I was mailing with one the top ALS doctor/researcher dr. Memede de Carvalho (he is the one who published the studies about benign fasciculations to ALS (rare but described in study), etc.)
He was so kind to answer a few my questions about EMG:
You assume the EMG technique is perfect to detect neurogenic changes, which is not the case. In particular, there are different techniques in EMG, mostly qualitative methods are aaplied which are less sensitive than using quantitative tools (such as MUP analysis, Single fiber EMG, Motor unit number estimation etc). Possibly in most quantitative tecnhiques would detect neurogenic changes. I think there is a group where fasciculations mirroring lower motor neurons hyperexcitability precedes neurgenic changes. Only one case has been clearly described showing this -attached.
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...with normal EMG ( 4 limbs) should mean no MND
But it was related to twitching and EMG..and if your EMG was clean and you already have weakness, I would say you are safe. Some PALS have clean EMG, but mostly those with bulbar form of the disease where LMN are not involved yet.
Usually with ALS, if one arm is affected, EMG picks up changes even in the other, clinically yet strong, arm.