||A 43 -year-old man presented with oscillopsia, dizziness, binocular vertical diplopia, and gait difficulties. He reported a six-month history of abdominal pain, anorexia and 50-pound weight loss, and a 3-month history of mood and cognitive changes. Torsional nystagmus, a left 4th nerve palsy (NP), unsteady gait, occasional head jerks, and intermittent facial grimacing were noted onexamination, and MRI brain/orbits was normal. Over weeks his symptoms worsened and he developed labile blood pressure and syncope. Repeat MRI, single-fiber EMG, myasthenic panel, celiac panel, TSH, cyanocobalamin, thiamine, RPR, copper, ESR, CRP, ANA, RF, ceruloplasmin, anti-ASO titers were normal. CSF and serum infectious, toxic, autoimmune and paraneoplastic panels were also within normal limits. He was referred to our institution 1 month after his initial assessment. Our evaluation demonstrated full ocular motility, a 6 prism-diopter left hypertropia (worse in right gaze and left head tilt, unchanged in upgaze and slightly less in downgaze), and spontaneous upbeat-torsional nystagmus with top poles beating toward the left ear and a milder left-beating component. Saccades, smooth pursuit, horizontal vestibulo-ocular reflex (VOR), VOR suppression and optokinetic nystagmus appeared unremarkable when taking into account the nystagmus. Afferent neuro-ophthalmic examination was normal. Serum anti-GAD antibody was positive (15 IU/mL, Normal <5.0 IU/mL). MRI brain and testicular ultrasound were normal. PET-CT showed increased FDG uptake in left medial rectus (MR), right lateral rectus (LR), and bilateral inferior recti (IR) muscles. Plasma exchange (PLEX) was initiated and his symptoms improved. Several days later, serum anti-dipeptidyl aminopeptidase-like protein 6 (DPPX) IgG returned as positive (titer 1:15.360), and the diagnosis of anti-DPPX-associated encephalitis was made. Screening for an underlying neoplasm was unrevealing, and treatment with rituximab was initiated. Nystagmus improved, and repeat PET-CT demonstrated resolution of the prior extraocular muscle (EOM) avidity.