BACKGROUND—Anecdotal evidence has previously suggested that retrobulbar local 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 development of ptosis and akinesia in other extraocular muscle groups were recorded and compared. The effects of retrobulbar injections were also studied on posterior orbital structures in 10 patients using low frequency ultrasound. RESULTS—The mean time of onset of ptosis was 4.76 seconds, an order of magnitude less than times recorded for akinesia in other muscle groups. Ultrasonography revealed a significant distension of the extraocular muscle cone during retrobulbar injection. CONCLUSION—Ptosis develops significantly more rapidly than other motor effects in retrobulbar anaesthesia and can therefore be taken as an indicator of accurate intraconal placement. Retrobulbar injection is associated with significant distension of the extraocular muscle cone.
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