You might have misread about the EMG. EMG tests are checking for PNS (peripheral nervous system) disorders, while MRIs of the brain and spine are checking for CNS (central nervous system) disorders.

While it may not seem it, there's a huge difference between the two even if the symptoms appear identical. And it's more than simply peripheral nerves being those that run throughout the body, and the central obviously being the 'hub' through which they communicate.

The central nervous system has one additional factor being the blood/brain barrier which is supposed to protect it. Outside of traumatic injury, almost all CNS disorders are autoimmune.

The PNS doesn't have any protection so to speak, aches from a cold are in a way a PNS disorder, but so are pinched nerves, scarring and the like, and it does have some of its own autoimmune diseases.

But for the CNS, it pretty much has to be autoimmune, trauma or spinal meningitis and like viruses to cause, nervous system issues stemming from the CNS. If it's autoimmune the only way to dx a CNS disorder is through MRI, CT scan and/or blood tests including the ever popular spinal tap.

It's next to impossible to dx PNS disorders from an MRI or CT scan, so doctors use EMG tests to see the health of the muscles and the strength of their signal to the CNS.

ALS, MS, Parkinson's, even Alzheimer's are CNS disorders and will likely show on an MRI, sometimes small lesions in MS can hide between the cuts or are only present in the spine, but still can be found on MRIs.

Hope this helps some