If patient has radicular symptoms and MRI shows nerve root compression from stenosis or disc, that may not always turn up on an EMG/NCS. In the case of radiculopathy, NCS has little value. It may show some decreased motor amplitude in affected distribution (ex. decreased motor fibular or motor tibial amplitudes in an L5 or S1 radiculopathy, or decreased median motor or ulnar motor for C8-T1 radiculopathy), however, this is not always the case. Sensory NCS is not affected as these lesions are preganglionic and the nerve from DRG to distal area where NCS is taking place is intact. For EMG, the first thing you'll see for an acute radiculopathy is reduced recruitment of motor unit potentials, followed by the development of spontaneous activity later, and large motor unit potentials even later. An EMG prior to 3 weeks of symptoms is not helpful really at all since there hasn't been enough time to actually have any of those findings develop. Now, just because the EMG/NCS is normal, doesn't mean that there isn't a radiculopathy, just means there is no electrodiagnostic/electrophysiologic evidence of a radiculopathy. If an acute radiculopathy without EMG/NCS evidence, with the predominant symptom being pain, would still try an epidural for pain relief. If symptoms are numbness/tingling or weakness, wouldn't do the injection necessarily. For weakness I might consider decompression.
I personally feel that EMG/NCS is most helpful for:
1. Determining electrophysiologic evidence of peripheral nerve or muscle diseases. Some of these are painful, which may go across Pain Medicine's door, but others present with numbness, tingling, weakness, cramping, fasciculations, atrophy, etc, and may not be typically seen by Pain.
2. As stated above, differentiating between 2 potentially overlapping suspected pathologies. By differentiating between 2, can better guide treatment.
3. Monitoring reinnervation after initiation of medication or after surgery. Example. CIDP patient diagnosed by EMG/NCS, started on immunosuppression medication, rechecked to see electrophysiologic response. Or, patient receives nerve graft for a muscle that previously had no motor units firing and to see if it takes and helps regain some function.
4. Disease progression monitoring. Example. ALS diagnosed via EMG/NCS but 6 months later patient is functionally worse. Can recheck EMG/NCS to see extent of disease progression, particularly if certain muscles are electrophysiologically beginning to be affected before visually being seen as affected.
Confirming "neuropathy" may be more of an academic thing, however, there are many causes of neuropathy that (A) progress differently and (B) present differently on EMG/NCS. A patient with diabetes with family history of charcot-marie-tooth may take some time to differentiate out which is causing the neuropathy. This will be key for progression of disease. Neuromuscular ultrasound is also being using to help look at specific nerves, in conjunction with EMG/NCS, to help more accurately diagnose NM conditions.
