Measurement of active motor threshold using a dynamometer during navigated transcranial magnetic stimulation in a postoperative brain tumor patient: technical note

Abstract Background Navigated transcranial magnetic stimulation (nTMS) is being used for different purposes in patients with brain tumors. However, the procedure requires a positive electrophysiological response. For patients with negative response in rest conditions, active motor threshold (AMT) may be used. However, sometimes it is difficult to obtain AMT measures due to inability of the patient to sustain steady muscle contraction. Herein, we describe a simple method by using a hand dynamometer to obtain AMT measures during nTMS session. Case description A 68-year-old woman underwent total removal of a right frontal lobe oligodendroglioma World Health Organization grade II, 15 years ago. Cranial magnetic resonance imaging during follow-up revealed local recurrence. In the postoperative period, she developed left upper limb paresis. A postoperative nTMS session was performed for motor electrophysiological evaluation. However, using the standard technique for AMT measurement, the patient was unable to perform sustained muscle contraction as required. A hand dynamometer was used. It allowed sustained muscle contraction for AMT measurement. A counter force for the index finger flexion, the hand support to stabilize hand joints and a numerical screen serving for both the examiner and the patient as a feedback parameter may explain the success obtained with this simple device. Conclusion Although more studies are necessary to validate the method, the hand dynamometer should be considered for patients unable to sustain muscle contraction during AMT measurement.