Why Negative EMG's?
 

I have a question that I've been wondering about for a week, especially in light of the fact that my own sfemg came back negative. My question is, why is it that some patients, particularly those with hereditary myasthenia gravis and, as Rach recently wrote, with sero-negative mg tend to have negative emgs and sfemgs?

I asked the neurophysiologist who did my sfemg why that test and the regular emg would come back negative when I clearly had a problem with either my nerves or muscles (my eyebrow can fall so far down my eye is almost covered and weakness in my lower facial muscles can cause my mouth to sort of curve up or pull down depending on, well, I don't know what!). I didn't really get an answer from him - he told me to see a plastic surgeon. Which besides being completely illogical left my question unanswered.
Anyway, I'm still left wondering. If someone could explain this in a not too master's degree in physiology way, I'd really appreciate it.

Also, is it true that if there is any weakness at all somewhere then the sfemg will show up even in a non-weak muscle? What about the regular emg? I was told that it would almost certainly be positive in a person with mg - around 90% of the time. Of course I've had two negative emg's . . .

Thanks for any help!

Ally

Hi Ally
 

Alice is the best person to answer this question for you.

In Japan in 2003 scientists there did a study on patients with seronegative MG and the SFEMG. They reached the conclusion that seronegative MG will not always show on a sfemg. Angela Vincent was part of this study. It was published in a medical journal, however its never really been embraced with open arms by the medical profession. I dont know why.

The rest of the answer I can't give you, but its something to do with the way the electrical current runs through the muscle. Im sure someone who hasn't got a bout of brain fog will be able to help you.

It is incredibly frustrating as SFEMG is still held up as the gold standard, which clearly it isn't.

Love
Rach

I'm not Alice but I'll run through this again if it's okay.

First, drugs can interfere with the SFEMG. Any steroids or other immunusuppressants. Mestinon. IVIG. Plasmapheresis.

Women's muscles fatigue more slowly than men's muscles do. This might be why women need to do exercise EMG's to show a decrement. When I had my borderline SFEMG, I walked around a lot before it.

There is the issue of "fat" being different on women than on men. There is skin, then fat and then the muscle. You know how certain things interfere with an electrical signal, like certain radio/micro-waves? Same deal with fat.

And then there's the expertise of the person doing the SFEMG.

In a SFEMG, the average "jitter" of a muscle is determined by really old studies, done like decades ago!!! So, for example, if your age is 40 and the average jitter of the frontalis/forehead muscles is 35 for your age, then they expect the jitter to go up quite a bit for it to be positive. What if your NORMAL jitter for that muscle is 15? Or what if it is 55? Then their conclusions are based on faulty assumptions.

Average is relative.

And here's a quote from one of Alice's posts on this.

In addition, if someone has CMS, the SFEMG may not be positive at all. And if it's limb-girdle CMS, a.k.a., DOK-7, then they need to test the legs/hips. That's only one example.

And if someone has a mitochondrial disease, then they have to do a very specialized muscle biopsy to know for sure. An EMG could be completely normal.

Doctors like to have "simple" equations and solutions and science is anything but simple. Time constraints and money are often their concern and not the science of a situation. It used to be that science came first.

And that is why a clinical exam, lots of other evidence like breathing tests, a neuro-ophthalmologist exam and other evidence is so important in diagnosing a patient.

Okay, that's it for me. Alice? Anyone else?

Annie

I really have nothing to add,

alice