Clinical Reasoning: Two see or not two see—Is it really double vision?

A 57-year-old right-handed woman presented to the emergency department with complaints of double vision and intractable nausea that began abruptly 2 days earlier. Her visual symptoms were characterized as seeing overlapping or separate horizontally or diagonally displaced objects. She had no history of headaches or stroke. Her cerebrovascular risk factors included hypertension, type II diabetes, coronary artery disease, and cigarette smoking. Her medications included clopidogrel, lisinopril, paroxetine, and oxycodone. Her family history was notable for late-onset ischemic heart disease in her parents with no first-degree relatives with early vascular disease. On examination, her blood pressure was 158/101 mm Hg, pulse rate was 87 bpm, and she was afebrile. She was alert and fully oriented. Her attention, recall of recent events, and general fund of knowledge were normal. Her speech was fluent and nondysarthric. Cranial nerve examination was notable for no dysconjugacy or nystagmus, but double vision predominately in the horizontal plane, in all directions of gaze. The diplopia persisted with monocular vision in each eye, and did not improve with a pinhole test. The degree of diplopia waxed and waned during the examination, with visual field extinction tests being difficult to perform reliably. Her pupils were equal with bilateral hippus. Visual fields were full to confrontation. Direct funduscopy revealed normal optic discs. She had a mild right hemiparesis with mild right arm and leg drift, but no facial asymmetry. There was mild hypesthesia over her right arm and leg and appendicular ataxia in her right arm that was worse with eyes open. She did not have extinction to double simultaneous sensory stimuli. Gait evaluation was deferred during her initial examination.