A quick-acting, quick-reversing method for blocking action potentials in peripheral nerves could be used in the treatment of muscle spasticity and pain. A high-frequency alternating-current (HFAC) sinusoidal waveform is one possible means for providing this type of block. HFAC was used to block peripheral motor nerve activity in an in vivo mammalian model. Frequencies from 10 to 30 kHZ at amplitudes of between 2 and 10 V were investigated. A complete and reversible motor block was obtained at all frequencies. The block threshold amplitudes showed a linear relationship with frequency, the lowest threshold being at 10 kHZ. HFAC block has three phases: an onset response; a period of asynchronous firing; and a steady state of complete or partial block. The onset response and the asynchronous firing can be minimized by using an optimal frequency-amplitude combination. In general, the onset response was lowest for the combination of 30 kHZ and 10 V.

High-frequency alternating current (AC) waveforms have been shown to produce a quickly reversible nerve block in animal models, but the parameters and mechanism of this block are not well understood. A frog sciatic nerve/gastrocnemius muscle preparation was used to examine the parameters for nerve conduction block in vivo, and a computer simulation of the nerve membrane was used to identify the mechanism for block. The results indicated that a 100% block of motor activity can be accomplished with a variety of waveform parameters, including sinusoidal and rectangular waveforms at frequencies from 2 kHz to 20 kHz. A complete and reversible block was achieved in 34 out of 34 nerve preparations tested. The most efficient waveform for conduction block was a 3-5 kHz constant-current biphasic sinusoid, where block could be achieved with stimulus levels as low as 0.01 microCphase(-1). It was demonstrated that the block was not produced indirectly through fatigue. Computer simulation of high-frequency AC demonstrated a steady-state depolarisation of the nerve membrane, and it is hypothesised that the conduction block was due to this tonic depolarisation. The precise relationship between the steady-state depolarisation and the conduction block requires further analysis. The results of this study demonstrated that high-frequency AC can be used to produce a fast-acting, and quickly reversible nerve conduction block that may have multiple applications in the treatment of unwanted neural activity.

Objectives The features and clinical applications of balanced-charge kilohertz frequency alternating currents (KHFAC) are reviewed. Preclinical studies of KHFAC block have demonstrated that it can produce an extremely rapid and reversible block of nerve conduction. Recent systematic analysis and experimentation utilizing KHFAC block has resulted in a significant increase in interest in KHFAC block, both scientifically and clinically. Materials and Methods We review the history and characteristics of KHFAC block, the methods used to investigate this type of block, the experimental evaluation of block, and the electrical parameters and electrode designs needed to achieve successful block. We then analyze the existing clinical applications of high frequency currents, comparing the early results with the known features of KHFAC block. Results Although many features of KHFAC block have been characterized, there is still much that is unknown regarding the response of neural structures to rapidly fluctuating electrical fields. The clinical reports to date do not provide sufficient information to properly evaluate the mechanisms that result in successful or unsuccessful treatment. Conclusions KHFAC nerve block has significant potential as a means of controlling nerve activity for the purpose of treating disease. However, early clinical studies in the use of high frequency currents for the treatment of pain have not been designed to elucidate mechanisms or allow direct comparisons to preclinical data. We strongly encourage the careful reporting of the parameters utilized in these clinical studies, as well as the development of outcome measures that could illuminate the mechanisms of this modality.

High frequency alternating current (HFAC) sinusoidal waveforms can block conduction in mammalian peripheral nerves. A mammalian axon model was used to simulate the response of nerves to HFAC conduction block. Sinusoidal waveforms from 1 to 40 kHz were delivered to eight simulated axon diameters ranging from 7.3 to 16 microm. Conduction block was obtained between 3 to 40 kHz. The minimum peak to peak current at which block was obtained, defined as the block threshold, increased with increasing frequency. Block threshold varied inversely with axon diameter. Upon initiation, the HFAC waveform produced one or more action potentials. These simulation results closely parallel previous experimental results of high frequency motor block of the rat sciatic and cat pudendal nerve. During HFAC block, the axons showed a dynamic steady state depolarization of multiple nodes, strongly suggesting a depolarization mechanism for HFAC conduction block.

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Electrical currents can be used to produce a block of action potential conduction in whole nerves. This block has a rapid onset and reversal. The mechanism of electrical nerve conduction block has not been conclusively determined, and inconsistencies appear in the literature regarding whether the block is produced by membrane hyperpolarization, depolarization, or through some other means. We have used simulations in a nerve membrane model, coupled with in vivo experiments, to identify the mechanism and principles of electrical conduction block. A nerve simulation package (Neuron) was used to model direct current (dc) block in squid, frog, and mammalian neuron models. A frog sciatic nerve/gastrocnemius preparation was used to examine nerve conduction block in vivo. Both simulations and experiments confirm that depolarization block requires less current than hyperpolarization block. Dynamic simulations suggest that block can occur under both the real physical electrode as well as adjacent virtual electrode sites. A hypothesis is presented which formulates the likely types of dc block and the possible block current requirements. The results indicate that electrical currents generally produce a conduction block due to depolarization of the nerve membrane, resulting in an inactivation of the sodium channels.

These results suggest that clinical KHFSCS may not function through direct activation or conduction block of dorsal column or dorsal root fibers. Although these results should be validated with further studies, the authors propose that additional concepts and/or alternative hypotheses should be considered when examining the pain relief mechanisms of KHFSCS.

Purpose-The purpose of this study was evaluate the potential of a second-generation implantable neuroprosthesis that provides improved control of hand grasp and elbow extension for individuals with cervical level spinal cord injury. The key feature of this system is that users control their stimulated function through electromyographic (EMG) signals.Methods-The second-generation neuroprosthesis consists of 12 stimulating electrodes, 2 EMG signal recording electrodes, an implanted stimulator-telemeter device, an external control unit, and a transmit/receive coil. The system was implanted in a single surgical procedure. Functional outcomes for each subject were evaluated in the domains of body functions and structures, activity performance, and societal participation.Results-Three individuals with C5/C6 spinal cord injury received system implantation with subsequent prospective evaluation for a minimum of 2 years. All 3 subjects demonstrated that EMG signals can be recorded from voluntary muscles in the presence of electrical stimulation of nearby muscles. Significantly increased pinch force and grasp function was achieved for each subject. Functional evaluation demonstrated improvement in at least 5 activities of daily living using the Activities of Daily Living Abilities Test. Each subject was able to use the device at home. There were no system failures. Two of 6 EMG electrodes required surgical revision because of suboptimal location of the recording electrodes.Conclusions-These results indicate that a neuroprosthesis with implanted myoelectric control is an effective method for restoring hand function in midcervical level spinal cord injury. Type of study/level of evidence-Therapeutic IV.Keywords functional electrical stimulation; neuroprosthesis; spinal cord injury; electromyography; activities of daily livingThe Ability To Grasp And Manipulate objects is an important factor in determining if the tetraplegic individual can regain independence and return to a functional role in society. Implantable functional electrical stimulation systems, or neuroprostheses, provide hand and arm function that is both versatile and transparent. These systems have evolved from the feasibility stage through clinical evaluation in an outpatient population to a Food and DrugCorresponding author: Kevin L. Kilgore, PhD, MetroHealth Medical Center, Hamann 601, 2500 MetroHealth Dr., Cleveland, OH 44109; e-mail: klk4@case.edu. NIH Public Access NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptAdministration (FDA)-approved medical product. 1 Neuroprostheses provide the user with increased independence, thus improving the opportunity for a better quality of life.A first-generation upper-extremity neuroprosthesis (NP) for hand control 2 was first implanted in 1986 3 and became known as the Freehand System. [4][5][6][7][8][9][10][11][12] The Freehand NP (NeuroControl Corp., Valley View, OH) used an 8-channel implanted receiver-stimulator with 8 epimysial or intramuscular forearm and hand electrodes, leads...