Reliability of intracortical inhibiton measured using threshold tracking technique

Background. Paired-pulse transcranial magnetic stimulation allows assessing intracortical inhibition. However, a high variability of motor evoked potential (MEP) amplitude is a limitation of its use. Therefore, a new threshold tracking technique became of particular interest, which is based on the measurement not of the amplitude, but of the change of test stimulus intensity required to induce a MEP of a given amplitude.
Aim. The assessment of absolute and relative reliability of short-interval intracortical inhibition (SICI) using threshold tracking technique in healthy volunteers.
Materials and methods. All healthy volunteers included into the study (n = 12) underwent diagnostic paired-pulse transcranial magnetic stimulation in two consecutive days. The procedure included registration of passive motor threshold; the registration of 30 MEPs with supra-threshold intensity and determination of SICI using threshold tracking technique. At the first day the procedure was performed twice (T1, T2), at the second day – once (T3). Standard error of the measurement (SEM) and SEM% were calculated to assess absolute reliability, and intra-class correlation coefficient – for the assessment of relative reliability.
Results. A good or excellent relative reliability were observed for SICI averaged at intervals 1.0–3.0 ms and 1.0–7.0 ms when assessed within a day or at different days. Relative reliability of SICI at separate interstimulus intervals varied in a wide range. SEM% was more than 10 % both for averaged SICI and SICI at all interstimulus intervals. Motor threshold had excellent reliability both assessed within a day and at different days, and low SEM values (5.6 % for Т1–Т2 and 4.39 % for Т1–Т3). High SEM and SEM% were observed for average MEP amplitude, it also had a moderate relative reliability when assessed within a day and a poor one – at different days.
Conclusion. Calculation of averaged SICI can be recommended in further studies because of its high reliability values.

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