Chiari Malformation Causing Downbeat Nystagmus in Lateral Gaze

This is a 20-yo-man who presented with oscillopsia in lateral gaze from downbeat nystagmus (DBN). In primary gaze, very subtle DBN was only noted with ophthalmoscopy, but in lateral gaze, prominent DBN was present. Other central ocular motor signs included gaze-evoked nystagmus (GEN) vertically, in addition to mild GEN in lateral gaze, with rebound nystagmus when returning from lateral to primary gaze (not shown in the video). The combination of DBN and GEN in right and left gaze can produce an oblique or diagonal nystagmus that beats down and out, also known as "side-pocket" nystagmus, which was present to a mild degree in this patient. Additionally, pursuit and VOR suppression were impaired (choppy) in all directions. MRI demonstrated that the cerebellar tonsils extended about 20 mm below the foramen magnum, consistent with a Chiari (type 1) malformation