Threshold curves for transcranial magnetic stimulation to improve reliability of motor pathway status assessment

Abstract

Objective To provide a new protocol for a simple determination of resting motor threshold (MT) and assessment of excitation-inhibition balance in motor cortex and pathways. Methods Navigated TMS was used to map cortical representation area of the FDI muscle bilaterally in ten healthy subjects. Reference MTs were determined using a threshold hunting paradigm. Subsequently, a novel stimulation protocol was applied which included 70 stimuli (7 intensities, sub- and suprathreshold). The “MT-curve” was constructed by computing the MTs with several threshold amplitudes with the novel protocol. The measurements were repeated. Sensitivity of the MT-curve to stimulus location was also tested. Results The reference MTs agreed with those determined with the novel protocol ( R = 0.96–0.99, p < 0.001). Based on coefficient of repeatability derived from non-parametric one-way ANOVA, the repeatability was good ( ρ AO = 0.929, p < 0.05). Generally, the mean difference between the repeated MT-curves was <3% of the maximum stimulator output. Coil movement 10 mm medially from the optimal stimulus location increased that difference to >7%. Conclusions The MTs derived using the MT-curve protocol concurred with the reference MTs. The MT-curve is highly reproducible and sensitive to the exact cortical location of stimulation. Significance The MT-curves provide a simple way to assess motor pathway status using a single stimulation train. This may be useful in the follow-up and monitoring of motor pathway recovery e.g. from stroke or trauma.