Comparison between conventional electrodes and ultrasound monitoring to measure TMS evoked muscle contraction

Abstract

BackgroundTranscranial Magnetic stimulation (TMS) is a non-invasive cortical stimulation method that has been widely employed to explore cortical physiology in health and a range of diseases. At the core of many TMS protocols is the measurement of evoked muscle contractions using surface electromyography (sEMG). While sEMG is appropriate for many superficial muscles such as abductor pollicis brevis (ABP) and first dorsal interosseous (FDI), there are situations where the study of less accessible muscles may be of interest. Peripheral ultrasound is a non-invasive method that could provide a solution. We explore the relationship between TMS evoked sMEP and TMS evoked muscle contractions measured with muscle ultrasound. We hypothesise that in a healthy population, we expect a positive correlation between EMG and ultrasound measures. MethodsIn 10 participants we performed a standard TMS recruitment curve and simultaneously measured MEP and peripheral muscle ultrasound (pUS). We targeted the following muscles: biceps (BI), first dorsal interosseous (FDI), tibialis anterior (TA) and the tongue (TO). ResultsWe report a very close relationship between the MEP and pUS contraction. Resting motor threshold (RMT) measurements and recruitment curves are consistent in sEMG and pUS. A key aspect of this work is the ability to examine clinically relevant muscles that are difficult to probe using surface EMG electrodes, such as the tongue. ConclusionWe find that TMS muscle contractions can be measured with muscle ultrasound in superficial and deep muscles, enable additional, previously hard to study muscles, to be investigated. This could be valuable for allowing TMS to be used to explore a new range of muscles in disorders such as ALS. In muscles less accessible by sEMG, such as the tongue, it may be possible to use pUS as an alternative output. This may be useful in conditions such as ALS and stroke that can differentially affect the tongue.