Effects of age, height, and sex on motor evoked potentials to magnetic stimulation: data from a large Italian cohort in a real-world clinical setting

Introduction: Motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) are known to be susceptible to several sources of variability. However, conflicting evidences on individual characteristics have been reported. We investigated the effect of age, height, and sex on MEPs of the motor cortex and spinal roots. Methods: A total of 587 subjects without any clinical and neuroradiological motor impairment were included. MEPs were recorded during slight tonic contraction through a circular coil applied over the “hot spot” of the first dorsal interosseous and tibialis anterior muscles, bilaterally. Central motor conduction time (CMCT) was estimated as the difference between the cortico-muscular latency and the peripheral motor conduction time (PMCT) by cervical or lumbar magnetic stimulation. Peak-to-peak MEP amplitude to cortical stimulation and right-left difference of each TMS index were also measured. Results: Tables 1 and 2 summarize the demographic features and the descriptive statistics, respectively. After Bonferroni correction, linear regression analysis (Table 3) showed that both MEP latency and PMCT at four limbs positively correlated with age and height. At upper limbs, a negative correlation of the same measures was also observed with sex. CMCT at four limbs and side-to-side differences did not correlate with any physical variable. Conclusion: Physical variables need to be considered for a more accurate MEPs comparison and meaningful interpretation. Both in clinical practice and research settings, patients and controls should be matched for age, height, and sex. Notably, CMCT was not influenced by any variable here considered, thus representing a stable and reliable TMS index of central motor conductivity.