Simple MEP Identification
Purpose
To provide information on how to identify MEPs in EMG traces and how to determine MEP status in simple scenarios. More challenging scenarios can be found here.
Overview
A patient is classified as MEP- if a MEP cannot be elicited at 100% MSO at rest and while attempting voluntary facilitation. All efforts need to be made to elicit a MEP if possible by using stimulation at 100% MSO, systematic coil positioning, and active bilateral facilitation.
A patient is classified as MEP+ if MEPs of any amplitude are observed in either ECR or FDI at an appropriate latency. This can be either at rest or during voluntary facilitation.
Check the following before classifying a patient as MEP+.
MEP latency
The latency of the MEP is the time between the magnetic stimulation and the onset of the MEP in the target muscle. The latency window for ECR is typically 15 – 25 ms and FDI latencies are typically 20 – 30 ms. These latency windows account for the effects of stroke. Latencies are also affected by the patient’s height. People who are below average height have latencies nearer the beginning of the window, and people who are above average height have latencies nearer the end of the window.
If MEPs are seen at appropriate latencies in either ECR or FDI then classify as MEP+ and provide a Good prediction.
If MEPs are present at a consistent latency but delayed by up to 10 ms:
- Consider the patient’s height, as described above.
- The patient may be appropriate for a ‘guarded’ Good prediction, if MEPs are seen just later than the typical latency window and height does not appear to be a factor. The prediction is ‘guarded’ because delayed MEPs might indicate more significant damage to the corticomotor pathways, and although a message can still get through to the hand and arm it may be more difficult for them to use their hand and arm in daily activities 3 months after stroke. So their Good outcome could be at the lower end of the range of ARAT scores for this category, or it could take them longer to reach their predicted outcome. In this situation, it’s still very important to focus on providing upper limb rehabilitation that maximises the patient’s function. The rehabilitation focus is the same for Good and ‘guarded’ Good
If responses are seen at random times across the patient’s EMG traces then this is indicative of sporadic motor unit firing. Motor units firing can be identified due to their consistent size and shape in EMG traces. While the shape can sometimes resemble an MEP, they usually appear at latencies which do not correspond with the expected MEP latency. If you suspect motor unit responses rather than MEPs are present then a patient needs to be classified as MEP-, providing all efforts have been made to elicit a MEP if possible. Examples of EMG traces with motor units firing can be found in the EMG Technique module.
MEP consistency
Patients can be classified as MEP+ based of a minimum of 1 MEP present within the appropriate window of an EMG trace. If a MEP is detected it is useful to stimulate the same coil position several times to see if MEPs are consistently elicited. No more than 5 MEPs are needed, and after this the TMS session can be concluded. The amplitude of MEPs will likely vary between traces and this is not a factor when determining MEP status.
Examples of simple MEP identification
Here are some examples that classify patients as MEP+ or MEP- based on the presence of a MEP in one or both muscle traces at an appropriate latency. Traces should be considered representative of all EMG traces for the patient.
A: Patient (5ft 8”) at rest, 73% MSO
A: A MEP is identifiable in both ECR (top trace) and FDI (bottom trace). The FDI MEP latency of 25 ms is slightly longer than ECR, as expected. This patient can be classified as MEP+.
B: Patient (5ft 4”) at rest, 87% MSO
B: A MEP is identifiable at an appropriate latency of ~18 ms in the ECR EMG trace. No MEP is identified in the FDI trace. This patient can be considered MEP+, as MEPs in either ECR or FDI muscles allow a MEP+ classification.
C: Patient (6ft 0”) 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. No MEPs are identified in either FDI or ECR EMG traces. The traces do not show muscle activity during facilitation due to severe paresis in the tested arm. This patient can be classified as MEP-.
D: Patient (5ft 6”) at rest, 62% MSO
D: A MEP is identifiable in both ECR and FDI EMG traces. There is a clear large ECR MEP at an appropriate latency. The FDI MEP occurs at an appropriate latency and is small, although note that MEP amplitude is not a determining factor for MEP status. This patient can be classified as MEP+. It is worth noting that they would be considered MEP+ based on FDI MEPs alone, if ECR MEPs were not present.
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 quizzes to assess your learning on simple 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 Simple 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.