Recovery of peripheral versus central nerves identified by saccadic velocity after abducens neuropathy.

The abducens is the motor nerve with the most substantial course, both within and outside the brain and it innervates only one muscle. Sixth nerve palsy affords an opportunity to compare recovery after central versus peripheral nerve damage by assessing the dynamics of abduction. Horizontal saccade peak velocities and durations in 14 patients with unilateral peripheral sixth nerve palsies (5 acute, 9 chronic) are compared with those in 5 patients with central sixth nerve palsies (2 acute, 3 chronic) and with those in 10 normal subjects. Acutely, abducting saccades in the paretic eye were slow in both central and peripheral palsies, as anticipated from weakness of the lateral rectus muscle. In chronic central palsies, abducting saccadic velocities remained reduced, but in chronic peripheral palsies, they increased to normal within the limited range of excursion. The chronically damaged peripheral nerve behaves like a high-pass filter in transmitting phasic velocity commands, whereas tonic position commands remain defective, accounting for limited abduction but normal velocities within the range of duction. In chronic central (fascicular) palsies, saccade velocities remain reduced. Impaired conduction from damage to central myelin or axons is more persistent in central palsies, consistent with limited regeneration within the brain. Recording of saccade velocities may aid the distinction of fascicular from peripheral palsies. Saccade speed is repaired in peripheral palsies, probably by remyelination, and perhaps also by central monocular adaptation of innervation selectively to the paretic eye in order to drive both eyes rapidly and simultaneously to a target in the paretic field of motion.