The estimation of short intra-cortical inhibition depends on the proportion of spinal motoneurones activated by corticospinal inputs

Abstract

Objective: The high variability of SICI limits its utility and by extension that of TMS in clinical neurophysiology. Non-linear summation of descending volleys due to heterogeneous motoneurone properties, on which MEP size depends, has not previously been implicated as an issue in SICI evaluation.

Methods: MEP size and SICI were normalised to the test MEP (mV), and as a percentage of M(max) to take account of the proportion of motoneurone pool activated by TMS. Two EMG systems, producing large and small MEPs, were used to determine how the normalisation affected MEPs of different amplitude.

Results: M(max) normalisation (i) counteracted the influence of recording conditions on the MEP size, (ii) revealed a significant influence of the test size on SICI (between medium and large MEPs), and of test size on the conditioning intensity (the larger the MEP, the stronger the SICI), and (iii) decreased the variability.

Conclusions: Data normalised to M(max) better reflect the motoneurone recruitment after SICI. To enhance reproducibility, MEP should be normalised to M(max). This adjusts for some of the non-linear properties at the spinal, and possibly, at cortical levels.

Significance: To reduce variability is important because TMS is becoming widely adopted and is being used in patients.