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Impulse Activity in Corneal Sensory Nerve Fibers after Photorefractive Keratectomy

From the Instituto de Neurociencias, Universidad Miguel Hernández-CSIC, San Juan de Alicante, Spain.

PURPOSE. To evaluate the changes in spontaneous and stimulus-evoked nerve impulse activity of corneal polymodal and mechanonociceptor sensory fibers of the cornea after photorefractive keratectomy (PRK).

METHODS. A central corneal ablation 6 mm in diameter and 70 µm in depth was performed with an excimer laser in both eyes of three anesthetized cats, after removal of the corneal epithelium. Single nerve fiber activity was recorded in these animals 12 to 48 hours after surgery. Activity in corneal nerve fibers with receptive fields (RFs) within and/or close to the wound, as well as with RFs far from the lesioned area, was studied. Incidence and frequency of spontaneous discharges and nerve impulse firing responses to mechanical (Cochet-Bonet esthesiometer) and chemical (CO₂ gas pulses) stimuli were studied.

RESULTS. The incidence of nociceptor fibers exhibiting ongoing activity (15/35 vs. 1/9) and the frequency of their spontaneous firing (0.25 ± 0.09 impulses [imp]/s versus 0.08 ± 0.08 imp/s) was higher in fibers with RFs within and/or bordering the wounded area than in those with RFs far away from the wound. Mechanical responsiveness of fibers with RFs within or nearby the ablated area was often reduced. In these fibers, CO₂ pulses evoked a lower-frequency impulse discharge (0.9 ± 0.2 imp/s inside, 2.3 ± 0.7 imp/s outside the wound). CO₂-evoked discharges recorded from fibers innervating the intact wound border were similar to those recorded in corneal fibers of intact cats.

CONCLUSIONS. The spontaneous impulse activity and the abnormal responsiveness shown by a part of the corneal nerve fibers innervating the injured cornea are presumably the neurophysiological substrate of the pain sensations experienced by human patients hours after PRK surgery.