Curved Oblique Saccades and Saccadic Slowing in a Patient with an Anti-GAD Mediated Posterior Fossa Syndrome

This is a patient who developed muscle spasms especially involving the muscles of the trunk in addition to a progressive gait disorder. Examination demonstrated slow saccades, slower horizontally than vertically, in addition to gaze evoked nystagmus with a side pocket pattern. Side pocket nystagmus is an oblique nystagmus that results is the combination of downbeat nystagmus in addition to gaze evoked nystagmus. For example, when looking to the right, instead of seeing pure right beating due to the gaze evoked nystagmus or pure down beating, the horizontal and vertical components combine to create an oblique trajectory that beats both down and laterally. However, her nystagmus was difficult to appreciate given how subtle the fast phases appeared. This was almost certainly a result of her slow saccades, which is a well described sign that can be seen with posterior fossa involvement due to an anti-GAD syndrome.; ; Finally, given the fact that her saccades were slower horizontally as compared to vertically, when asked to perform oblique saccades, for example looking up and to the right and then down and to the left, a curved trajectory can be seen. Initially the saccade is primarily vertical, and this is followed by the slower horizontal component to make it to the target. This discrepancy in the speed of vertical and horizontal saccades gives it a curved or L-shaped trajectory.