Periodic alternating nystagmus (PAN) is defined as abnormal horizontal eye movements whose directions periodically alternate. It is an unusual finding, except in congenital nystagmus, but has manifested in various disorders. Despite the many reports about PAN, the length of the cycles has not been discussed. In this paper, we report two cases of PAN, and demonstrate that there are two types of PAN which are defined by the length of cycle. One type is symmetric, the length of the cycle toward

more »

... right and/or left is regular, and the other is asymmetric, the cycle is irregular and the direction changes at random. Case 1 was a 38-year-old man with spinocerebellar ataxa 6 (SCA6) showing symmetric PAN. Brain MRI showed atrophy of the cerebellum. He suffered from oscillopsia and his cerebellar disequilibrium was severe. Case 2 was a 21-year-old man with congenital nystagmus showing asymmetric PAN. Because he was negative for CNS lesions, and specific nystagmus of pendular jerking type was seen, his disease was diagnosed as congenital nystagmus. The direction and cycle of PAN was irregular. The diseases and mechanism of PAN in these two patients may differ. PAN caused by central nervous disorders is strictly symmetric, while others are asymmetric.