Measuring the Contralateral Silent Period Induced by Transcranial Magnetic Stimulation

Protokoll

All procedures involving human participants have been performed in compliance with the institutional, national, and international guidelines for human welfare and have been reviewed by the local institutional review board.

1. Experimental procedures (Figure 1)

1. Select the muscle for positioning the electrodes.
   1. Ask the subject to put their hand over the table, in a prone position. Select the FDI muscle, localized between the first and second metacarpal osseous. To identify the FDI, ask the subject to abduct their index finger against resistance, keeping the rest of the hand still and laying on the table, while you are palpating the area.
   2. Expose the selected area. Use a disposable razor to shave the area to improve electrode contact with the skin, if necessary, and clean the area with alcohol pads to remove skin oils and other factors that could increase impedance. Certify that there is free skin to ensure contact with the electrode.
      NOTE: If evaluating lower limb activity, use the tibialis anterior (TA) muscle for electrode placement. It is localized on the lateral side of the tibia and lies near the superficies of the skin. It can be identified by ankle dorsiflexion.
2. Place the surface electromyography (EMG) electrodes
   1. With the area exposed and cleaned, apply the conductive gel to each electrode of the channel to ensure good impedance.
   2. Place the negative electrode on the belly of the FDI muscle (the center or the most prominent bulge of the muscle belly) and the positive on the distal interphalangeal joint, with an inter-electrode distance of at least 1.5 cm. Place the reference electrode (neutral) on the wrist, over the ulnar styloid process.
      NOTE: The presence of motor endpoints, muscle tendons, or other active muscles can impact the stability of the recordings, so it is important to avoid these locations. For the TA muscle, the electrodes should be placed at one-third on the line that connects the tip of the fibula and the tip of the medial malleolus. Provide a 20 mm distance between each electrode's pole and place the reference electrode in the ankle.
3. Determine the required muscle contraction force
   1. Use a digital pinch dynamometer and a quadrangular pyramid support to minimize mechanical distortions and elevate the sensitivity for minimal contractions.
   2. Place the dynamometer between the first and second fingers with the help of the pyramidal support. Ensure that the third, fourth, and fifth fingers lay still on the table, while the 1st and 2nd generate the forces of the pinching movement.
   3. With the fixed position, ask the participant to press the dynamometer with the first finger and the side of the pyramid with the index finger, squeezing the dynamometer-pyramid system with their maximum force and creating a strong contraction of the FDI muscle.
   4. Using that value as reference, determine the 20% of maximum force. The participant must practice maintaining the target at 20% of sustained contraction. Allow for variations from 15%-25% of maximum voluntary contraction (MVC).
      NOTE: Alternatively, in case of unavailability of a dynamometer that can catch the isolated muscle activity being investigated, use EMG feedback to standardize force. The recording software will measure the maximum peak-to-peak amplitude that corresponds to the subject's maximum force, and using that value as reference, will determine the 20% MVC. Subjects can receive visual and/or auditory clues of when 20% is achieved.
4. Identification of the initial location for hotspot searching
   1. Put a swim cap on the subject's head. All the reference points will be marked on it.
   2. Measure the sagittal circumference of the head from the nasion (the point between the forehead and the nose) to the inion (the most prominent point in the occipital region). Divide that value by two and mark that middle spot on the head.
   3. Mark the location of the patient's nasion, inion, the helix of both right and left external ears, and right and left supraorbital ridge. This is to certify that the cap has not slipped during the procedure, and/or that in future experiments it will be equally positioned on the patient's head.
   4. As described above, measure the tragus-to-tragus distance and add a mark halfway. Mark the intersection between them, a point identified as the vertex (Cz).
   5. From the vertex, move 5 cm laterally in parallel to the midsagittal line, on the contralateral side to the selected muscle. This mark approximately identifies the (M1), on the same coronal level as the hand motor cortex. Use this as the first spot to initiate the search for the hotspot.
   6. The hotspot is the area of the motor cortex where the lowest motor threshold is detectable. Set up a low intensity (e.g., 30% of maximum stimulator output [MSO]) and initiate the search by delivering multiple pulses to the first spot.
   7. Pursue with small intensity increments until identifying the lowest stimulus that detects an EMG-indexed response (i.e., MEP). For the delivery of the stimuli, angle the figure-of-eight coil at 45° in relation to the midsagittal line with the handle pointed toward the posterior of the patient.
   8. To ensure that the best spot was identified, move around the first spot and test the subsequent ~3 MEPs at 1 cm anterior, 1 cm lateral, 1 cm medial, and 1 cm posterior to it. Repeat this procedure as many times as needed for a consistent response; stick to the spot that elicits the largest MEP.
   9. Once the hotspot is found, mark that spot in the patient's head (swim cap). Use this location during this experiment and the potential follow-up visits. Be cautious so as not to cause discomfort to the subject due to extra pressure. Use both hands to support the coil on the subject's head.
5. Determine resting motor threshold (RMT)
   1. Estimate the motor threshold as the minimum intensity required to promote an MEP of a minimal detectable amplitude (usually at least 50-100 µV).
   2. To determine the motor threshold, apply ten consecutive stimuli at the hotspot and select the lowest intensity that produced an MEP with a peak-to-peak amplitude of at least 50 µV on the target muscle, in 50% of the trials.
      NOTE: This protocol can be done with the target muscle at rest (RMT) or during active contraction (active motor threshold [AMT]). Both can further be used as references for the suprathreshold TMS pulses. The acquisition of the AMT is more prone to variability because it relies on the standardization of MVC, which can be an issue for longitudinal studies with multiple assessments.
6. CSP protocol
   1. Deliver suprathreshold stimuli to elicit MEPs during tonic voluntary contraction of the target muscle.
   2. Deliver 10 stimuli with the stimulation intensity (SI) of 120% of the RMT with 10 s period in between them. During the application of the stimuli, ask the patient to maintain 20% of the maximum motor contraction of the target muscle, as practiced with the dynamometer.
   3. To ensure capturing the whole SP, certify that the EMG time window is long enough to capture up to 400 ms of EMG activity. Not infrequently - depending on the disease being studied - subjects might require higher SIs for a successful cSP to be obtained.

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Ergebnisse

Figure 1: Experimental steps. 1. Electrode placement on the belly of the FDI muscle 2. Positioning of the dynamometer between fingers. 3. Voluntary contraction of the target muscle to test the standardization of 20% MVC 4. Head measurements and TMS pulses for identification of the hotspot and the RMT (lowest stimuli that elicits an MEP of at least 50 mV in five out of ten trials) 5. CSP protocol, consistent of 10 pulses with 120% RMT spaced out by 10s, during sustained muscle contraction. In the bottom center figure, the small red rectangle represents a single TMS pulse and divides pre-TMS stimulus (sustained muscle contraction and background EMG activity) and the cSP recording. CSP is considered from the start of the MEP until the reemergence of EMG baseline activity, represented inside the blue rectangle. In the yellow rectangle, the MEP latency is shown.

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Materialien

Name	Company	Catalog Number	Comments
Alcohol pads	Medline		Preparation with 70% isopropyl alcohol
Conductive gel	Weaver and Company		Used on the electrode
Echo Pinch	JTECH medical	0902A302	Digital dynamometer.
Mega-EMG	Soterix Medical	NS006201	Digital multiple channel EMG with built in software.
MEGA-TMS coil	Soterix Medical	NS063201	8 shaped TMS coil
Mega-TMS stimulator	Soterix Medical	6990061	Single Pulse TMS
Neuro-MEP.NET	Soterix Medical		EMG software used to analyse the muscles eletrical activity.
Swim cap	Kiefer