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Ultrasound can identify entrapment and traumatic neuropathies, neurogenic tumors, and some hereditary PN abnormalities, and is particularly useful when the results of clinical examination and tests are unclear.
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Ultrasound can play an important role in choosing conservative or surgical treatment.
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In chronic PN compression, ultrasound examination can reveal a typical notch sign: a thinner PN at the site of compression with a fusiform enlargement of the nerve.
Peripheral nerve (PN) disorders are common, and they can usually be diagnosed on the basis of the clinical history and a careful physical examination. When further investigation is needed, PN disorders are usually evaluated with electrophysiology testing, and when imaging is needed, ultrasound or magnetic resonance imaging (MRI) can be used to show the PN and its internal structure.
The first articles about PN ultrasound were published in the mid-1980s, and since then, interest in this topic has grown, and the number of publications has increased. Ultrasound technology improved with high-frequency broadband transducers (up to 18 MHz) capable of high spatial and contrast resolution, allowing great definition of superficial soft tissues, including PNs.
Ultrasound and Electrophysiology
Electrophysiology testing is a functional, not anatomic, evaluation of the nerve. Ultrasound becomes a complementary examination because of its capacity to show the PN’s internal structure and the surrounding anatomic structures. Ultrasound assessment of PN pathology correlates well with electrophysiology results, and when used together with electrophysiology, the sensitivity for diagnosing PN entrapment syndrome is better than electrophysiology alone.
Nerve conduction measures reflect the function of the best-surviving nerve fibers, and values can remain almost normal even if only a few fibers remain unaffected by the pathology. Electrophysiology testing can yield up to 30% false-negative results for PN disorders. Electrophysiology testing cannot differentiate intraneural pathology from extraneural nerve compression, an important point because extraneural nerve compression has a worse prognosis if treated conservatively. In traumatic PN pathology, electrophysiology testing cannot differentiate axonotmesis from neurapraxia, and it can take weeks to differentiate neurotmesis from severe axonomesis.
Ultrasound can play an important role in choosing conservative or surgical treatment. Outcomes are better for early surgery for neurotmesis and for conservative treatment for less-severe lesions. Waiting more than 6 months before surgical intervention for PN pathology can jeopardize end-organ integrity.
Electrophysiology testing has other drawbacks. It provides no anatomic information about the PN, it cannot determine the exact anatomic location of the lesion, it is uncomfortable for the patient, and needle insertion in a small or deep muscle can be difficult.
Ultrasound and Magnetic Resonance Imaging
Compared with MRI, ultrasound of PNs offers some well-recognized advantages. It has higher resolution for small nerves, and it can show the whole length of most PNs of the upper limb and lower limb compared with the contralateral limb. Dynamic evaluation is easily performed, quick, inexpensive, and well tolerated by the patient.
Ultrasound Anatomy
In the typical PN architecture, each nerve fiber is surrounded by a very thin endoneurium. The nerve fibers are tightly grouped together in bundles called fascicles, and every fascicle is surrounded by a thin perineurium. The size of the fascicles can vary greatly in the same PN, and the number of fascicles is usually proportional to the nerve size; small nerves in distal extremities are often made of one fascicle. The loose connective tissue surrounding the fascicles, the internal epineurium, contains fat, connective tissue, blood, and lymph vessels. The external layer and outer limit of the PN is the external epineurium ( Fig. 11-1 A).
Ultrasound evaluation with a high-frequency probe (≥10 MHz) allows good visualization of the internal structure of the PN with its typical fascicular or honeycomb-like echotexture. The ultrasound image corresponds well with results of histological studies. In short-axis scanning, the multiple, round, hypoechoic structures correspond to the nerve fascicles, surrounded by hyperechoic tissue that corresponds to the internal and external epineurium layers, which are predominantly made of adipose tissue, collagen fibers, and small vessels.
On long-axis scanning, the fascicles are seen as multiple parallel, but discontinued hypoechogenic bands separated by the hyperechoic internal epineurium. The external layer of the PN, the external epineurium, can be slightly more hyperechoic than the internal epineurium (see Fig. 11-1 B and C). The loosely organized connective tissue around the fascicles (i.e., epineurium) protects the nerve against friction and trauma, especially in osteofibrous tunnels.
The sizes of the fascicles measured with ultrasound correlate well with values determined by histologic studies, but the number of fascicles seen with ultrasound is lower than actually exist. This underestimation is more important with lower-frequency probes (≤10 MHz). The endoneurium and perineurium are not visible on ultrasound.
Scanning Technique
Most of the PNs of the upper and lower limb, including the brachial plexus, can be evaluated by ultrasound. The quality of the ultrasound equipment and probe affect the ability to assess PNs with ultrasound ( Table 11-1 ). Some nerves are too small, too deep, or under bony surfaces and are not accessible to ultrasound. In those cases, ultrasound can sometimes show indirect signs of PN pathology, such as selective atrophy and fatty changes of the denervated muscles or extrinsic compression, by the presence of masses or anatomic variants on the known path of the PN. Experience with the sonographic appearance of normal and pathologic PNs and knowledge of the anatomy and clinical signs of PN entrapment syndromes are mandatory for successful PN ultrasound scanning.
| Upper Limb Nerves | Lower Limb Nerves | ||
|---|---|---|---|
| Regular Ultrasound Equipment | High-End Ultrasound Equipment | Regular Ultrasound Equipment | High-End Ultrasound Equipment |
| Radial nerve | Brachial Plexus | Femoral nerve | Saphenous nerve |
| Ulnar nerve | Suprascapular nerve | Sciatic nerve | Lateral cutaneous nerve |
| Median nerve | Musculocutaneous nerve | Tibial nerve | Obturator nerve |
| Axillary nerve | Common peroneal nerve | ||
| Posterior interosseous nerve | Calcaneal branch | ||
| Superficial cutaneous branch | Medial plantar nerve | ||
| Dorsal branch of ulnar nerve | Lateral plantar nerve | ||
| Motor and sensory branches | Deep peroneal nerve | ||
| Anterior interosseous nerve | Superficial peroneal nerve | ||
| Palmar cutaneous branch | Sural nerve | ||
| Digital nerves | |||
∗ Evaluation depends on the quality of the ultrasound equipment. Most peripheral nerves are visible with high-resolution ultrasound. Nerves in italic are hard to see, but ultrasound can show selective muscle denervation and atrophy of their motor branches when present.
Short-axis (transverse) scanning of the PN, looking for the nerve as a round or triangular shape and its typical fascicular pattern, is the preferred method for PN scanning. In long-axis scanning, it can be difficult to differentiate the PN from the surrounding soft tissues. We suggest use of a well-known anatomic landmark to locate the PN, and using the so called “elevator technique”, the PN is followed on a short-axis scan with rapid and repetitive up-and-down sweeping movements of the probe through the whole length of the PN in the limb.
The PN is less susceptible to the anisotropy artifact than tendons and muscles, making short-axis ultrasound studies of long segments of PN possible without dramatic changes in echo structure caused by probe movement. Unlike tendons or ligaments, the normal PN change its shape under probe pressure with slight displacement of the uncompressible fascicles inside the very loose and flexible surrounding epineurium tissue. PNs are gliding, mobile structures that allow the full range of motion of the limbs without stretching of nerves. This mobility can lead to subluxation or dislocation of PNs during movement, and in these cases, dynamic ultrasound scanning is very informative. There can be a normal change in the echogenicity of the PN when it passes through osteofibrous tunnels: it can appear slightly more hypoechoic and can lose its fascicular aspect because of tightly packed fascicles and less epineurium tissue due to the surrounding pressure in the tunnel.
Short-axis scans look for the hallmarks of PN pathology: a swollen nerve, lost of fascicular patterns, local thinning of the nerve with the notch sign, surrounding anatomic variants or masses extrinsically compressing the PN and ultrasound signs of selective denervated muscle. This qualitative ultrasound evaluation is compared with opposite side for most PNs. PN measurements with ultrasound are reliable, and quantitative measures with high-end equipment are possible for larger nerves.
The cross-sectional area (CSA) of the nerve on a short-axis scan is the best criterion for diagnosing PN pathology. The measurement is made at the point of maximal enlargement of the PN, and the CSA is calculated without including the most external hyperechoic rim of the PN (i.e., external epineurium) because it can be difficult to distinguish the limit between the outer border of the PN and the surrounding fat ( Fig. 11-2 ). The CSA seems to be more precise with the use of direct tracing than with the ellipse calculating method. Many studies have tried to establish normal CSA reference values for PN’s. The normal size of a PN can vary. The value seems to be strongly correlated with weight and body mass index of the patient, less correlated with age and height, and not correlated with gender or dominant side; correlation with ethnic group origin is unclear. We think comparison with the opposite side is mandatory in ultrasound evaluation of PN pathology. Studies have shown that a ratio of CSA measures (i.e., separate CSA measurements made at two sites on the same PN) may be more specific and sensitive for diagnosing PN entrapment than a single measurement of CSA.
Ultrasound-Detected Pathology
In compressive PN neuropathies, when pressure is applied for a short period, the result is usually a transient neurapraxia and a normal ultrasound study result. A normal PN identified by ultrasound examination correlates with a higher probability of functional recovery. In chronic compression, initial impairment of the microvasculature of the PN can lead to an ischemic reaction in the epineurium, venous congestion, and endoneural edema mimicking an intraneural microcompartment syndrome. In the early stages of chronic PN compression, intraneural edema can be reversible, which may explain the on and off symptoms associated with various degrees of the edema. With chronic edema, irreversible thickening and fibrotic changes of the epineurium lead to chronic compression of fascicles and nerve degeneration. It is not possible to differentiae PN edema from fibrosis, because both are hypoechoic on ultrasound.
In chronic PN compression, ultrasound examination can reveal a typical notch sign: a thinner PN at the site of compression with a fusiform enlargement of the nerve, which usually is 2 to 4 cm proximal to the compression site ( Fig. 11-3 ). The enlarged nerve is uniformly hypoechoic, loosing its normally fascicular echotexture because of a combination of crowded, edematous, and hypoechoic fascicles and the hypoechoic edema of the epineurium. The PN eventually recovers its normal size and fascicular form at a distance from the injured site ( Fig. 11-4 ). Evaluation of the intraneural edema with ultrasound can be profoundly impaired if the PN is incased in scar tissue, because scar tissue and edema have the same uniformly hypoechoic appearance.
The finding of an intraneural power Doppler signal is rare in cases of nonpathologic PNs. Any increased signal should be interpreted as pathologic, usually indicating severe and chronic PN compression.
Compressive Neuropathies
PNs, tendons, and vessels pass through osteofibrous tunnels that protect them and redirect their paths across synovial joints. The tunnels are common sites of compressive neuropathies, and most are accessible to ultrasound evaluation.
Along with the classic nerve entrapment syndromes, congenital anatomic variants and hereditary pathology can be risk factors for compressive neuropathies. Patients affected by hereditary neuropathy with liability to pressure palsy (HNPP) present with recurrent mononeuropathies from microtrauma, and ultrasound can show pathologic nerve enlargement and hypoechogenicity of symptomatic and nonsymptomatic nerves. Congenital anatomic variants such as accessory muscles can lead to compressive neuropathies, and they can mimic masses and compress adjacent PNs. The most common are anconeus epitrochlearis muscle at the cubital tunnel, an anomalous abductor digiti minimi muscle at Guyon’s tunnel ; abnormal insertion of the flexor digitorum or lumbrical muscles at the carpal tunnel ; and accessory soleus muscle and accessory flexor digitorum longus at the tarsal tunnel.
The presence of a proximal division on the median nerve in the forearm and a bifid median nerve at the level of the carpal tunnel is common (2.4% of the normal population) and a risk factor for carpal tunnel syndrome (CTS). The proximal division of the median nerve is often accompanied by a persistent median artery, and some investigators think carpal tunnel release surgery should include preoperative ultrasound examination to provide a better anatomic orientation ( Fig. 11-5 ).
Upper Limb Compressive Neuropathies
At the shoulder, the suprascapular nerve can be compressed, usually by a posterior articular cyst communicating with the glenohumeral articulation through a labral tear at the level of spinoglenoid notch (involving only the infraspinatus muscle) or at the level of the scapular notch (involving supraspinatus and infraspinatus muscles). The nerve is usually difficult to see, but ultrasound can show the compressing cyst and the selective muscle atrophy ( Fig. 11-6 ).
The axillary nerve can be compressed in the quadrilateral space in the posterior aspect of the shoulder by a mass, a fibrous band, or traction trauma. On ultrasound, identification of the axillary artery with power Doppler helps to localize the very small axillary nerve close to the inferior border of the teres minor muscle. Selective atrophy of the deltoid and teres minor muscles without associated tendon rupture favors the diagnosis of axillary nerve pathology.
The radial nerve can be injured, mostly by trauma, at the midhumeral level as it passes close to the bone in the radial groove of the humerus or as it passes through the lateral intermuscular septum of the arm. At the elbow level, the radial nerve divides in two terminal branches: the motor posterior interosseous nerve (PIN) and the sensory superficial radial nerve. The PIN, usually monofascicular on ultrasound, passes in a tunnel between the two bellies of the supinator muscle to reach the posterior aspect of the forearm. At the supinator muscle level, the PIN can be compressed by a fibrous band, an hypertrophic arcade of Frohse, recurrent vessels, or a radial head fracture, producing a painful syndrome over the lateral aspect of proximal forearm or a pure motor injury, with weakness of the fingers’ extensors muscles but usually sparing the wrist extensors (i.e., supinator syndrome). Ultrasound can show an enlarged PIN at the supinator level ( Fig. 11-7 ). The sensory superficial radial nerve branch is prone to external compression or penetrating trauma at the level the distal and radial aspects of the forearm, where it becomes subcutaneous, passing between the tendons of the brachioradialis and the extensor carpi radialis longus muscles. Called Wartenberg’s syndrome, this purely sensory deficit over the dorsoradial aspect of the wrist and hand can be revealed on an ultrasound examination by identification of a hypoechoic enlargement in continuity with this small nerve branch; it is a differential diagnosis for DeQuervain’s tenosynovitis.
The ulnar nerve is more commonly compressed at the elbow level in the cubital tunnel than at the wrist in Guyon’s tunnel. At the elbow, the nerve passes in a proximal bony tunnel (i.e., condylar groove) made by the medial epicondyle and the olecranium and bridged by the cubital tunnel retinaculum, also known as the Osborne fascia, which is usually not visible on ultrasound if not pathologic (see Fig. 11-4 B). The ulnar nerve continues in a distal fibrous tunnel (i.e., cubital tunnel) between the ulnar and humeral heads of the flexor carpi ulnaris muscle and passes under the arcuate ligament that joins those two muscles heads (see Fig. 11-4 C). Many causes of cubital tunnel syndrome have been described : bony spur, heterotopic ossification, thickening of the medial collateral ligament, accessory anconeus epitrochlearis muscle, hypertrophic medial head of triceps muscle, loose body, ganglion cyst, and fracture deformities of the elbow. On ultrasound examination of the cubital tunnel, the nonpathologic ulnar nerve is slightly larger in the tunnel than proximal and distal to the tunnel (i.e., ulnar nerve normal maximal CSA = 7.9 ± 3.1 mm 2 ), and 80% of the normal ulnar nerves lose their multifascicular form to become monofascicular in the tunnel, which should not be misinterpreted as a pathologic finding.
Ultrasound is easily performed and is accurate for diagnosing compressive neuropathies of the ulnar nerve at the elbow, with a sensitivity of 80% and a specificity of 91%. The pathologic ulnar nerve shows hypoechoic enlargement but rarely a focal thinning with notch sign of the nerve, perhaps because of its substantial mobility inside the cubital tunnel ( Fig. 11-8 ). Dynamic evaluation with passive flexion and extension of the elbow is important, because subluxation and dislocation of the ulnar nerve over the medial epicondyle is common in normal population but can also cause friction neuritis. To prevent nerve dislocation with the probe, we suggest the use ample amounts of gel and very light probe pressure on the skin during dynamic scanning.
