Peripheral Nerves

Peripheral nerves have a fascicular or “honeycomb” echotexture. This consists of the mixture of nerve fiber (hypoechoic) and connective tissue (hyperechoic) content within the nerve. Because there is little connective tissue within more central nerves (e.g., the cervical ventral rami of the brachial plexus), these nerves have a monofascicular or oligofascicular appearance on ultrasound scans.¹ Nerves that are surrounded by hypoechoic muscle are usually easier to visualize than nerves that are surrounded by hyperechoic fat because the nerve borders are more evident.

Peripheral nerves have a complex architecture. Nerves are like a plexus within themselves, with fascicles combining and recombining internally along the nerve path. Because of this intertwined network, the fascicle count varies along the nerve path. Nerve sections taken 2 mm apart can have different fascicular patterns.² The connective tissue content and fascicle count of peripheral nerves vary directly. That is, the amount of connective tissue is more abundant in multifascicular nerves.³ The connective tissue within nerves protects the fascicles from injury. Therefore, monofascicular nerves are more vulnerable to damage.

Identification of nerve fascicles is the basis of peripheral nerve imaging. With ultrasound, only a small subset of the total number of fascicles is imaged. In one study of ex vivo nerve specimens, only about one third the number of fascicles visible on light microscopy were visible on ultrasound scans.⁴ It is difficult for imaging technologies to resolve thin collagenous boundaries between adjacent fascicles. For these reasons, fascicular discrimination is a standard by which to judge nerve imaging quality.

Nerves can be round, oval, or triangular. The shape can change along the nerve path or with heavy probe compression. Nerve fascicles are always round, and therefore monofascicular nerves are normally round. Despite changes in shape that may occur, nerves have a relatively constant cross-sectional area along their path.

High ultrasound frequencies (10-15 MHz) provide better resolution of nerve fascicles.⁵ However, at lower frequencies (7-10 MHz), peripheral nerves are still visible as cordlike structures.⁵ Commercial nerve imaging presets of imaging quality controls have been developed that enhance detection of nerve fascicles.

Short-axis sliding (sliding the transducer along the known nerve path with the nerve viewed in short axis) is a powerful technique not only to identify small nerves with ultrasound but also assess the longitudinal distribution of local anesthetic along the nerve.

Among morphometric variables, the best correlate of nerve diameter is body height.⁶ The best correlate of nerve visibility on ultrasound scans is the extremity size.⁷

FIGURE 18-1 The cervical ventral rami of the brachial plexus viewed in short axis. This monofascicular echotexture is observed in more central nerves that contain little connective tissue.

FIGURE 18-2 Interscalene block demonstrating monofascicular echotexture before (A) and after (B) injection of local anesthetic.

FIGURE 18-3 Median nerve in the forearm viewed in short axis (A) and long axis (B) demonstrating fascicular or honeycomb echotexture. These views were obtained after injection of local anesthetic around the nerve.

FIGURE 18-4 Sciatic nerve in the thigh demonstrating echobright connective tissue content and compartmentalization into its tibial and common peroneal nerve components.

FIGURE 18-5 Excised ex vivo nerve specimen demonstrating fascicular echotexture in short-axis (A) and long-axis (B) views. Peripheral nerves viewed in long axis often exhibit parallel fascicles with coarse wavy echotexture.