February 28, 2004

Researchers Admit to High Incidence of Long-term Pain Following Refractive Surgery

Neural Basis of Sensation in Intact and Injured Corneas 

Experimental Eye Research 78 (2004) 513–525

Carlos Belmonte, M.  Carmen Acosta, Juana Gallar

Instituto de Neurociencias de Alicante, Universidad Miguel Herna´ndez–CSIC, Apdo correos 18, 03550 San Juan de Alicante, Spain

Abstract

A renewed interest in the characteristics and neural basis of corneal and conjunctival sensations is developing in recent years due to the high incidence of discomfort and altered sensitivity of the cornea following refractive surgery,next term use of contact lenses and dry eyes. Corneal nerves are functionally heterogeneous: about 20% respond exclusively to noxious mechanical forces (mechano-nociceptors); 70% are additionally excited by extreme temperatures, exogenous irritant chemicals and endogenous inflammatory mediators (polymodal nociceptors), and 10% are cold-sensitive and increase their discharge with moderate cooling of the cornea (cold receptors). Each of these types of sensory fibres contributes distinctly to corneal sensations. Mechano-nociceptors mediate, sharp acute pain produced by touching of the cornea. Polymodal nociceptors elicit the sustained irritation and pain that accompany corneal wounding; cold receptors evoke cooling sensations. Depending on the relative activation by the stimulus of each subpopulation of corneal sensory fibres, different subqualities of irritation and pain sensations are evoked. Corneal sensations can be explored experimentally in humans with a gas esthesiometer that applies controlled mechanical, chemical and thermal stimuli to the corneal surface. When the cornea is wounded, corneal nerves are excited and eventually severed in a variable degree and local inflammation is produced. Activated corneal nerves release neuropeptides (SP, CGRP) that contribute to the inflammatory reaction (neurogenic inflammation). They also become sensitized by local inflammatory mediators, such as prostaglandins or bradykinin and thus exhibit spontaneous activity, lowered threshold and enhanced responses to new stimuli. This leads to spontaneous pain and hyperalgesia. Nerves destroyed by injury soon start to regenerate and form microneuromas that exhibit abnormal responsiveness and spontaneous discharges, due to an altered expression of ion channel proteins in the soma and in regenerating nerve terminals. Presumably, this altered excitability is the origin of the lowered sensitivity and the spontaneous pain, dry eye sensations and other disaesthesias reported in patients following previous termrefractive surgery.next term

Author Keywords: pain; corneal nerves; ocular surface; sensitivity; conjunctiva; dry eye; corneal inflammation; photorefractive keratectomy; laser-assisted in situ keratomileusis; nerve injury

The complete article in Adobe PDF Format may be downloaded here:   http://journals.ohiolink.edu/local-cgi/send-pdf/040127155336425362.pdf  

 

Posted by Admin at February 28, 2004 02:20 PM