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Functional electrical stimulation (FES) is a technique that uses electrical currents to activate nerves innervating extremities affected by paralysis resulting from spinal cord injury (SCI), head injury, stroke and other neurological disorders. FES is primarily used to restore function in people with disabilities. It is sometimes referred to as Neuromuscular electrical stimulation (NMES).
Neurons are electrically active cells. Presence of the electric field in nervous tissue leads to the depolarization of neuronal lipid membranes and induces firing of action potential. FES takes advantage of this property of nerve cells. Passing electric current through nervous tissue can lead to adverse effects such as decrease in excitability or cell death due to thermal damage, electroporation, toxic products from electrochemical reactions or overexcitation of neurons. Special care should be taken in designing safe FES. Typical stimulation protocol that are used in clinical FES involves either AC or DC stimulation. For AC stimulation, a train of electric pulses is delivered. Biphasic, charged balanced pulses are employed as it ensures safety of electrical stimulation and minimizes adverse effects. Pulse width, charge per phase and frequency are the key parameters that define safety and effectiveness of FES. Furthermore, polarity of a biphasic pulse, which can be either cathodic first or anodic first, have different threshold for activation of nervous tissue. For peripheral nerves stimulation, cathodic first pulses have lower threshold and more efficient for charge delivery. For surface cortical simulation, where axons are perpendicular to the electrode surface, anodic first pulses are more efficient.
FES was initially referred to as Functional Electrotherapy by Liberson, and it was not until 1967 that the term Functional Electrical Stimulation was coined by Moe and Post, and used in a patent entitled, "Electrical stimulation of muscle deprived of nervous control with a view of providing muscular contraction and producing a functionally useful moment". Offner's patent described a system used to treat foot drop.
The first commercially available FES devices treated foot drop by stimulating the peroneal nerve during gait. In this case, a switch, located in the heel end of a user's shoe, would activate a stimulator worn by the user.
Injuries to the spinal cord interfere with electrical signals between the brain and the muscles, resulting in paralysis below the level of injury. Restoration of limb function as well as regulation of organ function are the main application of FES, although FES is also used for treatment of pain, pressure, sore prevention, etc.
Some examples of FES applications involve the use of Neuroprostheses that allow the people with paraplegia to walk, stand, restore hand grasp function in people with quadriplegia, or restore bowel and bladder function.
High intensity FES of the quadriceps muscles allows patients with complete lower motor neuron lesion to increase their muscle mass, muscle fiber diameter, improve ultrastructural organization of contractile material, increase of force output during electrical stimulation and perform FES assisted stand-up exercises.
FES is commonly used in foot drop neuroprosthetic devices.
In the acute stage of stroke recovery, the use of cyclic electrical stimulation has been seen to increase the isometric strength of wrist extensors. In order to increase strength of wrist extensors, there must be a degree of motor function at the wrist spared following the stroke and have significant hemiplegia. Patients who will elicit benefits of cyclic electrical stimulation of the wrist extensors must be highly motivated to follow through with treatment, After 8 weeks of electrical stimulation, an increase in grip strength can be apparent. Many scales, which assess the level of disability of the upper extremities following a stroke, use grip strength as a common item. Therefore, increasing strength of wrist extensors will decrease the level of upper extremity disability.
Patients with hemiplegia following a stroke commonly experience shoulder pain and subluxation; both of which will interfere with the rehabilitation process. Functional electrical stimulation has been found to be effective for the management of pain and reduction of shoulder subluxation, as well as accelerating the degree and rate of motor recovery. Furthermore, the benefits of FES are maintained over time; research has demonstrated that the benefits are maintained for at least 24 months.
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