Unilateral corneal insult also alters sensory nerve activity in the contralateral eye

Abstract

AbstractWhen the ocular surface of one eye is inflamed or injured, its corneal sensory nerve activity is modified, which in turn evokes ocular discomfort, irritation, and pain sensations referred to the affected eye. Some patients with unilateral ocular inflammation, infection, or surgery also report discomfort and pain in the contralateral eye. Are these sensations experienced at the fellow eye due to alterations of its sensory nerve activity?To test this hypothesis, we recorded in vitro the activity of corneal mechanonociceptor, polymodal nociceptor and cold thermoreceptor nerves of both eyes of guinea pigs previously subjected unilaterally to three different experimental inflammatory and lesion conditions: UV‐induced photokeratitis, microkeratome corneal surgery, and chronic tear deficiency caused by removal of the main lacrimal gland. Recordings from the eyes of naïve animals were used as control.Compared with the naïve guinea pigs, animals subjected to unilateral UV‐induced mild corneal inflammation, showed inhibition of the spontaneous and stimulus‐evoked activity of cold thermoreceptors, and increased activity (sensitization) of nociceptors affecting both the ipsilateral and the contralateral eye. Unilateral microkeratome surgery affected the activity of nociceptors mostly, inducing sensitization in both eyes. Chronic eye dryness by removal of the main lacrimal gland increased cold thermoreceptor activity in both eyes. These results show that after unilateral eye damage, corneal sensory nerve activity is altered in both eyes, although to a lesser degree in the contralateral eye.This is the first direct demonstration that unilateral corneal nerve insult, and especially ocular surface inflammation, functionally affects the activity of the different types of corneal sensory nerves in both the ipsilateral and contralateral eyes. The mechanisms underlying the changes of contralateral sensory nerve activity remain to be determined, although available data support the involvement of the immune system. The present results have two main implications: (a) contralateral eyes cannot be considered as a control in experimental designs of both preclinical and clinical studies; and, (b) clinicians must consider the convenience of treating both eyes of patients with unilateral ocular conditions to avoid pain and secondary undesirable effects in the fellow eye.