In vivo evidence of reduced nodal and paranodal conductances in type 1 diabetes

Progression of axonal excitability abnormalities with increasing clinical severity of diabetic peripheral neuropathy

OBJECTIVE

Diabetic peripheral neuropathy (DPN) is a frequent complication for persons with type 2 diabetes. Previous studies have failed to demonstrate any significant impact of treatment for DPN. The present study assessed the role of axonal ion channel dysfunction in DPN and explored the hypothesis that there may be a progressive change in ion channel abnormalities that varied with disease stage.

METHODS

Neurophysiological studies were conducted using axonal excitability techniques, a clinical method of assessing ion channel dysfunction. Studies were conducted in 178 persons with type 2 diabetes, with participants allocated into four groups according to clinical severity of neuropathy, assessed using the Total Neuropathy Grade.

RESULTS

Analysis of excitability data demonstrated a progressive and stepwise reduction in two parameters that are related to the activity of Kv1.1 channels, namely superexcitability and depolarizing threshold electrotonus at 10-20 ms (p < 0.001), and mathematical modelling of axonal excitability findings supported progressive upregulation of Kv1.1 conductances with increasing greater disease severity.

CONCLUSION

The findings are consistent with a progressive upregulation of juxtaparanodal Kv1.1 conductances with increasing clinical severity of diabetic peripheral neuropathy.

SIGNIFICANCE

From a translational perspective, the study suggests that blockade of Kv1.1 channels using 4-aminopyridine derivatives such as fampridine may be a potential treatment for DPN.

Perception threshold tracking: validating a novel method for assessing function of large and small sensory nerve fibers in diabetic peripheral neuropathy with and without pain

ABSTRACT

It remains unknown why some people with diabetes develop painful neuropathies while others experience no pain. This study aimed to validate a novel method for assessing the function of small sensory nerves in diabetes to further elucidate this phenomenon. The function of large and small nerves was assessed using a novel perception threshold tracking technique in 3 well-characterized groups (n = 60) with type 1 diabetes, namely, (1) painful diabetic peripheral neuropathy (T1DM + PDPN), (2) painless diabetic peripheral neuropathy (T1DM + DPN), and (3) no neuropathy (T1DM - DPN), and healthy controls (n = 20). Electrical currents with different shapes, duration, and intensities were applied by 2 different skin electrodes activating large and small fibers, respectively. The minimal current needed to activate the fibers were analyzed as the rheobase of the stimulus-response function. Nerve fiber selectivity was measured by accommodation properties of stimulated nerves. The rheobase of both fiber types were highest for T1DM + PDPN, followed by T1DM + DPN, T1DM - DPN, and healthy controls, indicating that the nerve properties are specific in individuals with diabetes and pain. There was an overall significant difference between the groups ( P < 0.01). The accommodation properties of stimulated fibers were different between the 2 electrodes ( P < 0.05) apart from in the group with T1DM + PDPN, where both electrodes stimulated nerves displaying properties similar to large fibers. Perception threshold tracking reveals differences in large and small nerve fiber function between the groups with and without diabetes, DPN, and pain. This indicates that the methods have potential applications in screening DPN and explore further the features differentiating painful from nonpainful DPN.

Sensory and motor axonal excitability testing in early diabetic neuropathy

OBJECTIVE

The aim of the present study was to gain insight into the pathophysiology of diabetic polyneuropathy (DPN) and examine the diagnostic value of sensory and motor axonal excitability testing.

METHODS

One hundred and eleven type 2 diabetics with and without DPN (disease duration: 6.36 ± 0.25 years) and 60 controls were included. All participants received a thorough clinical examination including Michigan Neuropathy Screening Instrument (MNSI) score, nerve conduction studies (NCS), and sensory and motor excitability tests. Patients were compared by the likelihood of neuropathy presence, ranging from no DPN (17), possible/probable DPN (46) to NCS-confirmed DPN (48).

RESULTS

Motor excitability tests showed differences in rheobase and depolarizing threshold electrotonus measures between NCS-confirmed DPN group and controls but no changes in hyperpolarising threshold electrotonus or recovery cycle parameters. Sensory excitability showed even less changes despite pronounced sensory NCS abnormalities. There were only weak correlations between the above motor excitability parameters and clinical scores.

CONCLUSIONS

Changes in excitability in the examined patient group were subtle, perhaps because of the relatively short disease duration.

SIGNIFICANCE

Less pronounced excitability changes than NCS suggest that axonal excitability testing is not of diagnostic value for early DPN and does not provide information on the mechanisms.

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.

Altered peripheral nerve structure and function in latent autoimmune diabetes in adults

AIM

The present study was undertaken to investigate mechanisms of peripheral nerve dysfunction in latent autoimmune diabetes in adults (LADA).

MATERIALS AND METHODS

Participants with LADA (n = 15) underwent median nerve ultrasonography and nerve excitability to examine axonal structure and function, in comparison to cohorts of type 1 diabetes (n = 15), type 2 diabetes (n = 23) and healthy controls (n = 26). The LADA group was matched for diabetes duration, glycaemic control, and neuropathy severity with the type 1 and type 2 diabetes groups. A validated mathematical model of the human axon was utilized to investigate the pathophysiological basis of nerve dysfunction.

RESULTS

The most severe changes in nerve structure and function were noted in the LADA group. The LADA cohort demonstrated a significant increase in nerve cross-sectional area compared to type 1 participants and controls. Compared to type 1 and 2 diabetes, measures of threshold electrotonus, which assesses nodal and internodal conductances, were significantly worse in LADA in response to both depolarising currents and hyperpolarising currents. In the recovery cycle, participants with LADA had a significant increase in the relative refractory period. Mathematical modelling of excitability recordings indicated the basis of nerve dysfunction in LADA was different to type 1 and 2 diabetes.

CONCLUSIONS

Participants with LADA exhibited more severe changes in nerve function and different underlying pathophysiological mechanisms compared to participants with type 1 or 2 diabetes. Intensive management of risk factors to delay the progression of neuropathy in LADA may be required.

Hypertension and Diabetes Are Associated With Clinical Characteristics in Patients Undergoing Microvascular Decompression for Hemifacial Spasm

OBJECTIVES

The aim of the present study is to investigate effect of hypertension and diabetes on neuroelectrophysiology, outcomes and complications in patients with hemifacial spasm (HFS) treated by microvascular decompression (MVD).

METHODS

From June 2014 to December 2016, 476 consecutive HFS patients who had undergone MVD were divided into 2 groups according to the presence or absence of comorbidities: diabetic group (n = 26) versus non-diabetic group (n = 450), and hypertensive group (n = 141) versus normotensive group (n = 335). Relevant patient data, including preoperative and postoperative neuroelectrophysiology, operative findings, outcome of MVD and complications, were collected and analyzed retrospectively over the 2-year follow-up period. The impact of hypertension and diabetes on the clinical features of HFS patients was investigated by using logistic regression models.

RESULTS

Brainstem auditory evoked potential (BAEP), postoperative prognosis and abnormal muscle response (AMR) were not different between any of the 2 groups. Preoperative positive AMR occurred more frequently in the nondiabetic group than diabetic group [OR = 0.202, P = 0.004], whereas hypertension was not independently predictive for neuroelectrophysiology in patients with HFS. Adjusted multivariate analysis indicated that hypertension was the only clinical factor associated with MVD-related complications [OR = 0.482, P = 0.007] and hearing impairment [OR = 0.28, P = 0.004] after various potential confounders were taken into account, whereas diabetes was not predictive for postoperative complications.

CONCLUSIONS

Diabetes is associated with low positive rate of preoperative AMR, thus weakening the predictive role of AMR for successful MVD. Hypertension may be an independent risk factor for hearing impairment after MVD.

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.

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.

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.

Methylglyoxal Disrupts Paranodal Axoglial Junctions via Calpain Activation

Nodes of Ranvier and associated paranodal and juxtaparanodal domains along myelinated axons are essential for normal function of the peripheral and central nervous systems. Disruption of these domains as well as increases in the reactive carbonyl species methylglyoxal are implicated as a pathophysiology common to a wide variety of neurological diseases. Here, using an ex vivo nerve exposure model, we show that increasing methylglyoxal produces paranodal disruption, evidenced by disorganized immunostaining of axoglial cell-adhesion proteins, in both sciatic and optic nerves from wild-type mice. Consistent with previous studies showing that increase of methylglyoxal can alter intracellular calcium homeostasis, we found upregulated activity of the calcium-activated protease calpain in sciatic nerves after methylglyoxal exposure. Methylglyoxal exposure altered clusters of proteins that are known as calpain substrates: ezrin in Schwann cell microvilli at the perinodal area and zonula occludens 1 in Schwann cell autotypic junctions at paranodes. Finally, treatment with the calpain inhibitor calpeptin ameliorated methylglyoxal-evoked ezrin loss and paranodal disruption in both sciatic and optic nerves. Our findings strongly suggest that elevated methylglyoxal levels and subsequent calpain activation contribute to the disruption of specialized axoglial domains along myelinated nerve fibers in neurological diseases.

Correlation between markers of peripheral nerve function and structure in type 1 diabetes

BACKGROUND

Clinical and experimental studies in patients with type 1 and type 2 diabetes have demonstrated changes in ion channel function and nerve structure. In this study, we investigated the relationship between axonal dysfunction and morphological change in diabetic polyneuropathy by using neuromuscular ultrasound and nerve excitability techniques. We also explored possible differences in this relationship between type 1 and type 2 diabetes.

METHODS

Nerve ultrasound and corresponding motor excitability studies were undertaken in 110 diabetes patients (50 type 1; 60 type 2) and 60 age-matched controls (30 for each group). Neuropathy severity was assessed by using total neuropathy score. Median and tibial nerve cross-sectional areas were measured at nonentrapment sites by using high-resolution linear probe.

RESULTS

Median and tibial nerve cross-sectional areas were significantly higher in diabetes patients compared with controls: type 1 (median = 7.6 ± 0.2 mm² vs 6.3 ± 0.1 mm² ; tibial = 14.5 ± 0.7 mm² vs 10.8 ± 0.3 mm² , P < .05) and type 2 (median = 9.1 ± 0.3 mm² vs 7.2 ± 0.1 mm² ; tibial = 18.5 ± 1.0 mm² vs 12.8 ± 0.5 mm² , P < .05). In the type 1 cohort, significant correlations were found between nerve cross-sectional area and excitability parameters including resting current-threshold slope (median: r = 0.523, P < .0001; tibial: r = -0.571, P = .004) and depolarizing threshold electrotonus at 90 to 100 ms (median: 0.424, P < .01; tibial: r = 0.435, P = .030). In contrast, there was no relationship between excitability values and nerve cross-sectional area in the type 2 cohort.

CONCLUSIONS

This study has identified correlation between markers of axonal membrane function and structural abnormalities in peripheral nerves of type 1 diabetes patients. The differential relationship in nerve function and structure between type 1 and type 2 diabetes provides clinical evidence that different pathophysiological mechanisms underlie the development of neuropathy in these patient groups.

Burning pain: axonal dysfunction in erythromelalgia

To gain insights into erythromelalgia disease pathophysiology, this study elucidated changes in peripheral axonal excitability and influences of temperature and mexiletine on axonal function.

Erythromelalgia (EM) is a rare neurovascular disorder characterized by intermittent severe burning pain, erythema, and warmth in the extremities on heat stimuli. To investigate the underlying pathophysiology, peripheral axonal excitability studies were performed and changes with heating and therapy explored. Multiple excitability indices (stimulus–response curve, strength–duration time constant (SDTC), threshold electrotonus, and recovery cycle) were investigated in 23 (9 EMSCN9A+ and 14 EMSCN9A−) genetically characterized patients with EM stimulating median motor and sensory axons at the wrist. At rest, patients with EM showed a higher threshold and rheobase (P < 0.001) compared with controls. Threshold electrotonus and current–voltage relationships demonstrated greater changes of thresholds in both depolarizing and hyperpolarizing preconditioning electrotonus in both EM cohorts compared with controls in sensory axons (P < 0.005). When average temperature was raised from 31.5°C to 36.3°C in EMSCN9A+ patients, excitability changes showed depolarization, specifically SDTC significantly increased, in contrast to the effects of temperature previously established in healthy subjects (P < 0.05). With treatment, 4 EMSCN9A+ patients (4/9) reported improvement with mexiletine, associated with reduction in SDTC in motor and sensory axons. This is the first study of primary EM using threshold tracking techniques to demonstrate alterations in peripheral axonal membrane function. Taken together, these changes may be attributed to systemic neurovascular abnormalities in EM, with chronic postischaemic resting membrane potential hyperpolarization due to Na⁺/K⁺ pump overactivity. With heating, a trigger of acute symptoms, axonal depolarization developed, corresponding to acute axonal ischaemia. This study has provided novel insights into EM pathophysiology.

Schwann cell interactions with axons and microvessels in diabetic neuropathy

The prevalence of diabetes worldwide is at pandemic levels, with the number of patients increasing by 5% annually. The most common complication of diabetes is peripheral neuropathy, which has a prevalence as high as 50% and is characterized by damage to neurons, Schwann cells and blood vessels within the nerve. The pathogenic mechanisms of diabetic neuropathy remain poorly understood, impeding the development of targeted therapies to treat nerve degeneration and its most disruptive consequences of sensory loss and neuropathic pain. Involvement of Schwann cells has long been proposed, and new research techniques are beginning to unravel a complex interplay between these cells, axons and microvessels that is compromised during the development of diabetic neuropathy. In this Review, we discuss the evolving concept of Schwannopathy as an integral factor in the pathogenesis of diabetic neuropathy, and how disruption of the interactions between Schwann cells, axons and microvessels contribute to the disease.