Ultrasonic localization of anaesthetic fluid using sub-Tenon's cannulae of three different lengths

To study the distribution of anaesthetic fluid during sub-Tenon's block by B-scan ultrasonography using cannulae of three different lengths.

A total of 30 patients undergoing routine phacoemulsification and lens implantation were studied after informed consent had been obtained. Ages ranged from 60 to 92 years and globe axial lengths from 21.50 to 27.00 mm. All were given a sub-Tenon's block and the patients were classified into three groups in which either a long, intermediate, or short cannula was used. B-scan ultrasonography was performed before administration of the anaesthetic agent, during injection, and 2 min after completion of the injection.

In all patients the optic nerve was identified. During the injection, anaesthetic fluid could be seen tracking behind the globe and opening up the posterior sub-Tenon's space (the previously described 'T'-sign). After 2 min very little or no fluid was seen, suggesting that it had dispersed into the surrounding tissues. There was no discernible difference in the distribution of fluid or in the quality of the block in the three groups.

B-scan ultrasonography can reliably identify the distribution of anaesthetic fluid during sub-Tenon's block. Anaesthetic agents can be successfully delivered into the sub-Tenon's space by long, intermediate, or short cannulae.