LAUSR

A 71-year-old man presented with a one-month history of binocular vertical diplopia. On examination, visual acuity was 20/25 OD and 20/20 OS. He had an 8-diopter right hypotropia in primary gaze that worsened with right gaze and with left head tilt, in keeping with a left superior oblique (SO) palsy. Cover testing demonstrated an upward refixation movement of the right eye, indicating preferential fixation with his paretic left eye. Contemporaneous eye movement photographs were not obtained; however, his binocular misalignments in the cardinal positions of gaze and with head tilts are recreated schematically in Panel A. The remainder of his physical examination was unremarkable. Magnetic resonance imaging (MRI) head with gadolinium disclosed a homogenously-enhancing left trochlear nerve lesion in the ambient cistern (Panel B, arrow) and multifocal T2 FLAIR hyperintensities in both cerebral hemispheres. The orbits and extraocular muscles were normal. An 18-F-fluorodeoxyglucose (F-FDG) PET (positron emission tomography)/CT (computerized tomography) was ordered to rule out sarcoidosis and histiocytic disorders, such as Erdheim-Chester disease. The PET/CT did not show hypermetabolism of the intracranial lesions or F-FDG avidity in regions typically affected by sarcoidosis and histiocytic disorders. The PET study did, however, demonstrate focally increased F-FDG avidity within the structurally-normal IR muscle of the paretic left eye. The PET was fused with MRI to better delineate the increased F-FDG uptake within the left IR (Panel C, arrow). Quantitatively, the total lesion glycolysis (TLG) was threefold higher in the left IR (1.7 SUVbw ml) (standardized uptake value [SUV], body weight [bw]) than in the right IR (0.5 SUVbw ml). The TLG asymmetry confirmed that the left IR was more metabolically active in fixation duress. Eventually, an intracranial biopsy was performed and diagnosed central nervous system light-chain amyloidosis. In most cases of SO palsy, the unopposed elevating action of the inferior oblique (IO) muscle produces hypertropia in the affected eye. However, up to 18% of patients with unilateral SO palsy prefer to fixate with their paretic eye. This preference can occur, for example, when the paretic eye has better visual acuity. In this substantial [Color figure can be viewed at www.annalsofneurology.org]