Challenging MEP Identification
Purpose
To provide information on how to identify MEPs in EMG traces and how to determine MEP status in challenging scenarios. Simple scenarios can be found here.
Examples of challenging MEP identification
Here are some examples that classify patients as MEP+ or MEP- based on their EMG traces that are complicated by delayed latency and/or the presence of biological and non-biological factors (see EMG page for more information). Traces should be considered representative of all EMG traces for the patient.
A: Patient (5ft 5”) performing active facilitation, 100% MSO
A: A MEP-like response is identified in the ECR EMG trace, but it occurs very late at ~70 ms. No MEP is identified in the FDI trace. Based on these traces the patient can be classified as MEP- as the ECR latency is not appropriate.
B: Patient (5ft 8”) performing active facilitation, 100% MSO
B: This patient had no MEPs at 100% MSO while at rest and is attempting active bilateral facilitation to increase the likelihood of eliciting a MEP. Evidence of low-level muscle activity is seen in the ECR trace. No MEP is identified in the ECR post-trigger trace. No MEP is identified in the FDI EMG trace. This patient can be classified as MEP- as no MEPs are identified in either muscle despite all efforts to elicit one.
C: Patient (5ft 7”) performing active facilitation, 100% MSO
C: This patient had no MEPs at 100% MSO while at rest and is attempting active bilateral facilitation to increase the likelihood of eliciting a MEP. Evidence of voluntary contraction is seen in both traces. A clear MEP is identified in the FDI trace at an appropriate latency (~20ms). There is no clear ECR MEP. This patient can be classified as MEP+ based on the presence of FDI MEPs.
D: Patient (5ft 6”) performing active facilitation, 100% MSO
D: This patient had no MEPs at 100% MSO while at rest and is attempting active bilateral facilitation to increase the likelihood of eliciting a MEP. Evidence of voluntary contraction is seen in ECR. A MEP is identified in the ECR EMG trace at an appropriate latency (~20ms), with EMG activity returning to that seen pre-trigger soon after. There is an FDI MEP that is just slightly later than the typical expected latency. This patient can be classified as MEP+, based on the presence of ECR MEPs.
If ECR MEPs were absent and FDI MEPs were present at about this same latency (~ 34 ms) the patient may be appropriate for a ‘guarded’ Good prediction. However it would be important to check MEP size and shape across other traces to rule out motor unit firing.
E: Patient (5ft 6”) at rest, 75% MSO
E: The FDI EMG trace (bottom) is clearly contaminated by electrical noise. However, a MEP is identifiable in the ECR EMG trace at an appropriate latency. This patient can be classified as MEP+, based on the presence of ECR MEPs. The EMG set-up for FDI should be checked for issues such as a loose cable, poor electrode conduction due to lack of contact with skin, or a faulty electrode.
F: Patient (5ft 9”) at rest, 67% MSO
F: A MEP is clearly identifiable in the FDI EMG trace at an appropriate latency. A smaller MEP is identified in the ECR EMG trace at an appropriate latency. The other fluctuations seen in both EMG traces are not MEPs. These are motor units firing sporadically. They can be identified by their consistent size and shape, and appearance at latencies which do not correspond with the expected latency for ECR or FDI. In both traces the motor units also fire prior to delivery of the TMS stimulus. This patient can be classified as MEP+, based on the presence of ECR or FDI MEPs.
G: Patient (5ft 7”) at rest, 77% MSO
G: It is difficult to determine whether there is a MEP present in the ECR EMG trace, due to the elongated stimulus artefact and downward drifting signal. The EMG set-up for ECR should be checked for issues such as a loose cable, poor electrode conduction due to lack of contact with skin, or a faulty electrode. A response is identified in the FDI EMG trace at an appropriate latency for a MEP. In addition, a similar-shaped small response is identified later in the trace (~ 75 ms) which may be a motor unit firing. Before classifying based on information from the FDI trace alone, it would be good to see whether other traces also have a response at 20 ms.
H: Patient (5ft 8”) at rest, 50% MSO
H: The above figure displays a series of ECR EMG traces, each of which is the result of a single stimulation. An ECR MEP is present in the first two and last two traces, and these can be seen in the green circles. You can be confident these are MEPs because their shape is similar and they all have a latency of ~18 ms, which is within the expected window for ECR MEPs. Throughout the traces there are a high number of motor units firing, some of which are highlighted in red circles. Although these responses vary in size, their consistent shape and appearance at latencies that don’t correspond to the expected ECR MEP latencies indicates they are motor units firing sporadically. There are many other motor units firing in these traces that haven’t been circled. The two responses in yellow are within the expected window for ECR MEP latencies, however the fact their shape is so similar to the motor units firing, and so dissimilar to the MEPs present in the first and last two traces, indicates they are likely also motor units firing sporadically.
I: Patient (6ft 1”) at rest, 80% MSO
I: The above image shows a series of FDI traces, and 4 of them have deflections at ~25 ms. The latency, shape, and amplitude of these deflections is nearly identical which suggests they may be motor units firing. However, these are likely to be MEPs rather than motor units firing for several reasons. First, they appear at ~25ms which is within the expected latency for FDI MEPs. Second, if the large deflections at ~25 ms were motor units we’d expect to see deflections of similar shape and amplitude appear at different latencies between traces but they are all consistent at ~25 ms. The small deflections at ~42 ms, ~60 ms, and ~75 ms are likely to be motor units though given their latencies. In order to be certain the deflections at ~25 ms were MEPs it’d be useful to reposition the participant’s hand and arm and see if the same responses appear, as well as increase the stimulus intensity by 10% to see if the shape or amplitude of the responses changes.
J: Patient (6ft 6”) at rest, 50% MSO
J: The above figure shows a series of FDI traces with severe electrical noise present. Looking at any of these traces individually, especially traces such as 21 and 29, may lead you to think no MEPs are present. However it’s only once viewing all these traces together you can see the very consistent nature of the deflections at ~38 ms which indicates this patient is MEP+. Even the upwards deflection in trace 21 at ~38ms is sharper than the surrounding electrical noise. Although 38 ms is reasonably late compared to the expected FDI latency range of ~20-30 ms, MEP latency can be delayed in stroke and depends on the person’s height so this is not an unreasonable latency.
MEPs typically become larger with increasing stimulus intensity so a good idea for subject J would be to increase the stimulus intensity another 10% to see if the MEPs become larger and more obvious amongst the electrical noise.
FAQs
I’ve finished the TMS test and I’m having trouble interpreting MEP status. I’m concerned about giving the patient an incorrect prediction. What should I do?
Take your time. If there is any difficulty identifying MEP status then it is best to wait until you are sure of the prediction before delivering it to the patient. This may mean letting the patient and their family know that you will return later to share the prediction. Reviewing and discussing the EMG traces with colleagues who are trained in TMS can help you determine MEP status. In addition, reviewing the MEP examples on this page or in the MEP identification quizzes below may also help your decision making. The TMS experts who train your staff may also be able to support you.
What should I do if there is a MEP in only one muscle, but the latency is a little later than I expect?
It is important to determine if a similar response is seen in other EMG traces at approximately the same latency. If this is the case, and the latency is only slightly outside the typical range then the patient may be classified as MEP+. Delivering a ‘guarded’ Good prediction may be most appropriate. If response latencies vary across traces this may indicate a random motor unit is firing, and that some responses are landing near the typical latency window by chance. Reviewing the shape of the response may help too, as responses due to motor unit firing tend to be very consistent in size and shape.
Quizzes
Click below for practice quiz to assess your learning on challenging MEP identification.
Once the practice quiz has been completed with at least 70% correct you will be emailed the link to the final quiz for the challenging MEP identification module.
If you pass the final quiz with at least 80% correct you will be emailed a certificate of completion. There is no limit to the number of attempts for the practice or final quizzes.
MEP Identification: Challenging