Impact of physical variables on motor evoked potentials: a TMS study from a large Italian cohort

Objective 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 in relatively small sample size have been reported (1). We investigated the effect of age, height, and sex on MEPs of the motor cortex and spinal roots on a large cohort. Material and 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 MEP cortical latency and the peripheral motor conduction time (PMCT) by cervical or lumbar magnetic stimulation. Peak-to-peak MEP amplitude to cortical stimulation and right-to-left difference of each parameter were also measured (2). Results After Bonferroni correction, general linear regression analysis showed that both MEP latency and PMCT at four limbs positively correlated with age and height. At lower limbs, a correlation of the same measures was also observed with sex (lower values in women). CMCT and side-to-side differences at four limbs did not significantly correlate with any physical variable. Discussion/Conclusions Physical variables need to be considered for a more accurate MEPs comparison and meaningful interpretation. Both in clinical practice and research setting, patients and controls should be matched for age, height, and sex. Notably, CMCT was not influenced by any factor here considered, thus representing a stable and reliable index of central motor conductivity.