Disorders of cranial nerves

The 12 cranial nerves are peripheral nerves except for the optic nerve which is a central nervous system tract. Disorders of particular note include the following: Olfactory (I) nerve—anosmia is most commonly encountered as a sequel to head injury. Third, fourth, and sixth cranial nerves—complete lesions lead to the following deficits (1) third nerve—a dilated and unreactive pupil, complete ptosis, and loss of upward, downward and medial movement of the eye; (2) fourth nerve—extorsion of the eye when the patient looks outwards, with diplopia when gaze is directed downwards and medially; (3) sixth nerve—convergent strabismus, with inability to abduct the affected eye and diplopia maximal on lateral gaze to the affected side. The third, fourth, and sixth nerves may be affected singly or in combination: in older patients the commonest cause is vascular disease of the nerves themselves or their nuclei in the brainstem. Other causes of lesions include (1) false localizing signs—third or sixth nerve palsies related to displacement of the brainstem produced by supratentorial space-occupying lesions; (2) intracavernous aneurysm of the internal carotid artery—third, fourth, and sixth nerve lesions. Lesions of these nerves can be mimicked by myasthenia gravis....