Rare frequency of downbeat positioning nystagmus in spinocerebellar ataxia type 31.

Spinocerebellar ataxia type 31 (SCA31) and spinocerebellar ataxia type 6 (SCA6) are the most frequent types of spinocerebellar degeneration in Japan. Previous reports described that it was difficult to distinguish SCA6 and SCA31 in clinical situations. There is not much difference except that the onset age of SCA31 is slightly higher than that of SCA6. Therefore we surveyed our medical records retrospectively, and then compared clinical symptoms of SCA6 and SCA31. As previously stated, the onset age of SCA31 is higher than that of SCA6. Gaze-evoked nystagmus is more frequent in SCA6 than in SCA31. The percentage in downbeat positioning nystagmus (DPN) is as high as 63% in SCA6. In contrast, DPN in SCA31 is rare and subtle. Our study suggests that the presence of DPN is an important sign that can differentiate SCA6 from SCA31 clinically.