The routine circular coil is reliable in paired-TMS studies.
OBJECTIVE
Motor cortex excitability can be measured by transcranial magnetic stimulation (TMS) using different coil types, but paired-TMS was originally devised with a figure-of-eight coil. We asked whether the most popular, circular coil was suited to the every-day assessment of cortical excitability, particularly paired-TMS indexes, and if it reduced the measurement error.


METHODS
We studied 12 right-handed, healthy subjects (34±7.6 years). Resting motor threshold (MT), cortical silent period (CSP), short-interval intracortical inhibition (SICI) at the 2, 3, 4 and 5 ms interstimulus intervals (ISIs) and intracortical facilitation (ICF) at the 14 and 16 ms ISIs were measured. Intrinsic variability of these indexes was evaluated in terms of Coefficients of Variation, to estimate the measurement error. This sequence was carried out both using a figure-of-eight coil over the hand motor area and a circular coil centred at the vertex. Testing was repeated 8-13 months later.


RESULTS
On average, MT, SICI and ICF did not show any statistically significant difference (p>0.05) when studied with the figure-of-eight as compared with the circular coil. CSP was significantly shorter (p=0.007) with the figure-of-eight coil. Using either coil did not affect measurement variability. There was no significant session-to-session group difference at any of the variables using either coil type.


CONCLUSIONS
Except for the CSP duration, the TMS testing and retesting of cortical excitability, particularly the paired-pulse indexes, did not vary significantly as a function of the coil used.


CONCLUSIONS
Routine circular coils can be used reliably in paired-TMS studies designed to measure longitudinal changes in cortical excitability though they do not reduce the measurement error.
Department of Neurology, Austin Health, Heidelberg, Victoria, Australia.
Radwa A B
RA
Badawy
Roberto
R
Tarletti
Marco
M
Mula
Claudia
C
Varrasi
Roberto
R
Cantello
D000328:Adult; D003627:Data Interpretation, Statistical; D004576:Electromyography; D004581:Electronics; D004867:Equipment Design; D019054:Evoked Potentials, Motor / Q000502:physiology; D005260:Female; D006225:Hand / Q000294:innervation; D006801:Humans; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000502:physiology*; D009434:Neural Pathways / Q000502:physiology; D015203:Reproducibility of Results; D050781:Transcranial Magnetic Stimulation / Q000295:instrumentation*