A 37-year-old right-handed Indigenous Canadian man presented with 2.5 years of progressive involuntary movements, gait instability, and hyperesthesia. He complained of constant, involuntary head and trunk movements, severe speech disturbance, dysphagia, blurred vision, and binocular diplopia in all directions. He took no regular medications and drank no alcohol. Medical history was noncontributory. Family history was notable for a great uncle who died in his fifties from an unspecified gait disorder.On examination, vital signs were normal. MMSE was 28/30, losing points for writing a sentence and copying a figure, largely due to involuntary limb movements. He had leftward square wave jerks on primary gaze, saccadic pursuit, bilateral horizontal gaze-evoked nystagmus, and impaired upgaze. He had significant dysarthria with staccato speech. The remainder of the cranial nerve examination was unremarkable. He had an 8-cm amplitude, 3-4-Hz frequency, rhythmic, oscillatory titubation of the head and trunk with rest and posture as well as a 3-cm amplitude, 3-4-Hz frequency, postural and action (up to 5 Hz) tremor of the hands and feet bilaterally. He had an exaggerated startle response without spontaneous myoclonus. Muscle bulk, tone, and power were otherwise normal without fasciculations. Muscle stretch reflexes were 2+ throughout. He had widespread hyperesthesia throughout his limbs and trunk, which precluded more detailed sensory testing. Coordination testing revealed bilateral, lower more than upper limb dysmetria. He had dysdiadokinesia and uncoordinated fine finger movements bilaterally, right more than left, and severe truncal ataxia when sitting upright that worsened when standing. Gait was broad based with lateropulsion. Questions for Consideration:1. Where does this process localize? 2. What clinical tests and investigations would you consider?GO TO SECTION 2