Miho Nakata

Hyperreflexia in axonal Guillain-Barré syndrome subsequent to Campylobacter jejuni enteritis.

We describe a patient with the acute motor axonal neuropathy (AMAN) form of Guillain-Barré syndrome (GBS), who showed generalized hyperreflexia. A 24-year-old man developed acute paralysis following Campylobacter jejuni enteritis. He showed exaggerated tendon reflexes with abnormal reflex spread to other segments, and was initially diagnosed as having post-infectious myelitis. Nerve conduction studies showed motor axonal degeneration (the AMAN pattern), and increased soleus H-reflex amplitudes. His serum was positive for IgG antibodies to gangliosides GM1b and GalNAc-GD1a. He was treated with plasmapheresis, resulting in rapid recovery. Hyperreflexia was still present 12 months after onset when muscle strength was completely normal. This case provides further evidence that patients with AMAN can develop increased motor neuron excitability, and possible mechanisms for the hyperreflexia are discussed.

Distal excitability changes in motor axons in amyotrophic lateral sclerosis

OBJECTIVE Previous axonal excitability studies in amyotrophic lateral sclerosis (ALS) have suggested that impaired potassium channel function could be responsible for the generation of fasciculations, but the ectopic activity arises predominantly from the motor nerve terminals. This study tested the hypothesis that dysfunction of potassium channels is more pronounced in the more distal parts of axons. METHODS Threshold electrotonus was used to compare accommodation at the motor point of abductor pollicis brevis, and at the wrist portion of the median nerve, between 22 patients with ALS and 19 normal subjects. As target responses for motor point stimulation, movement-related potentials were recorded using an accelerometer. RESULTS Compared to normal subjects, ALS patients showed greater threshold changes to depolarizing conditioning currents at both the motor point and wrist, suggesting less accommodation by potassium currents. Differences in the threshold electrotonus curves between the normal and ALS groups were much more prominent at the motor point than at the wrist. CONCLUSIONS In ALS, axonal potassium channels are impaired more prominently in distal portions of axons than at the nerve trunk, and this is consistent with evidence that fasciculations mostly arise from the nerve terminals. SIGNIFICANCE Excitability testing at the motor point provides additional information about the pathophysiology of ALS.

Markedly reduced axonal potassium channel expression in human sporadic amyotrophic lateral sclerosis: An immunohistochemical study

Fasciculations are characteristic features of amyotrophic lateral sclerosis (ALS), suggesting abnormally increased excitability of motor axons. Previous nerve excitability studies have shown reduced axonal potassium currents in ALS patients that may contribute to the hyperexcitability and thereby generation of fasciculations. To clarify changes in axonal ion channel expression in motor axons of ALS, we performed immunohistochemistry of potassium and sodium channels in the C7 and L5 ventral/dorsal roots obtained from five autopsy cases of sporadic ALS. Compared to controls, the immunoreactivity of potassium channels (Kv1.2) was markedly reduced in the ventral roots, but normal in the dorsal roots of all the ALS patients. Nodal sodium channel expression was not significantly different in ALS patients and control subjects. Our results show prominently reduced expression of axonal potassium channels, and provide the neuropathological and biological basis for decreased accommodative potassium currents in motor axons of ALS patients. The axonal hyperexcitability would lead to generation of fasciculations, and possibly enhances motor neuron death in ALS.

Nodal persistent Na+ currents in human diabetic nerves estimated by the technique of latent addition

OBJECTIVE To investigate the effects of hyperglycemia on persistent Na+ currents in human diabetic nerves, eliminating the factors of passive membrane properties as a factor. Previous studies show that strength-duration time constant of a nerve is shortened under hyperglycemia, suggesting reduced axonal persistent Na+ currents. However, the time constant is also affected by changes in passive membrane properties. Latent addition using computerized threshold tracking is a new method that can separately evaluate Na+ currents and passive membrane properties. METHODS Latent addition was used to estimate nodal Na+ currents in median motor axons of 83 diabetic patients. Brief hyperpolarizing conditioning current pulses were delivered, and threshold changes at the conditioning-test interval of 0.2 ms were measured as an indicator of nodal persistent Na+ currents. Seventeen patients were examined before and after insulin treatment. RESULTS There was an inverse linear relationship between hemoglobin A1c levels and threshold changes at 0.2 ms (P=0.02); the higher hemoglobin A1c levels were associated with smaller threshold changes. After insulin treatment, there was a significant improvement in nerve conduction velocities associated with greater threshold changes at 0.2 ms (P=0.03), suggesting an increase in persistent Na+ currents. The fast component of latent addition, an indicator of passive membrane properties, was not affected by the state of glycemic control. CONCLUSIONS Hyperglycemia could suppress nodal persistent Na+ currents, presumably because of reduced trans-axonal Na+ gradient or impaired Na+ channels, and this can be rapidly restored by glycemic control. SIGNIFICANCE Reduced nodal Na+ currents may partly contribute to the pathophysiology of human diabetic neuropathy.

Aldose reductase inhibition alters nodal Na+ currents and nerve conduction in human diabetics

Background: In diabetic nerves, activation of the polyol pathway via an aldose reductase and the resulting impairment of the Na+–K+ pump would lead to a decreased transaxonal Na+ gradient and thereby reduced nodal Na+ currents. Objective: To investigate whether the aldose reductase inhibitor (ARI) epalrestat improves nodal Na+ currents and nerve conduction in human diabetic neuropathy. Methods: The authors conducted a 6-month, open clinical trial with an ARI, epalrestat, in 30 patients with mild-to-moderate diabetic neuropathy. The latent addition technique and measurements of the strength-duration time constant were used to estimate nodal persistent Na+ currents in median motor axons. Excitability testing and extensive nerve conduction studies including F-wave analyses were performed before and 1 and 6 months after the initiation of treatment with oral epalrestat. Results: Within a month of the start of treatment, there was a significant improvement in nerve conduction, particularly in conduction times across the carpal tunnel and F-wave latencies. The results of latent addition (p < 0.05) and strength-duration time constant (p = 0.06) suggested increased nodal persistent Na+ currents. At 6 months, nerve conduction continued to improve. Conclusions: Aldose reductase pathway inhibition could rapidly increase nodal Na+ currents and thereby improve the slowing of nerve conduction, presumably because of a restoration of the membranous Na+ gradient.

Changes in excitability properties associated with axonal regeneration in human neuropathy and mouse Wallerian degeneration

OBJECTIVE The aim of this study was to investigate changes in excitability properties associated with axonal regeneration in human neuropathy and a mouse Wallerian degeneration model. METHODS Threshold tracking was used to measure axonal excitability indices such as strength-duration time constant (SDTC), threshold electrotonus, supernormality in median motor axons at the wrist of 13 patients with vasculitic neuropathy in their recovery phase, and in tibial motor axons at the ankle of mice with sciatic nerve crush. In the mouse model, excitability testing was performed 4, 8, 12, and 20weeks after the nerve crush. RESULTS In patients, there were longer SDTC, greater threshold changes at 0.2ms in latent addition, and greater threshold changes in depolarizing and hyperpolarizing threshold electrotonus, compared with controls. The pattern of changes in excitability indices was similar to those in experimental nerve crush, in which the indices remained abnormal for 20weeks after the crush. These changes suggest an increase in nodal persistent sodium currents, whereas multiple factors may also contribute to changes in excitability properties, such as axonal hyperpolarization, increased internodal resistance, and altered potassium currents. CONCLUSIONS Excitability properties in regenerating axons are characterized by increased nodal persistent currents with variable combination of changes in passive properties, membrane potential, and potassium currents. SIGNIFICANCE Increased persistent sodium currents are potential reasons for positive symptoms in patients with axonal neuropathy. Sodium channel blockers could be considered a treatment option.

The effects of mexiletine on excitability properties of human median motor axons

OBJECTIVE To investigate the effects of mexiletine, an analog of lidocaine, on excitability of human axons in vivo. METHODS Threshold tracking was used to measure multiple excitability indices (strength-duration time constant, rheobase, refractoriness, supernormality, and threshold electrotonus) in median motor axons of 20 patients with neuropathic pain or muscle cramping, before and 3 months after treatment with oral 300 mg mexiletine per day. RESULTS After treatment, there was a reduction in pain/muscle cramps, associated with decreased strength-duration time constants (P=0.01), increased rheobasic currents (P=0.06), and lower refractoriness (P=0.02), all of which were consistent with reduced nodal Na+ currents. Supernormality and threshold electrotonus did not change significantly. The changes in strength-duration properties suggest a decrease in persistent Na+ conductance. The lowered refractoriness after treatment might result from reduced transient Na+ currents, but the lack of change in supernormality and threshold electrotonus was not consistent with this hypothesis. CONCLUSIONS Oral mexiletine in a dosage of 300 mg daily suppresses persistent Na+ currents in human motor axons. SIGNIFICANCE Measurements of the excitability indices can be used for non-invasive assessment and monitoring of the effects of mexiletine in patients with neuropathic pain or muscle cramps.

Axonal potassium conductance and glycemic control in human diabetic nerves

OBJECTIVE To investigate the effects of hyperglycemia on axonal excitability and potassium conductance in human diabetic nerves. METHODS Threshold tracking was used to measure excitability indices, which depend on potassium channels (supernormality, late subnormality, threshold electrotonus, and a current/threshold relationship) in median motor axons of 96 diabetic patients. The effects of hyperglycemia on these indices were analyzed. RESULTS Among diabetic patients, higher serum hemoglobin A1c (HbA1c) levels were significantly associated with greater supernormality (P = 0.04) and smaller late subnormality (P = 0.02), suggestive of reduced nodal/paranodal potassium currents under hyperglycemia. Threshold electrotonus and current/threshold relationships did not correlate with HbA1c levels, but partly related with nerve conduction slowing. CONCLUSIONS Hyperglycemia could reduce nodal potassium conductances, possibly due to reduced membranous potassium gradient or suppression of potassium channels. In contrast, internodal potassium conductances may be determined by both metabolic factors and structural changes such as exposure of internodal channels by demyelination. SIGNIFICANCE Measurements of the excitability indices could provide new insights into nodal and internodal axonal membrane properties in human diabetic neuropathy, whereas multiple factors can affect especially internodal properties.

Anti-GQ1b antibody does not affect neuromuscular transmission in human limb muscle

Anti-ganglioside GQ1b antibody induces neuromuscular blocking on mouse phrenic nerve-diaphragm preparations. Several reports suggest that patients with this antibody show abnormal neuromuscular transmission in the facial or limb muscles, but limb muscle weakness is unusual in Miller Fisher syndrome that is often associated with anti-GQ1b antibody. To determine whether anti-GQ1b sera affect neuromuscular transmission in human limb muscles, axonal-stimulating single fiber electromyography was performed in the forearm muscle of seven patients with anti-GQ1b antibody. All showed normal jitter and no blocking. Anti-GQ1b antibody does not affect neuromuscular transmission in human limb muscles. The different findings in mouse and human may be explained by the extent of expression of GQ1b on the motor nerve terminals in the muscle examined.

Latent addition in human motor and sensory axons: Different site-dependent changes across the carpal tunnel related to persistent Na+ currents

OBJECTIVE To compare site-dependent changes across the carpal tunnel in axonal persistent Na+ conductances in motor and sensory axons. Positive sensory symptoms are prominent features in carpal tunnel syndrome, and a persistent Na+ current is a major determinant of axonal excitability. METHODS The technique of latent addition was used to estimate persistent Na+ currents in median motor and sensory axons at the wrist and palm of 10 normal subjects. Brief hyperpolarizing conditioning current pulses were delivered, and threshold change at the conditioning-test interval of 0.2 ms was measured as an indicator of persistent Na+ currents. RESULTS Threshold changes at 0.2 ms were greater in sensory than in motor axons at both the wrist and palm. In motor axons, the threshold changes were significantly smaller at the palm (mean, 4.9%) than at the wrist (10.0%). By contrast, the threshold changes were similar at the two sites of sensory axons (12.6 and 13.1%). The passive membrane time constant was similar for motor and sensory axons at the palm and wrist. CONCLUSIONS Nodal persistent Na+ conductances have substantial site-dependent changes decreasing distally across the carpal tunnel in median motor axons, but not in sensory axons. SIGNIFICANCE Whereas sensory axons generally have higher excitability than motor axons, the sensory-motor differences become more prominent across, and possibly at the carpal tunnel than the nerve trunk, and it is suggested that this contributes to the predominance of positive sensory symptoms in carpal tunnel syndrome.