Central Anatomy of the Fourth Nerve

The IVth or trochlear nucleus is located ventral to the central periaqueductal grey matter, dorsal to the medial longitudinal fasciculus (MLF) and medial to the oculosympathetic tract at the level of the inferior colliculus. The fascicles of the IVth nerve travel dorsally and caudally around the central grey matter and decussate at the roof of the aqueduct, known as the anterior medullary velum. Thus, a IVth nerve palsy that is nuclear in origin is contralesional, while associated ipsilesional signs include: hemi-ataxia (superior cerebellar peduncle), Horner's syndrome (oculosympathetic tract), internuclear ophthalmoplegia (INO - due to MLF lesion), or an ipsilesional or contralesional relative afferent pupillary defect that occurs without a decrease in acuity, color vision or a visual field defect, due to a lesion involving the brachium of the superior colliculus. Upbeat nystagmus has also been described as occurring with a central IVth nerve palsy, presumably due to a lesion involving the superior cerebellar peduncle (brachium conjunctivum), through which the anterior semicircular canal fibers travel. Redrawn and modified with permission from Wolters Kluwer and the American Academy of Neurology originally published in Gold DR, Shin RK, Galetta SL. Pearls & Oysters: Central fourth nerve palsies. Neurology. 2012;79(23):e193-6.