In vivo evidence for reduced ion channel expression in motor axons of 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.

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.

Drop-off in axonal regeneration along the length of a cross-face nerve graft: An experimental study in rats

INTRODUCTION

The average nerve graft length utilized in cross-face nerve grafting for reconstruction of facial nerve palsy is 20-22 cm. While the graft length is thought to be one of the greatest determinants of muscle strength, the mechanism through which this happens remains unknown. We studied changes in axonal regeneration along the length of a 2 cm cross-face nerve graft in a rat model. The hypothesis was that axon count would decrease along the length of the graft.

METHODS

A 2 cm nerve graft (sciatic nerve) was used as a cross-face nerve graft in 16 adult female, 210-250 g, Sprague Dawley rats. Thirteen weeks later, 5 mm nerve biopsies were taken at four sites: the facial nerve trunk (control), proximal graft, midpoint of graft (1 cm distal to coaptation) and distal graft (2 cm distal to coaptation). Retrograde nerve labeling with FluoroGold was performed at the biopsied nerve site and the facial motor nucleus was taken 1 week later. Microscopic imaging and manual counting of axons and labeled motor nuclei was performed.

RESULTS

Retrograde-labeled motor neuron counts were decreased at the midway point of the graft compared to the facial trunk (1517 ± 335 axons, Δ% = 92.5, p = .01) and even further decreased at the distal end of the graft (269 ± 293 axons, Δ% = 175.5, p = .006). Analysis of the nerve biopsies demonstrated no significant differences in myelinated axon count between the nerve trunk and over the length of the nerve graft (range 6207-7179 axons, Δ% = 14.5, p = .07).

CONCLUSION

In a rat model, the number of regenerating motor neurons drops off along the length of the graft and axon count is preserved due to axon sprouting. How this pattern correlates to ultimate muscle strength remains unknown, but this study provides insight into why shorter grafts may afford better outcomes.

Diagnostic accuracy of nerve excitability and compound muscle action potential scan derived biomarkers in amyotrophic lateral sclerosis

BACKGROUND AND PURPOSE

The lack of reliable early biomarkers still causes substantial diagnostic delays in amyotrophic lateral sclerosis (ALS). The aim was to assess the diagnostic accuracy of a novel electrophysiological protocol in patients with suspected motor neuron disease (MND).

METHODS

Consecutive patients with suspected MND were prospectively recruited at our tertiary referral centre for MND in Utrecht, The Netherlands. Procedures were performed in accordance with the Standards for Reporting of Diagnostic Accuracy. In addition to the standard diagnostic workup, an electrophysiological protocol of compound muscle action potential (CMAP) scans and nerve excitability tests was performed on patients' thenar muscles. The combined diagnostic yield of nerve excitability and CMAP scan based motor unit number estimation was compared to the Awaji and Gold Coast criteria and their added value was determined.

RESULTS

In all, 153 ALS or progressive muscular atrophy patients, 63 disease controls and 43 healthy controls were included. Our electrophysiological protocol had high diagnostic accuracy (area under the curve [AUC] 0.85, 95% confidence interval [95% CI] 0.80-0.90), even in muscles with undetectable axon loss (AUC 0.78, 95% CI 0.70-0.85) and in bulbar-onset patients (AUC 0.85, 95% CI 0.73-0.95). Twenty-four of 33 (73%) ALS patients who could not be diagnosed during the same visit were correctly identified, as well as 8/13 (62%) ALS patients not meeting the Gold Coast criteria and 49/59 (83%) ALS patients not meeting the Awaji criteria during this first visit.

CONCLUSIONS

Our practical and non-invasive electrophysiological protocol may improve early diagnosis in clinically challenging patients with suspected ALS. Routine incorporation may boost early diagnosis, enhance patient selection and generate baseline measures for clinical trials.

The Node of Ranvier as an Interface for Axo-Glial Interactions: Perturbation of Axo-Glial Interactions in Various Neurological Disorders

The action potential conduction along the axon is highly dependent on the healthy interactions between the axon and myelin-producing glial cells. Myelin, which facilitates action potential, is the protective insulation around the axon formed by Schwann cells and oligodendrocytes in the peripheral (PNS) and central nervous system (CNS), respectively. Myelin is a continuous structure with intermittent gaps called nodes of Ranvier, which are the sites enriched with ion channels, transmembrane, scaffolding, and cytoskeletal proteins. Decades-long extensive research has identified a comprehensive proteome with strictly regularized localization at the node of Ranvier. Concurrently, axon-glia interactions at the node of Ranvier have gathered significant attention as the pathophysiological targets for various neurodegenerative disorders. Numerous studies have shown the alterations in the axon-glia interactions culminating in neurological diseases. In this review, we have provided an update on the molecular composition of the node of Ranvier. Further, we have discussed in detail the consequences of disruption of axon-glia interactions during the pathogenesis of various CNS and PNS disorders.

FUS Alters circRNA Metabolism in Human Motor Neurons Carrying the ALS-Linked P525L Mutation

Deregulation of RNA metabolism has emerged as one of the key events leading to the degeneration of motor neurons (MNs) in Amyotrophic Lateral Sclerosis (ALS) disease. Indeed, mutations on RNA-binding proteins (RBPs) or on proteins involved in aspects of RNA metabolism account for the majority of familiar forms of ALS. In particular, the impact of the ALS-linked mutations of the RBP FUS on many aspects of RNA-related processes has been vastly investigated. FUS plays a pivotal role in splicing regulation and its mutations severely alter the exon composition of transcripts coding for proteins involved in neurogenesis, axon guidance, and synaptic activity. In this study, by using in vitro-derived human MNs, we investigate the effect of the P525L FUS mutation on non-canonical splicing events that leads to the formation of circular RNAs (circRNAs). We observed altered levels of circRNAs in FUS^P525L MNs and a preferential binding of the mutant protein to introns flanking downregulated circRNAs and containing inverted Alu repeats. For a subset of circRNAs, FUS^P525L also impacts their nuclear/cytoplasmic partitioning, confirming its involvement in different processes of RNA metabolism. Finally, we assess the potential of cytoplasmic circRNAs to act as miRNA sponges, with possible implications in ALS pathogenesis.

Consensus for experimental design in electromyography (CEDE) project: Single motor unit matrix

The analysis of single motor unit (SMU) activity provides the foundation from which information about the neural strategies underlying the control of muscle force can be identified, due to the one-to-one association between the action potentials generated by an alpha motor neuron and those received by the innervated muscle fibers. Such a powerful assessment has been conventionally performed with invasive electrodes (i.e., intramuscular electromyography (EMG)), however, recent advances in signal processing techniques have enabled the identification of single motor unit (SMU) activity in high-density surface electromyography (HDsEMG) recordings. This matrix, developed by the Consensus for Experimental Design in Electromyography (CEDE) project, provides recommendations for the recording and analysis of SMU activity with both invasive (needle and fine-wire EMG) and non-invasive (HDsEMG) SMU identification methods, summarizing their advantages and disadvantages when used during different testing conditions. Recommendations for the analysis and reporting of discharge rate and peripheral (i.e., muscle fiber conduction velocity) SMU properties are also provided. The results of the Delphi process to reach consensus are contained in an appendix. This matrix is intended to help researchers to collect, report, and interpret SMU data in the context of both research and clinical applications.

Neuroplasticity of peripheral axonal properties after ischemic stroke

OBJECTIVE

This study investigated how peripheral axonal excitability changes in ischemic stroke patients with hemiparesis or hemiplegia, reflecting the plasticity of motor axons due to corticospinal tract alterations along the poststroke stage.

METHODS

Each subject received a clinical evaluation, nerve conduction study, and nerve excitability test. Nerve excitability tests were performed on motor median nerves in paretic and non-paretic limbs in the acute stage of stroke. Control nerve excitability test data were obtained from age-matched control subjects. Some patients underwent excitability examinations several times in subacute or chronic stages.

RESULTS

A total of thirty patients with acute ischemic stroke were enrolled. Eight patients were excluded due to severe entrapment neuropathy in the median nerve. The threshold current for 50% compound muscle action potential (CMAP) was higher in paretic limbs than in control subjects. Furthermore, in the cohort with severe patients (muscle power ≤ 3/5 in affected hands), increased threshold current for 50% CMAP and reduced subexcitability were noted in affected limbs than in unaffected limbs. In addition, in the subsequent study of those severe patients, threshold electrotonus increased in the hyperpolarization direction: TEh (100-109 ms), and the minimum I/V slope decreased. The above findings suggest the less excitable and less accommodation in lower motor axons in the paretic limb caused by ischemic stroke.

CONCLUSION

Upper motor neuron injury after stroke can alter nerve excitability in lower motor neurons, and the changes are more obvious in severely paretic limbs. The accommodative changes of axons progress from the subacute to the chronic stage after stroke. Further investigation is necessary to explore the downstream effects of an upper motor neuron insult in the peripheral nerve system.

Axonal excitability changes in children with spinal muscular atrophy treated with nusinersen

Spinal muscular atrophy (SMA) is associated with developmental disruption of motor axons in ventral roots of the spinal cord alongside motor axon degeneration. The pathogenesis of peripheral axonal change during development is pertinent to understand treatment response. Nerve excitability techniques, stimulating the median motor nerve at the wrist, were utilised to investigate axonal change during neurodevelopment in 24 children with SMA, compared with 71 age-matched controls. Longitudinal axonal response to nusinersen treatment in 18 children was also investigated. Significant differences in axonal development were noted in the youngest children with SMA, signified by reduced compound muscle action potential (CMAP) (P = 0.030), higher axonal threshold (P = 0.016), rheobase (minimal current amplitude of infinite duration, required to generate an action potential) (P = 0.012) and greater changes in depolarising and hyperpolarising threshold electrotonus. Subexcitability increased in all children with SMA, compared to controls. With treatment, nerve excitability changes were observed prominently in young children, with increases in CMAP, reduction in axonal threshold, fanning-in of threshold electrotonus, increase in resting current-threshold slope and reduction in subexcitability. Whilst motor axons continue to mature in SMA, developmental delays in passive and active membrane properties occur especially in early childhood. Concurrently, motor axons actively undergo degeneration. Nusinersen restores the developmental trajectory of motor axons reducing degeneration, especially in children with early treatment initiation. Our findings move the field forward in understanding the developmental aspect of childhood-onset motor neurone diseases and changes in axonal function associated with disease modification. KEY POINTS: Pathomechanisms in spinal muscular atrophy involve concurrent neurodevelopmental and neurodegenerative processes. The greatest delays in maturation of the passive and active properties of the peripheral motor axon are seen in early childhood. Nusinersen facilitates developmental recovery of the motor axon whilst also reducing neurodegeneration. Axonal dysfunction is reversed with SMN repletion particularly when intervention occurs early in development.

Rate-dependent depression is impaired in amyotrophic lateral sclerosis

OBJECTIVE

We investigated rate-dependent depression (RDD) of the Hoffman reflex (H-reflex) in patients with amyotrophic lateral sclerosis (ALS), a degenerative disease with ventral horn involvement.

PATIENTS AND METHODS

In this case-control study, we enrolled 27 patients with ALS and 30 matched healthy control subjects. Clinical and electrophysiological assessments, as well as RDD in response to various stimulation frequencies (0.5 Hz, 1 Hz, 3 Hz and 5 Hz), were compared between groups. Multiple clinical and electrophysiological factors were also explored to determine any underlying associations with RDD.

RESULTS

The ALS group showed a significant loss of RDD across all frequencies compared to the control group, most notably following 1 Hz stimulation (19.1 ± 20.3 vs. 34.0 ± 13.7%, p = 0.003). Among factors that might influence RDD, the enlargement of the motor unit potential (MUP) showed a significant relationship with RDD following multifactor analysis of variance (p = 0.007) and Pearson correlation analysis (ρ =  - 0.70, p < 0.001), while various upper motor neuron manifestations were not correlated with RDD values (p > 0.05).

CONCLUSION

We report a loss of RDD in patients with ALS. The strong correlation detected between the RDD deficit and increased MUP suggests that RDD is a sensitive indicator of underlying spinal disinhibition in ALS.

TRIAL REGISTRATION

ChiCTR2000038848, 10/7/2020 (retrospectively registered), http://www.chictr.org.cn/ .

Chemical modulation of Kv7 potassium channels

The rising interest in Kv7 modulators originates from their ability to evoke fundamental electrophysiological perturbations in a tissue-specific manner. A large number of therapeutic applications are, in part, based on the clinical experience with two broad-spectrum Kv7 agonists, flupirtine and retigabine. Since precise molecular structures of human Kv7 channel subtypes in closed and open states have only very recently started to emerge, computational studies have traditionally been used to analyze binding modes and direct the development of more potent and selective Kv7 modulators with improved safety profiles. Herein, the synthetic and medicinal chemistry of small molecule modulators and the representative biological properties are summarized. Furthermore, new therapeutic applications supported by in vitro and in vivo assay data are suggested.

Axonal regeneration and sprouting as a potential therapeutic target for nervous system disorders

Nervous system disorders are prevalent health issues that will only continue to increase in frequency as the population ages. Dying-back axonopathy is a hallmark of many neurologic diseases and leads to axonal disconnection from their targets, which in turn leads to functional impairment. During the course of many of neurologic diseases, axons can regenerate or sprout in an attempt to reconnect with the target and restore synapse function. In amyotrophic lateral sclerosis (ALS), distal motor axons retract from neuromuscular junctions early in the disease-course before significant motor neuron death. There is evidence of compensatory motor axon sprouting and reinnervation of neuromuscular junctions in ALS that is usually quickly overtaken by the disease course. Potential drugs that enhance compensatory sprouting and encourage reinnervation may slow symptom progression and retain muscle function for a longer period of time in ALS and in other diseases that exhibit dying-back axonopathy. There remain many outstanding questions as to the impact of distinct disease-causing mutations on axonal outgrowth and regeneration, especially in regards to motor neurons derived from patient induced pluripotent stem cells. Compartmentalized microfluidic chambers are powerful tools for studying the distal axons of human induced pluripotent stem cells-derived motor neurons, and have recently been used to demonstrate striking regeneration defects in human motor neurons harboring ALS disease-causing mutations. Modeling the human neuromuscular circuit with human induced pluripotent stem cells-derived motor neurons will be critical for developing drugs that enhance axonal regeneration, sprouting, and reinnervation of neuromuscular junctions. In this review we will discuss compensatory axonal sprouting as a potential therapeutic target for ALS, and the use of compartmentalized microfluidic devices to find drugs that enhance regeneration and axonal sprouting of motor axons.

Neuropathy in sporadic inclusion body myositis: A multi-modality neurophysiological study

OBJECTIVE

Sporadic inclusion body myositis (sIBM) has been associated with neuropathy. This study employs nerve excitability studies to re-examine this association and attempt to understand underlying pathophysiological mechanisms.

METHODS

Twenty patients with sIBM underwent median nerve motor and sensory excitability studies, clinical assessments, conventional nerve conduction testing (NCS) and quantitative thermal threshold studies. These results were compared to established normal controls, or results from a normal cohort of older control individuals.

RESULTS

Seven sIBM patients (35%) demonstrated abnormalities in conventional NCS, with ten patients (50%) demonstrating abnormalities in thermal thresholds. Median nerve motor and sensory excitability differed significantly in sIBM patients when compared to normal controls. None of these neurophysiological markers correlated significantly with clinical markers of sIBM severity.

CONCLUSION

A concurrent neuropathy exists in a significant proportion of sIBM patients, with nerve excitability studies revealing changes possibly consistent with axolemmal depolarization or concurrent neuronal adaptation to myopathy. Neuropathy in sIBM does not correlate with muscle disease severity and may reflect a differing tissue response to a common pathogenic factor.

SIGNIFICANCE

This study affirms the presence of a concurrent neuropathy in a large proportion of sIBM patients that appears independent of the severity of myopathy.

Exciting Complexity: The Role of Motor Circuit Elements in ALS Pathophysiology

Amyotrophic lateral sclerosis (ALS) is a fatal disease, characterized by the degeneration of both upper and lower motor neurons. Despite decades of research, we still to date lack a cure or disease modifying treatment, emphasizing the need for a much-improved insight into disease mechanisms and cell type vulnerability. Altered neuronal excitability is a common phenomenon reported in ALS patients, as well as in animal models of the disease, but the cellular and circuit processes involved, as well as the causal relevance of those observations to molecular alterations and final cell death, remain poorly understood. Here, we review evidence from clinical studies, cell type-specific electrophysiology, genetic manipulations and molecular characterizations in animal models and culture experiments, which argue for a causal involvement of complex alterations of structure, function and connectivity of different neuronal subtypes within the cortical and spinal cord motor circuitries. We also summarize the current knowledge regarding the detrimental role of astrocytes and reassess the frequently proposed hypothesis of glutamate-mediated excitotoxicity with respect to changes in neuronal excitability. Together, these findings suggest multifaceted cell type-, brain area- and disease stage- specific disturbances of the excitation/inhibition balance as a cardinal aspect of ALS pathophysiology.

Simulating perinodal changes observed in immune-mediated neuropathies: impact on conduction in a model of myelinated motor and sensory axons

Immune-mediated neuropathies affect myelinated axons, resulting in conduction slowing or block that may affect motor and sensory axons differently. The underlying mechanisms of these neuropathies are not well understood. Using a myelinated axon model, we studied the impact of perinodal changes on conduction. We extended a longitudinal axon model (41 nodes of Ranvier) with biophysical properties unique to human myelinated motor and sensory axons. We simulated effects of temperature and axonal diameter on conduction and strength-duration properties. We then studied effects of impaired nodal sodium channel conductance and paranodal myelin detachment by reducing periaxonal resistance, as well as their interaction, on conduction in the 9 middle nodes and enclosed paranodes. Finally, we assessed the impact of reducing the affected region (5 nodes) and adding nodal widening. Physiological motor and sensory conduction velocities and changes to axonal diameter and temperature were observed. The sensory axon had a longer strength-duration time constant. Reducing sodium channel conductance and paranodal periaxonal resistance induced progressive conduction slowing. In motor axons, conduction block occurred with a 4-fold drop in sodium channel conductance or a 7.7-fold drop in periaxonal resistance. In sensory axons, block arose with a 4.8-fold drop in sodium channel conductance or a 9-fold drop in periaxonal resistance. This indicated that motor axons are more vulnerable to developing block. A boundary of block emerged when the two mechanisms interacted. This boundary shifted in opposite directions for a smaller affected region and nodal widening. These differences may contribute to the predominance of motor deficits observed in some immune-mediated neuropathies.NEW & NOTEWORTHY Immune-mediated neuropathies may affect myelinated motor and sensory axons differently. By the development of a computational model, we quantitatively studied the impact of perinodal changes on conduction in motor and sensory axons. Simulations of increasing nodal sodium channel dysfunction and paranodal myelin detachment induced progressive conduction slowing. Sensory axons were more resistant to block than motor axons. This could explain the greater predisposition of motor axons to functional deficits observed in some immune-mediated neuropathies.

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.

Warming nerves for excitability testing

INTRODUCTION

The aim of this study was to find the best method of warming the median nerve before excitability testing to a standard temperature.

METHODS

In 5 healthy subjects, the forearm and hand were warmed for 1 h to 37°C by infrared lamp, water blanket, or water bath. Recordings were performed before and during warming every 10 min. Excitability indices were fitted by exponential relations, thereby calculating the time needed to reach 95% of their asymptotic end value.

RESULTS

Distal motor latency, refractory period, and superexcitability at 10 ms changed exponentially with time. Warming by water bath took the shortest time (24 min); this was followed by warming by infrared lamp (34 min) and water blanket (35 min).

CONCLUSIONS

Warming by water bath is the quickest way. The other methods took only moderately more time. Future studies need to specify both warming method and warming time before excitability testing. Muscle Nerve, 2019.

Conduction block in immune-mediated neuropathy: paranodopathy versus axonopathy

BACKGROUND AND PURPOSE

Conduction block is a pathognomonic feature of immune-mediated neuropathies. The aim of this study was to advance understanding of pathophysiology and conduction block in chronic inflammatory demyelinating polyneuropathy (CIDP) and multifocal motor neuropathy (MMN).

METHODS

A multimodal approach was used, incorporating clinical phenotyping, neurophysiology, immunohistochemistry and structural assessments.

RESULTS

Of 49 CIDP and 14 MMN patients, 25% and 79% had median nerve forearm block, respectively. Clinical scores were similar in CIDP patients with and without block. CIDP patients with median nerve block demonstrated markedly elevated thresholds and greater threshold changes in threshold electrotonus, whilst those without did not differ from healthy controls in electrotonus parameters. In contrast, MMN patients exhibited marked increases in superexcitability. Nerve size was similar in both CIDP groups at the site of axonal excitability. However, CIDP patients with block demonstrated more frequent paranodal serum binding to teased rat nerve fibres. In keeping with these findings, mathematical modelling of nerve excitability recordings in CIDP patients with block support the role of paranodal dysfunction and enhanced leakage of current between the node and internode. In contrast, changes in MMN probably resulted from a reduction in ion channel density along axons.

CONCLUSIONS

The underlying pathologies in CIDP and MMN are distinct. Conduction block in CIDP is associated with paranodal dysfunction which may be antibody-mediated in a subset of patients. In contrast, MMN is characterized by channel dysfunction downstream from the site of block.