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Background: Fisher syndrome (FS) is characterized by a triad of ophthalmoplegia, ataxia, and areflexia. The lesion sites responsible for ataxia and ophthalmoplegia in FS require further exploration. The aim of this study was to determine involvement of the central nervous system in FS using 18F-fluorodeoxyglucose positron emission tomography (PET). Methods: Cerebral glucose metabolism of 10 patients with FS was compared with that of 60 age- and gendermatched normal controls using PET. For individual analyses, 15 age- and gender-matched controls were selected from the control group. Patients also underwent MRI of the brain and measurement of serum anti-GQ1b antibody. Results: Group analyses revealed increased metabolism in the cerebellar vermis and hemispheres, pontine tegmentum, midbrain tectum, left thalamus and right inferior frontal cortex (P<0.001, uncorrected). In contrast, the visual association cortices (Brodmann areas 18 and 19) showed decreased metabolism bilaterally. Individual analyses disclosed hypermetabolism in the cerebellar vermis or hemispheres (n=7), inferior frontal cortex (n=5) and brainstem (n=4, P<0.005, uncorrected). We also found a negative correlation between the cerebellar hypermetabolism and the interval from symptom onset to PET (r =-0.745, P=.0013). The hypermetabolism was normalized on follow-up PET with an improvement of the ophthalmoplegia and ataxia in one patient. Conclusions: These findings indicate an involvement of the central nervous system in FS, and the hypermetabolism in the cerebellum and brainstem suggests an antibody-associated acute inflammatory process as a mechanism of this autoimmune disorder. [ABSTRACT FROM AUTHOR]