Short interval intracortical facilitation: comparison between threshold-tracking and conventional amplitude recording methods.

OBJECTIVE

Among the cortical excitability measurements performed by transcranial magnetic stimulation, short-interval intracortical facilitation (SICF) is of clinical interest as a potential biomarker for cortical dysfunction in disorders such as Parkinson's disease. This study was undertaken to compare conventional amplitude measurements (A-SICF) with a published threshold-tracking alternative (T-SICF-1).

METHODS

A-SICF was measured as the increase in motor evoked potential (MEP) amplitude when a stimulus that evoked a 1 mV MEP was followed by a subthreshold conditioning stimulus. For T-SICF-1, facilitation was measured as the decrease in threshold for a 200 µV MEP when followed by a conditioning stimulus 95% of that threshold. A-SICF and T-SICF-1 were both recorded twice from 60 healthy volunteers, using 10 stimuli at each of 14 inter-stimulus intervals (ISI) from 1 to 4.9 ms. Twenty of the subjects were later tested with a new threshold-tracking protocol (T-SICF-2), with stimuli the same as A-SICF when there was no facilitation.

RESULTS

T-SICF-1 values were less variable than normalized A-SICF ones, but correlation between them over the range of ISIs was poor (R = -0.586). T-SICF-1 showed appreciable facilitation when A-SICF showed none. Correlation between T-SICF-2 and normalized A-SICF was much stronger (R = -0.896), but variability was no better.

CONCLUSIONS

Divergence of T-SICF-1 from A-SICF was attributed to the weaker test stimuli, stronger conditioning stimuli, and contamination by short-interval intracortical inhibition, so that unlike A-SICF and T-SICF-2, T-SICF-1 cannot be regarded as a good indicator of I-wave facilitation.

SIGNIFICANCE: T-SICF-1 is not recommended as an alternative to A-SICF.