Normative data of cortical excitability measurements obtained by transcranial magnetic stimulation in healthy subjects

Introduction: The assessment of cortical excitability (CE) measurements has been increasingly used in neuropsychiatric research. However, there is scant information on the normative values of these measurements, as well as the possible effect of hemisphere laterality, gender and age on these variables. Objectives: To obtain normative data for CE measurements by transcranial magnetic stimulation, to assess inter-/intra-investigator variability and the influence of sex, age and oral contraception use. Methods: A sample of 216 healthy volunteers matched according to age and gender was evaluated. Bilateral rest motor thresholds, motor evoked potentials (MEP), intracortical inhibition and facilitation were measured in the first dorsal interosseous muscle area representation of the primary motor cortex with a circular transcranial magnetic stimulation coil delivering biphasic pulses. Normative data were obtained for 200 participants (in a 1:1 male:female ratio) in a balanced proportion between five age groups (18-30; 31-40; 41-50; 51-60; > 60 years). Inter/intra-investigator variability was assessed in 20 healthy volunteers in two sessions performed within a 30-minute interval. Measurements were also performed in a subgroup of 16 healthy female volunteers, using oral contraception and during the menstrual phase. Results: Age had a dichotomous effect on CE measurements, providing significantly different normative data for subjects < 50 and > 50 years old, with smaller MEP's and intracortical inhibition in older individuals. There were no differences between genders or between left and right hemispheres. Also, CE parameters did not significantly differ with use of contraceptive treatment compared to the menstrual phase of the cycle. The inter-/intra-investigator reliability assessment showed some variability that may not be clinically significant. Conclusions: Age had a non-linear effect on CE. There were non-significant differences between genders, hemispheres or with use of oral contraceptives. There was good inter-/intra-investigator correlation for rest motor thresholds and motor evoked potentials while intracortical inhibition and facilitation had low correlations but acceptable reliability.