Dose-effect autonomic responses to ocular surface stimulation

Abstract

Introduction: The ocular surface is one of the most densely innervated superficial tissues of the human body supplied extensively by autonomic and sensory nerve fibres. Studies have shown that ocular surface sensory neurons respond to thermal, chemical and mechanical stimuli, but investigation into the functional response of the autonomic nervous system (ANS) to ocular surface stimulation is lacking. The ANS is cardinal to human functioning as it acts below the level of consciousness to regulate the internal organs of the body thus controlling secretory cells, smooth muscle and cardiac muscle. Within the eye the ANS is responsible for the control of pupillary reflexes, accommodation and regulation of blood flow, thus monitoring these mechanisms can provide information about ANS functionality. The primary objective of this research was to determine the response of the ANS to ocular surface stimuli by measuring pupil size, conjunctival blood flow and accommodation changes after the delivery of noxious and innocuous corneal stimuli.

Methods: A calibrated and computerised Belmonte pneumatic esthesiometer was used to determine detection thresholds (using ascending method of limits), and to randomly deliver mechanical and chemical stimuli from levels of detection threshold (100% threshold) to twice the threshold (200% threshold) in 50% steps, to the central cornea of 43 healthy subjects, aged 19 - 35 years. Statistical analyses were performed using SPSS (Chicago, SPSS Inc.) and p ≤ 0.05 was considered statistically significant. Chapter 3: 15 participants enrolled in this study. For each suprathreshold stimulus a spectrophotometer (Spectrascan650; Photoresearch Inc, Chatsworth, VA) was used to measure ipsi- and contralateral redness before and after delivery of the corneal stimulus, the change in redness represented the ocular vascular response to noxious stimuli. Conjunctival redness between the stimulated and unstimulated eye was analyzed using dependent t-tests. The effects of stimulus intensity and modality on conjunctival redness were analyzed using repeated measures ANOVA. Tukey HSD tests were used for all post hoc analyses. Chapter 4: 15 participants were enrolled in this study. For each suprathreshold stimulus, imaging of the stimulated and unstimulated eye was performed using two modified and calibrated Logitech c920 digital cameras (Logitech c920; Logitech International S.A., Newark, CA), for 4 seconds before (pre-stimulus capture) and 4 seconds after the delivery of the stimulus (post-stimulus capture). The data were processed with a custom segmentation algorithm to help identify the pupils and pupil diameter (average of horizontal and vertical measures) was measured using ImageJ software (NIH, Bethesda, MD). Pupil dilation response differences between the ipsi- and contralateral eye was analyzed using dependent t-tests. The effect of stimulus intensity, modality and sex of subjects were analyzed using repeated measures ANOVA. Chapter 5 13 participants were enrolled in Part A. For each suprathreshold stimulus the accommodative response at a sampling rate of 25Hz, over a 5 second period (while the subjects fixated on a high contrast (85%) color cartoon frame at 66cm) prior to (baseline) and after stimulus delivery was acquired with an eccentric infra-red (IR) photorefractor (Power Refractor, Multi-channel Systems, Reutlingen, Germany). The accommodative response for the left and right eye were averaged. Quantitative differences in accommodative response, stimulus intensity and modality were analyzed using repeated measures ANOVA. The data used in Part B were acquired from the same subjects in Part A. The pupil response (while the eyes were accommodating to a 66cm target) to ocular surface stimulation was acquired using the same device and methods as in Part A. Quantitative differences in pupil response, stimulus intensity and modality were analyzed using repeated measures ANOVA.

Results: Chapter 3 In mechanical and chemical stimulation experiments, the stimulated eye became redder than the unstimulated eye (all dependent t-test p > 0.05). On average, redness increased from baseline as the corneal stimulus intensity increased. This happened regardless of whether mechanical or chemical stimulation occurred (ANOVA p < 0.05). At 200% threshold, conjunctival redness was greater than all stimulus intensities (Tukey HSD, all p < 0.05). There was a difference between chemical and mechanical stimulation based on stimulus intensity (ANOVA p < 0.05), chemical stimulation produced greater conjunctival redness than mechanical stimulation at all stimulation levels (all Tukey HSD p < 0.05). Chapter 4: In mechanical and chemical stimulation experiments, there was no difference in pupil responses between the stimulated eye and the unstimulated eye, (all dependent T-test p > 0.05). On average, pupil diameter increased from baseline as the corneal stimulus intensity increased. This happened regardless of whether mechanical or chemical stimulation occurred (ANOVA p < 0.05). At 200% threshold, pupil diameter was greater than at all stimulus intensities (Tukey HSD, all p < 0.05). There was a difference in pupil diameter between male and female subjects based on stimulus intensity (ANOVA p < 0.05); females had greater pupil diameters than males at levels of 150% threshold and 200% threshold (all Tukey HSD p < 0.05). Chapter 5 Part A: On average, accommodation increased from baseline as the corneal stimulus intensity increased. This happened regardless of whether mechanical or chemical stimulation occurred (ANOVA p < 0.05). At 200% threshold, accommodation was greater than all stimulus intensities (Tukey HSD, all p < 0.05). There was no difference in accommodation between chemical and mechanical stimulation based on stimulus intensity. Part B: On average, pupil constriction response (during accommodation) was different between baseline and 200% threshold but there was no dose dependent pupil response to ocular surface stimulation. This happened regardless of whether mechanical or chemical stimulation occurred (ANOVA p < 0.05). There was no difference in pupil response between chemical and mechanical stimulation based on stimulus intensity (ANOVA p > 0.05).

Conclusion: Suprathreshold stimulation of the cornea appears to evoke dose dependent autonomic responses in the pupils, conjunctival vasculature and the accommodative mechanism. These autonomic measures are accessible, relative easy and cost effective to acquire. The components that respond to noxious corneal stimulation are linked in a homeostatic loop of complex sympathetic, parasympathetic and sensory neural control and therefore, understanding the characteristics of the local stimulus-response neural circuitry relating nociceptive stimuli to autonomic nervous functionality is important. It also promises the development of clinical procedures and instruments to better understand how these neural responses are impacted by pain.