Neuromuscular electrical stimulation (NMES) involves the use of a device that transmits an electrical impulse to the skin over selected muscle groups by way of electrodes. There are two broad categories of NMES. One type of device is used to treat muscle atrophy and stimulates the muscle when the individual is in a resting state. The second type, also known as functional electrical stimulation (FES), is used to enhance functional activity of neurologically impaired individuals.
NEUROMUSCULAR ELECTRICAL STIMULATORS FOR MUSCULAR DISUSE ATROPHY
NMES is used as a treatment modality for disuse atrophy due to a condition such as limb casting or hip replacement surgery, in which the nerve supply to the muscle is intact. The NMES device encompasses a portable stimulator with electrodes that are placed on the skin over a targeted muscle or muscle group. The current passes through the electrodes into the body, and the motor nerves are stimulated, causing a muscle contraction. The intensity and frequency of stimulation can vary based on the level of muscular function and response to treatment.
The use of NMES was studied by the Office of Health Technology Assessment (OHTA) at the Public Health Service in cases where neural supply (including brain, spinal cord, and peripheral nerves) to the muscle was intact or where other non-neurological reasons for disuse were causing atrophy. The report excluded clinical situations in which the cause of the disuse was considered permanent or was characterized by a nonreversible pathology. Based on their complete assessment of all relevant literature, OHTA concluded that the use of NMES for the treatment of disuse atrophy is considered effective therapy when the cause of the muscle disuse is not permanent and there is no nervous system involvement. In this situation, the treatment of disuse atrophy was considered a surrogate health outcome (i.e., an intermediate outcome as opposed to an eventual outcome, such as the ability to ambulate), because the treatment of disuse atrophy generally leads to a full recovery of function.
FUNCTIONAL ELECTRICAL STIMULATORS FOR THE TREATMENT OF NEUROMUSCULAR CONDITIONS
The second type of NMES is FES, which is used to enhance functional activity in neurologically impaired patients. The objective of FES is to activate targeted muscle groups to facilitate performance of functional activities (e.g., grasping utensils for feeding) or movements (e.g., ambulation). The use of FES has been primarily proposed for individuals with neurological conditions such as spinal cord injury (SCI), multiple sclerosis (MS), cerebrovascular accident (CVA), and cerebral palsy (CP).
An application of FES is to provide individuals with SCI with the ability to stand and walk. The FES device delivers a precise sequence of electrical pulses to trigger action potentials at selected nerves at the quadriceps (for knee extension), common peroneal nerve (for hip flexion), and the paraspinals and gluteals (for trunk stability). The individual uses a walker or elbow-support crutches for further support. The electrical impulses are controlled by a computer microchip attached to the individual's belt that synchronizes and distributes the signals. Additionally, there is a finger-controlled switch that permits activation of the stepping by the individual. The Parastep® Ambulation System (Therapeutic Alliances, Inc., Fairborn, OH) is the only such device to receive approval by the U.S. Food and Drug Administration (FDA) and is indicated to "enable appropriately selected skeletally mature spinal cord injured patients (level C6-T12) to stand and attain limited ambulation and/or management of spinal cord injury."
Other FES devices have been developed for individuals with foot drop. Foot drop is caused by weakness in the foot and ankle, which is activated by the peroneal nerve, resulting in decreased dorsiflexion and difficulty with ambulation. It can be caused by neurological conditions that include SCI, CVA, MS, and CP. WalkAide® and the Bioness NESS L300™ (Innovative Neurotronics, Inc., Bethesda, MD) are examples of FES devices for foot drop. The NESS L300™ consists of three components: a functional stimulation cuff, a gait sensor, and a wireless control device. The functional stimulation cuff is worn by the individual just below the knee and holds the stimulation device and electrodes. The gait sensor is worn inside the individual's shoe and automatically detects different walking speeds and surfaces and sends a signal to the functional stimulation cuff to adjust the stimulation to the peroneal nerve to allow the individual's gait to be adjusted. The control device allows the individual to turn the NESS L300™ on and off, as well as make minor stimulation adjustments.
Overall, small studies focusing on FES devices for foot drop have been performed but did not demonstrate that the use of such devices provided clinically significant improvement in ambulation. A crossover study with the use of FES and ankle foot orthosis in individuals with chronic post-stroke foot drop showed improved satisfaction with FES but no change in walking speed. In 2020, a meta-analysis by da Cunha et al. examined 14 crossover studies in post-stroke individuals using FES applied to the peroneal nerve compared to supervised exercise. The combination of FES along with supervised exercises improved gait speed, and showed positive finding of FES on improved ankle dorsiflexion, balance, and mobility. However, the authors warn that their results must be interpreted cautiously as the studies that comprised the meta-analysis were of low-quality evidence and had high heterogeneity. The single randomized controlled trial examining FES for foot drop in individuals with MS did not demonstrate a clinically significant benefit when compared with no stimulation or a program of exercise.
Tolerability and efficacy of FES was studied over a 3-month period in 10 children with hemiparetic CP who typically wore an ankle foot orthosis for the correction of foot drop (Meilahn, 2013). Gait analysis was performed, but quantitative results were not included in the report. Although half of the children improved gait velocity, mean velocity was relatively unchanged. Another study examined the acceptability and effectiveness of FES in 21 children with CP who had mild gait impairments and unilateral foot drop over a period of 3 months (Prosser et al., 2012). There was no significant change in other gait parameters, including walking speed. Both of the aforementioned studies, which should be considered preliminary, show no improvement in walking speed with the FES device. In addition, daily use decreased over the course of one trial. Studies in a larger number of individuals over a longer duration are needed to permit conclusions concerning the effect of the technology on health outcomes in individuals with foot drop caused by CP.
Another application of FES is to restore upper extremity functions such as grasp-release in individuals with CVA or SCI. An example of an FES device for the upper extremity is the NESS H200™ (Innovative Neurotronics, Inc., Bethesda, MD). It consists of two components: an orthosis and a control device. The orthosis fits on the individual's hand and forearm and maintains the wrist in a position to perform functional hand movements. It has electrodes that are in direct contact with the individual's forearm muscles. The control device has a microprocessor that sends electrical impulses from the control device to the electrodes and stimulates the forearm muscles, causing muscle activation and the ability to pinch, grasp, and release. Based on the review of available peer-reviewed published literature, the evidence for the use of FES devices for an upper extremity is limited by the small number of subjects and limited data demonstrating its utility outside of the clinical setting. The available evidence is insufficient to conclude that FES improves outcomes by providing some upper extremity function.