P 157 Detection radius of EMG for fasciculations: combined ultrasonographic-electromyographic analysis

Background Fasciculations are caused by spontaneous discharges of single motor units. In electromyography (EMG), they are characterized by single motor unit action potentials. Muscle ultrasonography is increasingly applied as a non-invasive screening tool for the examination of fasciculations. In the context of motor neuron disease, the sensitivity of muscle ultrasonography to detect fasciculations was superior to EMG. Limited detection radius of concentric needle EMG may explain this discrepancy, but a direct EMG-ultrasonography-correlation of single fasciculations has not been reported so far. In the current study, we aimed to define an “EMG detection radius” of fasciculations by simultaneous recording of EMG and muscle ultrasonography. Methods Parallel to concentric needle EMG, we recorded in-plane high-resolution videos of muscle ultrasonography showing the needle tip within the transverse section of the muscle. A total of 200 fasciculations were acquired during 50 recordings of 10 patients with motor neuron disease. Amplitude and duration of EMG fasciculation potentials were subsequently correlated to the corresponding ultrasonographic muscle displacement and the distance from the EMG needle tip. Results The onset of ultrasonographic muscle displacement was concurrent to its EMG fasciculation potential whereas the plateau phase and the backward movement occurred when the EMG had already returned to baseline. About 40% of fasciculations observed on ultrasonography were not detected in EMG and this included fasciculations located close to the needle tip. The amplitude of EMG-positive fasciculations was negatively correlated to their distance from the needle tip. However, the correlation differed significantly between different patients and between different muscles. Discussion Analysis of fasciculations on simultaneous EMG and ultrasonography recordings improves the understanding of the “EMG-detection radius” of fasciculations. A relevant portion of ultrasonographic fasciculations was not detected on EMG which may in part explain the superior sensitivity of ultrasonography. Different localization of neuromuscular endplates and of myofibrils along the longitudinal axis of the analyzed muscles might explain the occurrence of “EMG-negative” fasciculations. Alternatively, muscle remodeling or changes of axon terminals due to neurodegeneration may alter the sensitivity of EMG. Further studies are needed to classify fasciculations of other entities with this method.