Aims: To relate dynamic properties of eye movements to visual disability in a patient with chronic progressive external ophthalmoplegia (CPEO) due to mitochondrial DNA deletion.
Methods: Visual function was evaluated by neuro-ophthalmological examination and visual function index (VF-14). Eye movements were measured using the magnetic search coil technique.
Results: The patient denied diplopia or oscillopsia at rest or during locomotion. His range of eye movements was limited to <12 degrees horizontally, and <19 degrees vertically with smaller movements of the left eye. All classes of movements were impaired, but the eyes accelerated to over 1000 degrees /s/s at the onset of saccades and in response to brisk head rotations.
Conclusions: Diplopia and oscillopsia were suppressed, probably due to visual system adaptation during the slow disease progression. We postulate that relative sparing of pale global extraocular fibres accounted for preserved acceleration of saccades and vestibular eye movements.