Interaction of corneal nociceptive stimulation and lacrimal secretion

To investigate the interaction between corneal stimuli at different positions and tear secretion and to establish relationships between nociceptive stimuli detection thresholds and stimulated tearing.

Using a computerized Belmonte-esthesiometer, mechanical and chemical stimuli, from 0% to 200% of the threshold in 50% steps, were delivered (in random order) to the central and peripheral (approximately 2-mm inside the limbus) cornea during four separate sessions to 15 subjects. Immediately after each stimulus, tear meniscus height (TMH) was measured using optical coherence tomography to quantify the amount of lacrimal secretion, and subjects reported whether they felt tears starting to accumulate in their eyes. Thresholds (50% detection) for detection of tearing were estimated.

TMH increased with increasing stimulus intensity (P < 0.05), and the overall increase was higher with central stimulation than with peripheral stimulation (P < 0.05). The changes in TMH with threshold-scaled stimulus intensity depended on test location (P < 0.05) and stimulus modality (P < 0.05). The maximum intensity of mechanical stimulation of the central cornea induced the greatest TMH (all P < 0.05). For chemical stimulation, the stimulus intensity required to induce detectable tearing was higher than that required to detect a stimulus and higher in the periphery than at the center (all P < 0.05).

Noxious mechanical and chemical stimuli evoked measurable tear secretion, with central corneal mechanical stimulation evoking the strongest lacrimation reflex. Central mechanical corneal stimulation is the most effective stimulus-position pairing and appears to be the major sensory driving force for reflex tear secretion by the lacrimal functional unit.