Non-pharmacological stimulation of the autonomic nervous system and short-term changes in axial length

Abstract

Purpose : The choroid contains vascular and non-vascular smooth muscles innervated by the autonomic nervous system (ANS). We studied the effect of non-pharmacological stimulation on the ANS and the resultant short-term changes in the axial length (AxL) of the eye (reflecting changes in choroidal thickness), and secondary measures of pupil size (PS) and heart rate (HR).

Methods : Fifteen healthy individuals aged 22 ± 2 years participated, who were either myopic (n=7, mean SER -3.13 ± 1.13 D) or emmetropic (n=8, +0.09 ± 0.28 D). Four non-pharmacological techniques were used to stimulate the ANS, along with a fifth control condition, tested in randomized order, with a 5-minute wash-out between conditions. The cold pressor task (CPT) requires the participant to submerse their hand in ice cold water for up to 3 mins to induce mild controlled pain for sympathetic stimulation. The unilateral forced nostril breathing (UNFB) can be used to modify the balance of brain hemisphere activity and thereby the relative level of parasympathetic and sympathetic activity. Parasympathetic stimulation occurs through slow and deep breathing via the activity of the vagus nerve. Baseline measurements were first obtained, followed by repeated measurements during 3 mins of ANS stimulation. A Lenstar biometer was used for both AxL and PS measurements, while a fingertip pulse oximeter was used to monitor changes in HR.

Results : Multivariate analysis revealed a significant effect of ANS stimulation on AxL (p < 0.05). Pairwise comparisons showed a mean increase in AxL of 5 ± 1 μm for the right UFNB stimulus (increase in sympathetic) compared to the control stimulus (p = 0.04). The effect of refractive error group was not significant (p = 0.81). Pupil size also showed a significant effect of the ANS stimulation (p < 0.05), with the CPT increasing PS by 0.3 ± 0.1 mm compared to the control stimulus (p = 0.03). The overall correlation between changes in PS and AxL changes for all conditions was weak but statistically significant (ANCOVA, β = 3.4, p < 0.05) with a 1 mm increase in PS associated with a 3 μm increase in AxL. Analysis of HR did not reveal a significant effect of the ANS stimulation (p > 0.05).

Conclusions : Short-term ANS stimulation can influence AxL. The findings have implications for risk factors related to myopia development and for the control of confounding variables when taking choroidal thickness measurements.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.