description of the ability of each technique to provide neurophysiological insight into the complex pathophysiology of MND/ALS. However, it is important to recognise the strengths and limitations of each approach in order to clarify utility. These neurophysiological biomarkers have demonstrated reliability, specificity and provide additional information to validate and assess lower motor neuron dysfunction. Their use has expanded the knowledge about MND/ALS and enhanced our understanding of the relationship between motor units, axons, reflexes and other neural circuits in relation to clinical features of patients with MND/ALS at different stages of the disease. Taken together, the ultimate goal is to aid early diagnosis, distinguish potential disease mimics, monitor and stage disease progression, quantify response to treatment and develop potential therapeutic interventions.

Neuronal Circuit Dysfunction in Amyotrophic Lateral Sclerosis

The primary neural circuit affected in Amyotrophic Lateral Sclerosis (ALS) patients is the corticospinal motor circuit, originating in upper motor neurons (UMNs) in the cerebral motor cortex which descend to synapse with the lower motor neurons (LMNs) in the spinal cord to ultimately innervate the skeletal muscle. Perturbation of these neural circuits and consequent loss of both UMNs and LMNs, leading to muscle wastage and impaired movement, is the key pathophysiology observed. Despite decades of research, we are still lacking in ALS disease-modifying treatments. In this review, we document the current research from patient studies, rodent models, and human stem cell models in understanding the mechanisms of corticomotor circuit dysfunction and its implication in ALS. We summarize the current knowledge about cortical UMN dysfunction and degeneration, altered excitability in LMNs, neuromuscular junction degeneration, and the non-cell autonomous role of glial cells in motor circuit dysfunction in relation to ALS. We further highlight the advances in human stem cell technology to model the complex neural circuitry and how these can aid in future studies to better understand the mechanisms of neural circuit dysfunction underpinning ALS.

Nerve excitability measured with the TROND protocol in amyotrophic lateral sclerosis: a systematic review and meta-analysis

This meta-analysis assessed the 30+ nerve excitability indices generated by the TROND protocol to identify potential biomarkers for amyotrophic lateral sclerosis (ALS). A comprehensive search was conducted in multiple databases to identify human studies that tested median motor axons. Forest plot analyses were performed using a random-effects model to determine the pooled effect (Z-score), heterogeneity (I2), and Cohen's d for potential biomarker identification. Out of 2,866 studies, 23 studies met the inclusion criteria, incorporating data from 719 controls and 942 patients with ALS. Seven indices emerged as potential biomarkers: depolarizing threshold electrotonus (TEd) 90-100 ms, strength-duration time constant (SDTC), superexcitability, TEd 40-60 ms, resting I/V slope, 50% depolarizing I/V, and subexcitability (ranked by the magnitude of the difference between patients and controls from largest to smallest). In a sensitivity analysis focusing on patients with larger compound muscle action potentials (CMAPs), only four indices were potential biomarkers: TEd 10-20 ms, TEd 90-100 ms, superexcitability, and SDTC. Among the extensive range of 30+ excitability indices generated by the TROND protocol, we have identified seven indices that effectively differentiate patients with ALS from healthy controls. Furthermore, a smaller subset of four indices shows promise as potential biomarkers when the CMAP remains relatively large. However, most studies were considered to be at moderate risk of bias due to case-control designs and absence of sensitivity and specificity calculations, underscoring the need for more prospective diagnostic test-accuracy studies with appropriate disease controls.NEW & NOTEWORTHY This meta-analysis uncovers seven potential axonal excitability biomarkers for lower motor neuron pathology in ALS, shedding light on ion channel dysfunction. The identified dysfunction aligns with the primary pathology-protein homeostasis disruption. These biomarkers could fill a gap to detect presymptomatic spread of the disease in the spinal cord and monitor treatments targeting protein homeostasis and limiting spread, toward enhancing patient care.

Demystifying the spontaneous phenomena of motor hyperexcitability

Possessing a discrete functional repertoire, the anterior horn cell can be in one of two electrophysiological states: on or off. Usually under tight regulatory control by the central nervous system, a hierarchical network of these specialist neurons ensures muscular strength is coordinated, gradated and adaptable. However, spontaneous activation of these cells and their axons can result in abnormal muscular twitching. The muscular twitch is the common building block of several distinct clinical patterns, namely fasciculation, myokymia and neuromyotonia. When attempting to distinguish these entities electromyographically, their unique temporal and morphological profiles must be appreciated. Detection and quantification of burst duration, firing frequency, multiplet patterns and amplitude are informative. A common feature is their persistence during sleep. In this review, we explain the accepted terminology used to describe the spontaneous phenomena of motor hyperexcitability, highlighting potential pitfalls amidst a bemusing and complex collection of overlapping terms. We outline the relevance of these findings within the context of disease, principally amyotrophic lateral sclerosis, Isaacs syndrome and Morvan syndrome. In addition, we highlight the use of high-density surface electromyography, suggesting that more widespread use of this non-invasive technique is likely to provide an enhanced understanding of these motor hyperexcitability syndromes.

The rise and fall of fasciculations in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a devastating neurodegenerative disease with a median survival of 3 years from symptom onset. Accessible and reliable biomarkers of motor neuron decline are urgently needed to quicken the pace of drug discovery. Fasciculations represent an early pathophysiological hallmark of amyotrophic lateral sclerosis and can be reliably detected by high-density surface electromyography. We set out to quantify fasciculation potentials prospectively over 14 months, seeking comparisons with established markers of disease progression. Twenty patients with amyotrophic lateral sclerosis and five patients with benign fasciculation syndrome underwent up to seven assessments each. At each assessment, we performed the amyotrophic lateral sclerosis-functional rating scale, sum power score, slow vital capacity, 30-min high-density surface electromyography recordings from biceps and gastrocnemius and the motor unit number index. We employed the Surface Potential Quantification Engine, which is an automated analytical tool to detect and characterize fasciculations. Linear mixed-effect models were employed to account for the pseudoreplication of serial measurements. The amyotrophic lateral sclerosis-functional rating scale declined by 0.65 points per month (P < 0.0001), 35% slower than average. A total of 526 recordings were analysed. Compared with benign fasciculation syndrome, biceps fasciculation frequency in amyotrophic lateral sclerosis was 10 times greater in strong muscles and 40 times greater in weak muscles. This was coupled with a decline in fasciculation frequency among weak muscles of -7.6/min per month (P = 0.003), demonstrating the rise and fall of fasciculation frequency in biceps muscles. Gastrocnemius behaved differently, whereby strong muscles in amyotrophic lateral sclerosis had fasciculation frequencies five times greater than patients with benign fasciculation syndrome while weak muscles were increased by only 1.5 times. Gastrocnemius demonstrated a significant decline in fasciculation frequency in strong muscles (2.4/min per month, P < 0.0001), which levelled off in weak muscles. Fasciculation amplitude, an easily quantifiable surrogate of the reinnervation process, was highest in the biceps muscles that transitioned from strong to weak during the study. Pooled analysis of >900 000 fasciculations revealed inter-fasciculation intervals <100 ms in the biceps of patients with amyotrophic lateral sclerosis, particularly in strong muscles, consistent with the occurrence of doublets. We hereby present the most comprehensive longitudinal quantification of fasciculation parameters in amyotrophic lateral sclerosis, proposing a unifying model of the interactions between motor unit loss, muscle power and fasciculation frequency. The latter showed promise as a disease biomarker with linear rates of decline in strong gastrocnemius and weak biceps muscles, reflecting the motor unit loss that drives clinical progression.

Excitability of sensory axons in amyotrophic lateral sclerosis

OBJECTIVE

To evaluate the excitability of sensory axons in patients with amyotrophic lateral sclerosis (ALS).

METHODS

Comprehensive sensory nerve excitability studies were prospectively performed on 28 sporadic ALS patients, compared to age-matched controls. Sensory nerve action potentials were recorded from digit 2 following median nerve stimulation at the wrist. Disease severity was measured using motor unit number estimation (MUNE), the revised ALS Functional Rating Scale (ALSFRS-R) and the MRC scale.

RESULTS

There were no significant differences in standard and extended measures of nerve excitability between ALS patients and controls. These unchanged excitability measures included accommodation to long-lasting hyperpolarization and the threshold changes after two supramaximal stimuli during the recovery cycle. Excitability parameters did not correlate with MUNE, ALSFRS-R, APB MRC scale or disease duration.

CONCLUSIONS

This cross-sectional study has identified normal axonal membrane properties in myelinated sensory axons of ALS patients. Previously described sensory abnormalities could be the result of axonal fallout, possibly due to a ganglionopathy, or to involvement of central sensory pathways rostral to gracile and cuneate nuclei.

SIGNIFICANCE

These results demonstrate the absence of generalized dysfunction of the membrane properties of sensory axons in ALS in the face of substantial deficits in motor function.

Fatigue and activity dependent changes in axonal excitability in amyotrophic lateral sclerosis

BACKGROUND

While patients with amyotrophic lateral sclerosis (ALS) may complain of fatigue, the underlying mechanisms appear complex, with dysfunction of central and peripheral nervous systems independently reported as contributing factors. The aim of the present study was to further delineate the mechanisms underlying increased fatigability in ALS by measuring activity dependent changes in axonal excitability following a maximum voluntary contraction (MVC).

METHODS

Nerve excitability changes were recorded before and after an MVC of the abductor pollicis brevis in 16 patients with ALS and 25 controls.

RESULTS

In patients with ALS, there was a greater increase in threshold (36.5 (5.9)%; controls 19.6 (3.5)%; p<0.05) as a result of MVC, with reduction in the amplitude of the compound muscle action potential generated by a submaximal stimulus (ALS 49 (7.6)%; controls 41.0 (5.4)%). These changes were associated with an increase in superexcitability (ALS 65.1 (25.4)%; controls 42.3 (5.7)%) and reduction in strength-duration time constant (ALS 20 (4.9)%; controls 10 (2.5)%; p<0.01), indicative of axonal hyperpolarisation. The increase in threshold was more pronounced in patients with ALS with predominantly lower motor neuronal involvement.

CONCLUSIONS

Higher firing rates of surviving motor axons attempting to compensate for neurogenic weakness are likely to explain the greater activity dependent changes in ALS. As such, the present study suggests a further peripheral factor underlying the development of fatigue in ALS.

Excitability of human axons

The excitability of human axons can be studied reliably using the technique of threshold tracking, which allows the strength of a test stimulus to be adjusted by computer to activate a defined fraction of the maximal nerve or muscle action potential. The stimulus current that just evokes the target response is considered the "threshold" for that response. More useful than the resting threshold are other indices of axonal excitability derived from pairs of threshold measurements, such as refractoriness, supernormality, strength-duration time constant and "threshold electrotonus" (i.e. the changes in threshold produced by long-lasting depolarizing or hyperpolarizing current pulses). Each of these measurements depends on membrane potential and on other biophysical properties of the axons. Together they can provide new information about the pathophysiology underlying abnormalities in excitability in neuropathy.

Strength-duration properties and their voltage dependence as measures of a threshold conductance at the node of Ranvier of single motor axons

In a number of clinical studies, measurement of axonal strength-duration properties has been used to provide indirect insight into conductances at the node of Ranvier, particularly persistent Na(+) conductance. However, the specificity of any changes is limited because other factors can affect strength-duration behavior. The present study was undertaken to define the relationship between different strength-duration measures at rest and at different membrane potentials, and also to determine the limits within which strength-duration behavior can be used as a measure of nodal conductances. The strength-duration time constant (tau(SD)) and rheobase of 20 single motor units in the flexor carpi ulnaris were calculated from thresholds defined using threshold tracking. "True" rheobase and rheobasic latencies were measured using test stimuli of 100-ms duration. For ten units, the technique of latent addition was used to measure threshold changes directly attributable to nodal conductances, and for six units these were compared with strength-duration properties at different membrane potentials. The data indicate that measurements of tau(SD) and rheobase can provide sensitive indicators of conductances present at the node of Ranvier when membrane potential changes. There is a reciprocal relationship between tau(SD) and rheobase for single motor units at different membrane potentials, and this relationship may allow changes in tau(SD) due to depolarization and demyelination to be differentiated.

Reproducibility of indices of axonal excitability in human subjects

OBJECTIVES

Different indices of axonal excitability are now being measured in human subjects, both normal volunteers undergoing some test manoeuvre and patients with a variety of peripheral nerve disorders. The reproducibility of these indices has not previously been established, and was determined for cutaneous afferents in the median nerve of 12 healthy subjects, using threshold tracking techniques.

METHODS

Refractoriness and supernormality were determined as the change in stimulus current required to produce a predetermined target potential when conditioned by a supramaximal stimulus at appropriate conditioning-test intervals. Strength-duration time constant was calculated from the threshold currents using unconditioned test stimuli of 0.1 ms and 1.0 ms. The effects of changes in membrane potential on these indices was assessed by applying subthreshold DC currents (from 50% depolarizing to 50% hyperpolarizing), using the reciprocal of threshold (i.e., 'excitability') as an indicator of membrane potential. The intraindividual reproducibility was determined by repeating the study on each subject up to 10 times.

RESULTS

Refractoriness and supernormality were variable between subjects (mean +/- SD of 31.5 +/- 9.5% and 13.2 +/- 3.8%, respectively) and within subjects (coefficient of variation 0.2104 and 0.21849, respectively). TauSD showed even greater interindividual variability (499.2 +/- 115 micros) and intraindividual variability (coefficient of variation 0.2339). The slopes of relationships between each of the indices and axonal 'excitability' suggest that refractoriness is extremely sensitive to changes in excitability (0.9767 +/- 0.1907), tauSD less so (0.3766 +/- 0.1322), supernormality least (0.2223 +/- 0.1268).

CONCLUSIONS

Under controlled conditions, refractoriness is the most sensitive and least variable of the indices of axonal excitability. However, small decreases in temperature greatly increase refractoriness but have little effect on tauSD. Given that 3 indices reflect different biophysical mechanisms, nodal and internodal, greater insight into the functional state of peripheral nerve axons will come when there are coherent changes in all 3 indices.

Temperature dependence of excitability indices of human cutaneous afferents

The temperature dependence of different indices of axonal excitability (threshold, latency, refractoriness, supernormality, strength-duration time constant, and rheobase) was studied for cutaneous afferents of 8 healthy human volunteers using threshold tracking. Cooling from approximately 32 - approximately 22 degrees C dramatically increased the threshold for a conditioned potential evoked during the relatively refractory period (average increase 573%) but had little effect on the threshold for unconditioned potentials (increased by 4% with 0.1-ms test stimuli), strength-duration time constant (increased by 18%), or rheobase (decreased by 12%). Cooling increased the latency of the unconditioned test potential by 41%, but this slowing was small compared with the effect of cooling on the latency slowing attributable to refractoriness. This measure of refractoriness was initially 0.17 ms at a conditioning-test interval of 2 ms, and increased with cooling to 1.30 ms at the same interval. With cooling, refractoriness was both greater at any one conditioning-test interval and longer in duration, extending into intervals normally associated with supernormality. It is concluded that, although cooling affects all excitability indices to some extent, the most prominent feature is the increase in refractoriness. By contrast, strength-duration time constant is influenced little by temperature.