Rapid onset of ptosis indicates accurate intraconal placement during retrobulbar anaesthetic injection

Background-Anecdotal evidence has previously suggested that retrobulbar loc al anaesthetic (LA) injection is accompanied by the rapid onset of ptosis. Here the validity of this potentially valuable sign is tested. Methods-25 patients received a retrobulbar injection and the times for deve lopment of ptosis and akinesia in other extraocular muscle groups were reco rded and compared. The effects of retrobulbar injections were also studied on posterior orbital structures in 10 patients using low frequency ultrasou nd. Results-The mean time of onset of ptosis was 4.76 seconds, an order of magn itude less than times recorded for akinesia in other muscle groups. Ultraso nography revealed a significant distension of the extraocular muscle cone d uring retrobulbar injection. Conclusion-Ptosis develops significantly more rapidly than other motor effe cts in retrobulbar anaesthesia and can therefore be taken as an indicator o f accurate intraconal placement. Retrobulbar injection is associated with s ignificant distension of the extraocular muscle cone.