A woman in her 30s presented to the hospital with decreased vision in both eyes, confusion, and ataxia for 3 weeks. She had undergone gastric bypass surgery 3 months before presentation, which was complicated by a gastric ulcer and stricture. She had lost 34.5 kg since the procedure. Other medical history included hypothyroidism and thyroidectomy for thyroid cancer diagnosed 10 years ago. Her medication regimen included levothyroxine sodium and occasional bariatric chewable vitamins. She denied alcohol or tobacco use or drug abuse. Her best-corrected visual acuity was 20/200 OD and 20/400 OS. Her pupils were equally reactive, with no relative afferent pupillary defect. There was a mild abduction deficit in both eyes as well as prominent upbeating nystagmus. The amplitude of the nystagmus increased in upgaze and dampened with downgaze. She was unable to read the control plate of the Ishihara test. Findings from an anterior segment examination were unremarkable. Findings from a dilated fundus examination revealed disc edema in both eyes with peripapillary nerve fiber layer thickening and large intraretinal hemorrhages (Figure).