Measurement of axonal excitability: Consensus guidelines

Associations between acute glucose control and peripheral nerve structure and function in type 1 diabetes

AIM

To examine the associations between continuous overlapping net glycaemic action (CONGA), percentage time in hyperglycaemia (%HG) or normoglycaemia (%NG) and peripheral nerve structure and function in type 1 diabetes.

METHODS

Twenty-seven participants with type 1 diabetes underwent continuous glucose monitoring followed by corneal confocal microscopy and nerve excitability assessments. CONGA, %HG (> 10.0 mmol/l) and %NG (3.9-10.0 mmol/l) were correlated against corneal nerve fibre length and density in the central cornea and inferior whorl region, corneal microneuromas, and a nerve excitability score while controlling for age, sex, diabetes duration and HbA_1c .

RESULTS

An increase in CONGA [median 2.5 (2.0-3.1) mmol/l] or %HG (mean 46 ± 18%) was associated with a worse nerve excitability score (r = -0.433, P = 0.036 and r = -0.670, P = 0.0012, respectively). By contrast, greater %NG (51 ± 17%) correlated with better nerve excitability scores (r = 0.672, P = 0.0011). Logistic regression revealed that increasing %HG increased the likelihood of abnormal nerve function [odds ratio (OR) 1.11, 95% confidence interval (CI) 1.01-1.23; P = 0.037). An increase in CONGA and %HG were associated with worsening nerve conduction measures, whereas longer %NG correlated with improved nerve conduction variables. CONGA and %HG were associated with inferior whorl corneal nerve fibre length (r = 0.483, P = 0.034 and r = 0.591, P = 0.021, respectively) and number of microneuromas (r = 0.433, P = 0.047 and r = 0.516, P = 0.020, respectively).

CONCLUSIONS

Short-term measures of glucose control are associated with impaired nerve function and alterations in corneal nerve morphology.

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.

Myelin protein zero gene dose dependent axonal ion-channel dysfunction in a family with Charcot-Marie-Tooth disease

OBJECTIVE

The myelin impairment in demyelinating Charcot-Marie-Tooth (CMT) disease leads to various degrees of axonal degeneration, the ultimate cause of disability. We aimed to assess the pathophysiological changes in axonal function related to the neuropathy severity in hypo-/demyelinating CMT patients associated with myelin protein zero gene (MPZ) deficiency.

METHODS

We investigated four family members (two parents and two sons) harboring a frameshift mutation (c.306delA, p.Asp104ThrfsTer14) in the MPZ gene, predicted to result in a nonfunctional P₀, by conventional conduction studies and multiple measures of motor axon excitability. In addition to the conventional excitability studies of the median nerve at the wrist, we tested the spinal accessory nerves. Control measures were obtained from 14 healthy volunteers.

RESULTS

The heterozygous parents (aged 56 and 63) had a mild CMT1B whereas their two homozygous sons (aged 31 and 39 years) had a severe Dejerine-Sottas disease phenotype. The spinal accessory nerve excitability could be measured in all patients. The sons showed reduced deviations during depolarizing threshold electrotonus and other depolarizing features which were not apparent in the accessory and median nerve studies of the parents. Mathematical modeling indicated impairment in voltage-gated sodium channels. This interpretation was supported by comparative modeling of excitability measurements in MPZ deficient mice.

CONCLUSION

Our data suggest that axonal depolarization in the context of abnormal voltage-gated sodium channels precedes axonal degeneration in severely hypo-/demyelinating CMT as previously reported in the mouse models.

SIGNIFICANCE

Measures of the accessory nerve excitability could provide pathophysiological markers of neurotoxicity in severe demyelinating neuropathies.

Interrogating interneurone function using threshold tracking of the H reflex in healthy subjects and patients with motor neurone disease

OBJECTIVE

The excitability of the lower motoneurone pool is traditionally tested using the H reflex and a constant-stimulus paradigm, which measures changes in the amplitude of the reflex response. This technique has limitations because reflex responses of different size must involve the recruitment or inhibition of different motoneurones. The threshold-tracking technique ensures that the changes in excitability occur for an identical population of motoneurones. We aimed to assess this technique and then apply it in patients with motor neurone disease (MND).

METHODS

The threshold-tracking approach was assessed in 17 healthy subjects and 11 patients with MND. The soleus H reflex was conditioned by deep peroneal nerve stimulation producing reciprocal Ia and so-called D1 and D2 inhibitions, which are believed to reflect presynaptic inhibition of soleus Ia afferents.

RESULTS

Threshold tracking was quicker than the constant-stimulus technique and reliable, properties that may be advantageous for clinical studies. D1 inhibition was significantly reduced in patients with MND.

CONCLUSIONS

Threshold tracking is useful and may be preferable under some conditions for studying the excitability of the motoneurone pool. The decreased D1 inhibition in the patients suggests that presynaptic inhibition may be reduced in MND.

SIGNIFICANCE

Reduced presynaptic inhibition could be evidence of an interneuronopathy in MND. It is possible that the hyperreflexia is a spinal pre-motoneuronal disorder, and not definitive evidence of corticospinal involvement in MND.

Inhibition of Fast Nerve Conduction Produced by Analgesics and Analgesic Adjuvants-Possible Involvement in Pain Alleviation

Nociceptive information is transmitted from the periphery to the cerebral cortex mainly by action potential (AP) conduction in nerve fibers and chemical transmission at synapses. Although this nociceptive transmission is largely inhibited at synapses by analgesics and their adjuvants, it is possible that the antinociceptive drugs inhibit nerve AP conduction, contributing to their antinociceptive effects. Many of the drugs are reported to inhibit the nerve conduction of AP and voltage-gated Na⁺ and K⁺ channels involved in its production. Compound action potential (CAP) is a useful measure to know whether drugs act on nerve AP conduction. Clinically-used analgesics and analgesic adjuvants (opioids, non-steroidal anti-inflammatory drugs, ₂-adrenoceptor agonists, antiepileptics, antidepressants and local anesthetics) were found to inhibit fast-conducting CAPs recorded from the frog sciatic nerve by using the air-gap method. Similar actions were produced by antinociceptive plant-derived chemicals. Their inhibitory actions depended on the concentrations and chemical structures of the drugs. This review article will mention the inhibitory actions of the antinociceptive compounds on CAPs in frog and mammalian peripheral (particularly, sciatic) nerves and on voltage-gated Na⁺ and K⁺ channels involved in AP production. Nerve AP conduction inhibition produced by analgesics and analgesic adjuvants is suggested to contribute to at least a part of their antinociceptive effects.

Cold aggravates abnormal excitability of motor axons in oxaliplatin-treated patients

INTRODUCTION

Cold allodynia is often seen in the acute phase of oxaliplatin treatment, but the underlying pathophysiology remains unclear.

METHODS

Patients scheduled for adjuvant oxaliplatin for colorectal cancer were examined with quantitative sensory testing and nerve excitability tests at baseline and after the second or third oxaliplatin cycle at different skin temperatures.

RESULTS

Seven patients were eligible for examination. All patients felt evoked pain and tingling when touching something cold after oxaliplatin infusion. Oxaliplatin decreased motor nerve superexcitability (P < .001), increased relative refractory period (P = .011), and caused neuromyotonia-like after-activity. Cooling exacerbated these changes and prolonged the accommodation half-time.

DISCUSSION

The findings suggest that a combined effect of oxaliplatin and cooling facilitates nerve excitability changes and neuromyotonia-like after-activity in peripheral nerve axons. A possible mechanism is the slowing in gating of voltage-dependent fast sodium and slow potassium channels, which results in symptoms of cold allodynia.

Altered nerve excitability properties after stroke are potentially associated with reduced neuromuscular activation

OBJECTIVE

To determine limb differences in motor axon excitability properties in stroke survivors and their relation to maximal electromyographic (EMG) activity.

METHODS

The median nerve was stimulated to record compound muscle action potentials (CMAP) from the abductor pollicis brevis (APB) in 28 stroke subjects (57.3 ± 7.5 y) and 24 controls (56.7 ± 9.3 y).

RESULTS

Paretic limb axons differed significantly from non-paretic limb axons including (1) smaller superexcitability and subexcitability, (2) higher threshold during subthreshold depolarizing currents, (3) greater accommodation (S3) to hyperpolarization, and (4) a larger stimulus-response slope. There were smaller differences between the paretic and control limbs. Responses in the paretic limb were reproduced in a model by a 5.6 mV hyperpolarizing shift in the activation voltage of Ih (the current activated by hyperpolarization), together with an 11.8% decrease in nodal Na⁺ conductance or a 0.9 mV depolarizing shift in the Na⁺ activation voltage. Subjects with larger deficits in APB maximal voluntary EMG had larger limb differences in excitability properties.

CONCLUSIONS

Stroke leads to altered modulation of Ih and altered Na⁺ channel properties that may be partially attributed to a reduction in neuromuscular activation.

SIGNIFICANCE

Plastic changes occur in the axon node and internode that likely influence axon excitability.

Early sensory neurophysiological changes in prediabetes

AIMS/INTRODUCTION

To elucidate whether axonal changes arise in the prediabetic state and to find a biomarker for early detection of neurophysiological changes.

MATERIALS AND METHODS

We enrolled asymptomatic diabetes patients, as well as prediabetic and normoglycemic individuals to test sensory nerve excitability, and we analyzed those findings and their correlation with clinical profiles.

RESULTS

In nerve excitability tests, superexcitability in the recovery cycle showed increasing changes in the normoglycemic, prediabetes and diabetes cohorts (-19.09 ± 4.56% in normoglycemia, -22.39 ± 3.16% in prediabetes and -23.71 ± 5.15% in diabetes, P = 0.002). Relatively prolonged distal sensory latency was observed in the median nerve (3.12 ± 0.29 ms in normoglycemia, 3.23 ± 0.38 ms in prediabetes and 3.45 ± 0.43 ms in diabetes, P = 0.019). Superexcitability was positively correlated with fasting plasma glucose (r = 0.291, P = 0.009) and glycated hemoglobin (r = 0.331, P = 0.003) in all participants.

CONCLUSIONS

Sensory superexcitability and latencies are the most sensitive parameters for detecting preclinical physiological dysfunction in prediabetes. In addition, changes in favor of superexcitability were positively correlated with glycated hemoglobin for all participants. These results suggest that early axonal changes start in the prediabetic stage, and that the monitoring strategy for polyneuropathy should start as early as prediabetes.