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Neuromuscular Ultrasound of Mononeuropathies
One of the most common and useful applications of neuromuscular ultrasound is in the evaluation of mononeuropathies. Although mononeuropathies often result from entrapment of isolated peripheral nerves within specific anatomic fibro-osseous canals, there are a variety of other causes, many of which are structural in nature and can be easily visualized on ultrasound. Usually, a combination of clinical examination supplemented by electrodiagnostic (EDX) studies allows one to conclude that a patient’s condition is due to a problem with one nerve. If segmental demyelination can be demonstrated, the lesion can be localized to a specific segment of nerve. However, even with localization to a specific segment of nerve, it is still not known exactly what is causing the problem at that location . The differential diagnosis might include entrapment from various structural lesions, including narrowing of anatomical tunnels; fibrous bands and aponeuroses; ganglion and synovial cysts; tendon and tendon sheath abnormalities; abscesses, bone spurs, and increased callus; aneurysms; varices; tumors (both benign and malignant); infiltration (e.g., amyloid); and other causes.
It is not infrequent that EDX studies can localize the problem to one nerve but then cannot localize it to one specific segment of nerve. This occurs classically in axonal loss lesions. If the sensory nerve action potential of a nerve is abnormal, this denotes a lesion of the peripheral nerve at or distal to the dorsal root ganglion. Otherwise, the only other localization that can be determined is that the lesion is at or proximal to the most proximal muscle that is abnormal on needle electromyography (EMG) (see Chapter 16 ). For example, consider a patient with numbness of the thumb, index, and middle fingers, along with weakness of thumb abduction. One can correctly deduce that there is likely a problem with the median nerve. If the nerve conduction study shows an axonal loss pattern on median motor and sensory nerve conduction studies (low amplitudes, normal or slightly prolonged distal latencies, and normal or slightly reduced conduction velocities), with other nearby nerves being completely normal, a median neuropathy can be confirmed. If one then finds active denervation on needle EMG in the abductor pollicis brevis, flexor pollicis longus, and flexor carpi radialis, but normal findings in the pronator teres and in muscles supplied by the ulnar and radial nerves, the only localization that can be determined is that there is a median neuropathy at or above the takeoff to the flexor carpi radialis. As emphasized in Chapter 16 , one might be tempted to place the lesion between the flexor carpi radialis and the pronator teres. However, this is frequently incorrect as it is common in many neuropathies caused by external compression for some fascicles to be affected while others are spared. It is in this very situation that ultrasound can be especially helpful. Ultrasound has the ability to visualize the major upper extremity nerves from the lower brachial plexus down through the upper extremity. In the lower extremity, the sciatic nerve can be followed from the gluteal fold down to the thigh, where it divides into the common peroneal and tibial nerves. Anteriorly, ultrasound can readily visualize the femoral nerve as it travels in the thigh. Below the knee, the peroneal, tibial, sural, and superficial peroneal nerves can be visualized.
Indeed, one can make a reasonable argument that ultrasound should be employed as a useful complement to EDX studies in all mononeuropathies, since ultrasound can add structural information that EDX studies cannot determine. For example, one might think that ultrasound would add very little to the most common entrapment neuropathy, median neuropathy at the wrist. However, surprisingly, ultrasound can sometimes add very useful information in this situation. Not only can it confirm the lesion and localization, but it can also demonstrate that the median neuropathy is in some cases due to a structural lesion such as tenosynovitis, synovial hypertrophy, a nerve sheath tumor, a fibrolipomatous hamartoma, or anomalous muscles. It is especially important to perform ultrasound of the median nerve when the symptoms are more prominent in the nondominant hand, which might suggest an unusual structural lesion.
In addition, ultrasound is particularly helpful when looking at unusual mononeuropathies. Because these conditions are rare, and often controversial syndromes, ultrasound can be of particular importance. Among these syndromes are lesions of the tibial nerve at the tarsal tunnel (i.e., tarsal tunnel syndrome), the posterior interosseous nerve at the Arcade of Frohse (i.e., radial tunnel syndrome), the anterior interosseous nerve, the ulnar nerve at the wrist, and the median nerve in the region of the elbow (i.e., pronator syndrome).
Lastly, ultrasound is especially important in cases of mononeuropathies associated with trauma. Ultrasound can often be helpful in determining if the nerve is in continuity and, given enough time, can discern whether there is a stump neuroma or neuroma in continuity. In addition, especially when examining a nerve for injury after surgery following trauma, ultrasound can be helpful in assessing the possible etiologies. For example, in some cases, the nerve may be damaged by the original trauma. In other cases, the neuropathy may result from external compression, such as from a cast or, rarely, from surgical hardware.
Appearance of Normal Nerve
Peripheral nerves have a characteristic pattern on ultrasound—on transverse (short axis) sections, they have a “honeycomb” appearance ( Fig. 18.1 ). The actual fascicles are hypoechoic (dark), whereas the connective tissue is hyperechoic (bright). Perineurium surrounds the fascicles and the epineurium surrounds the entire nerve; both are hyperechoic. On longitudinal imaging, the bright epineurium defines the boundary of the nerve with parallel lines running inside, which represent the perineurium ( Fig. 18.2 ). Thus nerves have mixed echogenicity, with the fascicles being dark and the connective tissue being bright. When one applies color or power Doppler ultrasound to nerves, usually no blood flow is seen. The blood vessels that nourish peripheral nerves are usually quite small and beyond the resolution of Doppler. In most nerves, one can make out the normal fascicular architecture. The various fascicles may be slightly different in size.
Somewhat surprisingly, nerves are actually quite mobile in many areas. This can be easily demonstrated during passive and active motion while viewing the nerve on ultrasound. For instance, one of the most common times this mobility is seen when looking at the median nerve at the wrist while the patient alternatively flexes and extends their fingers. The median nerve normally slides easily among the tendons of the flexor digitorum sublimis.
Peripheral nerves often run within a neurovascular bundle ( Fig. 18.3 ). This is fortunate but also creates some confusion. It is fortunate because it is fairly easy to identify arteries on ultrasound with color Doppler, and in many cases, the nerve is close by. One classic pattern is to have one artery, one vein, and one nerve run together in a group. Another common pattern is to have one artery, two veins and one nerve run together in a group. However, it is important not to make the mistake of including blood vessels as part of the nerve when one measures the cross-sectional area (CSA) of the nerve. Veins are fairly easy to identify because slight probe pressure will typically collapse them. In general, arteries are also often easy to identify with the use of color or power Doppler. However, if the artery is small, and/or if the probe is perpendicular to the direction of blood flow, color Doppler may not be positive.
Scanning a Peripheral Nerve
When scanning a peripheral nerve, it is useful to have a standard starting point. By using a standard starting point, one can recognize the usual anatomic pattern of nearby tendons, muscles, blood vessels, and bones, as well as the nerve of interest. For instance, a standard starting point for the median nerve is the short axis view at the distal volar wrist crease. Just radial to the median nerve is the tendon of the flexor carpi radialis; further radial are the radial artery and veins; posteriorly are the tendons of the flexor digitorum sublimis and profundus; and to the ulnar side is the ulnar artery ( Fig. 18.4 ). Good starting locations for other common nerves include (1) the distal volar wrist crease on the ulnar side of the wrist for the ulnar nerve; (2) between the brachioradialis and brachialis muscles at the elbow for the radial nerve; (3) the lateral apex of the popliteal fossa for the sciatic nerve, where it bifurcates into the tibial and common peroneal nerves; (4) in the groove between the medial and lateral gastrocnemius muscles for the sural nerve; and (5) in the groove between the extensor digitorum longus and peroneus longus muscles for the superficial peroneal sensory nerve.
Once the nerve of interest is found, it can then be traced proximally and/or distally. It is often most helpful to move the probe fairly quickly as one follows the nerve, as the nerve becomes more conspicuous with motion. If one moves the probe too slowly, one can easily lose the nerve within a sea of other echoes. The ability to follow a nerve throughout its course is one of the major advantages of ultrasound in the evaluation of mononeuropathies.
Measurements
There are many measurements and other observations that are useful to make when assessing peripheral nerves ( Table 18.1 ). The most important measurement is the size of the nerve, specifically the CSA on transverse (short axis) imaging. In general, when nerves are entrapped, they tend to swell proximal to the site of entrapment, causing them to enlarge ( Fig. 18.5 ). There are many valid published normal values for the CSAs of most of the major nerves ( Table 18.2 ). However, recall from Chapter 9 on statistics and the interpretation of test results, all cutoff values result in a small but significant number of false-positives and false-negatives. Having said this, there are clearly cases where the CSA of a nerve is normal and other cases where it is unequivocally abnormal. For example, if the CSA of the ulnar nerve at the wrist is 6 mm 2 , it is very clear that the size is normal. Conversely, if the CSA is 25 mm 2 at the wrist, it is unequivocally enlarged. However, there are many borderline cases. For example, if a normal ulnar CSA at the wrist is up to 10 mm 2 , one should hesitate to call it abnormal if it is 11 mm 2 at the wrist, with no other findings. From a realistic point of view, 9 mm 2 , 10 mm 2 , and 11 mm 2 are the same number. Just as with using normal values for nerve conduction studies, the authors recommend that the reader not be overly rigid about specific cutoff values for nerve size.
Table 18.1
Measurement and Other Assessments of Peripheral Nerves on Ultrasound.
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AP , Anterior-posterior.
Table 18.2
Nerve Cross-sectional Area Reference Ranges.
Sources: (1) Cartwright MS, Shin HW, Passmore LV, Walker FO. Ultrasonographic reference values for assessing the normal median nerve in adults. J Neuroimaging . 2008;19(1):47–51. (2) Cartwright MS, Shin HW, Passmore LV, Walker FO. Ultrasonographic findings of the normal ulnar nerve in adults. Arch Phys Med Rehabil . 2007;88(3):394–396. (3) Cartwright MS, Passmore LV, Yoon JS, Brown ME, Caress JB, Walker FO. Cross-sectional area reference values for nerve ultrasonography. Muscle Nerve . 2008;37(5):566–571.
| Nerve | Site | Upper Limit of Normal (mm 2 ) | Side-to-Side Upper Limit Difference (mm 2 ) |
|---|---|---|---|
| Median | Wrist | 13.0 | 3.4 |
| Forearm | 10.7 | 2.6 | |
| Pronator teres | 11.0 | 2.8 | |
| Antecubital fossa | 13.2 | 4.3 | |
| Mid-arm | 13.1 | 3.0 | |
| Axilla | 9.7 | 3.5 | |
| Ulnar | Wrist | 8.1 | 2.6 |
| Forearm | 8.3 | 2.0 | |
| Distal elbow | 8.6 | 2.0 | |
| Elbow | 8.8 | 2.2 | |
| Proximal below | 9.3 | 1.8 | |
| Mid-arm | 8.3 | 1.6 | |
| Axilla | 8.6 | 1.8 | |
| Radial | Antecubital fossa | 14.1 | 5.0 |
| Spiral groove | 13.3 | 4.5 | |
| Musculocutaneous | Axilla | 11.9 | 4.2 |
| Vagus | Carotid bifurcation | 9.0 | 3.1 |
| Brachial plexus | Trunk | 11.1 | 4.5 |
| Sciatic | Distal thigh | 80.6 | 18.9 |
| Peroneal | Popliteal fossa | 20.9 | 9.5 |
| Fibular head | 17.8 | 4.9 | |
| Tibial | Popliteal; fossa | 55.9 | 15.7 |
| Proximal calf | 39.9 | 10.8 | |
| Ankle | 22.3 | 5.7 | |
| Sural | Distal calf | 8.9 | 2.6 |
Note: Reference values are based on mean + 2 standard deviations. Nerve area may increase with increased body mass index.
When measuring the CSA, one must pay attention to a few important points. First, the ultrasound probe must be perpendicular to the nerve ( Fig. 18.6 ). If it is not, it will visualize the nerve at an angle, which artificially increases the measured size of the nerve. The way to avoid this error is to first identify the nerve and then gently tilt the probe back and forth along the probe’s long axis (rocking the probe) to the point where the nerve appears to be the smallest. At that point, it is fairly certain that the ultrasound beam is perpendicular to the nerve. One should also assess anisotropy (see Chapter 17 ) to make sure the ultrasound beam is perpendicular to the nerve. One can assess the anisotropy of the nerve itself or the anisotropy of nearby parallel tendons. When there is marked anisotropy, the image will be darker, indicating that the probe is not perpendicular to the nerve. When anisotropy is minimized, the image will be brighter, indicating that the ultrasound beam is perpendicular to the nerve. The next step is to make the actual measurement. There are two ways to measure the CSA: this can be done electronically by either fitting an elliptical pattern over the nerve or manually tracing the outline of the nerve. The expert consensus is that manually tracing the outline of the nerve is more accurate. As one gets more practice with neuromuscular ultrasound studies, one will see that nerves can take on a variety of shapes and hence the reason that the tracing method is the more accurate. However, for the tracing method, it is important to trace just inside the hyperechoic border of the epineurium ( Fig. 18.7 ). This is key and is extremely important for reliability and for using published normal values.
In addition to the CSA, other measurements of size can be done. The anterior-posterior (AP) diameter can be measured on short axis imaging. This is most helpful for measuring very small nerves for which accurate measurement of the CSA is difficult or impossible. For instance, the posterior interosseous nerve as it runs between the deep and superficial heads of the supinator muscle is normally quite small ( Fig. 18.8 ). In this case, taking the AP diameter measurement using a simple electronic caliper can be more easily done than the CSA measurement. Another useful measurement, especially in median neuropathy at the wrist, is the “flattening ratio.” This is the ratio of the nerve width to the height on short axis imaging ( Fig. 18.9 ). To calculate the flattening ratio, two lines are drawn, measuring the maximum width and the maximum height. For example, a normal flattening ratio for the median nerve at the wrist is 3:1 or less. Another very helpful measurement for some mononeuropathies is the “swelling ratio,” which measures the ratio of the CSA at one location compared with another location on the same peripheral nerve. This measurement is somewhat akin to nerve conduction studies wherein the conduction velocity at one segment of a nerve is compared with the conduction velocity at another segment of the same nerve, either proximally or distally. Since swelling typically occurs just proximal to the site of entrapment, the CSA of the nerve just proximal to the site of entrapment can be compared with the CSA of the same nerve either proximally or distally. This has been studied best in the median nerve using the “wrist-to-forearm ratio” (WFR), wherein the CSA of the median nerve at the wrist is compared with the CSA in the forearm. Ratios greater than 1.4 are considered abnormal. Likewise, ratio measures for the ulnar nerve are commonly used where the maximal CSA at the elbow is compared with the maximal CSA in the forearm and the maximal CSA in the mid-arm. Ratios greater than 1.5 are considered abnormal.
The next important parameter to assess is echogenicity ( Fig. 18.10 ). When nerves swell adjacent to an entrapment, they also become hypoechoic. The reason that these changes occur is not clear, but may be due to intraneural edema or impaired axoplasmic flow. It is very common for nerves to become enlarged and dark just proximal to the entrapment point. Although assessment of echogenicity is subjective, it is a skill learned over time and an important assessment to make to help determine if the nerve is normal or not. Along with echogenicity is the assessment of fascicular structure ( Fig. 18.10 ). This is also subjective, a skill learned over time, and another important measure. Commonly, when a nerve is entrapped, there is also loss of the normal fascicular architecture ( Fig. 18.10 ). All fascicles become enlarged and hypoechoic. When this occurs, one can no longer discern one fascicle from another. Sometimes, one fascicle within a nerve becomes markedly enlarged ( Fig. 18.11 ). It is very important to be able to recognize this abnormal finding, as this pattern is highly characteristic of certain conditions, especially neuralgic amyotrophy (see Chapter 33 ).
The last measures commonly made on a peripheral nerve are vascularity and mobility. As noted previously, most nerves are not vascular on Doppler ultrasound. Increased vascularity of nerve is associated most frequently with inflammation, infection, or neoplasia. For instance, in the rare situation where a patient has a malignant nerve sheath tumor, the vascularity may be increased. If there is infection of tissues near or surrounding the nerve, the vascularity is commonly increased. There are also reports of increased vascularity in common entrapment neuropathies. However, in the authors’ experience, this is a rare. Mobility of a nerve can also be an important assessment in some neuropathies, especially the median nerve at the wrist and the ulnar nerve at the elbow. One of the significant advantages of neuromuscular ultrasound is its ability to examine structures during movement. In patients with median neuropathy at the wrist, the mobility of the nerve is often diminished with repetitive flexion and extension of the fingers, in contrast to the normal mobility of the median nerve at the wrist in individuals without carpal tunnel syndrome. Obviously, this assessment depends on the patient’s ability to cooperate and physically move the fingers. Regarding ulnar neuropathy at the elbow, abnormal mobility of the ulnar nerve can sometimes aid in the diagnosis. In some individuals, flexion of the elbow is associated with subluxation or frank dislocation of the ulnar nerve out of the groove, which can readily be demonstrated on ultrasound (see Chapter 22 ).
In cases of trauma, assessment of nerve continuity is key. If there is an acute nerve transection, one can often see the discontinuity and retraction of the two nerve endings. If the discontinuity is chronic, because of the physical transection of the nerve along with hemorrhage and fibrous tissue formation, it is very unlikely that the nerve can successfully grow back. In this case, the nerve will classically end in a hypoechoic ball of tangled nerve fibers, known as a stump neuroma ( Fig. 18.12 ).
In summary, for every peripheral nerve, one should assess its size, shape, echogenicity, fascicular architecture, and vascularity. In some nerves, the mobility should also be assessed.
Important Structural Abnormalities to Recognize
One of the major advantages of neuromuscular ultrasound in assessing mononeuropathies—beyond evaluating size, shape, echogenicity, fascicular structure, and vascularity—is its ability to look for structural abnormalities that can be the pathologic cause of the mononeuropathy. The ultrasonographer must be able to recognize the most common of these structural abnormalities ( Table 18.3 ).
Table 18.3
Important Structural Lesions to Identify on Ultrasound.
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