Muscles alive: Ultrasound detects fibrillations

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Abstract

Objective

Muscle ultrasound is capable of visualizing muscle movements. Recent improvements in ultrasound technology have raised the question whether it is also possible to detect small-scale spontaneous muscle activity such as denervation. In this study we investigated the ability of dynamic muscle ultrasound to detect fibrillations.

Methods

Eight patients with fibrillations were measured simultaneously by ultrasound and EMG to verify which movements on ultrasound examination corresponded to fibrillation potentials on EMG. The temperature dependency of ultrasound detected fibrillations and the observer agreement was assessed in five healthy subjects with focal denervation induced by botulinum toxin.

Results

Fibrillations appeared on ultrasound examination as small, irregularly oscillating movements within the muscle while the overall shape of the muscle remains undisturbed. Visibility of fibrillations with ultrasound decreased with lower temperatures, with a 32% decrease at 30 °C compared to 39 °C. The interobserver agreement was substantial with a kappa of 0.65 for experienced observers.

Conclusion

Fibrillations could be visualized with ultrasound. Consistent results could be obtained from trained observers. Care has to be taken to ensure an optimal muscle temperature to avoid false negative results, especially in distal muscles.

Significance

Visualization of fibrillations by muscle ultrasound opens the way for a new diagnostic application of this technique.

Introduction

Fibrillation potentials are spontaneous action potentials of individual muscle fibers that indicate the loss of contact with their innervating axon, and result in contraction of the fiber. For this reason fibrillations are a prominent symptom in acute or severe peripheral nerve disorders, but can also occur in several myopathies when muscle fibers are split or inflammation is present. As fibrillations are invisible on the outside they can currently only be detected by needle electromyography (EMG), which is an invasive and sometimes uncomfortable technique.

Previous studies have shown that muscle ultrasound is useful in the diagnostic evaluation of neuromuscular disorders, as it can show muscle atrophy and structural changes due to fibrosis or infiltration of fat (Walker et al., 2004, Pillen et al., 2008). Moreover, in contrast to other imaging techniques ultrasound can be used to visualize muscles movements such as voluntary contraction and fasciculations (Walker et al., 1990, Reimers et al., 1996, Chi-Fishman et al., 2004). Ultrasound appeared to be even more sensitive than needle EMG for detecting fasciculations, probably because of its larger sampling area (Walker et al., 1990, Reimers et al., 1996). Resolution and frame rate of ultrasound have improved over the years. This has raised the question if it is possible to detect smaller scale spontaneous muscle activity such as fibrillations. A previous report stated that this is not yet possible because with the current resolution individual muscle fibers can not be visualized (Scheel and Reimers, 2004). The axial resolution of muscle ultrasound with a 17 MHz probe is 100 μm at best (Cosgrove, 1992), which is larger than the diameter of a normal muscle fiber (40–60 μm) (Oertel, 1988). However, in a subsequent report ultrasound seemed to be capable of detecting fibrillations in patients with spinal muscular atrophy, with fibrillating muscle fibers appearing as “continuous, chaotic-like movements” (Van Baalen and Stephani, 2007).

Needle EMG studies have shown fibrillations to be temperature dependent, especially below 22 °C (Denys, 1991, Lee and Kwon, 1997). With ultrasound not only the amount of fibrillations but also the characteristics of the corresponding muscle twitch which also alters at different temperatures (Bennett, 1985) might influence its detectability. This means that temperature may not only influence the amount of fibrillation potentials but also the amount of tissue displacement, and this could further decrease the detection of fibrillations with ultrasound at lower temperatures.

In this study we aimed to confirm the ability of ultrasound to detect fibrillations. Intra- and interobserver agreement for the visual detection of fibrillations with ultrasound were assessed, and the detectabilitity of fibrillations at different temperature levels.

Section snippets

Simultaneous ultrasound and EMG measurements

Eight patients with EMG-proven fibrillations were included after written informed consent had been obtained. The study was approved by our local ethics committee. (Table 1) outlines the patient characteristics and selected muscles. Eleven muscles in total were measured simultaneously with EMG (Medelec Mystro MS20, Surrey, United Kingdom; concentric needle electrode) and ultrasound (Phillips IU-22, Eindhoven, The Netherlands, 5–17 MHz linear transducer). The EMG needle was inserted parallel to

True positives (n = 4)

Four out of 11 muscles showed fibrillations on ultrasound examination that were confirmed by EMG. Fibrillations were visible on the ultrasound video as small, irregularly oscillating movements of muscle tissue in all directions, while the overall shape of the muscle was preserved (Supplementary Video 1). In these four muscles EMG showed fibrillation potentials with an overall frequency of at least 5 Hz. In one of these muscles the fibrillations were clearly elicited by the EMG needle, as the

Discussion

This study shows that even with the current resolution muscle ultrasound is capable of visualizing fibrillations. Detection of fibrillations with ultrasound was possible with a substantial interobserver agreement and also the amount of fibrillations could reliably be assessed, indicated by the significant correlation between this amount and decrease of the CMAP amplitude in subjects with denervation caused by botulinum toxin.

Even though the resolution of ultrasound is still not that of a single

Disclosure

The authors report no conflicts of interest.

Acknowledgements

The authors would like to thank Maarten van Hal, Henny Janssen and Wilma Raymann, electrodiagnostic technicians, for their help in determining interobserver agreement.

References (17)

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