Saccade Dynamics in the Acute and Recovery Phase of Abducens Nerve Palsy

To examine the natural adaptive course of ocular motor system in unilateral abducens nerve palsy while addressing the scarce literature on saccade dynamics and natural adaptation.

Binocular horizontal eye movements were recorded from 18 healthy adults and 21 adults with unilateral abducens nerve palsy during the acute and chronic phases. Dynamics of the paretic and non-paretic eyes were compared, and the non-paretic eye dynamics were correlated with the respective prism diopters. Non-parametric tests were used for statistical comparisons.

The paretic eye, compared to the non-paretic eye, presented a slightly lower saccadic gain and velocity/amplitude ratio and a higher duration/amplitude ratio. The non-paretic eye, compared to healthy controls, showed consistent amplitude gain ( $$ 1) and a tendency for a higher duration/amplitude ratio. In the acute phase, when the non-paretic eye was covered, the paretic eye's amplitude ratio was lower and the duration/amplitude ratio decreased significantly. In the acute phase, a greater degree of esotropia in the paretic eye was associated with a lower amplitude gain and duration/amplitude ratio in the non-paretic eye.

During adaptation in abducens nerve palsy, the saccade duration of the paretic eye increased, and a similar tendency was observed in the non-paretic eye. This finding likely reflects a change in the "pulse-step" pattern and may be related to plastic changes in central structures, such as the cerebellum, that support learning processes.