Further evidence of corticospinal tract dysfunction in spinocerebellar ataxia type 6

Triple stimulation technique (TST) was performed with seven patients with genetically confirmed spinocerebellar ataxia type 6 (SCA6) and nine age-matched normal controls. Recordings of the CMAP were obtained from the right first dorsal interosseus (FDI) muscle. Transcranial magnetic stimulation (TMS) of the left motor cortex was applied to the contralateral scalp. Conventional TMS, central motor conduction time (CMCT) by F-wave method and the TST amplitude ratio (TST test/TST cond.) were investigated. The mean resting motor threshold and mean CMCT did not show significant differences between normal controls and patients, whereas the mean TST amplitude ratio was significantly smaller than in controls. An abnormal TST represents upper motor neuron loss, central axon lesions or conduction blocks or inexitability to TMS. Lack of any pathological change in corticospinal tract in SCA6 indicates that this abnormality may be caused by crossed cerebellar diaschisis (CCD) or a functional disorder in the brain caused by CACNA1A mutations.