This protocol allows for the neurophysiologic assessment of the lower extremity corticospinal tract post-stroke when gait and balance are of primary interest. The main advantage of this technique is that it allows an increased probability of eliciting a motor evoked potential in a population where many individuals do not show a resting response. This method can be used in any neurological population where investigating the neurophysiology of the corticospinal tract is of interest, especially as it relates to gait and balance.
Begin by visually inspecting the signal quality once the electrodes are attached. Then wrap the shanks with an elastic bandage to minimize any movement of the electrodes and the resultant artifact during testing. Open the data collection software and start a new trial to calibrate the dual top force plate.
Click start and begin an FP0 trial. Collect three to five seconds of data with no load on the force plate and then click stop. Once the force plate is calibrated, the participant has been registered to the image guidance system.
And the sEMG electrodes have been placed and tested for the signal quality, then fit them with a safety harness. Instruct the participant to step onto the force plate and standardize their foot placement with masking tape pre-applied to the force plate to signify the foremost position of the foot and medial edges of the feet equal distances from the midline. Attach the participant's safety harness to the ceiling support.
Place a rollator or similar device around the force plate to provide participants with something to steady themselves with during testing if needed. Measure and collect the participant's weight as they stand on the force plate by clicking start and selecting an FP static trial. Record two to five seconds worth of data and click stop to end the trial.
And standing on the force plates ensure that the data collection software displays two bar graphs, representing the weight or force under each of the participants feet. When the participant shifts their weight to one side, the bar graphs will change in height. If a participant unloads the weight on their legs to their arms, ensure the bar graph displayed changes color.
After a participant becomes comfortable standing with equal weight distributed between their legs, measuring the CMR can commence. Begin the neurophysiological assessments by identifying a stimulator intensity that produces consistent motor evoked potentials or MEPs. That is EMG signal amplitude greater than 50 microvolts in a visible cortical silent period in the active muscles in the target tibialis anterior and soleus muscle.
Test the periodic limb first by applying transcranial magnetic stimulation or TMS pulses to the lesion hemisphere. Begin by setting the TMS stimulator power level to 50%maximum stimulator output or a percent MSO by turning the output control knob. Apply a single pulse at 50%MSO to the middle grid point located just lateral to the longitudinal fissure by pressing the trigger button on the stimulator.
Applied two to three pulses with an inter stimulus interval of 5 to 10 seconds. If responses are not seen in the tibialis anterior and soleus, increase the stimulator power by 10%MSO by turning the output control knob and deliver two to three TMS pulses. If no responses are seen after increasing the stimulator to 60%MSO, again, increase the power by 10%MSO.
If no MEPs are elicited at 70%MSO, randomly select several grid points and apply TMS pulses to determine whether there is a response at the current power setting. If no responses are recorded at any grid point at the current 70%MSO, return to the initial target grid point. And continue to increase the stimulator power by increments of 10%MSO and apply two to three stimulations as previously described.
Once the stimulator power that produces a consistent response has been identified, begin identifying the hotspot. That is the scalp location that produces the largest response to the applied TMS pulses. Start a new hotspot trial by clicking start and selecting hotspot.
Apply a single pulse stimulation to each of the 15 grid points at the super threshold power level identified in the previous steps. Using the image guidance system, move the coil to the first grid point. Once the coil is in the proper position, apply the TMS pulse by pressing the trigger button on the stimulator unit.
Next, move the coil to the next grid location and apply another single TMS pulse. Continue until a single stimulation has been applied to each grid point and click on stop to end the trial. Examine the amplitudes of the sEMG signals recorded at each grid point.
Visually identify the grid points with the largest MEP amplitude recorded in the sEMG signals for each targeted muscle. The grid locations with the largest MEP amplitudes are the hotspots and will be used to measure the cortico motor response. Then to determine the motor threshold of the targeted muscle, use the simple adaptive parameter estimation by sequential testing or PEST.
Open the PEST program and set the initial stimulator intensity to the super threshold value used to identify the hotspot by typing the value into the box. Begin a new PEST trial clicking the start tab in the data collection software and select PEST. Using the initial percent MSO intensity displayed in the PEST program, apply a single TMS pulse to the identified target muscles hotspot.
Indicate in the PEST program that a response was observed in the muscles sEMG signal by typing Y or N.The PEST program will automatically calculate the next stimulation intensity. Adjust the stimulators power level to match the PEST program and apply another single TMS pulse. Continue this process until the PEST program determines the motor threshold, indicated by a change in color of the stimulation intensity.
And end the data collection trial by clicking on the stop tab. After the target muscles hotspot and motor threshold have been identified, begin the CMR evaluation. Set the intensity of the stimulator to 120%of the determined motor threshold.
Initiate a new trial and the data collection software by clicking on the start tab and select an MEP trial. Allow for 5 to 10 seconds between each stimulation. Record the evoked sEMG responses for offline analysis.
Allow the participant to rest ad libitum and for sufficient time between testing procedures to reduce the likelihood of the participant developing fatigue, affecting the results. Click on the stop tab after recording the MEPs to end the trial. Motor thresholds were measured in four separate muscles, mainly the paretic and non-paretic tibialis anterior and soleus muscles.
When individuals were presented with measurable motor thresholds in the sitting and standing positions, the measured thresholds in the standing position were found to be lower. When applying TMS pulses ensure the participant is distributing their weight on their legs equally. And that their weight is not being unloaded from their legs onto the arms.
In addition to single pulse TMS measures, paired pulse TMS can be performed in the standing position to assess intracortical inhibition and facilitation of the corticospinal tract.
