June 2018

Ankle-foot orthoses and functional electrical stimulation for foot drop in MS: Pluses, minuses, progress

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Assistive ambulation devices for the ankle can bolster walking speed and safety and lessen the risk of injury to the joint. But which of 2 technologies is best for your patient?

By Hank Black

People with multiple sclerosis (MS) have difficulty walking: Gait impairment, including the reduced ankle dorsiflexion of foot drop, is one of the most common indicators of disability early in the course of this progressive autoimmune disease of the central nervous system, affecting approximately 75% of people with MS.1 Assistive technology, such as ankle–foot orthoses (AFO) and functional electrical stimulation (FES), increases the safety of walking and the speed of ambulation (even then, only about one half of patients remain ambulatory 15 years after disease onset).2,3 Assistive technology also reduces the risk of injury to the knee and ankle and reduces the effort of ambulation.

How do these technologies work?

Ankle–foot orthoses. An AFO can effectively eliminate foot drop by maintaining the ankle in a neutral position, although it has disadvantages for some patients (described below). There are many types of AFOs, including solid and hinged devices. Carbon-fiber AFOs have been in use for almost a decade; they are lighter and more comfortable than older AFOs—although not strong enough for patients who have a severe level of weakness. Carbon-fiber AFOs do not usually require larger shoes; molded plastic AFOs do.

“An AFO can improve foot drop and is something many physical therapists or physicians prescribe as an inexpensive solution to improve walking,” according to Kathleen M. Zackowski, PhD, OTR, MSCS, Senior Director of Patient Management, Care, and Rehabilitation Research for the National Multiple Sclerosis Society and Assistant Professor in the Departments of Physical Medicine & Rehabilitation and Neurology at the Johns Hopkins University School of Medicine, Baltimore, MD. In her role, Zackowski looks for innovations to improve patients’ quality of life and influence the course of disease. She has found that AFOs and FES may offer benefits to people with MS.

An AFO “enables better walking by limiting ankle movement and keeping the foot in a static position, thus preventing it from dropping down and dragging with each step,” Zackowski noted. “Some AFOs allow a small degree of movement at the ankle, allowing the person to use the range of motion they have.” The benefits are that an AFO “can be implemented independently and used for walking day-to-day in any environment. Importantly, AFOs help maintain ankle stability and often reduce the risk of trips and falls.”

But AFOs can have disadvantages. “Many people don’t like that an AFO is often bulky and noticeable, making many people feel that this accentuates their disability to the public,” Zackowski said. An AFO “doesn’t fit easily into all types of shoes… [and] can feel heavy and uncomfortable, especially in the summer heat.”

The cost of an off-the-shelf AFO can start at less than $100. Many US insurers do not provide coverage, or provide minimal coverage, for an AFO in the United States, however—making access to these devices problematic for many MS patients.

There are benefits of, and a downside to, both AFOs and FES. Knowledge of the literature, experience, ongoing refinements in technology, and careful assessment of the individual patient will help you select the device that holds most potential.

Functional electrical stimulation. FES uses skin surface electrodes to provoke the peroneal nerve and stimulate the anterior tibialis muscle that controls ankle flexion. It utilizes either 1) a pressure switch or sensor in the shoe that is activated by heel-strike to synchronize with the swing phase (Odstock Medical Ltd., Salisbury, Scotland, UK, and Bioness, Inc., Valencia, CA) or 2) a swing-pedometer switch that senses the angle and velocity of the tibia (Innovative Neurotronics, Inc., Reno, NV). These devices have been found to protect the integrity of the knee joint and reduce the back pain that results from walking with an abnormal gait.

Lower-extremity clinicians and many people with MS view FES as a promising alternative to an AFO; researchers are hard at work to solidify the science behind the technology. FES systems for MS have been prescribed in the United Kingdom since 2009, often covered by National Health Service insurance.4 In the United States, Medicare and most other insurers do not cover such devices for MS.5,6 Advanced FES systems can cost $6,500 or more, according to several clinicians interviewed for this article. FES does not usually require adjustments to footwear.

From the literature: Benefits of assistive ambulatory technology

Photo courtesy of South Shore Neurologic Associates

A number of studies in recent years—most small and methodologically flawed, however—provide evidence of a beneficial effect of FES on the risk of falls and fear of falling,7,8 energy cost of gait,7,9-12 gait speed,13,14 and functional mobility.8,15

Large trials of FES for drop foot in MS are rare; a 2015 trial of MS and drop foot included 187 participants and found a significant improvement in walking speed both initially and after 20 weeks (27% average improvement). Overall functional walking was maintained or improved in 95% of responders.13

Linda (Miller) Renfrew, PT, MPhil, consultant physical therapist in MS at the Douglas Grant Rehabilitation Unit, Ayrshire Central Hospital, Ayrshire, Scotland, UK, and others have studied outcome measures with the MS population and assistive technology, in an effort to achieve more clarity in clinical decision-making. Speaking with LER, Renfrew said that “the most common outcome measure to assess with both FES and AFO has been gait speed. Whether it is the best, is still a matter of debate.” The debate hinges in part on the quality of psychometric properties.

Andreopoulou and colleagues16 called for more, higher-quality studies of walking measures in a 2018 review of walking tests evaluating AFO and FES use in MS. The researchers found 20 walking measures in 41 studies. The most commonly used were the 10-meter walk test, gait kinematics, and the Physiological Cost Index. However, only 10 measures reported psychometric information (internal consistency and test–retest reliability). Strong evidence levels were found for the Multiple Sclerosis Walking Scale-12 and the timed 25-foot walk test, and moderate levels for the 6-minute walk test and the 10-meter walk test.

In a 2017 review of 19 studies involving 490 patients with MS who had foot drop on the effect of FES on gait speed, Miller and co-workers17 found weakness for blinding in all studies, but clinically meaningful orthotic effects of FES in the 5-minute self-selected walk test.

Some of the same researchers, including Renfrew, delved further into the effects on gait speed as well as the oxygen cost of assistive technologies in the first randomized clinical trial14 to compare initial orthotic effects of AFO and FES for this population. The trial included 78 participants, divided by modality of treatment and tested with and without their device.

Results showed that AFO reduced walking speed—more so in a fast-walking subset ( > 0.8 m/s), whereas FES increased speed in slow walkers ( < 0.8 m/s), but in not fast walkers. Miller and co-workers previously found this differential effect was dependent on self-selected walking speed.18

The results of the Renfrew randomized clinical trial suggest that people with MS who walk slowly gain positive initial orthotic effects from FES but not from an AFO.14 This, the researchers say, may help patient compliance with FES by helping inform clinical decision-making and patient understanding. No effect was found on gait speed in the 25-foot walk test, a result attributed to very small changes observed in walking speed.14

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Outcomes involving patients’ perception of exertion and other experiences with assistive ambulation technology are also helping frame discussions. Khurana and colleagues7 employed several self-report questionnaires to help compare AFO and FES energy cost, efficiency, and effort in people with MS. In a preliminary, randomized, crossover study,7 researchers tested 20 subjects in 2 walk trials on different days (1 day with each prosthetic device). Subjects then completed 3 questionnaires, the Fatigue Severity Scale, the Falls Efficiency Scale, and the Borg Scale of Perceived Exertion.

Results suggest FES may allow many patients to perform activities of daily living with less perceived fatigue.7 The researchers also saw a greater reduction in ambulatory exertion in patients who have had their diagnosis longer or have a greater fear of falling, which, they say, may have implications for identifying those who might best benefit from FES for improving decreased activity levels due to fatigue and foot drop.19 For patients with a rapidly progressive form of MS, the sudden worsening of foot drop may prohibit further use of FES as an assistive option.

Because investigation often leads to more product development, qualitative studies should complement traditional quantitative studies to gain a broader perspective, according to UK researcher Catherine Bulley, PhD, BSc (Hons). “There’s a growing recognition of the need to learn from the person who is experiencing services or equipment,” she said.

Bulley, a reader (professor) in physical therapy at Queen Margaret University School of Health Sciences, Musselburgh, Scotland, was part of a team investigating the impacts of AFO and FES on MS-associated foot drop.20 Among the team’s findings from 2 small focus groups was that both AFO and FES devices showed positive and negative aspects, but that most difficulties often were outweighed by improvements in quality of life, such as the ability to participate in activities.

“It can be easy for professionals to decide they are the expert,” Bulley said, “rather than, while linking to quantitative research, test products such as foot-drop footwear with the attitudes of patients, care-givers, family members, and others in mind, beginning in the early phases of design and continuing throughout.” Bulley is now engaged in evaluating the use of social media to communicate with end-users.

Gauge the utility and practicality of providing assistive technology

For clinicians, the priority in providing an assistive device is patient safety in relation to trips and falls, according to Zackowski. “We also want to know which assistive technology for walking feels better to the person wearing it, and whether it actually improves their walking and is comfortable enough to use consistently,” she said.

Quick, practical walking tests can provide most of the necessary information to get started. “We always look for a high-tech solution, such as gait-lab evaluation, but the gold standard for therapists in clinic is the simple timed 25-foot walk test,” Zackowski said. “Foot drop is a functional problem that slows walking speed and increases the risk of falls.”

For foot drop, she said, many rehabilitation clinics have AFOs and at least 1 type of FES device that therapists use for an in-clinic trial. Device-naïve patients likely would try both devices to see how they feel. If the tingling sensation of FES is too strong, the PT can adjust the intensity.

“With persistence, many get used to it,” Zackowski said, “and don’t even know it is on during use.”

The expertise of a PT is often key to getting a comprehensive evaluation and help. “Away from metropolitan areas it may be hard to find an OT or PT who specializes in MS, according to Zackowski. “However, if you find a therapist interested in learning about MS and someone you feel comfortable working with, you may find that you can get the help you need. Patients should search out the resources in [their] area.”

Select, test, and fit an assistive device

Mobility early in the course of MS may not be highly affected by the disorder, and clinicians often get pushback when assistive technology is suggested. Peiting Lien, PT, who practices at The Johns Hopkins University, in a community outpatient center in Baltimore, MD, said patients often express reservations about trying assistive devices because of cosmetic concerns. Eventually, however, patients come to terms with how such devices may benefit them.

“Most of our patients go for an AFO,” Lien said, “both because even custom orthoses are far cheaper than most FES devices, and it’s quicker to go through the fitting process.”

Lien’s clinic provides patients with a scorecard of the pluses and minuses of both types of devices and a trial of off-the-shelf AFOs of both static and dynamic design. Patients who are interested in pursuing FES are referred to a company representative for fitting because the clinic’s involvement is limited by the cost of purchasing successive generations of equipment and by time constraints for follow-up adjustments.

Some patients seek help from charitable organizations and foundations to pay for FES. Anna Berry, PT, of the Andrew C. Carlos Multiple Sclerosis Institute at the Shepherd Center, Atlanta, GA, said patients may receive FES through a nonprofit funded by a grateful patient. “With this major advantage, most of our patients can walk out with an insurance-paid AFO as well as an FES, so they can switch to the AFO when needing additional stability, to rest their muscle from the electrical stimulation, or for convenience in traveling,” she said.

Depending on the patient’s disease status, the therapist explores his (her) understanding of ambulation in MS, as well as what kind of lifestyle they have and desire. The therapist also determines whether difficulty in dorsiflexion is the main reason for walking problems.

“Timing up the kinetic chain, such as weakness in hip flexors or hamstrings, or core instability, must be addressed first,” Lien said.

FES is contraindicated in pregnancy and epilepsy and when there is a history of cancer, Berry said. If a patient has a pacemaker, the manufacturer’s recommendations are followed. A history of ankle instability, such as from a previous sprain or a structural abnormality, may be a contraindication to FES, Berry added.

“We want to look at the patient holistically, including consideration of their type of work, lifestyle, fatigue level, and support system.”

Patient input is key, therapists say. Some patients cannot adjust to the intensity of electrical current or the complexity of the FES, or don’t have adequate use of their upper limbs to be able to wear it. Initial clinic visits include evaluation of muscle tone and ankle-joint range of motion and strength. A visual gait analysis is also performed: The PT observes the patient’s walking pattern, looking for weakness in the hip or knee. (Regarding the hip: Newer, multi-channel FES models may help with muscles controlling the knee, but neither an AFO or an FES helps hip weakness.) Manual evaluation of muscle tone is an essential part of the PT’s protocol, Berry said, because spasticity around the ankle joint, especially in the plantarflexor muscles, may be made worse with electrical stimulation of the targeted muscle.

An initial ankle stretch will determine if tone is going to fight against stimulation, according to Casie Turcotte, PT, of the University of Maryland Rehabilitation & Orthopedic Institute, Baltimore, MD. FES usually will not alleviate drop foot in that case, although anti-spasticity injections may allow it to be effective afterward, she said.

FES also is inappropriate if range of motion is inadequate for the foot to dorsiflex. “The stimulation wants to pull the foot up, but without enough ROM, will end up pulling the foot out or in, usually out,” Turcotte said.

Speed and fatigue are determined by timed walk tests of varying length (with and without a device), which also provide the therapist the opportunity to further observe gait patterns. Subjective questionnaires, such as the Modified Fatigue Impact Scale and the Borg Scale of Perceived Exertion during an endurance test, also are used to evaluate fatigue. Turcotte’s group uses the 4-minute and 6-minute walk tests, as well as the Timed Up and Go, or TUG, test that assesses balance and fall risk. Timed tests inform the PT whether a person can ambulate sufficiently fast with a device to be safe at home (0.4 m/s)21 or in the community (0.8 m/s).22

Therapists do provide an exercise program for affected lower-extremity muscles. However, if a person with MS has zero muscle strength in the tibialis anterior, the peroneal nerve will not innervate it, Turcotte said, adding, “In that case, there’s no exercise I can give to simulate what the FES system does.”

For a trial of FES, therapists walk along with the patient to manually control the parameters of the device. During the trial, Turcotte asks herself several questions: “Do I need to stimulate a second sooner? Is the foot ‘slapping’ the ground after it lands, so I need to hold onto the stimulation a little bit longer? Should I increase the intensity to increase muscle recruitment?”

Additional concerns

Disease stage is crucial in assessing which type of assistive device is suitable to the individual, Berry noted. If the patient is in a relapsing–remitting stage of MS, she said, FES may be pursued but, if the patient has entered the progressive or secondary progressive phase, in which case return of function is unlikely, it probably should not be pursued.

“We may not be sure of that, however,” Berry concluded, “so we begin as though [the patient] will gain back function.”

Cost–benefit analysis of providing an assistive device is eventually necessary in the work-up. Most health-care insurers, Turcotte said, deny coverage because they look at research that shows that both these technologies either reduce or eliminate foot drop—or fail to do so—after the standard 8-week trial.

“However, we see FES separates itself with an additional 8 weeks of use,” Turcotte said. “There may be more automaticity of gait and more symmetry. A neuroplastic effect seems to be at play as the pertinent muscle repetitively responds to the stimulation.”

Quality of life. Many people with MS who wear an FES system for foot drop get a psychological lift from seeing their foot dorsiflex normally once again. “It’s amazing to them to see this result,” Turcotte concluded. “It becomes a quality-of-life issue, together with how they can cover it up better than an AFO. They just feel better about themselves.”

Hank Black is a freelance medical writer based in Birmingham, AL.

REFERENCES
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