A comparison of three sub-Tenon's cannulae

To compare the quality of anaesthesia and complication rates between three sub-Tenon cannula of increasing length (anterior Greenbaum, mid Kumar-Dodds, and posterior Steven's sub-Tenon's cannulae).

A total of 150 patients undergoing cataract extraction were randomised to receive a sub-Tenon injection of 5 ml of 2% lidocaine with hyaluronidase with one of the three cannulae. The development of akinesia was assessed every 2 min over a 6-min period. Complications were also recorded.

There was no difference in the onset of akinesia, with 46, 50, and 46 patients achieving adequate akinesia within 6 min for the anterior, mid, and posterior groups respectively (P>0.05). There was an increase in retained lid opening with anterior compared to mid and posterior cannulae (P=0.0001). There was significantly less retained lid closure with the posterior compared to the mid or anterior cannulae (P<00001). The mean (range, SD) scores for pain during injection were 0.4 (0-5, 0.83), 1.2 (0-9, 1.96), and 1.1 (0-6, 1.19) for the anterior, mid, and posterior groups, respectively. These were not significantly different between the anterior and mid groups, or the mid and posterior groups (P>0.05), but there was significantly more pain on injection with the posterior compared to the anterior groups (P<0.01). All patients scored intraoperative pain as zero. There was significantly more chemosis in the anterior group (76%) compared to the mid (20%) and posterior (32%) groups (P<0.0001). There were significantly (P=0.0004) more conjunctival haemorrhages in the anterior group (56%) than the mid (20%) or posterior (20%) groups.

We have shown that all three cannulae provide high-quality anaesthesia with minor differences in retained muscle activity, chemosis, and haemorrhage rates.