Ocular motor nerve palsies: implications for diagnosis and mechanisms of repair.

Measurements of the dynamics of the eyes in ocular motor nerve palsies may aid diagnosis, characterize peripheral and central palsies, and reveal adaptive properties of the brain. Saccadic and vestibulo-ocular reflex (VOR) functions of patients with peripheral and central sixth, and peripheral third and fourth nerve palsies were studied by three dimensional magnetic field search coil oculography. Combined third and fourth cranial nerve microvascular ischaemic palsy in diabetes mellitus produced low ratios of intorsion to adduction amplitudes. Presumed isolated third nerve palsy caused higher ratios of adduction to intorsion and violations of Listing's law. The VOR in third, fourth, and sixth nerve palsies reveals adaptive equilibration of the action of paretic agonist and their non-paretic antagonist muscles in violation of Hering's law during head motion. Saccadic speeds in the field of paretic agonists are repaired in chronic peripheral palsies despite limited ductions, but remain reduced in central palsies. Limited intorsion with third nerve palsy is attributed to concurrent fourth nerve ischaemia in the distribution of the inferolateral trunk of the intracavernous carotid artery. Adaptive repair of the VOR after ocular motor nerve palsies reduces asymmetric retinal image slip and binocular disparity, and repair of saccadic velocity drives both eyes rapidly and simultaneously into the paretic field of motion.