Ultrasound in the diagnosis of mononeuropathy: Future directions

In this issue, Hooper and colleagues describe the ultrasonographic findings in a small series of patients with genetically proven hereditary neuropathy with liability to pressure palsies (HNPP) and highlight the increasing utility of diagnostic ultrasound (US) in the arena of neuromuscular disease. Until recently, electrodiagnosis (EDX) has been the primary diagnostic test for disorders of peripheral nerve and muscle. Magnetic resonance imaging, muscle biopsy, metabolic assays, and genetic testing are also integral to the workup of selected cases. In recent years, as highlighted by Walker and Cartwright in their recent editorial in this journal, advances in technology and the everincreasing availability of high-resolution, portable, relatively low-priced ultrasound (US) equipment has rapidly increased the use of this imaging modality in the diagnosis of both nerve and muscle disease. In the diagnosis of mononeuropathy, neuromuscular US can add significant diagnostic information that is not obtained with EDX alone by identifying potentially causative structural lesions such as ganglion cysts, tumors, synovitis, etc. In some cases, US can identify the site of entrapment when EDX is non-localizing, or it can localize the site of nerve transection. Although used most commonly in the diagnosis of entrapment neuropathies, neuromuscular US may also provide valuable information in diffuse or multifocal peripheral neuropathies. In this issue, Hooper et al. describe the ultrasonic findings for three common nerves (median, ulnar, and tibial) in 7 patients with genetically proven HNPP as compared with normal control subjects. They found focal enlargement of at least four or more nerves in 6 of 7 HNPP patients, whereas the remaining patient had no enlarged nerves. All sites of nerve enlargement were at typical sites of entrapment (median/wrist, ulnar/elbow, tibial/ankle), with the exception of a single patient in whom enlargement of the nerve was seen at all sites tested, including the median and ulnar nerves in the forearm. Two potential control subjects were excluded due to focal enlargement of the median and/or ulnar nerves, which they attributed to subclinical entrapment neuropathy. At this time, some practitioners of neuromuscular US believe there is adequate data available to support the use of US in isolation, at least in the diagnosis of median neuropathy at the wrist and ulnar neuropathy. Nonetheless, many unanswered questions remain regarding the role of US in detecting specific structural changes that may be seen in entrapment. Cross-sectional area (CSA) of the nerve in question appears to be the most reliable measurement, although it is not yet clear whether patients must serve as their own controls. Besides CSA, there are a number of other potentially useful parameters that have been considered. Swelling ratios are useful in focal neuropathies, because patients can serve as their own controls, eliminating the impact of possible confounding variables such as weight and height. The CSA of the nerve at the site of entrapment is compared with a non-affected portion of the nerve elsewhere. Nerve diameter and flattening ratios have been used in some studies in isolation or in addition to CSA. Some investigators have reported qualitative measurements such as nerve echogenicity, vascularity, mobility, or fascicular pattern; however, these variables are subjective and are currently limited by the lack of quantitative measures. The study by Hooper et al. highlights some of the challenges encountered when attempting to incorporate neuromuscular US into clinical practice. Generalized use of US is limited due to variability of normal values between EDX laboratories and study populations, the need for high levels of intraand interrater reliability, and the impact of body metrics on image quality and some quantitative measures. Some of the variability likely reflects the fact that US is highly operator-dependent, with a relatively flat learning curve, and that limited Correspondence to: A. J. Boon; e-mail: boon.andrea@mayo.edu