Abstract
Abstract: :
Purpose: To assess the duration, depth and recovery time of anaesthesia produced by the topical instillation of 0.5% proxymetacaine hydrochloride using the Non–Contact Corneal Aesthesiometer (NCCA). Methods: Seventeen Caucasian subjects were recruited (m=2, f=15, age=26±3.6). Subjects with any ocular condition known to affect corneal sensitivity (CS) were excluded. Ethical approval was obtained and subjects were asked to sign a consent form prior to participating. CS was assessed using the NCCA which stimulates the cold sensitive C fibres. All measurements were made on the right eye only. 20µl of 0.5% proxymetacaine hydrochloride or 20µl of saline was then instilled in either the right or left eye. Central CS was measured under four conditions: (P–P) proxymetacaine in both eyes, (P–S) proxymetacaine in the right eye and saline in the left eye, (S–S) saline in both eyes and, (S–P) saline in the right eye and proxymetacaine in the left eye. CS was measured before instillation and at 2, 5, 10, 15, 20, 30, 45, and 60 minutes post instillation. Subjects were further classified according to iris colour into two groups: brown or blue. Results: (1) For conditions P–P and P–S onset of anaesthesia was observed at 2 min post–instillation (P–P, t–test p=0.001; P–S, t–test p=0.002), with a maximum depth of anaesthesia at 15 min post–instillation (P–P, t–test p<0.001; P–S, t–test p<0.001), after which sensitivity began to recover. CS did not return to baseline levels at 60 min post–instillation (P–P, t–test p=0.0003; P–S, t–test p=0.0133). No significant difference in depth of anaesthesia was noted between p–p and p–s. (2) No change in CS was found in the right eye over the period of the trial, for conditions s–s and s–p. (3) No significant difference was found between the brown and blue iris colour sub–groups, at 2, 15, and 60 min (P–P, P–S, t–test p>0.05). Conclusions: (1) The anaesthetic effect of 0.5% proxymetacaine hydrochloride is more prolonged than the 20 minutes previously thought. (2) The maximum anaesthetic effect does not occur until 15 minutes after instillation. (3) The results suggest that 0.5% proxymetacaine affects Aδ and C nerve fibres in different ways. (4) The iris colour of the subject has no effect on the anaesthetic action of 0.5% proxymetacaine hydrochloride.
Keywords: innervation: sensation • pharmacology • cornea: clinical science