Adaptive neural mechanism for Listing's law revealed in patients with sixth nerve palsy.

PURPOSE During fixation and saccades, human eye movements obey Listing's law, which specifies the torsional eye position for each combination of horizontal and vertical eye positions. To study the mechanisms that implement Listing's law, the authors measured whether the law was violated in peripheral and central unilateral sixth nerve palsy. METHODS Twenty patients with peripheral (13 chronic, 7 acute) sixth nerve palsy, 7 patients with central sixth nerve palsy caused by brainstem lesions, and 10 normal subjects were studied with scleral search coils. With the head immobile, subjects made saccades to a target that moved between straight ahead and eight eccentric positions. At each target position, fixation was maintained for 3 seconds before the next saccade. To quantify violations of Listing's law, we measured ocular torsion during fixation and during saccades, and compared it with the torsion predicted by the law. The SD of the differences between the predicted and measured torsion was called Listing deviation. RESULTS Patients with central sixth nerve palsy had abnormal ocular torsion in both the paretic and nonparetic eyes, which violated Listing's law. During fixation, Listing deviation averaged 2.4 degrees in the paretic eye and 1.7 degrees in the nonparetic eye, compared with 0.8 degrees in normal control subjects (P < 0.05). During saccades, the Listing deviation averaged 2.7 degrees in the paretic eye, and 1.6 degrees in the nonparetic eye, compared with 0.8 degrees in normal control eyes (P < 0.05). Donders' law was also violated in both eyes of patients with central sixth nerve palsy. They showed an abnormally wide range of ocular torsion in any given gaze direction. In contrast, patients with acute peripheral palsy had abnormal ocular torsion only in the paretic eye. Listing deviation of the paretic eye averaged 2.3 degrees during fixation and 3.2 degrees during saccades (P < 0.05). Donders' law was obeyed in acute peripheral palsy. Patients with chronic peripheral sixth nerve palsy obeyed Listing's and Donders' laws during both fixation and saccades. CONCLUSIONS Patients with central unilateral sixth nerve palsy have abnormal ocular torsion in both eyes, demonstrating that brainstem circuits normally participate in the maintenance of Listing's law. Eye movements in patients with acute peripheral sixth nerve palsy violate Listing's law, whereas those in patients with chronic peripheral palsy obey it, indicating that neural adaptation can restore Listing's law, even when the eye muscle remains abnormal.