Toward an implantable functional electrical stimulation device to correct strabismus

Abstract

Purpose: To investigate the feasibility of electrically stimulating the lateral rectus muscle to recover its physiologic abduction ability in cases of complete sixth cranial (abducens) nerve palsy.

Methods: In the feline lateral rectus muscle model, the effects of a charge-balanced, biphasic, current-controlled stimulus on the movement of the eye were investigated while stimulation frequency, amplitude, and pulse duration was varied. Eye deflection was measured with a force transducer. Denervated conditions were simulated by injection of botulinum toxin A.

Results: Three chemically denervated and 4 control lateral rectus muscles were analyzed. In control lateral rectus muscles, the minimum fusion frequency was approximately 170 Hz, and the maximum evoked abduction was 27 degrees. The minimum fusion frequency was unchanged after 4 weeks of chemical denervation. Stimulation of chemically denervated lateral rectus muscle resulted in 17 degrees of abduction. For both innervated and chemically denervated lateral rectus muscle, frequencies greater than 175 Hz yielded very little increase in abduction. Modulating amplitude produced noticeable movement throughout the tested range (0.2 to 9 mA).

Conclusions: Results from the feline lateral rectus muscle showed that electrical stimulation is a feasible approach to evoke a contraction from a denervated lateral rectus muscle. The degree of denervation of the feline lateral rectus muscle was indeterminate. Varying the stimulation amplitude allowed greater eye movement. It is very likely that both frequency and amplitude must be modulated for finer control of static eye position.

FIG 1

System sketch of lateral rectus stimulation apparatus to align eye positions. Two types of feedback inputs are under consideration: A, ipsilateral medial-rectus muscle force, and B, contralateral eye position. In both systems, the input is processed by a computer that delivers the stimulus that drives a desired muscle response. The stimulus is adjusted based on the feedback of the input to fine-tune the stimulus.

FIG 2

A, A sleeve-type electrode was constructed from multistranded stainless-steel wire stitched into silicone mesh material. The bipolar electrode positioned one set of stimulating leads on the global surface of the lateral rectus and another set on the orbital surface of the lateral rectus. B, The electrode wraps around the lateral rectus longitudinally and applies stimulus across the width of the muscle. The side facing away from the muscle is insulated from the stimulation in an attempt to localize the stimulating current.

FIG 3

The experimental setup for the feline lateral rectus muscle used a force-displacement transducer in series with a spring (0.3 g/cm) to measure eye deflection generated by the stimulated lateral rectus muscle. Values of linear displacement were obtained using Hooke’s Law. A video camera also captured the movements against a ruler.

FIG 4

Results of stimulation experiments on normal lateral rectus muscle showed that both stimulation amplitude and frequency elicited a proportional response on ocular rotation. (A) Frequencies between 100 Hz and 250 Hz responded mostly to changes in amplitude from 1 mA to 3 mA. (B) Amplitudes between 2 mA and 8 mA responded mostly to frequencies of 100–175 Hz. There was no significant change in ocular rotation when the frequency was increased >200 Hz.

FIG 5

Results of stimulation experiments on paralyzed lateral rectus muscles showed that most of the muscle contraction occurred when using (A) frequencies between 100 Hz and 200 Hz and (B) amplitudes between 4 mA and 8 mA. The degree of paralysis was not defined.