Determination of motor threshold using visual observation overestimates transcranial magnetic stimulation dosage: safety implications.

OBJECTIVE: While the standard has been to define motor threshold (MT) using EMG to measure motor cortex response to transcranial magnetic stimulation (TMS), another method of determining MT using visual observation of muscle twitch (OM-MT) has emerged in clinical and research use. We compared these two methods for determining MT. METHODS: Left motor cortex MTs were found in 20 healthy subjects. Employing the commonly-used relative frequency procedure and beginning from a clearly suprathreshold intensity, two raters used motor evoked potentials and finger movements respectively to determine EMG-MT and OM-MT. RESULTS: OM-MT was 11.3% higher than EMG-MT (p<0.001), ranging from 0% to 27.8%. In eight subjects, OM-MT was more than 10% higher than EMG-MT, with two greater than 25%. CONCLUSIONS: These findings suggest using OM yields significantly higher MTs than EMG, and may lead to unsafe TMS in some individuals. In more than half of the subjects in the present study, use of their OM-MT for typical rTMS treatment of depression would have resulted in stimulation beyond safety limits. SIGNIFICANCE: For applications that involve stimulation near established safety limits and in the presence of factors that could elevate risk such as concomitant medications, EMG-MT is advisable, given that safety guidelines for TMS parameters were based on EMG-MT.

Modulation of motor cortex excitability in obsessive-compulsive disorder: an exploratory study on the relations of neurophysiology measures with clinical outcome.

Low-frequency repetitive transcranial magnetic stimulation (rTMS) to supplementary motor area (SMA) showed clinical benefit in obsessive-compulsive disorder (OCD). Here we tested whether clinical improvement was associated with enhanced cortical inhibition as measured by single and paired-pulse TMS variables. In 18 OCD patients receiving 4 weeks of either active or sham rTMS in a double-blind randomized trial, we assessed bilateral resting and active motor thresholds (RMT and AMT), cortical silent period (CSP), short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF). We tested correlations between changes in Yale-Brown Obsessive Compulsive Scale-Self-report (Y-BOCS-SR), Clinical Global Impression-Severity subscale (CGI-S) and cortical excitability measures. Active rTMS increased right hemisphere RMT whose change correlated with Y-BOCS-SR improvement. Baseline RMT hemispheric asymmetry, defined as the difference between left and right hemispheres RMT, and its normalization after active rTMS correlated with Y-BOCS-SR and CGI-S improvements. Active rTMS also increased right hemisphere SICI whose change correlated with Y-BOCS-SR and CGI-S at week 4, and with normalization of baseline RMT hemispheric asymmetry. Treatment-induced changes in cortical excitability measures are consistent with an inhibitory action of SMA rTMS on dysfunctional motor circuits in OCD. Correlations of neurophysiology measures with therapeutic outcome are supportive of the role of SMA in the modulation of OCD symptoms.