Paraneoplastic downbeat nystagmus associated with cerebellar hypermetabolism especially in the nodulus

•Downbeat nystagmus occurred as a manifestation of paraneoplastic syndrome.•Paraneoplastic downbeat nystagmus was associated with cerebellar hypermetabolism.•Dysfunction of the cerebellar nodulus appears responsible for downbeat nystagmus.

A 52-year-old man with vertigo and imbalance for two weeks showed spontaneous downbeat (DBN), horizontal gaze-evoked, and positional apogeotropic nystagmus along with severe limb and truncal ataxia. Gadolinium-enhanced brain MRI was normal, but whole body and brain 2-deoxy-2-[F18]fluoro-d-glucose-positron emission tomography revealed hypermetabolism in the right lower lobe of the lung and the cerebellum, especially in the nodulus. The lesion in the lung was confirmed as mixed cell carcinoma. Paraneoplastic DBN may be associated with cerebellar hypermetabolism, especially in the nodulus.