To implement electrical stimulation in the paretic orbicularis oculi muscle in the future, a thorough investigation of its electrical stimulation characteristics in human subjects is essential. The electrical stimulation should be capable of inducing lid closure without causing pain or other adverse effects. Therefore, understanding suitable electrical current settings and identifying the ideal application site for the electrical stimulus is of paramount importance.
Electrical stimulation settings needed to induce sufficient eye closure of the orbicularis oculi muscle
Complete lid closure, both during the day and at night, is vital for preventing corneal exposure keratopathies. Additionally, the protective Bell phenomenon, which safeguards the cornea while the lid is closed, occurs only during complete or intended lid closure. Studies have demonstrated that electrical stimulation of the orbicularis oculi muscle can induce eye closure in human subjects [
11‐
15,
21]. In animal models, lid closure elicited by electrical stimulation shows an approximately linear response, with greater electrical stimuli resulting in greater palpebral fissure closure [
19‐
21]. Similarly, an increase in pulse width leads to increased lid closure. However, once the electrical pulse width exceeds a certain limit, a decrease in lid closure is observed. This effect might be attributed to long biphasic pulses, which activate different muscle groups with each pulse phase, leading to interference between stimuli [
19].
Other studies have found that a pulse train of several pulses leads to more natural-looking and functional results compared to a single pulse stimulation of the orbicularis oculi [
10,
19,
26]. Various single pulse and pulse train stimulation patterns have been tested on human subjects, as listed in Table
1. Pulse train stimulation induces contraction at lower pulse amplitude ranges and has been shown to reduce electric current by up to 40% compared to single pulse stimulation [
15,
19,
26]. In animal models, Sachs et al. demonstrated that the effect of orbicularis oculi stimulation reaches its plateau at 10 pulses per pulse train [
19]. The observed phenomenon of better lid closure in pulse train stimulation than in single pulse stimulation is believed to be due to wave summation and tetanic contraction [
10].
Table 1
Settings of electrical stimulation for the orbicularis oculi muscle in different human studies
[Reference] | | | | in ms | | | | | |
McDonnall D, Guillory KS, Gossman MD [ 14] | 2009 | Patients with facial nerve palsy, N = 6 | Biphasic pulse trains | 5 | 0.4 ± 0.1 V | 60 ms pulse trains at 50 Hz | Implantation of 4 platinum microwires 1 mm into the upper eye lid, 2 mm above lid margin | Incomplete | • Not systematically reported • 4/5 patients with significant motor response, prior to reaching highest discomfort level |
Pulse train with interferential stimulation | | | Interferential stimulation frequency of 30 Hz | Surface electrodes over orbicularis oculi muscle | Incomplete | • Not systematically reported • 2/3 patients with complete functional blink below maximal discomfort level |
Frigerio A, Cavallari P [ 11] | 2012 | Healthy volunteers, N = 10 | 10 pulses | 0.8 | 26 V | Two initial pulses (with dynamic pulse range) 250–300 Hz, then carrier frequency 75–200 Hz | Surface electrodes along zygomatic branch of the facial nerve | Complete | • Not systematically reported • Painful Stimulation at frequencies lower than 100 Hz |
Marcelli E, Cavallari P, Frigerio A, et al. [ 10] | 2013 | Healthy volunteer, N = 1 | 1 pulse | 2 | 8 mA | | Surface electrodes on temporal canthus of the eye | Complete | • Not systematically reported • Painful, longer than normal reflex eye blink in the contralateral healthy eye |
10 pulses | 2.0 | 3.5 mA | 200 Hz |
10 pulses | 1.0 | 3.5 mA | 200 Hz |
10 pulses | 0.5 | 4.0 mA | 200–400 Hz |
Frigerio A, Heaton JT, Cavallari P, et al. [ 15] | 2015 | Patients with facial nerve palsy, N = 40 | Pulse trains with shorter intervals between the first two pulses | 0.4–1 | 6.2–7.8 mA (mean 7.2 mA); identical voltage for the whole pulse train | 100–150 Hz | Surface electrodes along zygomatic branch of the facial nerve | Complete in 55% (22/40) | • 2/40 (5%) patients aborted trial at 8 mA because of discomfort • 16/22 patients (which achieved full eye closure) reported pain • Synkinesis at 12-week follow-up reported (not directly related to electrical stimulation) |
Ilves M, Lylykangas J, Rantanen V, et al. [ 12] | 2019 | Healthy volunteers, N = 24 | Pulse train (duration 80 ms) | 0.4 | 2.5–5 mA (mean 3.6 mA) | 250 Hz | Surface electrodes above orbicularis oculi muscle | Complete in 22/24 (91.7%) patients | Pain scale from 1 (no pain) to 9 (severe pain): • Mean pain rating for current threshold to elicit eye twitch: 1.8 (SD 1.5, n = 16) • Mean pain rating for current threshold to elicit eye blink: 3 (SD 1.7, n = 20); |
Mäkelä E, Venesvirta H, Ilves M, et al. [ 13] | 2019 | Patients with facial nerve palsy, N = 24 | Pulse train (duration 80 ms) | 0.4 | 2.3 mA ± 0.9 (SD) and 4.7 mA ± 2.2 (SD) | 250 Hz | Surface electrodes above orbicularis oculi muscle | Incomplete | Pain scale from 1 (no pain) to 9 (severe pain): • Mean pain rating 4.3 (± 2.6 SD) |
In contrast to physiological eyelid closure, single pulse stimulation can cause painful and prolonged eyelid closure as well as induce reflex blinking in the contralateral eye [
10]. Frigerio et al. also suggested that introducing dynamic frequencies in pulse train stimulation may be beneficial, particularly by shortening the interval between the first two pulses. They observed that dynamic frequencies resulted in a faster peak acceleration of the eyelid compared to using a constant frequency between all the pulses, leading to a 15% reduction in stimulation, which they attributed to the dynamic sensitivity of motoneurons [
11,
15].
Electrode placement for orbicularis oculi muscle stimulation
Electrode placement for electrical stimulation of the orbicularis oculi muscle in human subjects has been previously described. The most commonly utilized locations for electrode placement are along the zygomatic branch of the facial nerve at the temporal orbit rim [
10,
11,
15], or above the orbicularis oculi muscle [
12‐
14]. In both cases, complete eyelid closure was observed [
10‐
12,
15].
In a study by McDonnall et al. four microwires were implanted 1 mm into the upper eyelid to stimulate the orbicularis muscle; however, no eyelid closure was achieved [
14]. Due to the thin nature of the orbicularis oculi muscle in humans, implanting electrodes into the muscle belly can lead to the possibility of missing the muscle. Moreover, bleeding or edema resulting from electrode implantation could impede electrical stimulation. A separate study revealed a positive correlation between BMI and electric stimulation amplitude levels for forehead and cheek movements. The authors attributed this correlation to the fact that people with higher BMI tend to have more fat tissue, requiring higher currents to elicit muscle movement [
12]. However, no such study has been conducted for the orbicularis oculi muscle. To avoid complications such as bleeding or edema, the use of surface electrodes is a suitable alternative, which do not require surgery for implantation.
So far, different electrode placement settings for direct orbicularis oculi stimulation have only been compared in animal models. Zhang et al. [
21] conducted a study in rabbits to investigate the distribution of horizontal and vertical electrode arrays along the orbicularis oculi muscle and analyzed the electrical stimulation characteristics. They noted that the electric distribution field of the orbicularis oculi muscle has an oval shape, with a faster current conduction along the muscle fibers in the horizontal array compared to across muscle fibers in the vertical electrode array [
21]. Therefore, to efficiently stimulate the orbicularis oculi with minimal current and maximal energy efficiency, the electrode placement should consider these differences in current distribution [
21].
In another study, Somia et al. [
22] tested lid closure in dogs by stimulating in a single electrical field and multiple-channel stimulation. For single-field electrical stimulation, they inserted two electrodes into the orbicularis oculi muscle, creating a horizontal electrical field along the orbicularis oculi fibers. In multiple-channel stimulation, they inserted four electrodes in the upper and four in the lower lid, thus creating four separate electrical fields along the orbicularis oculi muscle fibers [
22]. They reported a significant reduction in stimulation intensity required to elicit eye twitch with multiple-channel field stimulation compared to single-field stimulation. Moreover, complete eye closure was only observed in cases of multiple-channel stimulation [
22].
To date, there is no study comparing the effects of different electrode application sites on lid closure when orbicularis oculi is stimulated in humans. Most studies in humans have either focused on stimulation of the peripheral nerve branch leading to the orbicularis oculi [
11,
15] or direct stimulation of the orbicularis oculi muscle [
10,
12‐
14], without making a comparison between the two approaches. When stimulation of the orbicularis oculi occurs by stimulation of the peripheral facial nerve, localizing the exact anatomical region of the nerve can be difficult.
If a future prosthetic device were to target the orbicularis oculi directly, it would be ideal to stimulate both the lower and the upper orbicularis oculi. The palpebral part of the orbicularis oculi in the lower lid plays a crucial role in the tear drainage system. For patients with a negative Bell phenomenon, stimulating the lower lid may be essential to minimize the risk of exposure to keratopathy.
In the design of such a prosthetic device, it should be considered that the palpebral part of the orbicularis oculi is shorter in the lower eyelid than in the upper eyelid, potentially requiring less electrical stimulation for sufficient contraction [
34]. Additionally, the tarsus lying beneath the orbicularis oculi in both the upper and lower eyelids could serve as a protective shield for the eye against repetitive electrical stimulation. Multiple-channel stimulation may be beneficial to achieve muscle contraction with minimal stimulation intensity, as discussed above [
22]. Studies are needed to assess the ideal placement sites for electrodes in humans and to investigate potential adverse effects.