Implementation of Listing's Law in Patients with Unilateral Sixth Nerve Palsy

Twenty-seven patients with unilateral sixth nerve palsy and 10 normal subjects were studied. Informed consent was obtained from each subject. Patients with diplopia of less than 4 weeks’ duration were classified as having acute palsy; all others were designated here as having chronic palsy. MRI with enhancement was performed for patients under 50 years of age and for those with neurologic signs. CT with contrast was obtained in patients with ischemic risk factors and those over 50 years of age. If the CT was normal, patients were followed at about 3 months. Those without improvement at 3 months and those with an abnormal CT were further investigated with MRI. Eye positions were measured with magnetic search coils while patients followed a laser target at 1 m with one eye covered. The laser was programmed to appear in nine different target positions, arranged in a 3 × 3 square. With head immobile, subjects made saccades to a target that moved between straight ahead and 8 eccentric positions. At each target position, fixation was maintained for 3 seconds before the next saccade. Using fitted functions, we quantified violations of Listing’s law by comparing the ocular torsion in each recorded eye position to the torsion predicted by the law. The standard deviation of the differences between the predicted and measured torsion was called Listing deviation.