Changes in Tear Secretion Induced by Selective Stimulation of Corneal and Conjunctival Sensory Receptors

Abstract

Abstract: : Purpose: To measure changes in tear secretion evoked by moderate and strong mechanical, chemical and cold stimulation of corneal and conjunctival sensory receptors. Methods: Twelve eyes from six healthy subjects (four women and two men, mean age=30.5±1.8 years) were used. McMonnies test was performed in order to know if there were any symptom related to dry eye. Tear secretion was measured in both eyes using the Schirmer's test I in normal conditions and after application to the center of the cornea or the temporal conjunctiva with a gas esthesiometer (Belmonte et al., 1999) the following stimuli: gas pulses of moderate (170 ml/min) and high intensity (260 ml/min) (mechanical), gas pulses containing 80% CO2 in air (chemical) and cooled gas pulses decreasing corneal temperature in-1°C (cooling) and –4.5°C (strong cold). Strong or moderate stimuli were known to evoke respectively high and low levels of irritation (Acosta et al., 2001a and b). Stimulation consisted in 3 pulses of 3 sec duration, applied sequentially. Immediately after the stimulus, the Schirmer strip was applied on the temporal tarsal conjunctiva of the lower lid for 5 min. Results: The volume of tear secretion increased significantly (p<0.05, paired t-test) after strong mechanical, chemical and cold stimulation of the cornea, but not of the conjunctiva. Moderate mechanical and cold stimuli applied to the cornea or the conjunctiva did not change significantly the volume of tear secretion. Conclusions: Reflex tear secretion resulting of corneal stimulation seems to depend predominantly on the stimulation of polymodal nociceptors while stimulation of mechano-nociceptors and cold receptors was less effective. Selective stimulation of conjunctival nociceptors or cold receptors was not sufficient to evoke an increase in tear secretion.

Keywords: cornea: tears/tear film/dry eye • conjunctiva