Tendons may be confused with nerves that have a hyperechoic appearance. This artifact confusion most commonly occurs while performing sciatic nerve blocks at the popliteal fossa or during selective nerve blocks in the arm and forearm. The normal sonographic image of a tendon is groups of hyperechoic dots mixed in among a hypoechoic background. Nerves often have fascicles with a hypoechoic center (Fig 8.1A), which may also be apparent in a long-axis view (Fig 8.1B). In Figure 8.1B, notice the epineurium of the nerve and the hypoechoic fascicles within the nerve. The primary distinction is that tendons have hyperechoic continuous fibrils, whereas nerves have fascicles. These fascicles have a hyperechoic border caused by the perineurium and hypoechoic center caused by the axons. Tendons may also show greater anisotropy than nerves, showing a greater change in echogenicity when toggling the probe. Moreover, a tendon should be seen taking origin from a muscle or inserting on a bone. This can be seen by scanning along the length of the structure in question. Finally, a tendon should move in the image when the appropriate muscle/joint is contracted (Fig. 8.1C-E).

Larger blood vessels are not usually mistaken for nerves. In fact, most regional anesthesiologists use vascular landmarks to help identify many nerves. Smaller blood vessels are easier to confuse with nerves, especially in the short-axis view. Color-flow Doppler is invaluable in helping to distinguish small blood vessels from nerves. This problem is frequently encountered while performing brachial plexus nerve blocks (Figs. 8.2 and 8.3). If in doubt, use Doppler to ensure that the nerve in question is truly a nerve.

Lymph nodes may be flat or oval and either hypoechoic or hyperechoic. Lymph nodes can be differentiated from nerves by their lack of fascicles. They are most commonly seen in axillary, femoral, sciatic, and interscalene blocks (Fig. 8.4).

Muscles have either fusiform or pennant structures. Fusiform muscles have fascicles with a parallel orientation that converge toward the tendon at the end of the muscle. Pennate muscles have fascicles that appear more featherlike in relation to their aponeurosis or tendon. Muscles have a relatively hypoechoic appearance with some hyperechoic structures (connective tissue or fat) embedded in the muscle. Some practitioners liken this appearance to a “starry night.” The ratio of hypoechoic to hyperechoic muscle is based on the ratio of connective tissue to fascicle within each muscle. In the long axis, muscle may have a featherlike appearance (pennate muscle), and it is usually well organized. Intramuscular tendons and aponueroses are best imaged in the short axis. It may be difficult to distinguish muscle from nerve when performing gluteal or popliteal sciatic nerve blockade (Fig 8.5). Small muscles may have the appearance of a small vessel or nerve. Use of nerve stimulator can usually allow the practitioner to differentiate nerves from muscles.