Fixating with the Paretic Eye: Evidence of “Fixation Duress” on <scp>PET</scp> Imaging

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.[Color figure can be viewed at www.annalsofneurology.org] An 18-F-fluorodeoxyglucose (18F-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 18F-FDG avidity in regions typically affected by sarcoidosis and histiocytic disorders. The PET study did, however, demonstrate focally increased 18F-FDG avidity within the structurally-normal IR muscle of the paretic left eye. The PET was fused with MRI to better delineate the increased 18F-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.1 This preference can occur, for example, when the paretic eye has better visual acuity. In this substantial minority of SO palsy patients, fixation with the paretic eye requires continuous activation of the eye's sole remaining depressor, the inferior rectus (IR) muscle – a phenomenon called “fixation duress.”2 Innervation to the ipsilateral elevator muscles diminishes accordingly, which results in decreased innervation to the contralateral elevator muscles as dictated by Hering's law of equal innervation. Therefore, the combination of reduced innervation to the elevator muscles and intact innervation to both depressor muscles causes the healthy contralateral eye to develop a secondary hypotropia when the paretic eye is in fixation duress.2 A common clinical mistake in the examination of diplopia is to assume that the deviated eye must always be the paretic one. This is simply not true. Secondary deviations of the healthy eye may occur as a natural consequence of the fixation duress, a physiologically active process as exemplified by our patient's PET/CT. Furthermore, diplopia localization techniques that rely on determining which eye sees a “true” (macular) image and which a “false” (peripheral) image are likewise unreliable, as the paretic eye may be fixating. Instead, clinicians examining a patient with diplopia should opt for techniques of alternate cover testing or Maddox rod testing,3 which do not depend on which eye is used to fixate and thus remain reliable for localization. To our knowledge, this is the first report with PET data that demonstrates and quantifies the metabolic changes in extraocular eye muscles during fixation duress. The increased 18F-FDG avidity within the left IR indicates glucose hypermetabolism as a physiologic response to its continuous activation when maintaining fixation in SO palsy.4 Using validated diplopia examination techniques, clinicians can accurately localize the paretic extraocular muscle, and avoid being misled by fixation duress and secondary deviations in patients fixating with their paretic eye. No funding or sponsorship was received in connection with this manuscript. C.L., A.B., and A.F. contributed to study concept and design. C.L., J.R., and A.F. contributed to data acquisition and analysis. C.L, J.R., A.B., and A.F. contributed to drafting manuscript or preparing figures. Nothing to report.

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