Help Me with Dennervation potentials..

[latinman]

Hi guys, I saw this pt last week and my attending doesn't seem to think that dennervation was taking place in two of his muscles. I was able to see the persistent regularity of the + sharp waves and fib potentials. This was not the case in muscles out of the myotomal level.

My attending was dismissing the morphology, frequency, etc. just becase the spontanoues activity wouldn't keep going on forever. Almost every site I would sample would have denervation going for at least 10 to 15 seconds. However, it would eventually go away. There was a neurogenic recruitment.

Do you have any good articles that dispell this myth about how dennervation doesn't keep going on indefinitely? Thank you very much!!

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[latinman]

Please help me out!

 

[RUOkie]

It is very hard to comment on this without a recording/video of the waveforms. Sometimes new electromyographers overinterpret things. I certainly used to. Especially recruitment pattern. If the potentials were very regular, could this have been endplate potentials?

If your attending discounted what you were seeing/hearing, hit record on the machine, and reinterpret it later. Use this as a chance to pick his/her brain. Ask him to define 1+/2+/3+/4+ in terms of fibs/psw.

Try reading Chapter 6 in Dumitru (at least it is Ch 6 in the 1st ed) on Needle Electromyography, and Chapter 11 (Pitfalls) especially the sections on Fibrillations and Positive Sharp waves. (p442 in the 1st Ed.)

If you have a disagreement with your staff. Challenge him/her in a non confrontational way. Pretend you are dumb, and keep asking questions until you get it. You may find that the staff is not that good. If that is the case, save all your waveforms and save the EMG (on most current machines it is real easy to save them as a video file) and go over the waveforms with a staff doc that you know is good at EMG.



When I have students/residents with me doing an EMG I schedule 90min a study (normally a single limb takes 15-20min) because my job is to teach!

 

[PMR 4 MSK]

Could have just been extended insertional activity, and/or needle movement you didn't notice from being new at this. Increased IA is not neccesarily denervation.

 

[Ludicolo]

How long was the patient symptomatic? Early in the course of denervation, like in an acute radic, you can see increased insertional activity with one or more unsustained trains of PSWs (But PMR 4 MSK is right – IA doesn’t necessarily mean denervation. You gotta interpret your findings in the proper clinical context). So if you are seeing reduced recruitment, with IA, and normal MUP morphology, you may be catching the patient transitioning from an acute to a subacute process. For 1+ fibs or more, you need persistent activity.

That said, 10-15 seconds is a long time, too long to be analyzing the same potential (I know you’re still learning so at this stage it’s ok). When you overanalyze it becomes more tempting to overcall. But if what you saw truly was a fib, IMHO 10 seconds counts as persistent activity.

One thing to consider during your needle exam: are your findings reproducible? That is – if you encounter a suspected area of denervation, move the needle away from that area then move it back. You should see the same pattern of denervation, firing at the exact same frequency.

Alternate explanations: you may have seen atypical endplate spikes. These appear to fire regularly, but they generally tend to fire at a higher frequency than fibs. Or perhaps you were in a zone of fibs, but during the exam your needle may have drifted away from the denervated area, due to subtle muscle contractions on the part of either you or the patient.

 

[latinman]

Hi everyone. Thank for your replies. I think by now I feel pretty confortable with my EMG's. It is just the attending that I have who I think is a nutcase.

Anyway clinically this patient had findings consistent with nerve root involved. In other words, the muscle strength for that myotome was decreased comparing side to side. Pt also with Dermatome impairment confined to the same level. This had been going on for 6 weeks which allowed enought time for wallerian degeneration.

Every site that I examined would light up like a christmas tree. By this I mean + Fibs, and Positive Sharp waves. I know when I see + sharp waves. They have this slow phase of repolarization back to baseline. There is no way a motor unit can mimick that.

The muscles not confined to that myotome were quiet. You could also appreciate the neurogenic potential. 6 Hz was the frequency with huge motor units. I just think I am going to have a rough semester with this Attending who always calls into question my clinical judgement.