What are the third cranial nerve nuclear and fascicular lesions in multiple sclerosis (MS)?

Updated: Feb 21, 2019
  • Author: Fiona Costello, MD, FRCP; Chief Editor: Hampton Roy, Sr, MD  more...
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The oculomotor (third) nerve nuclear complex is located in the midbrain at the level of the superior colliculus. [42] Of the four paired subnuclei within this complex, the most medial innervates the superior rectus muscle. This is the only portion of the oculomotor nucleus that sends its axons to the opposite eye. [42] A nuclear third nerve lesion causes bilateral superior rectus weakness, bilateral ptosis, and, often, ipsilateral deficits reflective of third nerve fascicle dysfunction. [40] In the case of a third nerve palsy, a fascicular lesion may cause partial or complete deficits in the following functions: ipsilateral elevation (superior rectus and inferior oblique muscles), depression (inferior rectus), and adduction of the eye (medial rectus); upper eyelid elevation; and pupillary constriction. [42] In rare cases, these lesions may be highly selective and may cause weakness of a single muscle, which can lead to diagnostic confusion.

The oculomotor nerve runs in close proximity to numerous midbrain structures; therefore, features of a third nerve palsy can be associated with other neurological deficits, including contralateral hemiparesis (Weber syndrome), contralateral tremors (or chorea, athetosis) (Benedict syndrome), and contralateral cerebellar ataxia (Nothnagel syndrome). [44] Lesions of the dorsal midbrain in individuals with third nerve palsies may also manifest as paralysis of upgaze, light-near dissociation of the pupils, skew deviation, lid retraction, and convergence-retraction nystagmus. [44]

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