Needle EMG diagnosis

Abstract

The purposes of the needle EMG examination are as follows ; 1. To identify the origin of the clinically observed weakness : Neurogenic, myopathic or central. 2. To clarify whether the abnormal condition is in the chronic or active phase. 3. To clarify the distribution of the pathologic lesion : regional or systemic. 4. To detect the abnormal condition that cannot be shown without EMG study, such as myotonic discharges in hyper–kalemic periodic paralysis.

The spontaneous activities are divided into two groups ; one originates from muscle fiber potentials, another from motor unit potentials. The muscle fiber origin spontaneous activities include fibrillation potential, positive sharp wave, myotonic discharge and complex repetitive discharge, etc. The motor unit potential origin spontaneous activities are represented as fasciculation potential, myokymic potential, and etc. These potentials or discharges have specific firing patterns and waveforms.

The most important role in the EMG diagnosis is to evaluate firing pattern of the motor unit potentials in the voluntary contraction. The motor unit potentials fire in a “semi-rhythmic” pattern and the onset–firing rate in smallest contraction is 4 to 5Hz. The recruitment–firing rate of 2nd motor unit is usually less than 11Hz. Firing of the single motor unit exceeds 15Hz strongly supports the motor unit loss ; reduced recruitment. The poor activation of motor unit potentials suggests the central weakness. The full interference in apparent weakness suggests the myopathic condition.

Needle EMG can detect the abnormalities objectively, based on the specific firing patterns of spontaneous potentials and MUPs.