Prolonged corneal anaesthesia by proxymetacaine hydrochloride detected by a thermal cooling stimulus

To assess the duration, depth and recovery time of anaesthesia produced by 0.5% proxymetacaine hydrochloride (proparacaine), using a thermal cooling stimulus.

Seventeen non-contact lens-wearing subjects were recruited (mean age=26+/-3.6 years, range=23-39; blue iris=8, brown iris=9). Central corneal sensitivity was measured in the right eye of each patient to establish a baseline, before 20microl of either 0.5% proxymetacaine hydrochloride (p) or 0.9% unpreserved saline (s) was instilled under four experimental conditions (right eye-left eye): p-p, p-s, s-p, s-s. Corneal sensitivity was re-measured at 2, 5, 10, 15, 20, 30, 45 and 60min post-instillation.

The onset of anaesthesia was observed at 2min (Wilcoxon, p<0.001), with the maximum anaesthesia occurring at 15min (Wilcoxon, p<0.001). Recovery of corneal sensitivity to baseline levels did not occur by 60min (Wilcoxon, p<0.001). No difference in onset time, depth of anaesthesia, or recovery time was noted between the blue and brown iris subjects (Mann-Whitney, p>0.05).

Although the anaesthetic effect of 0.5% proxymetacaine hydrochloride continues for more than 60min, this finding does not alter current clinical practice. The extended duration, however, is of relevance to studies that use corneal anaesthesia to investigate the role of corneal nerves in the blink mechanism.