Selective Surface Electrostimulation of the Denervated Zygomaticus Muscle

Abstract

This article describes a first attempt to generate a standardized and safe selective surface electrostimulation (SES) protocol, including detailed instructions on electrode placement and stimulation parameter choice to obtain a selective stimulation of the denervated zygomaticus muscle (ZYG), without unwanted simultaneous activation of other ipsilateral or contralateral facial muscles. Methods: Single pulse stimulation with biphasic triangular and rectangular waveforms and pulse widths (PW) of 1000, 500, 250, 100, 50, 25, 15, 10, 5, 2, 1 ms, at increasing amplitudes between 0.1 and 20 mA was performed. Stimulations delivered in trains were assessed at a PW of 50 ms only. The stimulation was considered successful exclusively if it drew the ipsilateral corner of the mouth upwards and outwards, without the simultaneous activation of other ipsilateral or contralateral facial muscles. I/t curves, accommodation quotient, rheobase, and chronaxie were regularly assessed over 1-year follow-up. Results: 5 facial paralysis patients were assessed. Selective ZYG response in absence of discomfort and unselective contraction of other facial muscle was reproducibly obtained for all the assessed patients. The most effective results with single pulses were observed with PW ≥ 50 ms. The required amplitude was remarkably lower (≤5 mA vs. up to 15 mA) in freshly diagnosed (≤3 months) than in long-term facial paralysis patients (>5 years). Triangular was more effective than rectangular waveform, mostly because of the lower discomfort threshold of the latter. Delivery of trains of stimulation showed similar results to the single pulse setting, though lower amplitudes were necessary to achieve the selective ZYG response. Initial reinnervation signs could be detected effectively by needle-electromyography (n-EMG). Conclusion: It is possible to define stimulation parameters able to elicit an effective selective stimulation of a specific facial muscle, in our case, of the ZYG, without causing discomfort to the patient and without causing unwanted unspecific reactions of other ipsilateral and/or contralateral facial muscles. We observed that the SES success is strongly conditioned by the correct electrode placement, which ideally should exclusively interest the area of the target muscles and its immediate proximity.

Electrical stimulation alters muscle morphological properties in denervated upper limb muscles

“…Our assumption about the stimulation parameters (shape form and amplitude) is consistent with the findings of Arnold and colleagues [20] , who studied ES on denervated facial muscles. When stimulating facial muscles, selectivity is of utmost importance to avoid synkinesia, which would have a counterproductive effect on facial expressions.…”

“…The rise of triangular pulses is flatter, and the energy applied per pulse is half that of rectangular pulses. This may be one reason why patients experience stimulation in triangular pulses as more comfortable than that of rectangular pulses [20] . When setting up the stimulation parameters, we opted for the greatest possible selectivity with a stimulation amplitude calculated as a function of size.…”

Selective Electrical Surface Stimulation to Support Functional Recovery in the Early Phase After Unilateral Acute Facial Nerve or Vocal Fold Paralysis

“…The process of muscle atrophy seems to be very variable ( 21 ). It seems that facial muscle degeneration is very slow as electrical stimulation of denervated facial muscles is possible even after years ( 22 ).…”

“…On the other hand, the amplitude needed for functional activation of denervated muscle fibers is 10–100 times higher than the amplitude needed for activation of nerve fibers. Hence, it is much more difficult to selectively stimulate a denervated muscle without side effects ( 22 , 26 , 36 , 37 ).…”

Preservation, reconstruction, and rehabilitation of the facial nerve

“…Die zumeist irreversible Atrophie und Fibrose der Gesichtsmuskulatur werden den Erfolg eines Versuchs der Reinnervation mit einer Nervennaht erheblich beeinträchtigen. Wie beschrieben, sollte mit Elektrostimulation der individuelle funktionelle Zustand der mimischen Muskulatur bestimmt werden [ 24 ]. Eine Empfehlung für die Zeit dazwischen, also bei einer Denervationszeit zwischen 2 und 12 Monaten, ist am schwierigsten abzugeben und muss individuell betrachtet werden.…”