To the Editor:
Ota et al.1 concludes that, when using ultrasound guidance (US), the anterior approach to sciatic nerve block is performed as easily and successfully as the posterior approach. The authors stated that, in elderly individuals, sciatic nerve identification is less successful because of muscle atrophy in which fascia may not be distinguishable with US imaging. Additionally, the authors implied that in obese patients, the sciatic nerve is not clearly visualized because of its deep anatomic location. On the basis of above considerations, I would like to comment on a few issues.
Ultrasound imaged muscle bundles are seen as hypoechoic zones, whereas the perimysium and aponeurosis are seen as hyperechoic structures. In the case of muscle atrophy, “hypoechoic” muscle bundles degenerate, whereas perimisium and aponeurosis remain intact. The atrophic muscles, depicted as hyperechoic structures, reflect US energy, thus decreasing the ability of the US beam to penetrate in deeper tissues.2,3
In obese patients, because of deep anatomic location of nerves, the US beam travels a greater distance, resulting in beam attenuation. In addition, other factors may affect imaging quality through fat, which are as follows: 1) Exaggerated attenuation, i.e., the adipose tissue, has a nonlinear relationship to frequency as opposed to the usually assumed linear relationship in most biological tissues; 2) Phase aberration of the sound field because of uneven speed of sound in the irregularly-shaped adipose layers. This is due to differing speeds of sound in the overlying, nonhomogeneous tissue above the focus of the transducer; and 3) Reflection because of mismatch of acoustic impedance at the fat/muscle interfaces. When the US beam crosses a boundary between muscle layer and fat, a portion of energy is reflected back to the transducer because of different acoustic velocity between the two tissues (pure fat 1450 m/s and muscle 1580 m/s).4–6
In these cases, image quality may be improved by using different technical approaches which reduce speckling, clutter, or other acoustic artifacts.7 Advanced US imaging techniques, such as compound and harmonic imaging, improve the image because of a reduction of these artifacts. For example, harmonic imaging reduces phase aberration artifacts from overlying tissue and compound imaging reduces similar artifacts by averaging multiple scan lines from different directions.7,8 Additionally, compression of fat, location of the fat in the focus of the transducer, and large beam width of the US signal may improve imaging quality through fat.4,9
Among the major US innovations of recent years, 3D US is the ideal tool to avoid the limitations affecting the diffusion and reliability associated with traditional US.10 However, many studies are required to ascertain its utility in the imaging of nerve structures.
In conclusion, obesity and muscle atrophy mainly increase the number of reflective interfaces not only leading to more echoes but also decreasing incident sound available to penetrate deeper tissues, such as nerves, vessels, or other targeted structures.
Theodosios Saranteas, PhD
2nd Department of Anesthesiology, School of Medicine
University of Athens, Attikon Hospital
Athens, Greece, EU
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