Downbeat Nystagmus and Cerebellar Atrophy

This is a 40-year-old man with 2 years of progressive ataxia and oscillopsia. On examination, he had downbeat nystagmus (DBN), an ocular motor finding that is usually (but not always) associated with flocculus/paraflocculus dysfunction, which causes overaction of the anterior canal (upward or anti-gravity) pathways relative to posterior canal (downward or gravity) pathways. This results in a continuous slow upward phase and subsequent fast downward phase, causing the DBN. In his case, there was a history of a testicular seminoma (s/p resection) several years prior to presentation, and during the initial work-up he was found to have a CSF lymphocytic pleocytosis, but had unrevealing paraneoplastic panels (serum and cerebrospinal fluid), brain MRI and body PET/CT-these exams were repeated several times over 1-2 years. There was no clear response to steroids and IVIG, and for his DBN, trials of 4-aminopyridine and chlorzoxazone were ineffective. After 2 years, significant cerebellar atrophy was apparent on his MRI.