By Anne-Caroline Dupont, PhD
Realistic Optimism FES may not allow for leaping buildings in single bounds, but it can provide more voluntary movement and function for clients with paralysis. Since its first medical use early in the 1960s,1 functional electrical stimulation (FES) has held much hope for people with disabilities. Some believe it to be a technological miracle that will permit all paralyzed people to move in the same manner as they did before their accident. However, physicians, therapists, researchers, and patients know that this is not so. This technology has permitted researchers to make strides in giving paralyzed individuals more voluntary movement and function. However, FES systems, although showing great promise for the future, are not a cure-all for many paralyzed-muscle challenges. The Basic Principles Functional electrical stimulation relies on the electrical stimulation of healthy nerves to activate muscles over which an individual no longer has voluntary control. Because nerves are activated at much lower electrical charges than muscles themselves, it is much more practical to stimulate nerves than to activate muscles directly.2 Moreover, by stimulating a nerve in one location, an entire muscle can be activated. For these reasons, FES is suitable for people who have had an injury to the central nervous system, because healthy nerves remain. People who have suffered a stroke or a spinal cord injury (SCI) are good candidates for FES, but FES systems are not indicated for individuals with peripheral nerve diseases or damage (such as amyotrophic lateral sclerosis and multiple sclerosis). As well, FES works best on healthy, although possibly weakened and atrophied, muscles. This technology is not as appropriate for people affected by muscle diseases, such as muscular dystrophy. FES systems rely on two state-of-the-art technology advances: a control system, which provides a way to determine when and how the stimulation is applied, and a stimulation system, which is based on the devices that can deliver the electrical pulses. Research in both these areas has been going strong for years. In order for FES to be useful, not only do the muscles need to be activated, but they need to be activated in a sequence that is logical and controlled by the user. Because the brain’s messages are not reaching these muscles anymore, an external controller is needed. Control Strategies The control strategy chosen for each application depends on the complexity of the application and how much control is needed. In some applications, only on and off controls are needed—this is an open-loop system, whose patterns of electrical pulses do not change because there is no feedback mechanism. In more complex applications, constant feedback to the system is needed to adjust either the level or sequence of stimulation or both. These require sensors and closed-loop controllers that are sophisticated; in these systems, the way that a muscle is stimulated will depend on other factors, such as the angle of a joint or the force produced by a muscle. One of the simplest applications is muscle strengthening. This is sometimes done as a preparatory step before use of a more complex system if the targeted muscles are very weak and prone to fatigue.3 For muscle strengthening, the muscles are typically activated at a fixed level for a certain amount of time at each session. As the sessions progress, the period and stimulation level are increased.4 For this straightforward application, a simple toggle switch is sufficient to turn the device on and off. For more complex applications, such as FES-assisted standing, a closed-loop system may be required. Movement at any joint generally needs to be compensated by a muscle contraction to maintain the body in its proper standing position. This system may have to analyze the deviations from the proper angles and then decide how much to stimulate the muscles in order to return the joint to a better angle.4 Between these two scenarios is an infinite variety of muscle contraction sequences that can produce different movements such as opening and closing the hand, moving the elbow joint, and flexing the ankle. For such applications, one part of the body is often used to control the paralyzed muscles. For example, people with a spinal cord injury around the C6-C7 level can often control (to some extent) their wrist movement but not hand closing and opening. Some system designs have incorporated wrist angle switches where extension of the wrist to a predetermined trigger angle initiates hand closing. Wrist flexion operates similarly for hand opening.5,6 Stimulation Systems FES is applied to nerve supplying a muscle using electrodes. Electrodes are used in pairs so that electric current flows from one to the other. Electrode systems are numerous but they can be filed in four categories: surface electrodes, percutaneous electrodes, implanted multichannel stimulators, and radio-controlled micro-devices. Surface electrodes are placed on the skin in the proximity of the area to stimulate. The two electrodes are placed on the skin so that when current runs from one to the other through the skin and underlying tissue, it excites nerves as well. Such systems are attractive because they are simple. However, they can have many disadvantages. Besides the less-than-convenient donning and doffing procedures, surface stimulation cannot target specific muscles well, especially if the muscles are deep—muscles that are superficial to the targeted muscles are also stimulated. In addition, cutaneous nerves can be stimulated. This creates unpleasant sensation and can even be painful. When patients cannot feel the sensations, this is not a problem but the high currents can also cause skin burns, particularly if the electrodes are not well attached. Despite these limitations, surface electrode systems are still the first choice of therapists and physicians when choosing a temporary stimulation system for short-term therapy, or testing whether a patient would be a good candidate for a more permanent stimulation system. With the other systems, the electrodes can be placed close to the nerve of a given muscle, where relatively weak stimulation currents can activate that nerve specifically without spread to other muscles and cutaneous nerves. Thus, these systems can provide selective control of individual muscles without undesirable sensations. They differ in the invasiveness of the procedures required to implant their internal components. Percutaneous electrode systems consist of implanted electrodes with one percutaneous (exiting through the skin) lead for each electrode. The percutaneous leads connect to an external stimulator that can take many forms and can be portable. Because only small electrodes and percutaneous leads have to be implanted, the surgical intervention is simple, minimally invasive, and relatively inexpensive. The most important difficulties of such a system are those associated with the leads because they can break and are a route to infection. As well, some patients find them cosmetically unacceptable. Because of these shortcomings, fully implanted systems are often preferred, particularly for chronic applications. Fully implanted multichannel stimulator systems comprise electrodes, leads, and a stimulation circuitry that are all implanted under the skin. In an application such as a hand motion controller, several electrodes can be implanted in the hand and forearm, with leads being passed subcutaneously to the stimulator that is implanted in the chest area. The stimulator may receive power by a radio-frequency magnetic field during use or it may have a battery that can be recharged occasionally by such a magnetic field. Fully implanted systems circumvent problems associated with percutaneous electrodes because there are no visible leads. The implanted components do not run outside of the body, thus reducing the risk of infection. Moreover, this stimulation system is practically invisible when the external components are not in place. However, surgeries, lead repairs, and lead replacements tend to be invasive and expensive. These systems are often used for people whose disabilities are considered permanent (such as SCI patients) and for functional applications, ie, movement, as opposed to therapeutic applications like increasing muscle strength. New technology has permitted the development of modular, leadless micro-devices that can be implanted individually in one or more muscles. These very small (about 2-3 mm diameter by 12-20 mm in length), self-contained devices emit stimulation current locally so that no leads are required. Power and commands are conveyed to the devices via a radio-frequency magnetic field created by an external antenna coil placed on the skin in the vicinity of the devices. Because they have no leads, no problems will be caused by wire breakage. The implantation is simple because the devices are so small; they can be injected through a large needle in an outpatient procedure. The downside of such systems is that an external coil must be positioned close to the implants whenever they are in use. These systems are just starting to be used in clinical trials for diverse clinical problems, including shoulder subluxation in stroke victims, disuse atrophy in osteoarthritis patients,7, 8 and urinary urge incontinence. Applications It might seem that with a sufficient number of electrodes and stimulation channels and the right control system, all SCI and stroke patients would be running marathons soon. However, even walking with FES faces substantial hurdles. First, we have to realize that walking is incredibly complicated. To use FES to control and power walking would require an amazingly sophisticated control system that must detect and respond to a multitude of volitional and environmental cues such as uneven pavements, wind force and direction, walking speed, grade of slopes, and internal variables such as joint angles, body segment positions and velocity, and muscle force. Second, the number of electrodes and channels needed is rather large because of the many specialized muscles that operate around the various joints. For most SCI patients, stimulation would be required to move not only the feet, legs, and thighs, but also the trunk (for body stability) and, in some instances, the arms as well. The complexity of the control and stimulation systems is not only immense, but it would require a multitude of external sensors, as well as a power source that can deliver power to all the electrodes for long enough for the person to walk at least a short distance without loss of battery power. This equipment is likely to be heavy and cumbersome to carry while walking. Safety poses another set of hurdles. FES systems use electrode materials and stimulation parameters that do not cause damage to the nerves and muscles. However, they cannot prevent muscle fatigue. When muscles fatigue, they stop contracting regardless of neural input. With an FES hand-grip system, the risks to the patient when muscle fatigue sets in are minimal: the drop of a cup, perhaps spilling a hot beverage. However, the incidence of fatigue with hand-grip systems is small because these systems are typically used for short periods during the day. The risks of injury when a paralyzed person walks with an FES system could be more significant. When the stimulated muscles experience fatigue, the patient could fall and break a bone. Even if the fallen person were not hurt, he or she may not be able to stand up from that position without help. In order to be well accepted by the disabled population, a new rehabilitative technology needs to be better than currently available alternatives. Thus, an FES walking system must be at least as reliable, convenient, safe, and fast as the average wheelchair—a formidable challenge for researchers. There are, however, a wide variety of applications to which researchers want to apply FES. Several of these are simple enough to go from research laboratories into clinics within the next few years. They include bowel and bladder management, assisted coughing, and joint movements to prevent contractures. Meanwhile, the technologies for delivering and controlling FES will steadily improve, enabling ever more ambitious clinical applications. Relatively simple FES systems are already being used by many patients. A few quadriplegics have been implanted with hand-control systems and so far the outcomes have been positive.9 FES-assisted leg cycling exercisers and standing systems have been used by paralyzed patients wanting to improve their cardiovascular health and increase muscle bulk in their lower extremities.4,10 FES of the tibialis anterior has helped hemiplegic stroke patients improve their gait by preventing foot-drop.11 Neuromuscular electrical stimulation offers great hope to improve the functional state of paralyzed individuals. Simple systems are already in use and research is making significant progress in both control and stimulation strategies and designs. In this early stage of development, however, we must avoid excessive enthusiasm for complex applications. I have often listened to an acquaintance excitedly telling me that a paraplegic had walked on TV. This person did not realize that it had been done in the controlled environment of the laboratory, when the patient was supported by either a body harness or parallel bars. This does not mean that it can be done in the streets. More patient awareness of what is realistically feasible would be welcome. But looking into the future, FES research combined with advances in methods to repair the central nervous system promises a more independent future for individuals who have had spinal cord injuries or strokes. Anne-Caroline Dupont, PhD, is a research scientist with the Al. E. Mann Institute for Biomedical Engineering at the University of Southern California (USC) in Los Angeles. Her research involves clinical trials of injectable radio-controlled stimulators as well as new applications with functional electrical stimulation. She is an instructor in the regulatory science master’s program at USC. She can be reached at adupont@usc.edu. References 1. Liberson WT, Holmquest HJ, Scott D, Dow M. Functional electrotherapy, stimulation of the peroneal nerve synchronized with the swing phase of the gait of hemiplegic patients. Arch Phys Med. 1961;42:101-105. 2. Eichhorn KF, Schubert W, David D. Maintenance, training and functional use of denervated muscles. J Biomed Eng. 1984;6: 205-211. 3. Gordon T, Mao J. Muscle atrophy and procedures for training after spinal cord injury. Phys Ther. 1994;74:56-66. 4. Taylor PN, Ewins DJ, Fox B, Grundy D, Swain D. Limb blood flow, cardiac output and quadriceps muscle bulk following spinal cord injury and the effect of training for the Odstock functional electrical standing system. Paraplegia. 1993;31:303-310. 5. Prochazka A, Gauthier M, Wieler M, Kenwell Z. The bionic glove: an electrical stimulator garment that provides controlled grasp and hand opening in quadriplegia. Arch Phys Med Rehabil. 1997;78:608-614. 6. Popovic D, Stojanovic A, Pjanovic A, et al. Clinical evaluation of the Bionic Glove. Arch Phys Med Rehabil. 1999;80:299-304. 7. Dupont AC, Bagg SC, Creasy JL, et al. Clinical trials of BIONs™ injectable neuromuscular stimulators. In: Proceedings of the Sixth Annual Conference of the International Functional Electrical Stimulation Society; June 16-20, 2001; Cleveland. 8. Dupont AC, Richmond FJR, Loeb GE. Electrical stimulation via BIONs: present and future applications. Presented at: Technology and Persons with Disabilities Conference; March 2001; Los Angeles. Available at: www.csun.edu/cod/conf2001/proceedings/0037dupont.html 9. Kilgore KL, Peckham PH, Keith MW, et al. An implanted upper-extremity neuroprosthesis: follow-up of five patients. J Bone Joint Surg. 1997;79:533-541. 10. Stein RB, Gordon T, Jefferson J, et al. Optimal stimulation of paralyzed muscle after human spinal cord injury. J Appl Physiol. 1992;72:1393-1400. 11. Stein RB, Gordon T, Wieler M, Belanger M. Functional recovery: after the regeneration stops. Adv Neurol. 1997;72:365-375.
