A 15-month-old male presented with a one-day history of acute onset, continuous oscillating movement of his right eye. He had received his one-year immunizations four days prior and had a four-day history of a febrile viral respiratory tract infection. Pregnancy was unremarkable. He had severe iron deficiency anemia (MCV 66, Hb 65) and was developmentally delayed, as he was unable to stand independently and was non-verbal. His head circumference was 49 cm (95th percentile) and his weight was at the 25th percentile. On physical examination, continuous horizontal large amplitude pendular nystagmus of the right eye at a frequency of 3-4 Hz was observed. No nystagmus was observed in the left eye, even on funduscopic examination. The child could fixate targets in all four quadrants with both eyes independently, and could fixate and track small objects with both eyes independently suggesting no significant visual field defect or visual loss. Dilated funduscopic examination was normal, extra ocular movements were full, pupils were equal and reactive and there was no relative afferent pupillary defect. The remainder of the neurological examination was normal. There was no head bobbing or anomalous head position and no stigmata of neurofibromatosis type 1. Magnetic resonance imaging of the brain (Figure) demonstrated a 2 cm x 1.6 cm x 1.2 cm suprasellar enhancing mass involving the optic chiasm, hypothalamus, mamillary bodies and superior pituitary stalk. There was no extension into the pituitary fossa or the optic nerves and no ventricular enlargement. A biopsy of the mass was obtained and revealed histology consistent with a low-grade pilocystic astrocytoma (World Health Organization (WHO) grade 1), consistent with a diagnosis of chiasmal glioma.