Cortical excitability threshold for distal limb muscles in transcranial magnetic stimulation. Method - Normative values

Abstract

Transcranial magnetic stimulation was carried out in 30 healthy subjects, aged 20 to 55 and 156 to 180 cm tall, in whom cortical excitability threshold was determined for musculus (m.) abductor digiti minimi (ADM) and m. extensor digitorum brevis (EDB). The lowest stimulus intensity causing in the muscle a reproducible potential of an amplitude of 0.1 mV was adopted as the threshold value. The intensity of the stimulus was noted as a percentage of maximum stimulator output, defined as 100% (Magstim 200). In the stimulation of cortical areas for ADM, a circular coil (2.0T-530 V/m) was used while the in case of EBD, a double-cone coil (1.4T - 660 V/m) was used. The threshold was noted while the muscle was relaxed and during weak voluntary contraction. The application of the double-cone coil allowed us to obtain for foot muscles similar threshold values as for hand muscles with a circular coil. It was confirmed that the threshold declines with voluntary contraction of the muscles under investigation. Normative values were prepared, with the norm, limits of the norm, outlier values and pathological markers being distinguished.