Proceedings of The Physiological Society

University of Oxford (2011) Proc Physiol Soc 23, PC296

Poster Communications

The effect of coil type and navigation on the reliability of transcranial magnetic stimulation

M. K. Fleming1, I. O. Sorinola2, S. F. Roberts-Lewis1, D. J. Newham1, J. H. Bergmann3,1

1. Centre of Human & Aerospace Physiological Sciences, King's College London, London, United Kingdom. 2. Division of Health and Social Care Research, King's College London, London, United Kingdom. 3. Medical Engineering Solutions in Osteoarthritis Centre of Excellence, Imperial College London, London, United Kingdom.


Transcranial magnetic stimulation (TMS) is a widely used non-invasive technique for assessing neural mechanisms underlying motor control and recovery from neurological dysfunction. Temporal stability of measurements is important since parameters such as resting motor threshold (RMT), motor evoked potential (MEP) amplitude, short latency intracortical inhibition (SICI) and facilitation (ICF) are often assessed at multiple time-points. The aim of this study was to investigate reliability of TMS parameters for three coil systems: hand-held circular and figure-of-eight coils and a novel, low cost navigated figure-of-eight coil. Stimulus response (SR) curves, SICI and ICF were studied in the right first dorsal interosseus muscle of 10 healthy human adults after obtaining written informed consent. Each coil system was tested twice per subject over three sessions. SR curves were constructed by delivering 10 stimuli at each of 5 stimulation intensities (90,100,110,120 and 130% RMT). MEP sum was determined by summing the average MEP amplitudes obtained from each of the stimulation intensities. SICI and ICF were investigated by delivering a sub-threshold conditioning stimulus 2.5 ms (SICI) or 12.5 ms (ICF) prior to the supra-threshold test stimulus (Kujirai et al., 1993) and expressing the conditioned MEP amplitude as a percentage of the non-conditioned MEP amplitude. Reliability was assessed using Bland and Altman analyses (Bland and Altman, 1986) and intraclass correlation coefficients (ICCs). Differences between coils were assessed using repeated measures analysis of variance (ANOVA). Cortical excitability, assessed using MEP amplitude at 120% RMT (MEP120) and MEP sum, showed moderate to good reliability for the hand-held and navigated figure-of-eight coils (ICCs 0.55-0.81), but poor reliability for the circular coil (ICCs 0.09 & 0.48). Reliability for SICI was good for all coil systems when an outlier was removed (ICCs 0.87-0.93), but poor for ICF (ICCs < 0.3). The circular coil had a higher RMT than the hand-held figure-of-eight coil (paired t-test: p = 0.016) and a higher MEP120 than the navigated figure-of-eight coil (paired t-test: p = 0.004). These results demonstrate that figure-of-eight coils can be used confidently to investigate cortical excitability over time. ICF measurements should be interpreted with caution. The low cost, easy to use navigation device enables tracking of the position of the coil and subject in real time and frees the experimenter without compromising reliability. The results help guide the choice of coil system for longitudinal measurements of motor cortex function.

Where applicable, experiments conform with Society ethical requirements