An 18-year-old man presented with an upper respiratory tract infection that persisted for a week without improvement. Because of this infection, he went to an emergency department and was prescribed antibiotics for presumed sinusitis. Five days later, he developed painless horizontal diplopia and returned to the emergency department. Findings from a computed tomographic scan showed diffuse sinusitis that was procedurally drained by an otolaryngologist. However, this treatment did not resolve the diplopia and he was subsequently referred to a neurologist. Additional evaluation included a repeated computed tomographic scan that showed no infectious involvement of the orbits or petromastoid complexes (Figure). Findings from magnetic resonance imaging of the patient’s brain and orbits did not reveal intracranial lesions or extraocular muscle myositis. Results of lumbar puncture and cerebrospinal fluid testing, including opening pressure, cell count, and glucose and protein levels, were in the reference range. The erythrocyte sedimentation rate and C-reactive protein levels were also in the reference range. Findings from neurologic examination did not reveal paresthesias, difficulty with gait, or abnormal tendon reflexes.