Sensitivity Across the Ocular Surface—Fundamental Findings and Clinical Applications
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Current understanding of sensitivity and sensation experienced across the ocular surface remains limited. This project explored the regional variation of corneal sensitivity and transducer function, interaction of sensory and autonomic nerves in the lacrimal functional unit, and the ocular surface sensitivity in Dry Eye and with silicone hydrogel (SH) lens wear. Experiments were undertaken, using Belmonte esthesiometer to deliver pneumatic mechanical, chemical and thermal stimuli and Cochet-Bonnet esthesiometer for tactile stimuli, to the cornea and conjunctiva. Psychophysical methods were used to determine the thresholds of stimulus detection, and the magnitude of sensations to suprathreshold stimulation was estimated assuming Steven’s power law. Additionally, tear secretion in response to corneal sensory input was determined by tear meniscus height measured using Optical Coherence Tomography. Sensitivity to pneumatic cool and mechanical stimuli varied slightly across the cornea while chemical sensitivity was not different between regions. The transducer function was also similar between central and peripheral cornea but different between stimulus modalities. In comparison, the reflex tearing response to suprathreshold stimuli was greater with central corneal stimulation. Also, corneal and conjunctival hypersensitivity was found in the dry eye symptomatic group, and it appeared to be associated with symptom severity, tear film stability and corneal epitheliopathy. Refitting with SH lenses after an initial no-lens interval led to increased conjunctival pneumatic mechanical sensitivity, while corneal tactile sensitivity showed a decrease. In addition, corneal staining induced by certain lens-solution combination appeared to be accompanied by increased corneal and conjunctival sensitivity. In conclusion, the position-invariant corneal sensitivity to pneumatic mechanical, chemical and thermal stimuli suggests that the distribution of human corneal sensory fibres may be more homogeneous than previously hypothesised. The mechanisms mediating the sensory aspect of corneal nociception may be similar across the cornea, while, perhaps due to the importance of the visual axis, the tear reflex response to central and peripheral cornea seems to be driven by different neural circuitry, perhaps at the higher levels of the sensory processing pathway. It appears that alteration in sensory processing of the ocular surface occurs in Dry Eye and accompanies SH lens-solution-induced corneal staining. This altered sensitivity seems to be more prominent in the conjunctiva than in the cornea.