Dynamic aspects of trochlear nerve palsy.

Trochlear nerve palsy leads to kinematic aberrations of both the paretic and the unaffected eye. During dynamic head roll, the rotation axis of the covered paretic or unaffected eye deviates inward, while the rotation axis of the viewing paretic or unaffected eye aligns with the line of sight; this convergence of rotation axes increases with gaze moving in the direction of the unaffected eye. During downward saccades, the trajectories of both eyes curve towards the unaffected side; these curvatures increase when the head is rolled to the affected side and gaze directed to the unaffected side. Hence, during both vestibular evoked and saccadic ocular movements, the unaffected eye shows similar kinematic aberrations as the paretic eye. While aberrations of the paretic eye can be explained by decreased force of the superior oblique (SO) muscle, aberrations of the unaffected eye may be due to increased force parallel to the paretic SO in the unaffected eye in accordance with Hering's law. This law, which forms the basis of conjugate eye movements, also seems to govern eye displacements in unilateral eye muscle palsy.