Cortical excitability revealed by motor evoked potential, cortical silent period and conduction time in spinocerebellar ataxias type 1, type 2 and idiopathic sporadic cerebellar ataxia : a transcranial magnetic stimulation study

• Autosomal dominant cerebellar ataxias are characterized by their underlying genetic defect and are referred to as spinocerebellar ataxias (SCAs 1-23). The clinical classification of the SCA has been difficult owing to variations and overlapping of the clinical signs. The aim of this study was to compare cortical motor evoked potential (MEP), central motor conduction time (CMCT) and cortical silent period (CSP) duration in SCA patients in Serbia, namely in genetically  homogenous groups of ataxia patients with type 1, type 2 and IDCA (idiopathic sporadic cerebellar ataxia). We examined 29 patients, 16 with the diagnosis of SCA 1, 6 SCA 2 and 7 IDCA patients. Eight healthy control subjects were gender and age matched. Transcranial magnetic stimulation (TMS) was used to investigate parameters of cortical excitability such as: motor threshold (MT) and MEP, CSP and CMCT. MT was established at rest, MEP was calculated as the area in the rectified EMG recording. CSP was evoked by 30% suprathreshold stimulation while subjects activated FDI muscle with contraction of 30% of their MVC. CMCT was calculated as a difference between the shortest MEP latency after cortical and after cervical stimulation (in the region of C5-C6). Results show that MT was increased in all ataxia patient groups, compared to control subjects. CMCT has significant increase in SCA 1 patients. CSP in IDCA patients is significantly longer then in SCA 1, SCA 2 and control subjects, while no difference was found between SCA 1, SCA 2 and control. MEP duration was significantly increased in all ataxia groups compared to control in relaxed muscle. Due to the cerebellar influence on the cortico-spinal system through control of inhibitory cortical interneurons, could be assumed that different categories of ataxia patients have disturbed cerebellar inhibitory influence to the various degrees. It might be possible that SCA 1 prominent abnormalities in cortical excitability originate from expansion of damage from cerebellum to some other cerebellar and brain structures.

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