In recent years, numerous studies have reported the effects of functional electrical stimulation (FES) in the management of foot drop. These various trials have spanned several patient populations and multiple outcome domains. An understanding of the recent findings associated with the use of this technology will enable clinicians to more accurately respond to patient inquiries and better anticipate the associated potential benefits. The purpose of this article is to review the current literature, investigating the effects of FES in the treatment of foot drop across different patient populations.
For the purposes of this review, publications were included if they were published between January 2006 and September 2009. The dates were chosen to limit the review to modern FES systems that are currently used in clinical practice. The review was limited to studies investigating single channel, surface FES systems intended for sustained daily use, including the Odstock Dropped Foot Stimulator (ODFS) (Odstock Medical Ltd, Salisbury, Wiltshire, UK), NESS L300 (NESS) (Bioness Inc, Valencia, CA), and WalkAide (WA) (Innovative Neurotronics, Austin TX). Thus, studies reporting on either multiple channel systems or systems with implanted electrodes were omitted. In addition, articles reporting on combined modalities, such as the use of FES in combination with body weight-supported treadmill training, were also excluded. Within these criteria, 11 articles were identified. Eight articles reported on studies in which the majority of the subjects presented with chronic hemiplegia because of stroke or brain injury. The three remaining articles reported on FES outcomes among patients with multiple sclerosis (MS).
The subjects for the trials reviewed in this section were primarily patients with chronic hemiplegia secondary to stroke. However, several authors broadened their inclusion criteria, so that limited numbers of subjects with brain injury, MS, and spinal cord injury were also included. For the purposes of this section, the findings of the various trials were organized according to the outcome domains, including velocity, physiological cost index (PCI), unconventional walking surfaces and tasks, endurance walking, “therapeutic” or “carry-over” effects, patient-reported data, dynamic balance and gait symmetry, and FES versus ankle-foot orthosis (AFO). Thus, individual trials reporting across multiple outcomes domains are treated in multiple subsections.
VELOCITY AND PCI
The findings within this section are limited to performance values over short distances at self-selected walking speeds and on flat surfaces. The effects of FES interventions on gait velocity are summarized in Table 1. The immediate effects of FES on self-selected walking velocity were examined by Sheffler et al.,1 reporting a nonsignificant trend toward improved velocity with FES compared with no device immediately after study subjects were fit with ODFS systems. However, the improvements observed with the FES intervention were not as great as those observed when subjects performed the same outcome measure with their previously prescribed AFOs.
The same year, Stein et al.2 reported on self-selected gait velocity and the associated PCI values observed within a cohort made up predominantly of patients with chronic hemiparesis as observed during a 10-m walk test (10MWT). In this multicenter trial, baseline velocity values were collected before the introduction of any FES intervention. These were then compared against values collected with the FES intervention 3, 6, and 12 months after subjects had been fit with WA systems. Authors reported increases in walking speed with the FES intervention of 15%, 32%, and 47% at the 3-, 6-, and 12-month follow-ups, respectively. Of relevance, sample size decreased appreciably at each successive follow-up, suggesting a possible bias in which those subjects who benefited the most from the intervention remained in the trial longer. Calculated PCI values suggested a nonsignificant trend toward reduced energy consumption at the 3-month follow-up, and an average energy cost reduction of 20% among the subpopulation who remained in the trial for the full year.2
A later trial by Laufer et al.3 supported these earlier findings. Following a cohort made up exclusively of patients with chronic hemiplegia, authors collected velocity data using the 10MWT with no intervention. Follow-up data were collected while the subjects used their NESS systems at both 8 weeks and 1 year after the introduction of the FES intervention. Improvements in gait velocity of 29% and 52%, respectively, were reported.
Finally, Burridge et al.4 reported on the differences in gait velocity as measured with the 10MWT test, with and without FES intervention on subjects who had used the ODFS system for a minimum of 3 months but an average of 2 years. The cohort was mixed, made up predominantly of patients with chronic hemiparesis including 35% representation of patients with MS. Among these legacy users, gait velocity was observed to be 14% faster with the FES intervention than without it. In contrast, the authors found no treatment effect with FES on PCI.
UNCONVENTIONAL WALKING SURFACES, ENDURANCE WALKING, AND ASSOCIATED PCI
This section is devoted to all velocity observations not addressed in the previous section and any associated PCI observations. As such, it will include ambulation across variable surfaces, while negotiating turns, obstacles and stairs, and extended walking tests. The findings are summarized in Table 2.
Reporting on their cohort cited earlier, Stein et al.2 also recorded gait velocity on a 10-m figure of eight track, measuring the patients' ability to turn in both directions and sustain continuous walking for 4 minutes. Predictably, the observed gait velocity values were lower than those of the same cohort when performing a traditional 10MWT. However, the subjects realized a 14% improvement in their sustained gait velocity values around the figure of eight track when the test was administered with the FES intervention 3 months after the subjects received WA systems.
Citing subjective data from an earlier study suggesting that improvements in “confidence when walking” were more important than gait speed,5 Burridge et al.4 created a 10-m uneven walkway in their laboratory (Appendix). Reporting on the legacy cohort cited earlier, the authors reported gait speed across this modified walking surface. Increases in walking velocity with the FES intervention on the uneven surface were greater than increases observed on the standard 10MWT, reported at 23% and 14%, respectively. Further, although the use of FES failed to appreciably affect PCI data with the standard 10MWT, its usage on the uneven surface decreased PCI averages by 13%. This finding led the authors to hypothesize that testing FES systems in traditional laboratory sittings with even floors may underestimate their effects.
The effects of the ODFS in negotiating various environmental terrains were also recently examined by Sheffler et al.1,6 By using the modified Emory Functional Ambulation Profile7 (mEFAP), authors reported an immediate, statistically significant improvement in gait velocity over carpet and nonsignificant improvement trends on the obstacles and stair components (Appendix). As with data regarding gait velocity on a hard flat surface cited earlier, the velocity improvements observed with FES immediately after its introduction were less than those observed when patients ambulated with their previously prescribed orthoses.1 A second, related article by the same authors will be presented in the Therapeutic or Carry-Over Effects section.
The efficacy of FES in negotiating such environmental terrains, both immediately after FES application and after sustained usage, has also been described.3,8 Reporting on the same cohort of patients with chronic hemiplegia described earlier,3 Hausdorff et al.8 evaluated gait velocities during a 10MWT test over carpet and an “obstacles” task, both of which were modified from similar tasks in the mEFAP (Appendix). The authors reported both immediate and continued improvements in gait speeds within both domains. Gait speed over carpet increased immediately by 37% and by 59% after 8 weeks of NESS usage. Similarly, velocities during the obstacles task increased by 24% and by 44%, respectively.8 A follow-up article reported continued velocity increases of 75% and 58% in the carpet and obstacles tasks, respectively, after 12 months of sustained usage.3
Along with their assessments across variable terrains, the same authors recorded sustained gait velocity values as recorded during the 6-minute walk test (6mWT).3,8 Hausdorff et al.8 reported a 17% increase in sustained walking velocity immediately after the application of the NESS system, which increased to 34% after 8 weeks of sustained use. In a follow-up article on the same cohort, Hausdorff et al. reported a 47% increase in walking velocity after 12 months of sustained NESS usage.3
THERAPEUTIC OR CARRY-OVER EFFECTS
By using the term “orthotic effects” to describe the benefits observed when subjects were wearing and using the FES systems at the time of evaluation, three of the articles cited thus far have also reported on what have been described as the “therapeutic,” “carry-over,” or “training” effects of the various FES systems. These latter effects are those observed after sustained FES usage but in the absence of its simultaneous use at the time of observation.
In addition to their observations on the orthotic effect of the WA system, Stein et al. also observed improvements in gait velocities at follow-up evaluations without the FES intervention. Specifically, improvements of 11%, 24%, and 31% were recorded 3, 6, and 12 months after the introduction of the FES system.2 However, as observed earlier, the sample size of the cohort decreased appreciably at each successive follow-up, suggestive of a possible sampling bias, and statistical analysis of these observations was not presented.
After their description of the immediate efficacy of the ODFS in the gait velocities observed during the various components of the mEFAP,1 Sheffler et al.6 reported on two case subjects who completed the mEFAP without the FES intervention on two occasions. The first occurred before any exposure to the technology, whereas the second occurred after 4 weeks of sustained usage of the FES system. Both subjects demonstrated improved performance values for each individual components of the mEFAP, with the magnitude of those improvements ranging from 3% to 30%.6
Finally, the observations of Laufer et al.3 on the various gait speeds observed during the 10MWT, 6mWT, obstacles, and carpet walking domains were repeated for the same cohort in the absence of active NESS usage after 12 months of acclimation to the technology. Relative to the baseline performance values obtained before any intervention, statistically significant improvements were observed across all four domains. Although the velocity values recorded in this condition were less than those obtained during the same time period with active FES usage, they approximated the obtained with active FES usage after 8 weeks of acclimation to the intervention.
In addition to the empirical outcomes regarding the efficacy of the various FES systems, several articles also collected and reported the patient-generated feedback. Some of these insights came from the use of previously published, validated questionnaires. Others were derived from simple surveys created and administered by their various authors to supplement clinical insights.
Reporting on their previously described cohort of patients with chronic hemiplegia, Laufer et al. administered the short version of the Stroke Impact Scale9 and the participation domain of the Stroke Impact Scale,10 both self-report questionnaires designed to assess the physical function and social participation, respectively, in the community life of patients with hemiparesis.11 Regarding physical function, short version of the Stroke Impact Scale Scores were observed to increase by 18% after 8 weeks of sustained usage of the NESS systems. Similarly, self-reported participation domain of the Stroke Impact Scale Scores increased by 25% during the same period. Both improvements were reported as statistically significant and are suggestive of increased levels of community integration. In addition to these early gains in function and participation, scores on the same indices 10 months later still suggested further improvements in both domains.11
Supplementing their observations on gait velocities and PCI values over flat and uneven surfaces, Burridge et al.4 generated individual perception scores for each subject by soliciting responses to a seven-item questionnaire. These perception scores were then plotted against each individual's stimulated:not stimulated PCI ratio on the uneven surface or the relative effect on PCI with and without the FES intervention. A negative relationship was established (r = 0.58), suggesting that those participants who expended less effort with the FES intervention perceived a greater benefit with the intervention. When these same perception scores were plotted against stimulated:not stimulated speed ratios on uneven surfaces, no strong relationship was evident. Thus, these perception values support the idea that the influence of the ODFS systems on physiologic costs when negotiating uneven surfaces was valued more than its effect on velocity.
In addition to their findings as assessed with the mEFAP, Sheffler et al.1 supplemented their observations with the immediate impressions of their study cohort on the ODFS system. Twelve of the 14 subjects expressed a preference for the ODFS over their AFOs for daily ambulation. Similar findings were observed in a study reporting on the NESS system in which all 15 study subjects expressed a preference for FES over AFOs for daily ambulation.12 In addition, participants subjectively reported improvements in muscle movement, gait pattern, and strength.12 Increased mobility, decreased effort, safety, and ease of use were similarly identified with use of the WA system in the work by Stein et al.2 By using their own questionnaire, Hausdorff et al.8 reported similar benefits with the NESS system. Specifically, subjects agreed with statements, identifying the FES system to be comfortable, convenient, and safe.8 In addition, subjects reported increased physical activity and greater confidence in walking on slopes and uneven surfaces.8
DYNAMIC BALANCE AND GAIT SYMMETRY
Observing that studies focused on gait velocity and energy cost may not fully assess the impact of FES interventions on gait and functional abilities, additional data were collected on the same cohort of patients with chronic hemiparesis that has been reported on throughout this review.3,11 Specifically, authors collected the temporal gait parameters of stride times and percent swing durations while subjects performed the 6mWT.8 These were then used to calculate each subject's stride time variability13 and gait asymmetry index14 (Appendix). The former has been linked to fall risk, whereas the later has been associated with both fall risk and poor balance. Further data were collected immediately after the application of the NESS system, after a 4-week period of increasing usage and after 8 weeks of NESS usage. Authors reported an immediate reduction in stride time variability of 23%. This value continued to improve by 27% and 33%, respectively, after 4 and 8 weeks of NESS use. Similarly, the swing asymmetry index immediately improved by 28% with the introduction of the FES systems, reaching a 45% improvement after 8 weeks.8
FES VERSUS AFO
There have been two recent comparisons between FES interventions and the more traditional modality of AFOs. In the first of these, cited earlier, subjects with chronic stroke completed the five timed tasks of the mEFAP in three conditions: no intervention, using a previously prescribed AFO, and with the ODFS system immediately after its receipt and training. Although gait velocities with the FES intervention were improved in every walking task relative to no intervention, they were lower than those observed when subjects performed the same tasks with their existing AFOs.1
Examining a subgroup of the chronic hemiplegic population attributed to Laufer et al., each of whom regularly used an AFO as prescribed by a physiatrist, Ring et al.12 explored the differences between the NESS system and AFOs relative to several gait performance parameters. The type of AFO used by the individual study subjects was variable, including both off-the-shelf and custom, articulated devices. After a 4-week adaptation period in which subjects used both interventions as they increased their daily use of the FES intervention, there were no appreciable differences between the two conditions with respect to gait speed, the calculated gait asymmetry index or swing time variability. After an additional 4 weeks of exclusive FES usage, significant improvements were observed in all indices compared with those observed with AFO use, with the exception of gait speed where the appreciable improvement observed with the FES intervention fell slightly short of statistical significance.12
SUMMARY: CHRONIC HEMIPARESIS
Recent findings support the assertion that FES systems facilitate the improvements in gait velocity across both smooth and irregular surfaces in patients with chronic hemiparesis. These improvements have been observed immediately after the application of the various technologies but may be further augmented with extended usage. The “theurapeutic” effects of such systems across variable walking surfaces and tasks have been broadly supported in the recent literature. Patient-derived data indicate high-acceptance rates and may provide further insights into what aspects of FES usage are perceived to be the most beneficial. In addition to its effects on velocity, FES intervention has been associated with improved symmetry and rhythmicity of gait. Any empirical advantages of FES systems over AFOs seem to require acclimation to the FES intervention.
Recent years have seen the addition of three clinical trials evaluating the effectiveness of FES in the treatment of drop foot secondary to MS.
Contemporary evidence suggests that among patients with more severe MS who have experienced continued usage of an FES foot drop system, sustained preferred walking speeds are significantly faster with the technology than without it.15 Specifically, among a cohort of patients with predominantly secondary progressive MS, all of which required some form of walking aid (i.e., one cane, two canes, or rolling walkers) and who had used FES for at least 6 months (with an average usage time of 3 years, 5 months), average preferred walking speeds during a 5-minute walking trial were significantly faster with their FES than without it. The effect of the FES on sustained gait speeds within this cohort was quite variable. Although the average improvement in speed was 15%, this difference ranged from 0.0% to 44.1% for individual subjects. It should be noted that even with the FES, the mean sustained walking speed for the study subjects was less than half of that observed in a cohort of unaffected, matched controls.14
Similarly, among the same subjects, physiological costs of ambulation were significantly lower with the FES than without. As with differences in preferred walking speeds, differences in physiological costs were variable, averaging a 12% reduction, but ranging from a 19.2% increase to a 27.9% reduction.15
A separate clinical trial further substantiated the efficacy of FES with respect to its orthotic benefit on subjects with secondary progressive MS during both short-timed (10 m) and long-timed (3 minute) walking tests with sustained FES usage over 18 weeks.16 Several interesting findings are noteworthy. First of all, the level of physical debilitation of the subjects in this trial is perhaps best recognized by the need to modify the originally intended outcome measure, the 6mWT, to a shorter 3-minute walk test, as the former proved too demanding in pilot trials. Among this cohort, the orthotic benefit of the FES intervention over short distances (10 m) resulted from the maintenance of gait speed with the FES in contrast to a slight deterioration of gait speed in the absence of FES over time. In contrast, the orthotic benefit during evaluations of walking endurance was evidenced in substantial improvements in the distance covered during 3 minutes with FES.16
In contrast, recent findings suggest that among those patients with a less debilitating form of MS, short-term use of an FES system does not seem to improve performance on outcome measures characterized by short distances (i.e., less than 10 m).17 Specifically, among a cohort of patients with a relapsing remitting form of MS, after 4 weeks of acclimation to regular use of an FES unit, subjects were assessed in their performance of a timed 25-foot walk test and the five-timed components of the mEFAP. The only significant improvements observed were in the “stairs” component of the mEFAP. Values observed with and without the FES intervention for both the 25-foot walk test and the remaining components of the mEFAP failed to suggest any substantial velocity benefits with the FES over the short distances examined.17
Despite these limitations, 10 of the 11 subjects preferred ambulation with the FES unit when compared with no device, and 9 of the 11 subjects preferred ambulation with the FES unit when compared with an AFO, suggesting possible clinical benefits outside of those evaluated by the selected outcome measures.17 Also worthy of note, the authors of this last trial observed that approximately 50% of all subjects screened for enrollment for the FES trial were ultimately excluded. Leading causes were the presence of genu recurvatum, equinovarus ankle positioning, limited passive range of motion at the ankle, and peripheral neuropathy—all of which, the authors observed, may be adequately controlled by an AFO.
SUMMARY: MULTIPLE SCLEROSIS
Recent evidence supports the efficacy of an orthotic effect associated with the use of FES during both short-distance and endurance walking events among patients with more debilitating MS. In contrast, the use of FES by less debilitated patients with MS was not shown to increase the speed of short-distance walking events relative to performances without it. The benefit of FES over endurance walking events in less debilitated patients has not been evaluated. To the limited extent that it has been investigated, there is no evidence of a therapeutic or carry-over effect associated with FES among the more debilitating forms of MS. This effect has not been investigated among more functional walkers with MS.
With respect to MS, recent clinical trials have confined their outcome measures to timed walking events and physiological cost indicators. The effect of FES on reducing trips and falls, general stability and balance, confidence and fatigue have not been investigated. Further, with the exception of the subjective opinions collected by Sheffler et al., head-to-head comparisons between FES and AFO interventions have not been undertaken in the MS community.
APPENDIX: REFERENCED OUTCOME MEASURES
10-M UNEVEN WALK
Eight small strips of wood, measuring 2-cm high and 5-cm wide were placed randomly underneath a 10-m length carpet to simulate walking in an outdoor environment with unpredictable, uneven surfaces.4
MODIFIED EMORY FUNCTIONAL AMBULATION PROFILE
Previously published and validated timed outcome measure created for assessment of functional ambulation across five defined domains, which are follows:7
- 5MWT: 5-m walk on hard floor.
- 5MWT over carpet: 5-m walk on carpeted surface.
- Timed up and go: rise from chair, walk 3 m, turn, and return 3 m to resume a seated position.
- Obstacles: standardized course in which subjects step over bricks and negotiate a turn around a garbage can.
- Stair: standardized ascent or descent of four steps.
STRIDE TIME VARIABILITY
Defined as 100 × (standard deviation of stride time/mean stride time), where lower values suggest less variability in stride time.13
GAIT ASYMMETRY INDEX
Defined as 100 × ([swing time paretic − swing time nonparetic]/[sing time paretic + swing time nonparetic]), where a gait asymmetry index of 0 indicates a perfectly symmetrical gait.14
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