Bilateral Assessment of the Corticospinal Pathways of the Ankle Muscles Using Navigated Transcranial Magnetic Stimulation

This protocol is critically important because it standardizes an approach with a very difficult task of detecting motor-evoked potentials in the distal lower extremity muscles. The main advantage of this technique is that it consolidates the literature associated with lower extremity stimulation, which often focuses only on the tibialis anterior and uses a variety of techniques. We've studied this technique in neurologically healthy individuals as well as those with stroke.

However, the importance of the technique is in standardizing lower extremity assessments. The technique allows for the opportunity to identify the gaps in our understanding of motor control during gait and revisits research that did not allow for the control of interfering factors. Demonstrating the procedure will be John Kindred, PhD, a postdoctoral scholar, and Brian Cence, a research coordinator, both from our research laboratory.

Begin by uploading the subject's MRI files into a neuronavigation system. Then, manually co-register the MRI to anterior and posterior commissures, so that the Montreal Neurological Institute Atlas can be used. Next, reconstruct the skin and full curvilinear brain model by adjusting the bounding box around the skull and brain tissues.

Identify four anatomical landmarks using the skin model, including the tip of the nose, nasion, and supratragic notch of the right and left ear. Now, place a rectangular grid over the leg motor area in each hemisphere. Position the centered row of the grid at the center and over the gyrus of the leg motor cortical area.

Then, position the medial column of the grid parallel and adjacent to the medial wall of the ipsilateral hemisphere. This grid will be used to find the hot spot. For motor mapping, use larger grids, either by adding more spots or increasing the distance between spots as needed.

Perform placements of electrodes while the subject is in a standing position. First, prepare the areas where each electrode will be placed by first shaving and then lightly exfoliating any dead skin cells and oils using alcohol swabs. Next, ask the subject to lift their toes upwards and then place the electrode at the upper third of the line between the head of the fibula and medial malleolus.

Do this bilaterally for placement on the tibialis anterior. Now, attach electrodes bilaterally on the lateral soleus. Ask the subject to raise their heel and then place the electrode at the lower third of the line between the lateral femoral condyle and lateral malleolus below the gastrocnemius muscle belly.

Also, attach the ground reference passive electrode either on the patella or lateral malleolus bilaterally or unilaterally, depending on the EMG acquisition unit in use. Test the electrode placement by asking the subject to either dorsiflex or plantarflex the ankle in an upright posture while displaying the raw EMG signal of all muscles tested on a computer screen. If an electrode is misplaced, remove and replace it until clear detectable EMG bursts with minimal background noise are seen.

Then, with the subject seated with their muscles at rest, test the signal quality by discharging a few TMS pulses while the coil is held away from the subject. The most crucial step in this technique is verifying that the EMG signal is as clear of noise as possible. Failure to do this adequately makes data analysis incredibly difficult.

Next, check that the baseline signal for each EMG channel is close to zero. If noise is present in a channel, remove the corresponding electrode and repeat the skin preparation procedures. If the noise is still present, adjust the reference electrode's position and replace the electrolyte gel.

Once a good signal is verified, wrap all electrodes using light foam pre-wrap tape to hold them in place and to reduce motion artifact from the EMG. Now, seat the subject in a chair and, to ensure consistent feet placement across subjects, secure both feet in walking boots that allow the ankle range of motion to be adjusted to a specific position and provide resistance during tonic voluntary action. Also, adjust both hip and knee angles to avoid subject discomfort and instruct the subject to remain still throughout the experiment.

Begin tonic voluntary activation testing by first determining the maximum voluntary isometric contraction of each muscle bilaterally. For each motion, instruct the subject to maximally contract the contralateral examined muscle four times. Next, verify the position of the motion capture camera by placing the subject tracker, pointer, and the coil tracker in its capture volume space.

Then, perform the subject image registration by placing the tip of the pointer on the four anatomical landmarks. Now, determine the hot spot of both muscles bilaterally. First, find the suprathreshold intensity by applying a single stimulus over the centered spot next to the interhemispheric fissure.

Next, start at low intensity and gradually increase the TMS intensity by five percent increments until reaching an intensity that elicits a motor-evoked potential with a peak to peak amplitude greater than 50 microvolts in all contralateral examined muscles or three consecutive stimuli and repeat for each muscle. Apply one TMS pulse on each spot of the grid. Then, transfer the amplitude values of each spot for all contralateral muscles in a spreadsheet and sort amplitude from high to low.

Identify the hot spot of the contralateral tibialis anterior and soleus muscles as a location in the grid with the largest amplitude and the shortest latency. Select the grid spot in the neuronavigation system that corresponds to one of the muscle's hot spots. Next, set the initial intensity and step size at 45 and six percent maximum stimulator output.

Then, use an adaptive threshold hunting method for resting motor threshold determination. Do this twice for each muscle and use the average for the sunsequent corticomotor response assessment. Now, to assess bilateral corticomotor response during rest, select the grid spot in the neuronavigation system that corresponds to the examined muscle's hot spot.

Prior to each stimulus, instruct the subject to stay still and relax the examined muscles bilaterally and monitor the activity of all muscles using a real time visual feedback. Apply ten single TMS pulses at 120%of resting motor threshold of the examined muscle. If any muscle is active before or after TMS, discard that trial and apply an additional single pulse.

Repeat this until 10 waveforms for each contralateral examined muscle at rest have been collected. Next, assess the corticomotor response during tonic voluntary activation bilaterally. Select the same grid spots in the neuronavigation system that were used during resting conditions.

Ask subjects to contract the examined muscle at approximately 15%maximum muscle activity value and apply 10 single TMS pulses at 120%of resting motor threshold. Ask them to keep the displayed smooth moving line of the examined muscle within the two horizontal cursors and to sustain that contraction at that level for a few seconds. When tibialis anterior is the examined muscle, ask subjects to pull slightly up against the boot straps for the leg contralateral to stimulated hemisphere.

When soleus is the examined muscle, ask subjects to push slightly down against the boot on the contralateral leg. Monitor the muscle activity of the active and resting muscles using real time visual feedback. If the examined muscle's activity is below or above the predetermined range, or if any other muscle is activated, discard the stimulus and apply an additional single pulse.

Collect 10 trials while the examined muscle is activated at the predetermined range. This figure shows bilateral tibialis anterior and soleus hot spots. Here, bar plots represent the mean resting motor threshold of two assessments for each muscle, while the values below denote the number of stimuli applied.

The dashed line indicates the intensity used for the hot spot hunting. This figure shows the bilateral responses of tibialis anterior and soleus when their hot spot was stimulated during rest. The bilateral EMG average waveform of each muscle is shown.

If a motor-evoked potential was present, the values of the peak to peak amplitude and latency are presented. Here, we see bilateral responses when the hot spots were stimulated during tonic voluntary action. The EMG of the bilateral muscles were collected while the examined contralateral muscle was activated at about 15%maximum voluntary isometric contraction.

Additional analysis can be conducted to help answer questions regarding the cortical spinal tract post-stroke since the data in the protocol are collected in agonist and antagonist muscle groups bilaterally. Research continues using this technique to determine the individual contributions of multiple factors to corticomotor responses in order to increase our overall understanding of the motor control of walking. It should be noted that there are risk factors due to the high magnetic field that should be accounted for by screening subjects prior to this procedure.