Literature ReviewUltrasonographic Evaluation of Peripheral Nerves
Introduction
Ultrasonography (US) remains one of the most time-efficient and cost-effective diagnostic imaging modalities of peripheral nerves. Historically, the role of US in the diagnosis of nerve injury was limited, largely because of low-frequency transducers. However, with technological advancements in equipment, including the development of high-frequency ultrasound transducers and refinements in scanning techniques, high-resolution imaging of relatively small peripheral nerves is possible with resolutions that are higher than is achievable with clinical magnetic resonance imaging (MRI) scanners. Although electrophysiologic studies provide important diagnostic data in evaluating the relative location and degree of nerve dysfunction, they are limited in their ability to identify morphologic changes associated with a particular type of nerve injury. US can reliably provide this information, and it does so in a painless manner, compared with electrodiagnostic studies.
Compared with MRI, US provides images that are of higher resolution. The axial in-plane resolution of a 10-MHz probe is approximately 150 μm.1 In comparison, approximate resolution of a common clinical MRI is 450 μm.2 Clinical imaging transducers can reach frequencies up to 18 MHz, with further improvements in resolution. US is therefore superior in visualizing the ultrastructure of individual nerves and in evaluating small-caliber nerves such as digital nerves. US also allows for assessment of nerves near artificial implants, which may compromise MRI resolution.
Support for the use of US compared with MRI comes from a retrospective study by Zaidman et al.,3 in which 53 patients with mononeuropathies or brachial plexopathies underwent both US and MRI. Among the 46 patients with neuropathology diagnosed by surgical (n = 39) or clinical/electrodiagnostic (n = 14) evaluation, US detected neuropathology more frequently than MRI, with a sensitivity of 93% versus 67%. Specificity of both studies was similar (86%). MRI techniques optimized for nerve imaging, such as magnetic resonance neurography and diffusion tractography, may improve the sensitivity of MRI detection of lesions of the peripheral nervous system.4, 5, 6, 7, 8 In a multicenter study of 204 patients,9 the combined use of US and MRI yielded an overall sensitivity of 76% and a specificity of 96% in detecting brachial plexus disease.
A unique feature of US, and an important advantage over MRI, is the ability to perform dynamic examination of nerves portably. This feature allows its use in both the clinic and the operating room setting. Another important advantage of US is that the acquisition of US images is more time efficient and cost efficient than MRI, especially when evaluating a nerve over a long anatomic region (e.g., the entire length of an extremity), and can be repeated easily. However, new three-dimensional anatomic MRI sequences allow for better profiling of longer stretches of nerves.10 For patients with contraindications to MRI (e.g., a cardiac pacemaker), US is ideally suited because there are essentially no contraindications to its use. US is also favorable for patients who are claustrophobic and who may not tolerate MRI. US is preferred by patients, even over MRI, as seen in a recent assessment comparing patient satisfaction in patients with full-thickness rotator cuff tears undergoing evaluation by US or MRI; unlike with MRI, patients experience minimal pain or discomfort and receive real-time feedback from the examiner with US.11
Despite these advantages, US is not without limitations. The size of the patient, the depth of the nerve, and the presence of bone between the probe and nerve may limit adequate visualization. Also, it does not provide a comprehensive anatomic view of the affected area, as is obtained with MRI. Importantly, US requires specialty training in acquisition and interpretation of images; the quality of the information obtained is operator dependent. In this article, the use of US is described in the evaluation of multiple peripheral nerve diseases, including trauma, neoplasia, infection, and compression, as well as additional nerve-related clinical applications such as regional anesthesia.
Section snippets
Normal US Appearance
Evaluation via US anatomically spans a large extent of a nerve course. For example, in the US survey of the brachial plexus, evaluation can be performed from the spinal nerves and trunks of the brachial plexus to the digital nerves and is limited only by the user's knowledge of regional anatomy and topography. Some general landmarks include the brachial artery in the arm for the median nerve, the ulnar artery at the wrist for the ulnar nerve, the medial epicondyle at the elbow for the ulnar
Traumatic Nerve Injuries
In 1941, Seddon20 classified nerve injuries based on 3 main types of nerve fiber injury: neurapraxia, axonotmesis, and neurotmesis. Neurapraxic injury is verified on US as a swollen nerve with a hypoechoic appearance; however, axonotmetic and neurotmetic injuries are difficult to reliably distinguish using US and often require surgical exploration with intraoperative electrophysiologic assessment of the damaged nerve segments. Both neuromas-in-continuity and stump neuromas can be visualized as
Tumors
In general, US adequately shows various nerve sheath tumors, including schwannomas, neurofibromas, perineuriomas, and malignant peripheral nerve sheath tumors. Mass lesions are identified as relatively well-demarcated lesions of varying echogenicity. Although no imaging modality can definitively differentiate between schwannomas and neurofibromas, the US finding of a nerve eccentrically entering a mass is typically seen only in schwannomas, which allows for differentiation between neurofibromas
Infective Lesions
Infective nerve lesions, such as leprosy, can also be detected by US. In such lesions, nerve enlargement and edema occur with loss of fascicular architecture and increased vascularity of the perineurium and endoneurium.40, 41, 42 These findings may be apparent even before the onset of significant clinical nerve dysfunction. Early identification of nerve impairment can reduce disability associated with this disease.43
Entrapment Neuropathies
Some of the most common entrapment neuropathies, such as carpal tunnel syndrome (CTS) and cubital tunnel syndrome, can be reliably identified via US. Although the diagnosis of most entrapment neuropathies relies heavily on the clinical history and physical examination, imaging studies such as US can play a major role in challenging cases when the clinical diagnosis is not clear. The US imaging hallmark of an entrapped nerve is the hypoechoic swelling of the nerve just proximal to the site of
Additional Clinical Applications
In addition to diagnosis of peripheral nerve diseases, US has several other nerve-related clinical applications based on the ability to localize structures and guide interventions. Compared with conventionally administered nerve blocks reliant on palpable anatomic landmarks, ultrasound-guided administration of regional anesthesia permits direct visualization of neural structures, identification of anatomic variants, and monitoring of spread of local anesthetic.65 For brachial plexus blockade,
Conclusions
US is a useful modality in the evaluation of almost all peripheral nerve diseases. US provides a cost-effective, time-efficient, and well-tolerated method to assess the anatomic course of a nerve and identify any structural abnormalities and sites of compression. The unique ability of the modality to assess an entire peripheral nerve in minutes and to dynamically evaluate a nerve, along with its lack of contraindications, are also important advantages over MRI. This tool is valuable to the
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2019, Journal of Hand SurgeryCitation Excerpt :The characteristic findings of chronic nerve entrapment include nerve swelling proximally and sometimes distally with an abrupt transition to a narrower caliber and flattened contour at the site of compression, deemed the “notch” sign (Fig. 3). On ultrasound, there is hypoechoic blurring of the normal fascicular “honeycomb” pattern (Fig. 6).6 On MRN fluid-sensitive sequences, the nerve develops a conspicuous hyperintensity adjacent to the compressive lesion, proximal fascicular enlargement, and, in severe cases, hyperintensity along the distal course of the degenerating nerve and denervated muscle (Fig. 7).3
Conflict of interest statement: The authors declare that the article content was composed in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.