Alternative Stimulation Intensities for Mapping Cortical Motor Area with Navigated TMS

Area of Science:

• Neuroscience
• Neurosurgery
• Medical Imaging

Background:

• Navigated transcranial magnetic stimulation (nTMS) is increasingly used for pre-operative mapping of functional motor areas.
• Current practice often uses suprathreshold stimulation intensities relative to the patient's resting motor threshold (rMT), but lacks consensus on optimal intensity selection.
• Variability in stimulation intensity selection can influence the accuracy and reproducibility of motor mapping.

Purpose of the Study:

• To compare the efficacy of different stimulation intensities for nTMS motor mapping.
• To investigate the impact of using an upper threshold (UT) versus fixed percentages of rMT on motor representation size and variability.
• To determine the optimal stimulation intensity for reliable pre-surgical motor area delineation.

Main Methods:

• The study involved 12 healthy volunteers undergoing nTMS mapping of their left motor cortices.
• Motor evoked potentials were recorded using stimulation intensities of 110% rMT, 120% rMT, and an upper threshold (UT) derived from the Mills-Nithi algorithm.
• Representation sizes and their inter-individual variance were analyzed across the different stimulation intensity conditions.

Main Results:

• The upper threshold (UT) was significantly lower than 120% rMT but not significantly different from 110% rMT.
• Motor representation areas were significantly smaller with 120% rMT compared to both UT and 110% rMT.
• The variance in representation size was lowest with UT, suggesting reduced inter-individual variability.

Conclusions:

• The upper threshold (UT) appears to be a suitable stimulation intensity for nTMS motor mapping.
• Utilizing UT may provide more consistent and reliable delineation of functional motor areas compared to fixed suprathreshold intensities.
• This approach could minimize inter-individual variations in motor mapping, benefiting pre-surgical planning and radiosurgery.