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F. G. Velez, J. Isobe, H. Lee, S. Patnode, J. W. Judy, A. L. Rosenbaum; In-vivo Functional Electrical Stimulation of Feline Lateral Rectus Muscle: Relationship Between Stimulation Parameters and Eye Movement in Normal Muscle. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2840.
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Complete sixth-nerve palsy results in esotropia and limits abduction of the affected eye. Current treatments include extraocular vertical rectus muscle transposition in combination with weakening the medial rectus muscle. However this approach results in limited recovery of abduction. Our goal is to functionally electrically stimulate the lateral rectus (LR) muscle to recover physiologic abduction ability of the LR.
All procedures adhered to the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research, and were approved by the Chancellor’s Animal Research Committee of the University of California, Los Angeles. We investigated the effects of frequency, amplitude, and cathodic pulse duration on the movement of the eye using the feline LR muscle model. A computer automatically applied 1-second bursts of electrical stimuli of a randomized combination of three parameters to the LR muscle of an anesthetized feline. There was a minimum rest period of 10 seconds between each stimulus. The computer recorded output from a force transducer attached to the LR-sclera insertion point through a spring. Eye movement was inferred using measured force and Hooke’s Law.
Minimum fusing frequency occurred at approximately 180Hz. Eye movement had relatively little dependence on pulse duration. Frequencies below 150Hz were able to modulate eye movement. However, frequencies higher than 175Hz produced very little change in eye movement. Amplitude produced noticeable movement throughout the tested range (0.2-9mA).
In the feline LR muscle, varying amplitude allowed greater eye movement. It is very likely that both frequency and amplitude must be modulated to fine-tune a particular LR response. We determined that 200Hz results in a smooth, non-fatiguing response for a 1-second stimulus.
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