N140 Latency as a Novel Biomarker for the Evaluation of Somatosensory dysfunction in Patients with Spinocerebellar Ataxia Type 2

Introduction: The N140, a long latency component of the somatosensory evoked potentials (LL-SEPs) related to passive attention, could be a valuable complement to short latency evoked potentials (SL-SEPs) for the evaluation of somatosensory pathways in patients with spinocerebellar ataxia type 2 (SCA2). There are no studies of N140 in patients with SCA2. Objective: To characterize LL-SEPs and SL-SEPs in patients with SCA2 and healthy controls. Methods: We enrolled 28 patients with SCA2 and 34 age-matched healthy controls. LL-SEPs (N140) were recorded from the Cz electrode referenced to linked mastoids (Cz-A1+A2) following cutaneous stimulation of the second finger. SL-SEPs (N20) were obtained using standard procedures. Clinical assessment in patients included the Scale for the Assessment and Rating of Ataxia (SARA) and measurement of CAG repeat length. Results: Compared to controls, SCA2 patients exhibited significantly prolonged mean latencies for both the N140 and N20 components. While the mean amplitude of the N20 was significantly reduced in patients, the amplitude of the N140 did not differ between groups. Notably, both N140 and N20 latencies showed significant positive correlations with SARA scores. Conclusion: The N140 potential is a valuable complement to the N20 for evaluating somatosensory dysfunction in SCA2 for two reasons: (1) the relative ease of recording N140 presents a practical benefit, and (2) more importantly, N140 probe the function of higher-order cortical networks involved in the cognitive processing of somatosensory information—networks not assessed by N20. The prolonged N140 latency observed here likely reflects the defective thalamo-cortical and cerebello-cortical interactions that are characteristic of SCA2 pathology.