The ultrasonographic wrist-to-forearm median nerve area ratio in carpal tunnel syndrome
Introduction
Median neuropathy due to compression at the wrist, commonly known as carpal tunnel syndrome (CTS), is one of the most common indications for electrodiagnostic testing. A recent study performed in the United Kingdom revealed an incidence of CTS of 280.6 per 100,000 patient visits to general practitioners per year (Latinovic et al., 2006). As the population ages, and with obesity and diabetes mellitus becoming more prevalent, the incidence of CTS is likely to increase. Accurate diagnosis of CTS is important in guiding care and preventing possible disability and morbidity. Currently, electrophysiologic testing is generally considered to be the gold standard for the diagnosis of patients with a clinical presentation suggestive of CTS. Nerve conduction studies are highly specific (Jablecki et al., 2002) but may not be diagnostic in 10–25% of patients with clinical evidence of CTS depending on the severity of disease and the type of nerve conduction techniques used (Duncan et al., 1999, Jablecki et al., 2002, Preston, 2002). Newer techniques are reported to yield sensitivities near 98% but are not yet widely employed (Löscher et al., 2000).
Peripheral nerve ultrasonography is emerging as a promising diagnostic tool for entrapment neuropathies, particularly CTS. Several publications have specifically addressed measurement of median nerve area at the wrist as a means of diagnosing CTS (Buchberger et al., 1991, Hammer et al., 2006, Wong et al., 2004, Yesildag et al., 2004, Ziswiler et al., 2005). This is based on the premise that the median nerve is enlarged proximal to the site of compression in the carpal tunnel, which is supported by gross pathologic findings at the time of surgery (Tuncali et al., 2005). The nerve area is typically measured ultrasonographically by tracing the circumference of the median nerve in the proximal carpal tunnel at the level of the pisiform bone and calculating its area, although there is variability in the methods used among the published reports (Koyuncuoglu et al., 2005).
Normal limits for median nerve area have varied among reports, ranging from 7 to 9.4 mm2 (Hammer et al., 2006, Walker, 2004, Werner et al., 2004); the values for diagnosing CTS range from 9 to 15 mm2 (Beekman and Visser, 2003). Much of this variability is due to differences in measurement techniques, along with differences between populations studied.
Ultrasonography provides a simple, non-invasive means of visualizing peripheral nerve pathology, and its role in the diagnosis of CTS is promising. However, simple measurements of median nerve area at the wrist may not represent the optimal ultrasonographic parameter for diagnosis. For example, a patient with demyelinating hereditary sensorimotor neuropathy might have diffuse enlargement of all nerves (Martinoli et al., 2002). A single measurement at the wrist could result in a false positive ultrasound diagnosis of CTS. Given that recent data have shown normal median nerve area to be the same at the wrist and in the forearm (Cartwright et al., 2006),we hypothesize that a ratio of values at these sites may provide an alternative means of diagnosis. Given these findings, the ratio of median nerve area at the wrist as compared to the forearm (WFR) should approach 1:1. With median nerve enlargement at the wrist, as in CTS, a larger ratio is expected. This ratio should be less affected by the effects of variables such as weight and gender, as patients serve as their own controls. A ratio would also be less affected by differences in measurement technique.
The idea of using an ultrasonographic ratio as a diagnostic parameter for CTS is not entirely new, although our methods are. Keberle et al., 2000 calculated a “swelling ratio” between the median nerve area at the pisiform bone and distal radioulnar joint, and found it to be significantly different between 19 wrists in 10 controls and 15 CTS wrists in 8 patients. However, these sites are often technically difficult to image and prone to error, as described by the authors. By measuring at the distal wrist crease and a point 12 cm proximal in the forearm, easily reproducible sites not susceptible to significant imaging artifact were selected. In this study, we collected and compared preliminary data on the WFR of median nerve area in patients with electrodiagnostic evidence of CTS and healthy controls.
Section snippets
Methods
This prospective study compared patients seen in the Duke University Medical Center Electromyography Laboratory who had electrodiagnostic evidence of CTS to asymptomatic volunteers. Patients <18 years of age or having had a prior carpal tunnel release procedure were excluded. Women known to be pregnant were also excluded, as CTS during pregnancy may have a different etiology than CTS in other circumstances. The exclusion of pregnant women was not a factor in the study results, as none presented
Results
Characteristics of the control group and CTS patients are listed in Table 1. Eighteen patients comprised the control group. Forty-five patients with CTS were examined with ultrasound. One patient with CTS was excluded because accurate forearm median nerve measurements could not be obtained due to morbid obesity. Both arms were imaged in each control and the side with the largest WFR was used for calculations. In patients with CTS, the WFR on the least electrodiagnostically affected side was
Discussion
The best means of confirming the clinical diagnosis of CTS has long been a subject of debate. Physical examination findings, various electrodiagnostic tests, and improvement after surgery have all been touted. Most recently, median nerve sonography has joined the list of potential diagnostic tools. As previously discussed, measurements of median nerve enlargement at the wrist have been promising, but somewhat difficult to incorporate into clinical practice because of variability in results.
In
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