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

Purpose

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

Methods

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 20 μl 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 60 min post-instillation.

Results

The onset of anaesthesia was observed at 2 min (Wilcoxon, p < 0.001), with the maximum anaesthesia occurring at 15 min (Wilcoxon, p < 0.001). Recovery of corneal sensitivity to baseline levels did not occur by 60 min (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).

Conclusions

Although the anaesthetic effect of 0.5% proxymetacaine hydrochloride continues for more than 60 min, 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.