Supranuclear eye movement dysfunction in mitochondrial myopathy with tRNA(LEU) mutation

A patient with multiple neurological deficits and biopsy-proven mitochondrial myopathy with mutation of tRNA(LEU) at nucleotide 3243 was referred for eye movement evaluation. He had restricted range of voluntary motions in all directions and full range of eye movements on passive rotation of head while fixating a visual target. Eye movement recordings revealed decreased horizontal and vertical saccadic velocities and markedly decreased smooth pursuit gain in both directions. The vestibulo-ocular reflex showed gain abnormalities with many saccadic intrusions on the smooth reflex response. His brother, with similar mutation, was clinically asymptomatic. However, his eye movement recordings revealed slow horizontal saccadic velocities leftward and normal saccadic velocities rightward in both eyes as well as in upward and downward direction. Smooth pursuit and vestibulo-ocular reflexes were within normal limits. Although eye movement abnormalities are seen commonly in mitochondrial myopathies, the exact mechanism is not known. Our cases suggest supranuclear dysfunction as one of the mechanisms for ophthalmoparesis.