Multiple sclerosis (MS) is a chronic, progressive disease of the central nervous system, believed to be caused by an autoimmune process, and resulting in demyelination and axonal loss in the brain, spinal cord, and optic nerves. MS affects approximately 400,000 individuals in the US alone and 2.5 million people worldwide. A decline in neurologic function, most notably in coordination, strength, tone, cognition, vision, sensation, and volitional control of bowel/bladder are hallmark characteristics of the disease, leading to reduced quality of life (QoL) and decreased participation in activities. Of primary concern to the MS population is impaired mobility, as it is the most visible disability and because of its profound impact on daily life.1,2
Gait disturbance is present in a large number of persons with MS (pwMS) and has been identified as one of the most disabling features of this neurologic disease. Compared with healthy controls, pwMS demonstrate decreased walking speed, decreased stride length, increased cadence, reduced active lower extremity range of motion (ROM), and increased variability in gait parameters.3–9
One of the more common gait pattern abnormalities demonstrated by pwMS is foot drop, caused by weakness of muscles responsible for ankle dorsiflexion and spasticity of the ankle plantarflexors. The ability to clear the foot by maintaining active dorsiflexion during the swing phase of the gait cycle is compromised in individuals with foot drop. Therefore, foot drop causes decreased gait efficiency and gait instability, leading to unwanted stumbles and falls. As a result, pwMS develop compensatory strategies including pelvic obliquity, hip hiking, and hip abduction with circumducted gait pattern to preserve foot clearance.
Treatment modalities to address foot drop include stretching, exercise, rehabilitation, orthotics, and assistive devices. The goals of treatment regardless of the intervention are to improve gait efficiency and safety, and overall improve the gait pattern to reduce musculoskeletal stress from altered biomechanics. The standard of care for foot drop has been the use of an ankle–foot orthosis (AFO). A more recently developed alternative to the AFO is functional electrical stimulation (FES).
Functional Electrical Stimulation for Foot Drop
The term FES refers to applying electrical current to a peripheral nerve via transcutaneous, percutaneous, or implanted electrodes, which in turn triggers muscles contractions with the goal of improving balance and gait. In the case of the FES application to foot drop, the electrical stimulation is applied to the common peroneal nerve, recruiting muscles controlled by both the deep and superficial peroneal nerves, and resulting in dorsiflexion and eversion of the ankle. The stimulation is synchronized with the gait cycle, so that it occurs during the swing phase of gait, and stops during the stance phase. FES devices generally include a power source (usually batteries), a stimulation unit, electrodes, and a mechanism to turn the stimulation on and off depending on the phase of the gait cycle. Various designs have been developed: wired versus wireless; tilt sensor on the leg versus heel switch. Commercially available FES systems for foot drop include the Odstock Dropped Foot Stimulator (ODFS®, Odstock Medical Limited, Salisbury, UK), the WalkAide® system (Innovative Neurotronics Inc., Austin, TX, US), the Bioness NESS L300® Foot Drop System (Bioness Inc., Valencia, CA, US), and the MyGait® system (Ottobock, Duderstadt, Germany). A majority of the published research in MS has focused on the ODFS and WalkAide devices. To date, only one head-to-head trial of these two devices on energy cost and walking speed in pwMS has been conducted.7 All of these systems provide transcutaneous stimulation via surface electrodes on the skin. From this point on, we will use the term, FES, when referring to transcutaneous FES for foot drop.
In 1960, Liberson et al.19 investigated the immediate benefits of using electrical stimulation to produce ankle dorsiflexion during the swing phase of the gait cycle in hemiplegic patients. The investigators found an immediate, positive effect on walking performance once the device was turned on, commonly referred to as the orthotic effect. The orthotic benefit, or “on-off” effect of FES on gait, has been well-documented in the MS population.3–9,13,14,16,20–24 More recently, the term training, or therapeutic, effect has been applied to changes in walking performance after regular, prolonged FES use when gait is evaluated without the device.14 We will refer to this type of effect as training effect throughout this article. Ultimately, the training effect reflects an improvement in motor ability of the affected limb over time without the assistance of FES. Additionally, the total orthotic effect of FES (defined as the change in walking speed with FES at follow-up assessment compared with walking speed at baseline without FES8) has been reported in pwMS.8,16,24
One potential explanation for the training effect of FES is the promotion of neuroplasticity with repetitive daily stimulation, producing a cumulative effect over time.23,25 Evereart et al.23 observed the effects of using FES for several months in both nonprogressive and progressive disorders of the central nervous system and found a significant increase in motor voluntary contraction and motor end plate potential. The large increase in electrophysiologic parameters observed suggests strengthening of the residual corticospinal pathways and activation of motor-related areas of the cortex, regardless of the neurologic condition.23 However, evaluating the long-term training effects of FES (or any assistive device) in the MS population is complicated by the progressive accumulation of disability over time compared with nonprogressive conditions such as stroke.14 Therefore, data supporting the training effect of peroneal nerve stimulation has been mainly explored in the stroke population.
A review of the pertinent literature on the effects of FES for foot drop in individuals in MS was conducted. Evidence pertaining to the efficacy of FES on various outcome measures is presented in this review article and summarized in Tables 1, 2, and 3