“See that low amplitude response in the nerve? Classic.”
“So… that’s how we know it’s a motor neuron problem?”
“Yes.”
“Ah. So if it were a muscle problem, the amplitude would be HIGHER than normal to compensate, right?”
“What? No. Then it would still be lower than normal.”
“Oh. Huh. So.. the EMG doesn’t tell you whether it’s a motor neuron or muscle problem?”
“Nope.”
“Then what question is the EMG supposed to answer, exactly?”
“You got it.”
“… I’m sorry, I don’t follow. Do you just use the EMG to localize the lesion? Is that the purpose?”
“Well, you can. But the clinical exam really localizes it for you, right? Phalen’s sign, tinel’s sign, the patient’s actual symptoms, etcetera. This is just confirmatory.”
“I’m sorry. I guess I’m just asking these questions wrong. What I mean to say is – in the politest possible way – why would anyone ever order an EMG?”
“Beats me.”
Action Potential 
EMG’s are remarkably useful. Either this guy is playing games with you, or he’s a moron. Either way he shouldn’t be on the teaching service. Myopathic vs. neuropathic EMG changes are pretty damn obvious.
Grumpy’s right. Neuropathic motor units on needle EMG are generally higher amplitude than normal. It’s a little disturbing that someone doing an EMG would say those things.
Well, damn! THAT’S WHAT I THOUGHT.
I asked him if there would ever be a situation where you’d be unable to make a diagnosis without an EMG, and he said no. Maybe he just hates EMGs.
Work with a physiatrist (PM+R) and you’ll get a very different answer. Every one I’ve ever done a rotation with has *loved* EMGs, largely because up here that’s their big money-maker 😛
where the clinical presentation is clear, emg may not be needed. but where the presentation is conflicting or complex, electrodiagnostic testing is very helpful.
My EMG was about $1000 for 45 minutes of work, so I can see why someone would want to perform them. The guy who did mine had an interesting approach: at the end of the appointment, he asked, “That wasn’t so bad, was it?” It’s very good psychology to have the patient leave thinking, “That wasn’t so bad.” Because really, if you think about it, having someone electrocute you and stick needles in you isn’t something you’d agree to do again (except there’s this little voice saying, “that wasn’t so bad…”)