High-dose pharmaceutical-grade biotin in patients with demyelinating neuropathies: a phase 2b open label, uncontrolled, pilot study

BACKGROUND

We proposed to investigate high-dose pharmaceutical-grade biotin in a population of demyelinating neuropathies of different aetiologies, as a proof-of-concept.

METHODS

Phase IIb open label, uncontrolled, single center, pilot study in 15 patients (three groups of five patients) with chronic demyelinating peripheral neuropathy, i.e. chronic inflammatory demyelinating polyradiculoneuropathy, anti-myelin-associated glycoprotein neuropathy and Charcot-Marie-Tooth 1a or 1b. The investigational product was high-dose pharmaceutical-grade biotin (100 mg taken orally three times a day over a maximum of 52 weeks. The primary endpoint was a 10% relative improvement in 2 of the following 4 electrophysiological variables: motor nerve conduction velocity, distal motor latency, F wave latency, duration of the compound muscle action potential. The secondary endpoints included Overall Neuropathy Limitations Scale (ONLS) score, Medical Research Council (MRC) sum score, Inflammatory Neuropathy Cause and Treatment (INCAT) sensory sum score, 10-m walk test, 6-min walk test, posturography parameters, and nerve excitability variables.

RESULTS

The primary endpoint was reached in one patient. In the full population analysis, some secondary endpoints parameters improved: MRC score, INCAT sensory sum score, 6-min walk distance, strength-duration time constant, and rheobase. There was a positive correlation between the improvement in the 6-min walk distance and the strength-duration time constant. Regarding the safety results, 42 adverse events occurred, of which three were of severe intensity but none was considered as related to the investigational product.

CONCLUSIONS

Even if the primary endpoint was not met, administration of high-dose pharmaceutical-grade biotin led to an improvement in various sensory and motor parameters, gait abilities, and nerve excitability parameters. The tolerance of the treatment was satisfactory.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT02967679; date 2016/12/05.

[Effects of repetitive transcranial magnetic stimulation on neuronal excitability and ion channels in hindlimb unloading mice]

Weightlessness in the space environment affects astronauts' learning memory and cognitive function. Repetitive transcranial magnetic stimulation has been shown to be effective in improving cognitive dysfunction. In this study, we investigated the effects of repetitive transcranial magnetic stimulation on neural excitability and ion channels in simulated weightlessness mice from a neurophysiological perspective. Young C57 mice were divided into control, hindlimb unloading and magnetic stimulation groups. The mice in the hindlimb unloading and magnetic stimulation groups were treated with hindlimb unloading for 14 days to establish a simulated weightlessness model, while the mice in the magnetic stimulation group were subjected to 14 days of repetitive transcranial magnetic stimulation. Using isolated brain slice patch clamp experiments, the relevant indexes of action potential and the kinetic property changes of voltage-gated sodium and potassium channels were detected to analyze the excitability of neurons and their ion channel mechanisms. The results showed that the behavioral cognitive ability and neuronal excitability of the mice decreased significantly with hindlimb unloading. Repetitive transcranial magnetic stimulation could significantly improve the cognitive impairment and neuroelectrophysiological indexes of the hindlimb unloading mice. Repetitive transcranial magnetic stimulation may change the activation, inactivation and reactivation process of sodium and potassium ion channels by promoting sodium ion outflow and inhibiting potassium ion, and affect the dynamic characteristics of ion channels, so as to enhance the excitability of single neurons and improve the cognitive damage and spatial memory ability of hindlimb unloading mice.

A test to determine the site of abnormal neuromuscular refractoriness

Objective

The relative refractory period (RRP) of motor axons is an important parameter in nerve excitability tests of the recovery cycle (RC). Abnormalities may have a site in the axonal membrane, the neuromuscular junction, or in a dysfunction of the muscle. We aimed in this study to determine the site of abnormality, using a modified protocol of the conventional RC test, whereby an additional supramaximal stimulus is added at the same interstimulus interval as in RC recordings (RCSM).

Methods

Twenty-four healthy subjects aged 37.8 ± 2.4 years (mean ± SE) were examined with median nerve excitability testing using RC and RCSM protocols at normal temperature (34.1 ± 0.2 °C). The recordings were repeated in 12 subjects after selective cooling of the thenar muscle (25.2 ± 0.7 °C) and in 12 subjects after cooling the nerve trunk at the wrist (24.9 ± 0.3 °C).

Results

After cooling the nerve, RRP measured with RC and RCSM were prolonged similarly (medians by 1.8 ms, and 2.1 ms respectively). In contrast, cooling the muscle prolonged RRP measured with RC (by 1.3 ms), but did not significantly prolong RRP measured with RCSM. RRPs measured by RC and RCSM were significantly different when cooling was at the muscle (P = 5.10^-4), but not when cooling was at the nerve (P = 0.57).

Conclusions

A difference between RC and RCSM indicates abnormal excitability distal to the axonal membrane under the stimulating electrode.

Significance

Combining RCSM with the conventional RC protocol should help to localize the site of abnormal neuromuscular refractoriness.

Effect of exenatide on peripheral nerve excitability in type 2 diabetes

OBJECTIVE

To assess the effect of exenatide (a GLP-1 receptor agonist), dipeptidyl peptidase-IV (DPP-IV) inhibitors, and sodium-glucose co-transporter 2 (SGLT-2) inhibitors on measures of peripheral nerve excitability in patients with type 2 diabetes.

METHODS

Patients receiving either exenatide (n = 32), a DPP-IV inhibitor (n = 31), or a SGLT-2 inhibitor (n = 27) underwent motor nerve excitability assessments. Groups were similar in age, sex, HbA_1c, diabetes duration, lipids, and neuropathy severity. An additional 10 subjects were assessed prospectively over 3 months while oral anti-hyperglycaemic therapy was kept constant. A cohort of healthy controls (n = 32) were recruited for comparison.

RESULTS

Patients receiving a DPP-IV or SGLT-2 inhibitor demonstrated abnormalities in peak threshold reduction, S2 accommodation, superexcitability, and subexcitability. In contrast, patients treated with exenatide were observed to have normal nerve excitability. In the prospective arm, exenatide therapy was associated with an improvement in nerve function as patients demonstrated corrections in S2 accommodation, superexcitability, and subexcitability at follow-up. These changes were independent of the reductions in HbA_1c following exenatide treatment.

CONCLUSIONS

Exenatide was associated with an improvement in measures of nerve excitability in patients with type 2 diabetes.

SIGNIFICANCE

Exenatide may improve peripheral nerve function in type 2 diabetes.

Early surgery improves peripheral motor axonal dysfunction in acute traumatic central cord syndrome: A prospective cohort study

OBJECTIVE

To investigate the impact of early vs. delayed surgical decompression on peripheral motor axonal dysfunction following acute traumatic central cord syndrome (ATCCS).

METHODS

Both axonal excitability testing and motor unit number estimation (MUNE) were performed in 30 ATCCS patients (early- vs. delayed-surgical treatment: 12 vs. 18) before operation and 28 healthy subjects. Axonal excitability testing was repeated 3-5 days and 1-year after operation, and MUNE was re-evaluated 1-year after operation.

RESULTS

Preoperatively, an obvious modification in membrane potentials was observed in ATCCS patients that mostly coincided with depolarization-like features, and MUNE further revealed reduced motor units in tested muscles (P < 0.05). Unlike delayed-surgical cases, early-surgical cases showed recoveries of most measurements of axonal excitabilities soon after operation (P < 0.05). Postoperative one-year follow-up demonstrated that greater motor unit numbers in tested muscles were obtained in early-surgical cases than in delayed-surgical cases (P < 0.05).

CONCLUSIONS

ATCCS has adverse downstream effects on peripheral nervous system, even in the early stage of ATCCS. Early surgical treatment can ameliorate both excitability abnormalities and motor unit loss in distal motor axons.

SIGNIFICANCE

Optimizing axonal excitability in the early phases of ATCCS may alleviate peripheral nerve injury secondary to lesions of upper motor neuron and improve clinical outcomes.

Altered sensory nerve excitability in fibromyalgia

BACKGROUND/PURPOSE

To investigate nerve excitability changes in patients with fibromyalgia and the correlation with clinical severity.

METHODS

We enrolled 20 subjects with fibromyalgia and 22 sex and age-matched healthy subjects to receive nerve excitability test and nerve conduction study to evaluate the peripheral axonal function.

RESULTS

In the fibromyalgia cohort, the sensory axonal excitability test revealed increased superexcitability (%) (P = 0.029) compared to healthy control. Correlational study showed a negative correlation between increased subexcitability (%) (r = -0.534, P = 0.022) with fibromyalgia impact questionnaire (FIQ) score. Computer modeling confirmed that the sensory axon excitability pattern we observed in fibromyalgia cohort was best explained by increased Barrett-Barrett conductance, which was thought to be attributed to paranodal fast K⁺ channel dysfunction.

CONCLUSION

The present study revealed that paranodal sensory K⁺ conductance was altered in patients with fibromyalgia. The altered conductance indicated dysfunction of paranodal fast K⁺ channels, which is known to be associated with the generation of pain.

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.

Peripheral nerve maturation and excitability properties from early childhood: Comparison of motor and sensory nerves

OBJECTIVE

Understanding of maturational properties of sensory and motor axons is of central importance for determining the impact of nerve changes in health and in disease in children and young adults.

METHODS

This study investigated maturation of sensory axons using axonal excitability parameters of the median nerve in 47 children, adolescents and young adults (25 males, 22 females; age range 1-25 years) and compared them to concurrent motor studies.

RESULTS

The overall pattern of sensory maturation was similar to motor maturation demonstrating prolongation of the strength duration time constant (P < 0.001), reduction of hyperpolarising threshold electrotonus (P = 0.002), prolongation of accommodation half-time (P = 0.005), reduction in hyperpolarising current-threshold slope (P = 0.03), and a shift to the right of the refractory cycle curve (P < 0.001), reflecting changes in passive membrane properties and fast potassium channel conductances. Sensory axons, however, had a greater increase in strength duration time constant and more attenuated changes in depolarising threshold electrotonus and current-threshold parameters, attributable to a more depolarised resting membrane potential evident from early childhood and maintained in adults. Peak amplitude was established early in sensory axons whereas motor amplitude increased with age (P < 0.001), reflecting non-axonal motor unit changes.

CONCLUSIONS

Maturational trajectories of sensory and motor axons were broadly parallel in children and young adults, but sensory-motor differences were initiated early in maturation.

SIGNIFICANCE

Identifying the evolution of biophysical changes within and between sensory and motor axons through childhood and adolescence is fundamental to understanding developmental physiology and interpreting disease-related changes in immature nerves.

Neu-horizons: neuroprotection and therapeutic use of riluzole for the prevention of oxaliplatin-induced neuropathy-a randomised controlled trial

TRIAL DESIGN

Peripheral neuropathy is a commonly reported adverse effect of oxaliplatin treatment, representing a significant limitation which may require discontinuation of effective therapy. The present study investigated the neuroprotective potential of riluzole in patients undergoing oxaliplatin treatment in a randomised-controlled trial comparing riluzole and placebo-control.

METHODS

Fifty-two patients (17 females, 58.1 ± 12.7 years) receiving oxaliplatin treatment were randomised into either a treatment (50 mg riluzole) or lactose placebo group. The primary outcome measure was the total neuropathy score-reduced (TNSr). Secondary outcome measures include nerve excitability measures, 9-hole pegboard and FACT-GOG NTX questionnaire. Patients were assessed at baseline, pre-cycle 10 or 12, 4-week and 12-week post-treatment.

RESULTS

Both the treatment and placebo groups developed objective and patient reported evidence of neurotoxicity over the course of oxaliplatin treatment, although there were no significant differences across any parameters between the two groups. However, across follow-up assessments, the treatment group experienced greater neuropathy, represented by a higher TNSr score at 4-week post-chemotherapy of 8.3 ± 2.7 compared with 4.6 ± 3.6 (p = 0.032) which was sustained at 12-week post-treatment (p = 0.089). Similarly, patients in the treatment group reported worse symptoms with a FACT-GOG NTX score of 37.4 ± 10.2 compared with 43.3 ± 7.4 (p = 0.02) in the placebo group at 4-week post-treatment.

CONCLUSION

This study is the first to provide an objective clinical investigation of riluzole in oxaliplatin-induced peripheral neuropathy employing both functional and neurophysiological measures. Although the recruitment target was not reached, the results do not show any benefit of riluzole in minimising neuropathy and may suggest that riluzole worsens neuropathy associated with oxaliplatin treatment.

Motor excitability measurements in early stage familial amyloid polyneuropathy: The influence of tafamidis treatment

OBJECTIVE

To test motor fiber excitability in early affected patients with transthyretin (TTR)-type familial amyloid polyneuropathy (TTR-FAP) before and during tafamidis treatment.

METHODS

We examined the left median nerve of 21 healthy-matched controls and 10 early affected TTR-FAP patients using the automated threshold-tracking program, QTRAC. TTR-FAP patients were tested one day before the initiation of tafamidis treatment, 3 and 6 months later.

RESULTS

The drug was well-tolerated in all patients; there was no drop-out. No statistical difference was found between healthy controls and TTR-FAP patients at study entry. On treatment, both stimulus intensity for 50% of the maximal motor response and rheobase increased significantly from entry to the last evaluation at 6 months (P<0.05). Strength duration time constant decreased significantly from the 3rd to the 6th month of evaluation (P<0.05). There was also a "fanning-out" effect on the late depolarization phase (TEd 90-100ms) as well as a shortened relative refractory period from study entry to the 6th month of evaluation.

CONCLUSIONS

Threshold-tracking of median nerve motor fibers is not a helpful technique for the early diagnosis of TTR-FAP patients. Tafamidis was well-tolerated. We observed possible membrane hyperpolarization during treatment. Threshold tracking can contribute to documenting the action of new drugs to treat neuropathies. Tafamidis may change nerve electrical properties by reducing the burden of amyloid fibrils.

Modulation of sensory nerve fiber excitability by transcutaneous cathodal direct current stimulation

OBJECTIVE

To assess the lasting effects on sensory nerve membrane excitability of transcutaneous peripheral nerve stimulation with cathodal direct currents (pDCS).

METHODS

We performed pDCS in 10 healthy subjects with the active electrode placed over the distal right forearm and the reference electrode on the back of the right hand. We used 5×5cm rubber electrodes and the current applied was 2.5mA during 15min. Three pDCS sessions were performed on the same day: first, a baseline stimulation was performed, followed by a sham stimulation and lastly a cathodal stimulation. Median sensory nerve excitability measurements were performed at baseline and immediately after each pDCS session using the TRONDNF nerve excitability protocol of the QTRAC program (measurement on the second finger).

RESULTS

The protocol was completed and well tolerated in all subjects. RRP (relative refractory period) and refractoriness at 2.5ms were significantly different across the three study conditions, with a significant increase of RRP immediately following cathodal stimulation compared with baseline assessment (mean 4.2 versus 5.3, P=0.002). Other measurements were not modulated by the intervention. Sham-stimulation did not change axonal excitability.

CONCLUSIONS

Cathodal pDCS stimulation increased RRP of sensory fibers, but no other consistent long-lasting effect was observed. This finding might suggest a reduction of sensory fiber excitability induced by cathodal pDCS.

Relative contributions of diabetes and chronic kidney disease to neuropathy development in diabetic nephropathy patients

OBJECTIVE

Chronic kidney disease (CKD) caused by diabetes is known as diabetic kidney disease (DKD). The present study aimed to examine the underlying mechanisms of axonal dysfunction and features of neuropathy in DKD compared to CKD and type 2 diabetes (T2DM) alone.

METHODS

Patients with DKD (n = 30), CKD (n = 28) or T2DM (n = 40) and healthy controls (n = 41) underwent nerve excitability assessments to examine axonal function. Neuropathy was assessed using the Total Neuropathy Score. A validated mathematical model of human axons was utilised to provide an indication of the underlying causes of nerve pathophysiology.

RESULTS

Total neuropathy score was significantly higher in patients with DKD compared to those with either CKD or T2DM (p < 0.05). In DKD, nerve excitability measures (S2 accommodation and superexcitability, p < 0.05) were more severely affected compared to both CKD and T2DM and worsened with increasing serum K⁺ (p < 0.01). Mathematical modelling indicated the basis for nerve dysfunction in DKD was an elevation of extracellular K⁺ and reductions in Na⁺ permeability and the hyperpolarisation-activated cation current, which was similar to CKD.

CONCLUSIONS

Patients with DKD manifested a more severe neuropathy phenotype and shared features of nerve dysfunction to that of CKD.

SIGNIFICANCE

The CKD, and not diabetes component, appears to underlie axonal pathophysiology in DKD.

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.

Nerve excitability changes related to muscle weakness in chronic progressive external ophthalmoplegia

OBJECTIVE

To explore potential spreading to peripheral nerves of the mitochondrial dysfunction in chronic progressive external ophthalmoplegia (CPEO) by assessing axonal excitability.

METHODS

CPEO patients (n=13) with large size deletion of mitochondrial DNA and matching healthy controls (n=22) were included in a case-control study. Muscle strength was quantified using MRC sum-score and used to define two groups of patients: CPEO-weak and CPEO-normal (normal strength). Nerve excitability properties of median motor axons were assessed with the TROND protocol and changes interpreted with the aid of a model.

RESULTS

Alterations of nerve excitability strongly correlated with scores of muscle strength. CPEO-weak displayed abnormal nerve excitability compared to CPEO-normal and healthy controls, with increased superexcitability and responses to hyperpolarizing current. Modeling indicated that the CPEO-weak recordings were best explained by an increase in the 'Barrett-Barrett' conductance across the myelin sheath.

CONCLUSION

CPEO patients with skeletal weakness presented sub-clinical nerve excitability changes, which were not consistent with axonal membrane depolarization, but suggested Schwann cell involvement.

SIGNIFICANCE

This study provides new insights into the spreading of large size deletion of mitochondrial DNA to Schwann cells in CPEO patients.

Fingolimod attenuates experimental autoimmune neuritis and contributes to Schwann cell-mediated axonal protection

Background

Fingolimod, a sphingosine-1-phosphate receptor modulator with well-described immunomodulatory properties involving peripheral immune cell trafficking, was the first oral agent approved for the treatment of relapsing remitting multiple sclerosis. Analogous immunomodulatory treatment options for chronic peripheral autoimmune neuropathies are lacking.

Methods

We tested fingolimod in the animal model of experimental autoimmune neuritis in Lewis rat. Six to eight-week-old female rats were immunized with P2 peptide and from this day on treated with fingolimod. Histology of the sciatic nerve was done to analyze T cell and macrophage cell count, intercellular adhesion molecule (ICAM) and amyloid precursor protein (APP) expression, as well as apoptotic Schwann cell counts.

Results

Preventive oral treatment with 0.1 mg/kg up to 3 mg/kg fingolimod once daily dissolved in rapeseed oil completely ameliorated clinical neuritis signs. It reduced circulating peripheral blood T cells and infiltrating T cells and macrophages in the sciatic nerve, whereas at the same time, it preserved blood-nerve barrier impermeability. Most importantly, fingolimod showed beneficial properties on the pathogenic process as indicated by fewer apoptotic Schwann cells and a lower amount of amyloid precursor protein indicative of axonal damage at the peak of disease course.

Conclusions

Taken together, orally administered low-dose fingolimod showed an impressive immunomodulatory effect in the rat model of experimental autoimmune neuritis. Our current observations introduce fingolimod as an attractive treatment option for neuritis patients.

Cytokine cascades induced by mechanical trauma injury alter voltage-gated sodium channel activity in intact cortical neurons

BACKGROUND

Traumatic brain injury (TBI) triggers both immediate (primary) and long-term (secondary) tissue damages. Secondary damages can last from hours to days or even a lifetime. Secondary damages implicate several mechanisms, including influence of inflammatory mediators, mainly cytokines, on excitability of ion channels. However, studies should further explore the effects of inflammatory cytokines on voltage-gated sodium channels (VGSCs) and excitability in distal intact neurons.

METHODS

Mixed cultures of mouse cortical astrocytes and neurons were subjected to mechanical injury (trauma) to mimic TBI in vitro. Expression of various cytokines in these cultures were measured by real-time polymerase chain reaction and enzyme-linked immunosorbent assay. A trauma-conditioned medium with or without brain-derived neurotrophic factor (BDNF) was added to mouse primary cortical neurons for 6 and 24 h to mimic combined effects of multiple inflammatory cytokines on VGSCs. Spike behaviors of distal intact neurons were examined by whole-cell patch-clamp recordings.

RESULTS

Mechanical injury in mixed cortical neuron-astrocyte cultures significantly increased expression levels of multiple cytokines, including interleukin (IL)-1β, IL-6, tumor necrosis factor-α, monocyte chemoattractant protein-1, chemokine (C-C motif) ligand-5, IL-10, and transforming growth factor-β1, at 6 and 24 h after injury. Incubation in trauma-conditioned medium increased functional VGSCs in neuronal membranes and Na⁺ currents. Enhanced VGSCs were almost completely abolished by BDNF, and reinforcement of Na⁺ currents was also reduced in a dose-dependent manner. BDNF (30 ng/mL) also significantly reversed reduced neuronal cell viability, which was induced by medium conditioned at 6 h. At 6 and 24 h, trauma-conditioned medium significantly increased spike frequency but not spike threshold.

CONCLUSIONS

In TBI, the combined effect of inflammatory cytokines is directly involved in VGSC, Na⁺ current, and excitability dysfunction in distal intact neurons. BDNF may partly exert neuroprotective effects by maintaining balance of VGSC function in distal intact neurons.

An oral NaV1.8 blocker improves motor function in mice completely deficient of myelin protein P0

Mice deficient of myelin protein P0 are established models of demyelinating Charcot-Marie-Tooth (CMT) disease. Dysmyelination in these mice is associated with an ectopic expression of the sensory neuron specific sodium channel isoform NaV1.8 on motor axons. We reported that in P0+/-, a model of CMT1B, the membrane dysfunction could be acutely improved by a novel oral NaV1.8 blocker referred to as Compound 31 (C31, Bioorg. Med. Chem. Lett. 2010, 20, 6812; AbbVie Inc.). The aim of this study was to investigate the extent to which C31 treatment could also improve the motor axon function in P0-/-, a CMT model with a much more severe neuropathy. We found that the progressive impairment of motor performance from 1 to 4 months of age in P0-/- could be acutely reversed by C31 treatment. The effect was associated with an improvement of the amplitude of the plantar CMAP evoked by tibial nerve stimulation. The corresponding motor nerve excitability studies by "threshold tracking" showed changes after C31 consistent with attenuation of a resting membrane depolarization. Our data suggest that the depolarizing motor conduction failure in P0-/- could be acutely improved by C31. This provides proof-of-concept that treatment with oral subtype-selective NaV1.8 blockers could be used to improve the motor function in severe forms of demyelinating CMT.

Effect of fampridine on axonal excitability in multiple sclerosis

OBJECTIVE

To investigate the effects of fampridine on nerve excitability, the present study utilized peripheral axonal excitability techniques in 18 MS patients receiving treatment with fampridine.

METHODS

Studies were performed at baseline and repeated 3months after institution of fampridine at standard dosing.

RESULTS

Following treatment with fampridine there were significant changes in axonal excitability for those parameters associated with fast K(+) channels that shifted towards normal control values. Specifically, increases were noted in the peak superexcitability of recovery cycle (fampridine, -25.6±1.6%; baseline -22.8±1.7%; p<0.004), peak depolarizing threshold electrotonus (fampridine, 69.1±1.0%; baseline 67.0±1.4%; p<0.004), and depolarizing threshold electrotonus between 40 and 60ms after onset of depolarization (fampridine, 52.8±1.3%; baseline 49.9±1.4%; p=0.02).

CONCLUSION

The present study has established that fampridine at standard doses exerts effects on peripheral nerve function that may be mediated by reduction of fast K(+) conductances.

SIGNIFICANCE

Modulation of fast K(+) conductances by fampridine may contribute to the improvement observed in MS symptoms including motor fatigue.

Pathophysiology of motor dysfunction in a childhood motor neuron disease caused by mutations in the riboflavin transporter

OBJECTIVE

Brown-Vialetto-Van Laere (BVVL) syndrome is a progressive motor and sensory neuronopathy secondary to mutations in SLC52A2 encoding the riboflavin transporter type 2 (RFVT2). The phenotype is characterized by early childhood onset hearing loss and sensory ataxia followed by progressive upper limb weakness, optic atrophy, bulbar weakness and respiratory failure. To gain further insight into disease pathophysiology and response to riboflavin supplementation, the present study investigated whether axonal ion channel or membrane abnormalities were a feature of BVVL.

METHODS

Axonal excitability studies and clinical assessments were prospectively undertaken on six patients with BVVL secondary to riboflavin transporter deficiency type 2 (age range 10-21 years) at baseline and after 12 months of riboflavin (1000 mg daily) therapy.

RESULTS

At baseline, depolarizing and hyperpolarizing threshold electrotonus was 'fanned out' and superexcitability was increased, while the resting current-threshold gradient and refractoriness were significantly reduced in BVVL patients when compared to controls. Mathematical modeling suggested that functional alterations of myelin underlay these findings with an increase in myelin permeability. Riboflavin therapy resulted in partial normalization of the axonal excitability findings, paralleled by maintenance of muscle strength.

CONCLUSIONS

The present study established that abnormalities in myelin permeability at the paranode was a feature of BVVL and were partially normalized with riboflavin therapy.

SIGNIFICANCE

This study reveals a novel pathophysiological process for motor nerve dysfunction in BVVL. It also indicates that nerve excitability studies may be further developed in larger cohorts as a potential biomarker to identify treatment response for BVVL patients.

Relative sparing of the second lumbrical muscle in carpal tunnel syndrome is not associated with regional differences in axonal membrane potential

OBJECTIVE

Regional differences in nerve resting membrane potential have been associated with susceptibility to entrapment neuropathy. The aim of this study was to test whether the different susceptibilities to carpal tunnel syndrome (CTS) of median nerve motor axons supplying the second lumbrical (L2) and abductor pollicis brevis (APB) muscles could be explained in this way.

METHODS

Computerized nerve-excitability testing was used to examine the excitability properties of the median motor axons of both L2 and APB in 24 healthy volunteers.

RESULTS

Although some excitability measurements differed between the L2 and APB motor axons, estimates of resting membrane potential (RMP) by model fitting indicated no significant difference between the two groups.

CONCLUSION

Differences in RMP cannot account for the relative sparing of L2 axons in severe CTS.

SIGNIFICANCE

L2 sparing in CTS most likely has an anatomical rather than a biophysical basis.

Peripheral nerve axonal excitability studies: expanding the neurophysiologist's armamentarium

Nerve excitability studies have emerged as a recent novel non-invasive technique that offers complementary information to that provided by more conventional nerve conduction studies, the latter which provide only limited indices of peripheral nerve function. Such novel tools allow for the assessment of peripheral axonal biophysical properties that include ion channels, energy-dependent pumps and membrane potential in health and disease. With improvements in technique and development of protocols, a typical study can be completed in a short period of time and rapid measurement of multiple excitability indices can be achieved that provide insight into different aspects of peripheral nerve function. The advent of automated protocols for the assessment of nerve excitability has promoted their use in previous studies investigating disease pathophysiology such as in metabolic, toxic and demyelinating neuropathies, amyotrophic lateral sclerosis, stroke, spinal cord injury and inherited channelopathies. In more recent years, the use of nerve excitability studies have additionally provided insights into the pathophysiological mechanisms underlying cerebellar disorders that include stroke and familial cerebellar ataxias such as episodic ataxia types 1 and 2. Moreover, this technique may have diagnostic and therapeutic implications that may encompass a broader range of neurodegenerative cerebellar ataxias in years to come. In the foreseeable future, this technique may eventually be incorporated into clinical practice expanding the currently available armamentarium to the neurophysiologist.

Axonal excitability in X-linked dominant Charcot Marie Tooth disease

OBJECTIVE

We investigated peripheral nerve function in X-linked Charcot-Marie-Tooth disease type 1 (CMTX1), and considered the functional consequences of mutant connexin-32.

METHODS

Twelve subjects (9 female, 3 male) were assessed clinically, by nerve conduction and excitability studies. A model of myelinated axon was used to clarify the contributing changes.

RESULTS

All subjects had abnormal nerve conduction. Excitability studies on median nerve axons showed greater threshold changes to hyperpolarising currents, with "fanning out" in threshold electrotonus, and modest changes in the recovery cycle. Modelling suggested shortening of internodal length, increase in nodal fast potassium currents, shift of the voltage activation hyperpolarisation-activated cyclic-nucleotide-gated channels, and axonal hyperpolarisation. Plotting threshold versus extent of hyperpolarising threshold change in threshold electrotonus distinguished the CMTX1 patients from other chronic demyelinating neuropathies reported in the literature except hereditary neuropathy with pressure palsies (HNPP).

CONCLUSIONS

Some measures of axonal excitability are similar in CMTX1 and HNPP (though not the recovery cycle), but they differ from those in other chronic demyelinating neuropathies. The findings in CMTX1 are consistent with known pathology, but are not correlated to neuropathy severity.

SIGNIFICANCE

The findings in CMTX1 could be largely the result of morphological alterations, rather than plasticity in channel expression or distribution.

Evidence for a causal relationship between hyperkalaemia and axonal dysfunction in end-stage kidney disease

OBJECTIVE

Potassium (K(+)) has been implicated as a factor in the development of uraemic neuropathy. This study was undertaken to investigate whether hyperkalaemia plays a causal role in axonal dysfunction in end-stage kidney disease (ESKD).

METHODS

Median motor nerve excitability studies were undertaken in four haemodialysis patients during a modified dialysis session. The serum K(+) level was "clamped" (fixed) for the first 3h of dialysis, whilst allowing all other solutes to be removed, this was followed by dialysis against low dialysate K(+) for a further 4 h. Blood chemistry and nerve excitability studies were undertaken prior to, during and following dialysis. Results were compared to results from the same patients during routine dialysis sessions.

RESULTS

All patients demonstrated significant nerve excitability abnormalities reflective of nerve membrane depolarization in pre-dialysis recordings (p<0.01). After the 3 h clamp period, serum K(+) remained elevated (5.0 mmol/L) and nerve excitability remained highly abnormal, despite the significant clearance of other uraemic toxins. In contrast, studies undertaken during routine dialysis sessions demonstrated significant improvement in both serum K(+) and nerve function after 3 h.

CONCLUSIONS

The current study has established a causal relationship between serum K(+) and axonal membrane depolarization in haemodialysis patients.

SIGNIFICANCE

From a clinical perspective, strict K(+) control may help improve nerve function in ESKD.