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I enjoyed reading the anatomical discussion about the third cranial nerve presented by Huwez et al in a recent issue of the journal,1 and I wish to make a few relevant anatomical and clinical points.
All the extraocular muscles are innervated by an ipsilaterally located subnucleus, with the exception of the superior rectus muscle, which is innervated by a contralateral subnucleus.2
The particular relationship between the third cranial nerve and posterior cerebral artery (above), the superior cerebellar artery (below), and the posterior communicating artery (parallel) deserve further comments, especially when clinicians are faced by an acquired isolated third nerve palsy in adults. As the pupillary fibres in the third cranial nerve are located dorsally and peripherally, a dilated pupil is frequently an early sign of a compressive lesion. An aneurysm at the junction of the posterior communication artery and internal carotid artery is a common cause.3 Actually, around 30% of all third nerve palsies are caused by aneurysms, especially posterior communicating aneurysms.4
Other causes include compression or infiltration by neoplasm, infections, large dolichoectatic vessels, or shifted supratentorial structures. Occasionally it may be seen in generalised polyneuropathy (Miller-Fisher variant).3