The optic (second cranial) nerve
This important nerve mediates the sense of sight. Arising from the retina, it passes back through the optic canal at the apex of the orbit. It is closely related to the sphenoid sinus and may lie uncovered by bone within the sinus. It may be damaged by injuries to the eye and orbit, and it is at risk during certain forms of sinus surgery. The most important of these is functional endoscopic sinus surgery (FESS), where injury has been reported by surgeons working in the sphenoid sinus. Injury to the nerve in this situation will cause blindness on the affected side. Injuring the optic nerve during endoscopic sinus surgery implies substandard technique and is not defensible. The optic nerve may also be injured during craniofacial resections for tumours of the ethmoid sinuses, which again indicates inadequate surgical technique unless the surgeon was dealing with an extensive tumour which was invading the optic canal. The optic nerve is routinely divided during removal of the eye when this is carried out as part of an extended, radical maxillectomy; under such circumstances blindness is, of course, inevitable on the operated side.