51 Third cranial nerve palsy
Salient features
History
• Diplopia in all directions except on lateral gaze to the side of the third nerve lesion (because the lateral rectus muscle supplied by the sixth cranial nerve is intact)
• Painful onset (berry aneurysm or aneurysmal dilatation of the intracavernous part of the carotid artery causing third nerve palsy)
Examination
• Unilateral ptosis (from paralysis of the levator palpebrae superioris)
• Dilated pupil reacting slowly or incompletely to light (paralysis of the constrictor of the pupil)
• Paralysis of accommodation (from involvement of ciliary muscle)
• Squint and diplopia resulting from weakness of muscles supplied by the third cranial nerve (superior, inferior, medial recti and inferior oblique). The eye will be in the position of abduction (i.e. down and out) if the fourth and sixth nerves are intact
• Diplopia may not be obvious until the affected eyelid is elevated manually.
• Exclude associated fourth cranial nerve lesion (supplies the superior oblique) by tilting the head of the patient to the same side – the affected eye will intort if the fourth cranial nerve is intact. Remember superior oblique intorts the eye (SIN). Inferior oblique externally rotates the eye.
