Patient with Duane's syndrome of the left eye. Unable to abduct (pull out) the left eye when tracking along the horizon (due to inability to use the lateral rectus muscle of the left eye). No limitation of adduction (pulling left eye in towards nose). No retraction.
However, when in superior or inferior gaze, due to the combined action of the inferior and superior obliques (respectively), abduction beyond midline is achieved.
(Inferior oblique muscle elevates, abducts and extorts.)
(Superior oblique muscle depresses, abducts and intorts.)
This patient demonstrated the best ability to abduct the left eye when in up-gaze. She shows the potential to train eye movements past midline by developing the action of the oblique muscles to compensate for the lack of effectivity of the lateral rectus muscle.
By keeping a target single near the full extent of her binocular up-left gaze, she can develop some motor ability to extend her left eye past the midline. Then, when tracking the target down towards the horizon, apply effort to maintain single vision. This extends the physical ability of the muscles. It also invites the system to alter the nerve signal sent to abducting muscles of left eye and the paired adducting muscles of right eye, to help maintain single binocular vision.
Notice that the left eye was able to extend past midline in upgaze, but that the right eye was over-shooting when aiming up/left, rather than pairing with the direction of the left eye. This is because extending the left eye past midline required a very strong neural signal; the equivalent signal to the right eye was much more effective: produced much more abduction. By introducing fusion (holding single vision) during muscle exercises, the neural signals will adjust and balance, providing a greater ability to maintain coordination of the eyes into up-left gaze, and potentially into straight-left gaze.
