The risk of optic nerve injury in retrobulbar anesthesia: a comparative study of 35 and 40 mm retrobulbar needles in 12 cadavers.

PURPOSE: This study was designed to demonstrate the increased risk of optic nerve injury by the 40 mm needle when fully inserted into the orbit.

METHODS

Retrobulbar anesthesia needles 35 and 40 mm long were inserted into the orbits of 12 well-embalmed cadavers, as for typical retrobulbar anesthesia. The needle was seen directly through a fenestration of the orbital roof and by dissection of the orbital structures overlying the optic nerve.

RESULTS

In all orbits the 40 mm needle reached and in seven cases significantly pushed against the optic nerve and could obviously penetrate its sheaths. The 35 mm needle could just slightly touch the outer optic nerve sheath only in two cases.

CONCLUSIONS

We conclude that the 40 mm retrobulbar needle should not be fully inserted into the orbit and the 35 mm retrobulbar needle must be used with caution.