Functional electrical stimulation

Functional Electrical Stimulation: A Revolutionary Approach to Neurorehabilitation

Imagine a world where paralysis is not an insurmountable barrier, and the human body can be coaxed back into motion through a simple yet powerful technology—functional electrical stimulation (FES). This innovative method uses low-energy pulses to artificially generate movements in paralyzed individuals. But how does it work? And what makes FES such a game-changer in neurorehabilitation?

How Does Functional Electrical Stimulation Work?

FES technology was originally developed for neuroprostheses but has since evolved into a short-term therapy aimed at restoring voluntary function. It works by stimulating neurons and nerves, which can then activate muscles or other nerves. This process is akin to playing a conductor with an orchestra; the electrical pulses are like the baton guiding the symphony of muscle contractions.

Stimulating Nerves for Movement

The technology stimulates both motor and sensory nerves, allowing for complex tasks such as grasping, reaching, and walking. In a reflex, sensory nerves stimulate afferent nerves to evoke a coordinated muscle contraction in response to nerve stimulation. When a nerve is stimulated, an action potential propagates towards both ends of the axon, resulting in orthodromic propagation to the muscle and antidromic propagation to the central nervous system.

Designing Safe FES Devices

FES devices must be designed safely to avoid adverse effects such as cell death or thermal damage. The choice of electrodes is crucial; surface electrodes are noninvasive but often less effective due to impedance issues, while subcutaneous electrodes have limitations, including a higher risk of infection with percutaneous electrodes. Implantable electrodes can offer higher stimulation selectivity but require an invasive surgical procedure and carry a risk of infection.

Stimulation Protocols for FES

Typical stimulation protocols used in clinical FES involve trains of electric pulses, with biphasic, charged balanced pulses being employed as they improve safety and minimize adverse effects. Pulse duration, amplitude, and frequency are regulated by the device to ensure optimal results.

Historical Context and Applications

The term ‘functional electrical stimulation’ was coined in 1967 by Moe and Post, and the first commercially available devices treated foot drop by stimulating the peroneal nerve during gait. Since then, FES has been utilized for various purposes, including muscle strengthening, reaching and grasping, and bipedal locomotion.

Neuroprosthetics and Beyond

Kralj’s approach was extended into the Parastep FES system, which employs digital signal processing and received FDA approval in 1994. The system reduces fatigue by adjusting pulse-width and rate, enabling walking times of 20-60 minutes and distances up to 450 meters. Reported medical benefits include restored blood flow and bone density preservation.

Complexity and Customization

The complexity of FES systems is defined by the properties of the stimulation pulse trains and number of channels used during stimulation. Electrodes are placed over quadriceps muscles and peroneal nerves for neuroprosthetic stimulation, allowing users to control with pushbuttons or canes/crutches.

Alternative Approaches

An alternative approach is the Compex Motion neuroprosthesis, which stimulates all relevant lower limb muscles without peroneal nerve stimulation. Hybrid assistive systems also apply active and passive braces for added stability. Neuroprostheses for walking have limitations, including difficulty with hip flexion. Implanted systems offer better muscle selectivity, but hybrid systems with exoskeletons are promising alternatives.

Research and Benefits

Cyclic electrical stimulation increases isometric strength of wrist extensors in stroke recovery patients, with noticeable improvements after 8 weeks. The treatment also decreases upper extremity disability levels by improving grip strength. Functional electrical stimulation (FES) has been shown to be effective for managing pain, reducing shoulder subluxation, and accelerating motor recovery in post-stroke upper limb rehab.

Case Studies and Applications

A systematic review compared three types of FES: manual FES, BCI-FES, and EMG-FES. The study found that patients who used FES had more functional benefits than those who did not use FES. FES has been shown to be effective in treating drop foot by compensating for the lack of dorsiflexion during the swing phase of gait.

Conclusion

FES is a powerful tool that can transform lives, offering hope and mobility to those who have faced significant challenges. From spinal cord injuries to stroke recovery, FES has proven its worth in various applications. As research continues to advance, the potential for FES to revolutionize neurorehabilitation remains vast.