Saccade dynamics in the recovery phase of abducens nerve palsy

INTRODUCTION: Despite the fact that abducens nerve palsy (ANP) is the most common ocular motor palsy, the literature on the respective saccade dynamics, both in the paretic (PE) and non-paretic eye (nPE), is scarce. AIMS AND METHODOLOGY: The aim of this study was to examine the maximum velocity, duration and accuracy of horizontal saccades, in individuals with unilateral ANP, and to compare them with normal controls. Binocular horizontal eye movements were recorded at 5º, 10º and 15º, using an infrared corneal reflection device from 21 adults with microvascular unilateral ANP during the acute and the chronic phase of the palsy, as well as 18 healthy adults. Non-parametric tests were used for statistical comparisons. RESULTS: The PE, when compared to the nPE, presents a slightly lower saccadic amplitude and velocity/amplitude ratio and a higher duration/amplitude ratio. The nPE, compared to the healthy eye (HE) of the control group, showed consistently amplitude gain >1 while the velocity/amplitude ratio did not differ in either session. The duration/amplitude ratio tended to be higher in the nPE. The prism dioptres of the PE did not appear to correlate with any parameter tested (amplitude gain, velocity/amplitude ratio, duration/amplitude ratio) of the open nPE, but the amplitude ratio was statistically lower during the first session when the nPE was kept covered and the duration/amplitude ratio decreased significantly. CONCLUSIONS: One of the main findings of the study is the increase in saccade duration during adaptation of ANP. Specifically, the nPE performed orthometric saccades with a longer duration than healthy controls. Given that the motor command reaches the ocular muscles by neural discharges with a "pulse-step" pattern, any adaptation reflects in a change of this pattern. Cerebellar learning leads to an increase in the pulse width of the neural discharge. This idiosyncratic response may be related to plastic changes in central structures that serve learning processes such as the cerebellum. Further research could provide more insight into the cerebellar plastic processes involved in the saccadic adaptation.