Changes in axonal excitability of primary sensory afferents with general anaesthesia in humans

Sensory nerve conduction studies in infants, children and teenagers - An update

Objective

Nerve conduction studies (NCS) in children remain technically challenging and depend on the cooperation of the child. Motor NCS are not compromised by analgosedation but data for sensory NCS are lacking. Here, we ask whether sensory NCS is influenced by analgosedation. We also compare the present data with NCS studies from the 1990s regarding anthropometric acceleration of the contemporary paediatric population.

Methods

Sensory NCS of the median nerve and sural nerve were performed in 182 healthy subjects aged 1 to 18 years during general anaesthesia and in 47 of them without analgosedation.

Results

Sensory NCS was not influenced by midazolam or propofol. The sensory nerve action potential (SNAP) amplitude and the nerve conduction velocity (NCV) of the sural nerve as well as the SNAP of the median nerve show no significant age dependence in age range 1-18 years. The sensory NCV of the median nerve increased age-dependent.

Conclusions

In clinical practice, analgosedation can be used for diagnostic NCS. Sensory NCS data show no relevant secular trend over the last 30 years. Differences due to technical inconsistency predominate.

Significance

Analgosedation can improve diagnostic quality of sensory NCS in children. Additionally, we provide sensory NCS values from a large pediatric cohort.

Excitability and the safety margin in human axons during hyperthermia

Abstract  Hyperthermia challenges the nervous system's ability to transmit action potentials faithfully. Neuromuscular diseases, particularly those involving demyelination have an impaired safety margin for action potential generation and propagation, and symptoms are commonly accentuated by increases in temperature. The aim of this study was to examine the mechanisms responsible for reduced excitability during hyperthermia. Additionally, we sought to determine if motor and sensory axons differ in their propensity for conduction block during hyperthermia. Recordings of axonal excitability were performed at normal temperatures and during focal hyperthermia for motor and sensory axons in six healthy subjects. There were clear changes in excitability during hyperthermia, with reduced superexcitability following an action potential, faster accommodation to long-lasting depolarization and reduced accommodation to hyperpolarization. A verified model of human motor and sensory axons was used to clarify the effects of hyperthermia. The hyperthermia-induced changes in excitability could be accounted for by increasing the modelled temperature by 6°C (and adjusting the maximum conductances and activation kinetics according to their Q10 values; producing a 2 mV hyperpolarization of resting membrane potential), further hyperpolarizing the voltage dependence of Ih (motor, 11 mV; sensory, 7 mV) and adding a small depolarizing current at the internode (motor, 20 pA; sensory, 30 pA). The modelling suggested that slow K(+) channels play a significant role in reducing axonal excitability during hyperthermia. The further hyperpolarization of the activation of Ih would limit its ability to counter the hyperpolarization produced by activity, thereby allowing conduction block to occur during hyperthermia.

Activation of axonal Kv7 channels in human peripheral nerve by flupirtine but not placebo - therapeutic potential for peripheral neuropathies: results of a randomised controlled trial

BACKGROUND

Flupirtine is an analgesic with muscle-relaxing properties that activates Kv7 potassium channels. Kv7 channels are expressed along myelinated and unmyelinated peripheral axons where their activation is expected to reduce axonal excitability and potentially contribute to flupirtine's clinical profile.

TRIAL DESIGN

To investigate the electrical excitability of peripheral myelinated axons following orally administered flupirtine, in-vitro experiments on isolated peripheral nerve segments were combined with a randomised, double-blind, placebo-controlled, phase I clinical trial (RCT).

METHODS

Threshold tracking was used to assess the electrical excitability of myelinated axons in isolated segments of human sural nerve in vitro and motoneurones to abductor pollicis brevis (APB) in situ in healthy subjects. In addition, the effect of flupirtine on ectopic action potential generation in myelinated axons was examined using ischemia of the lower arm.

RESULTS

Flupirtine (3-30 μM) shortened the relative refractory period and increased post-conditioned superexcitability in human myelinated axons in vitro. Similarly, in healthy subjects the relative refractory period of motoneurones to APB was reduced 2 hours after oral flupirtine but not following placebo. Whether this effect was due to a direct action of flupirtine on peripheral axons or temperature could not be resolved. Flupirtine (200 mg p.o.) also reduced ectopic axonal activity induced by 10 minutes of lower arm ischemia. In particular, high frequency (ca. 200 Hz) components of EMG were reduced in the post-ischemic period. Finally, visual analogue scale ratings of sensations perceived during the post-ischemic period were reduced following flupirtine (200 mg p.o.).

CONCLUSIONS

Clinical doses of flupirtine reduce the excitability of peripheral myelinated axons.

TRIAL REGISTRATION

ClinicalTrials registration is NCT01450865.

[Intraoperative electrophysiological monitoring with evoked potentials]

During the last 30 years intraoperative electrophysiological monitoring (IOEM) has gained increasing importance in monitoring the function of neuronal structures and the intraoperative detection of impending new neurological deficits. The use of IOEM could reduce the incidence of postoperative neurological deficits after various surgical procedures. Motor evoked potentials (MEP) seem to be superior to other methods for many indications regarding monitoring of the central nervous system. During the application of IOEM general anesthesia should be provided by total intravenous anesthesia with propofol with an emphasis on a continuous high opioid dosage. When intraoperative MEP or electromyography guidance is planned, muscle relaxation must be either completely omitted or maintained in a titrated dose range in a steady state. The IOEM can be performed by surgeons, neurologists and neurophysiologists or increasingly more by anesthesiologists. However, to guarantee a safe application and interpretation, sufficient knowledge of the effects of the surgical procedure and pharmacological and physiological influences on the neurophysiological findings are indispensable.

Propofol decreases the axonal excitability in rat primary sensory afferents

AIMS

The aim of this present study was to investigate the changes of peripheral sensory nerve excitability produced by propofol.

MAIN METHODS

In a recently described in vitro model of rodent saphenous nerve we used the technique of threshold tracking (QTRAC®) to measure changes of axonal nerve excitability of Aβ-fibres caused by propofol. Concentrations of 10 μMol, 100 μMol and 1000 μMol were tested. Latency, peak response, strength-duration time constant (τSD) and recovery cycle of the sensory neuronal action potential (SNAP) were recorded.

KEY FINDINGS

Our results have shown that propofol decreases nerve excitability of rat primary sensory afferents in vitro. Latency increased with increasing concentrations (0μMol: 0.96 ± 0.07ms; 1000μMol 1.10 ± 0.06ms, P<0.01). Also, propofol prolonged the relative refractory period (0μMol: 1.79 ± 1.13ms; 100 μMol: 2.53 ± 1.38ms, P<0.01), and reduced superexcitability (0 μMol: -14.0±4.0%; 100μMol: -9.5 ± 5.5%) and subexcitability (0μMol: 7.5 ± 1.2%; 1000μMol: 3.6 ± 1.2) significantly during the recovery cycle (P<0.01).

SIGNIFICANCE

Our results have shown that propofol decreases nerve excitability of primary sensory afferents. The technique of threshold tracking revealed that axonal voltage-gated ion channels are significantly affected by propofol and therefore might be at least partially responsible for earlier described analgesic effects.