EMG and NCS are both types of electrodiagnostic tests. NCS stands for nerve conduction study and uses electrodes placed on the surface of the skin that record how well the nerve conducts (transmits) a signal when the person administering the tests “triggers” the nerve. EMG stands for electromyography and uses needles inserted into a muscle. The needles are used to measure how well the muscle responds to its nerve signal when the person flexes/moves the muscle. These two are often confused with one another with NCS often being referred to as an EMG. While EMG is often performed in conjunction with NCS in CMT, the two are not the same test, and NCS is the standard electrodiagnostic test for CMT.
In CMT, when symptomatic, detectable abnormalities are typically present on NCS, and sometimes on EMG. Demyelinating CMT (CMT1) typically has fairly uniform nerve conduction; each nerve tested will show nearly the same result. Some subtypes, such as CMT1A, for example, will have detectable abnormalities even when not symptomatic and from an incredibly young age. Axonal CMT (CMT2), however, exhibits significant variability between nerves tested. For example, the peroneal nerve in the lower left leg might show conduction results that are clearly consistent with axonal CMT, yet the medial nerve in the right forearm might show fairly normal conduction, especially if it’s early in the disease course. For these reasons, it’s imperative to test nerves in both arms and both legs.
What constitutes a normal NCS result is unfortunately subjective to the person who is interpreting the data. For example, somebody who specializes in dementia might look at a report and see that everything is normal. Yet, somebody who specializes in CMT might look at the same report and see a clear CMT case. Another variable to consider is the nerves that were tested.
If the nerves in only one arm or one leg were tested and the results fall within normal ranges, but no other nerves were tested, despite clear CMT symptoms, the test was too limited to yield an overall comprehensive result potentially indicative of CMT.
In CMT, when symptomatic, and the nerves in both legs and both arms were tested and the results are within normal ranges, it might not be CMT. The keywords are “when symptomatic” and “when both legs and arms are tested.” A condition called Small Fiber Neuropathy (SFN) has symptoms remarkably similar to CMT. A significant difference between the two is that SFN causes no detectable abnormalities on NCS when symptomatic.
Can EMG/NCS results be normal, and still have CMT? For the reasons discussed, and the reasons stated with distinction, yes, this is possible. When this happens, it lends itself to the difficulties doctors can have with diagnosing CMT.
