LISTINGS PLANE DEPENDENCE ON ALTERNATING FIXATION IN A STRABISMUS PATIENT

Listing's law of the eye is one of the best studied findings in motor control, but its functional meaning is still incompletely understood a nd its status in neurological disorders and in strabismus is almost en tirely unknown, We investigated the mechanisms underlying Listing's la w and its possible clinical relevance. The dual magnetic search coil t echnique was used to record three-dimensional binocular eye movements in a stereoblind strabismic patient with good visual acuity in both ey es and capable of voluntarily alternating fixation. This technique yie lded an accurate, objective and simultaneous measure of ocular misalig nment in three dimensions and showed that the squint angle depended on which eye was fixating. Saccadic eye movement data throughout the ocu lomotor range were used to fit Listing's plane, Listing's primary posi tion and the thickness of the plane for each eye mere calculated for t hree different fixation conditions. For comparison, control measuremen ts were taken from four normals. In the patient, no large deviations f rom normal values for the thickness of Listing's plane and the confide nce limits of the Listing primary position were found, The most remark able abnormality was that the orientation of Listing's plane depended on which eye was fixating. Both the change in ocular misalignment and the shift of Listing's primary positions observed when changing fixati on are probably linked to accommodation-related vergence. Despite repe ated surgery at early age, the patient had well-defined Listing planes for both eyes, but their alignment during left-eye fixation was abnor mal. The obedience to Listing's law may reflect a strategy which minim izes muscular effort in each eye separately. The abnormal fixation-con dition dependence is probably due to an aberrant coupling with vergenc e. (C) 1997 Published by Elsevier Science Ltd.