Dashing Dan's Diplopia

Dan, a 25-year-old man, requested evaluation for diplopia of 2 days duration. He had been well, except for a one week episode of vertigo (diagnosed as "labyrinthitis") the year before, until he awoke with horizontal diplopia, worse on left lateral gaze. He had no other symptoms.
 

The general physical examination was normal. On neurological examination, positive findings were restricted to abnormalities of eye movement. Movement to the right was normal, as was movement up and down. On attempted left lateral gaze, however, the right eye did not move past the midline and coarse nystagmus appeared in the left eye. Convergence was normal.

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