Monocular transcorneal electrical stimulation induces ciliary muscle thickening in contralateral eye

Transcorneal electrical stimulation (TES) is used as therapy for retinal diseases such as retinitis pigmentosa (RP) and was suggested for assessing retinal sensitivity by determining phosphene thresholds, subjective luminance impressions caused by retinal stimulation. Further applications concerned the accommodation process, revealing an improved accommodative amplitude in presbyopic eyes after TES treatment. The respective changes of the ciliary muscle (CM), the structure most important for near vision, during TES are yet unknown. In a pilot study, we aimed to assess whether monocular TES leads to morphological and functional CM changes and whether central accommodation control is affected. Ten healthy, near-emmetropic adults participated in the trial (4 females, age 26.3 ± 3.6 years). Using a wavefront and a stimulus generator, a biphasic square-wave stimulus (2 s positive and 6 s negative amplitude) of 0 μA average current was produced and transferred to the eye by means of a Dawson-Trick & Litzkow electrode. Prior to the stimulation, an individual determination of phosphene thresholds served to define individual TES current amplitudes, which ranged between 60 and 100 μA. Optical coherence tomography (OCT) imaging of the right eye's temporal ciliary muscle was performed before and during ipsi- as well as contralateral monocular TES in randomized order in the morning and afternoon of the same day. During imaging, subjects fixated a target at 4 m distance and refraction was simultaneously recorded via eccentric infrared photorefraction. OCT images were assessed using previously published custom-developed software, allowing the definition of selective CM thickness (CMT) readings, and plotting of continuous CMT profiles along the muscle border. CMT profiles revealed that both stimulations, on the ipsi- and contralateral eye, induced a thickening of the CM compared to the non-stimulated state. The selective CMT readings confirmed a significant increase with ipsi- (31 ± 30 μm; p = 0.010) and contralateral (25 ± 16 μm; p = 0.001) TES. However, refraction during far vision was not significantly affected by either stimulation (ipsilateral [n = 5]: median Δ_w/-w/o = 0 D; contralateral [n = 7]: Δ_w/-w/o = 0.13 D). Pupil size on average increased during TES, but without reaching significance (ipsilateral [n = 5] median Δ_w/-w/o = 0.23 mm, contralateral [n = 7] Δ_w/-w/o = 0.39 mm). Ipsilateral CM thickening could be explained by local changes within the stimulated ciliary muscle, such as increased blood flow or interstitial fluid rise induced by TES. However, the CMT increase in the right eye when TES was performed contralaterally, on the left eye, indicates an involvement of the central control circuit of accommodation. Further possible explanations for this finding are a synchronization of neuronal activities in the visual pathway, the release of vasoactive neuropeptides, or effects on the central blood pressure regulation. Given a neuromodulation effect on the CM function, TES might have implications for children with accommodation insufficiencies and as additional therapy in myopia control management, e.g. in combination with multifocal contact lens treatment. Our study is important for the clinical application of TES, and the outcome might add crucial knowledge to the current understanding of the accommodation process and inform research and treatment of both myopia and presbyopia.