A Novel Scalp Acupuncture-based Method to Target the Hand Motor Hotspot for Non-invasive Brain Stimulation

I study non-invasive brain stimulation for motor recovery after stroke. I'm trying to understand its mechanism and improve its effectiveness. A key challenge is quickly and accurately localizing the optimal stimulation side when neuroimaging tools are not available.

To begin, ensure that the participant is seated upright in a comfortable position, with both arms resting naturally. Prepare a tape measure and a marker for the M1 flexor digitorum superficialis hotspot localization. Using a tape measure, align the zero mark with the glabella, which is the midpoint between the eyebrows.

Ensure that the tape is held straight and level across the scalp. Now, stretch the tape measure to reach the inion, which is the tip of the external occipital protuberance. Confirm that the tape is held firmly and straight between these two points.

Identify the midpoint between the glabella and the inion along the longitudinal line. From this midpoint, measure 0.5 centimeters posteriorly and mark the location with a visible dot using the marker. Next, align the tape measure between the left and right preauricular points, or the apex of the ears.

Ensure the tape is level and mark the midpoint of this transverse line with a visible dot. Using the marked midpoint as a reference, adjust the point identified earlier laterally if required to ensure that the final upper motor point is centered on the transverse midpoint. For the lower motor point, align the zero mark of the tape measure to the lateral end of the eyebrow on the left side of the face.

Stretch the tape from the lateral end of the eyebrow to the inion, maintaining a straight diagonal line across the scalp. Then, identify the point where the diagonal line intersects the anterior hairline and mark this intersection clearly with a visible dot, defining it as the lower motor point. Align the zero mark of the tape measure with the upper motor point and extend the tape measure in a straight line to reach the lower motor point.

Record the total distance between the upper motor point and the lower motor point. Calculate the point located at 2/5 of the measured distance from the upper motor point, and mark this point clearly with a visible dot, defining it as the AC hotspot. For validation, place bipolar surface electromyography electrodes over the belly of the right flexor digitorum superficialis muscle, and set the sampling frequency to two kilohertz on the recording system.

Identify and mark the C3 site according to the International 10-20 Electroencephalography System. After marking the FDS hotspot, position the transcranial magnetic stimulation coil tangentially over the scalp at the AC site, with the handle directed backward at a 45-degree angle. Set the device to begin at 30%of the maximum transcranial magnetic stimulation output and deliver a single pulse of stimulation.

Now, gradually increase the stimulation intensity to determine the resting motor threshold. Then, set the final stimulation intensity to 120%of the resting motor threshold and record a representative motor evoked potential at the AC site. Fit the electroencephalography cap according to the standard protocol, ensuring that it is properly aligned with the scalp landmarks.

Position one electrode directly over the target site, either the AC or C3 location. Place four reference electrodes evenly spaced around the target site to ensure stable current distribution, and apply conductive gel to all electrode sites to minimize impedance. Now, check the impedance of all electrodes using the stimulator's built-in impedance meter, ensuring that all values are below two kiloohms before proceeding.

Set the stimulation parameters on the stimulator to two milliamperes intensity and 21 hertz frequency for a total duration of 20 minutes. Then, turn on the high-definition transcranial electrical stimulation device, and gradually ramp the current intensity to two milliamperes over a period of 30 seconds. Continuously monitor the participants'reactions during the 20-minute stimulation session, and check for any signs of discomfort, skin irritation, or unusual sensations.

Finally, after 20 minutes, turn off the stimulation device. Carefully remove all electrodes from the scalp and clean the electrode sites thoroughly. The motor-evoked potential waveforms from the AC and C3 sites were obtained at baseline and after stimulation.

At baseline, the peak-to-peak amplitude at the AC site was greater than that at the C3 site. Following high-definition transcranial electrical stimulation, a significant increase in motor-evoked potential amplitude was observed at the AC site, whereas no change was detected at the C3 site. We proposed a new approach to quickly localize the hotspot site for forearm muscle neuromodulation.

Our protocol is based on empirical evidence, requires minimal equipment, and can be translated to clinical practice easily. My future research will focus on applying these protocols and findings to stroke upper limb motor recovery.