How does Functional Electrical Stimulation work? An fMRI study of cortical activations during ankle dorsiflexion

Background and aim - Foot drop is one of the commonest gait impairments associated with Central Nervous System (CNS) lesions. Peripheral Functional Electrical Stimulation (FES) is often used to treat foot drop as active orthosis. However, some hemiplegic patients treated with FES have a beneficial effect on foot drop that outlasts the period of stimulation (the carryover effect). This suggests that FES induces some plasticity mechanisms in CNS reorganization that allows maintaining recovery of motor control, but little is known about its central mechanism of action. We hypothesise that the interaction of (i) antidromic firing induced by FES and (ii) voluntary effort could induce changes at the level of the cortex which persist beyond FES stimulation. Here we have used fMRI to study this interaction in healthy volunteers before going on to apply the paradigm to chronic stroke patients before and after an FES-based treatment for foot drop. Methods – A 2x2 event-related fMRI study design with voluntary effort [V] and FES [F] as factors was performed with Ankle DorsiFlexion (ADF) as motor task (FV = FES-induced ADF voluntarily accompanied by the subject; FP = FES-induced ADF, subject is asked to remain relaxed; V = voluntary ADF; P = subject is asked to stay relaxed, an operator dorsiflexes his/her ankle). The experimental set-up was composed of a 1.5 T MRI scanner, a motion capture system and an electrical stimulator as previously described (Gandolla et al., 2011). 16 healthy subjects underwent the acquisition. Up to now, 4 chronic stroke patients were included in the study; patients’ assessment included Medical Research Council scale index, Modified Ashworth Scale index, 6 minutes walking test, gait analysis, dynamic EMG and ankle range of motion as well. Patients are currently undergoing an FES-based treatment for one month. Results – Relative increases in brain activity were seen in controlateral sensorimotor cortex (lower limb area) for all 4 conditions in healthy subjects. We were particularly interested in the interaction of our experimental factors: (FV-V)-(FP-P). Using this contrast, we were able to demonstrate that activity induced by FES-antidromic stimulation in both primary motor and sensory cortices is increased by concurrent voluntary ankle dorsiflexion (Fig. 1). We are now interested in understanding whether the presence of this positive interaction is a prerequisite for a successful carryover central effect after FES treatment in stroke patients. Conclusions - Our results demonstrate that FES can modulate activity in primary motor and sensory cortices, an effect which may undelay its clinical mechanism of action. We now hypothesise that this is the mechanism by which FES leads to a clinically meaningful carryover effect. Furthermore, presence of this interaction should predict a positive recovery of voluntary motor control. Ultimately this could help to identify best responders and open a window about new strategy to facilitate functional recovery.