Investigation in Variability and Repeatability of Corneal Sensation in a Normal Population and Contact Lens Wearers

The aim of this study is to gain more physiological knowledge about ocular surface sensation (corneal sensitivity), with application of three different concepts employing different types of stimuli for triggering a response from the pain sensitive nerve endings in the superficial cornea: 1) tactile method: a round plastic nozzle (2 mm diameter) is applied to the ocular surface with a defined, low force for a duration of 100ms. 2) liquid jet method: a liquid jet (isotonic saline) of a temperature to match ocular surface temperature is applied to the ocular surface with low pressure and low volume, from a distance of 15 mm.3) commercially available Cochet Bonnet esthesiometer (nylon thread). The study group will be divided into three groups of individuals: two groups with different types of contact lenses (CL) and one without CL, as sensitivity changes are thought to occur with CL wear. A very interesting research question is to find out, if such sensitivity differences can be detected with the nature of the stimuli applied in this study.

Current knowledge about human corneal sensitivity is limited, as applied methods for ocular surface sensation measurement are limited with regards to reproducibility / accuracy.

Corneal sensitivity represents a neurological response from the free nerve endings within the epithelium. They are sensitive to mechanical, electrical, chemical or thermal stimuli and hence have a protective function for the cornea. Corneal nerves play an important role in cell growth and proliferation of epithelial cells, wound healing and repair. In experimental studies, corneal denervation has been reported to result in epithelial changes: increased permeability, decreased proliferation, changed appearance and delayed wound healing. Therefore, intact corneal innervation is required to maintain the integrity of a normal corneal epithelium. Corneal sensory nerves are believed to play an important role in maintaining the resting tear flow, as their afferent impulses from the ocular surface lead to a reflex response, best described by the lacrimal functional unit: an integrated system comprising the ocular surface tissues (cornea, corneal limbus, conjunctiva, conjunctival blood vessels, and eyelids), the tear secreting components (main and accessory lacrimal glands, meibomian glands, conjunctival goblet, and epithelial cells), and the sensory and motor nerves that connect them.

Current knowledge about ocular surface sensitivity is insufficient, as currently available measurement possibilities lack repeatability and accuracy. Before a new instrument can be developed, more research is required, in order to find a suitable concept for precise sensitivity measurement. For this purpose, two new different concepts with different / new stimulus types will be applied repeatably on healthy eyes in this study. The aim is to find out more about how corneal sensory fibres react to different types of stimuli (liquid / tactile / nylon thread) and how this can be consciously perceived by the individual. Is it possible to generate a stimulus that delivers a repeatable and reliable response within a useful stimulus force range which allows an interpretation / evaluation of normal / expected activity of superficial nerve fibres in the cornea? The study group will be divided into three groups of individuals: two groups with different types of contact lenses (CL) and one without CL, as sensitivity changes are thought to occur with CL wear.