Re: Explanation of an EMG

Posted By aradia on October 08, 1998 at 18:08:40:

In Reply to: Re: Explanation of an EMG posted by CCF MD mdf on October 08, 1998 at 00:30:39:






Hi,
I am writing to see if you can explain the results of an emg.  Here are my questions:
1)  How does the amplitude count in grading the emg? ex: normal mean of 5, 10, 15, etc
in the areas of peroneal, posterior tibial, post tib f wave, and sural.  Is it
the higher the amplitude to stimulate better or the lower?
2)  How does conduction velocity play a role?
3)  How does distal latency in milliseconds play a role?
4)  How does distance in cm play a role in the emg grading?
5)  What are within normal values for the peroneal, posterior tibial, post tib f wave, an
sural for ea:  amplitude, conduction velocity, latency, and distance?
6)  Is it common not to print the actual emg, as you would an ekg to keep in
the file to back up the report?
I want to thank you in advance for answering these questions for me.  They will
give me a better understanding of the test.
Aradia
=
1. By amplitude, it is not clear if you mean the amplitude of the motor unit potential (MUP) on the needle exam or the compound motor action potential (CMAP) or sensory nerve action potentials (SNAP) obtained on the nerve conduction studies. You mentioned several nerves, so I assume you are talking about CMAPs and SNAPs. The answer is: there is a range of normal amplitudes established for each nerve-muscle combination under certain testing conditions, which may vary a bit by lab. The examiner looks for the amplitude of the response (not the amplitude required to stimulate the nerve) and decides if it is over threshold for normal (OK) or less than threshold (too low). There is no established meaning for "too high." The amount of juice required to stimulate the nerve adequately is a function of positioning, skin conductance, temperature, and other factors and isn't really useful diagnostically.
2. Conduction velocity is calculated after the examiner measures the amount of time the signal takes to get from one part of the nerve to another. This is done in a systematic fashion, so the length of the nerve segment is defined. The distance traveled (usually in cm) divided by the time it took (usually msec) is the conduction velocity. Slowed conduction velocity often implies loss of myelin coating around nerve bundles, but this is not the only interpretation. As with any measured or calculated quantity, there are errors which can crop up and a single number cannot be taken out of context of the other data available.
3. Distal latency is related to the above. The recording electrodes are stuck with adhesive over the skin overlying the belly of the muscle (the most distal point), and the stimulator probe is placed at a specified distance from that, so the amount of time it takes from the shock to the bump in the recorded voltage over the muscle (the CMAP) is the distal latency. Prolonged latency (as above) often implies demyelination.
4. I think this was answered above. I suspect you are reading items off a nerve conduction study report in front of you. The distance between stimulating points and recording points (or between a distal and more proximal stimulating point) is always specified, so one can make sense of the latency in milliseconds by computing conduction velocities.
5. The normal values vary from lab to lab. For your specific EMG/NCS examiner, you should request the information and pencil it in next to the distal latency and amplitude figures. Don't worry about "normal" values for the distances (see explanation above).
6. It's common to issue a report consisting of just a table of numbers. The actual tracing of nerve conduction studies is often retained in a file folder or chart in the EMG lab itself, but we usually don't burden the referring doctor with the extra paperwork. Tracings of MUPs in needle exams are less often printed. That is done more "on the fly" and depends on experience and expertise of the examiner.
7. One editorial comment: interpretation of ANY test (EMG/NCS, MRI, EEG, you name it) requires context from the clinical information. The doctor should have established a hypothesis about what your symptoms and exam findings might mean, and the test then goes about answering that question (sometimes the test is designed to exclude a diagnosis).
I hope this helps. CCF MD mdf.

Hi,
Thank you for explaining the test to me.  It was very informative.  I was reading the information off of a report.  Regarding the actual file to go with the report, they never printed it out.  The doctor has not established a hypothesis or has not shared it with me.  I did ask for an explanation of the test, and was not given one.  Lower right calf numbness, reaching into right foot and the inner lower right
leg were the symptoms, which has now spread to the left foot, and outer left calf and ankle.  Large numb area on upper left back extending to mid-lower level.  No nerve impingement.  Results of emg were :  peroneal (amplitude) 5, (conduction velocity) 44, distal latency 4.9, posterior tibial (amplitude) 15, (conduction velocity) 46, distal latency 4.8, post tib f wave 51, sural (amplitude) 17, distal latency 1.2.
I have no frequencies for the needle insertion, but it was normal.  Are the other figures within normal range?  The numbness keeps spreading, am very frustrated.
Thank you