The pitfall of NCS/EMG

Abstract

There are various pitfalls for the electrodiagnostic tests including nerve conduction studies (NCS) and needle EMG. Failure to recognize them would easily lead us to misdiagnosis.

Regarding motor–nerve conduction studies (MCS), one of the most important facts is that the amplitude of the compound muscle action potential (CMAP) is supposed to be proportional to the muscle power, i.e. manual muscle testing (MMT). The dissociation between the CMAP amplitude and MMT would become an important clue to the diagnosis. For example, a normal CMAP amplitude in a weak muscle suggests either conduction block (CB) at proximal side, central weakness or other conditions. “MMT is 5, but CB is observed” is always wrong. An insufficient stimulation is a frequent cause of this error, which is especially apt to occur for Erb's point stimulation. If this occurs, a false CB between the Erb's point and arm/axilla would be observed. Unnecessarily strong stimulation at the wrist may cause a spread from the median to ulnar nerve, giving a false impression of CB. Martin–Gruber anastomosis, an anomalous branch from the median to ulnar nerve, is a well–known pitfall, which may give a false impression of CB between the wrist and elbow in the ulnar nerve MCS.

Reagrding needle EMG, correct identification of the spontaneous activities is the most important key to diagnosis. We must be able to distinguish fibrillation potentials and positive sharp waves from the end–plate spikes or voluntary motor unit potentials (MUPs). In voluntary activities, the famous formula that the high–amplitude MUPs suggest neurogenic changes, whereas the low–amplitude MUPs suggest myopathic changes is not always correct. Notably, high–amplitude MUPs are often observed in myopathy. Recruitment pattern is more important for diagnosis.