Functional Electrical Stimulation for Return of Function After Stroke

NMES Applications

The primary purpose of most upper-limb applications of FES after stroke is therapeutic, to improve the extent of arm and hand recovery so the upper limb can, at a minimum, be useful in performing bimanual tasks. Three NMES paradigms have been used for upper-limb motor relearning: cyclic NMES, triggered NMES, and proportionally controlled NMES. The main feature that distinguishes these paradigms is the method by which the intensity of electrical stimulation is controlled.

Cyclic NMES activates the paretic muscles according to an on/off cycle, with the timing of the cycle, the number of repetitions, and maximum intensity of stimulation preset by a therapist ( ). Surface electrodes are typically placed over the motor points of the finger and wrist extensors. Elbow extensors or shoulder muscles may also be targeted in some patients. When the device is turned on, stimulation elicits repeated muscle contractions, and therefore repeated arm or hand movement lasting several seconds at a time. Cyclic NMES requires no input from the patient, who has no role in controlling the stimulation. The approach is indicated for persons with some or no residual motor function.

Triggered NMES requires input from the patient or therapist for the stimulation to be delivered. Electromyograph (EMG)-triggered stimulators prompt the patient to produce a suprathreshold EMG signal by attempting to contract the paretic muscle, at least partially ( ). If and when the amplitude of the EMG signal exceeds a preset threshold, the stimulator turns on and delivers a preset intensity of stimulation to the target muscle(s) for a preset duration. After several seconds the stimulation turns off and the cycle repeats. This approach is indicated for patients who can partially activate a paretic muscle but are unable to generate sufficient muscle contraction for adequate exercise or function. Because stimulation and the corresponding cutaneous and proprioceptive feedback to the brain coincides with the patient’s own efforts to produce the elicited movement, it is thought that this method may be more effective in promoting neurological changes leading to better recovery. Alternatively, accelerometers or other body-worn sensors can provide methods of triggering stimulation ( ). Switch-triggered stimulators have push buttons that are either cabled to or integrated with the stimulator to trigger stimulation. The push buttons may be operated by a therapist ( ) or the patient ( ). Push buttons give the therapist or patient control of the initiation and duration of stimulation, which makes it more feasible to incorporate NMES into task practice. Goal-oriented task practice is a hallmark of effective therapy ( ); using NMES to assist task practice may lead to better outcomes than would be achieved with NMES modalities like cyclic NMES or EMG-triggered NMES, which cannot be easily used to assist task practice.

Proportionally controlled NMES is distinguished from cyclic and triggered NMES methods in that the intensity of the NMES is not preset but rather controlled in real time by the patient via a control strategy that translates the patient’s desired movement into stimulation intensities. Contralaterally controlled functional electrical stimulation (CCFES) is a proportionally controlled NMES approach where the intensity of stimulation to the paretic finger and thumb extensors is proportionally controlled by an instrumented glove worn on the opposite (contralateral) hand ( ). With the glove, the patient is able to control the degree of opening of the affected hand and can practice using it in task-oriented therapy ( Fig. 94.1 ). Other researchers are using EMG signals from the impaired upper limb to deliver proportionally controlled NMES in accordance with the patient’s motor intention ( ). Proportionally controlled NMES may be more efficacious than other NMES methods because it capitalizes on the principle of intention-driven movement, linking the patient’s motor commands (“I try to open my hand…”) to the stimulated movement (“…and it opens…”) and the resulting proprioceptive feedback to the brain (“…and I feel it open when I try to open it”). Giving the patient control of the timing and intensity of stimulation maximizes the degree of repeated synchronization of motor intention (central neural activity), stimulated motor response (peripheral neural activity), and afferent feedback (proprioceptive and cutaneous) to the brain. Such restoring of the motor–sensory circuit (sensorimotor integration) may promote Hebbian-type neuroplasticity that may underlie motor recovery ( ).

A recent review of 31 randomized controlled trials (RCTs) concluded that there is strong evidence that NMES applied in the context of task practice improves upper-extremity function in subacute and chronic stroke ( ). This is corroborated by a recent systematic review with metaanalysis of 18 RCTs (9 were upper-limb studies), which concluded that NMES-assisted task training has a large effect on upper-limb activity compared with training alone ( ). The most recent guidelines published by the American Heart Association recommend NMES in combination with task-specific training for stroke rehabilitation ( ). Generally, benefits of NMES are greatest in patients who have moderate to mild impairment and are less than 2 years poststroke ( ).

Only a few studies have directly compared electrical stimulation modalities. One such study of 122 subacute (≤6 months) stroke survivors found no significant differences between cyclic NMES, EMG-triggered NMES, and submotor threshold sensory stimulation in their effect on upper-limb function ( ), a finding that confirmed previous smaller studies ( ). A recent study of 80 chronic (>6 months) patients found that CCFES improved hand dexterity more than cyclic NMES ( ), which agrees with earlier studies in subacute patients ( ). This finding suggests that the method by which NMES is delivered can affect outcome, with proportionally controlled stimulation likely to have the most positive impact.