The use of ultrasonography for the diagnosis and evaluation of musculoskeletal entities has been rapidly increasing.1 One of these is the approach to peripheral neuropathies and entrapment syndromes of peripheral nerves. In this regard, ultrasonography provides a real-time examination of the nerve and its surrounding anatomic structures. The most common peripheral neuropathy is carpal tunnel syndrome (CTS). The median nerve in the carpal tunnel lies between the transverse carpal ligament volarly and the flexor tendons and carpal bones dorsally. The syndrome is thought to be caused by compression of the median nerve within its surrounding structures inside this narrow anatomical space. Therefore, the investigation of possible ultrasonography morphological changes in the carpal tunnel contents should be considered useful for the diagnosis of the syndrome.1
Ultrasonography measurements of the flattening ratio of the median nerve, anterior bowing and thickening of the flexor retinaculum, and the cross-sectional area (CSA) of the median nerve have been proposed as alternatives to electrodiagnostic testing for the diagnosis of CTS.2 Of these, measurement of the CSA of the median nerve within the carpal tunnel at the level of the pisiform, from the inner border of the echogenic epineurium surrounding the fascicles with outer epineurium excluded, has been the most researched.3 Studies that have attempted to indirectly examine the CSA using the ellipsoid formula have shown lower diagnostic accuracy3 because of the irregular shape of the median nerve; however, this formula has been widely preferred because of the high reproducibility between experienced and inexperienced observers.4 Additional characteristic parameters, such as palmar bowing of the flexor retinaculum and flattening of the median nerve, can aid the ultrasonography diagnosis of CTS.5 However, few validated quantitative scoring systems have been created for evaluating these parameters as reference standards for CTS.6
In CTS, median nerve enlargement is believed to result from large myelinated fibers at the periphery of the fascicles, interfascicular epineural fibrosis, and/or perineural thickening under chronic nerve compression.7 A higher CSA of the median nerve at the carpal tunnel inlet and a higher flattening ratio at the level of the hamate were seen in CTS wrists compared with controls. In a meta-analysis, a CSA greater than or equal to 9 mm2 was found the best single diagnostic criterion, with a sensitivity of 87.3% and a specificity of 83.3%.8 Other authors proposed measurement of the median nerve's swelling as a parameter for the diagnosis of CTS.9–11 This parameter is calculated as a ratio between the forearm and wrist and has been shown to reduce the rate of false-negative results from 37% to 2% over a single measurement of the CSA at the pisiform.9,10 An alternative measurement of swelling as the absolute change in the CSA between the forearm and carpal tunnel region has also shown high diagnostic accuracy and a 96% to 100% sensitivity.11
To evaluate the diagnostic role of ultrasonography and to compare it with nerve conduction studies (NCS) for CTS, the authors performed this study in patients with a clinical diagnosis of CTS and hypothesized that ultrasonography provides a diagnosis similar to that of NCS for CTS.
Materials and Methods
The authors studied 96 patients/hands with wrist and hand pain, numbness, tingling, and/or burning in the median nerve distribution of the hand, a positive Phalen test (73 patients), and a positive Tinel sign (65 patients). With the presumptive diagnosis of CTS, the patients were referred by their treating orthopedic surgeons (E.K., V.K., A.F.M.) to a physiatrist (E.M.) for an NCS evaluation and a radiologist (D.D.) for ultrasonography measurements. Both have approximately 10 years of experience in their specialty and were blinded to the findings of the other examiner. In all study patients, both hands were examined with both methods; in case patients were experiencing clinical symptoms in both hands, the most affected hand (according to the patients' symptoms) was used for evaluation and data analysis. Patients were excluded if they had previous treatments for CTS with splinting or surgery, a history of wrist fracture or surgery, and/or clinical diagnosis of mononeuropathy, polyneuropathy, or radiculopathy. All patients gave written informed consent for their data to be included in this study. This study was approved by the institutional review board/ethics committee of the authors' institution.
Nerve Conduction Studies Evaluation
All patients underwent median and ulnar nerve sensorimotor NCS using an electromyography (EMG) device in normal room temperature. Standard techniques of supramaximal percutaneous stimulation with a constant current stimulator and surface recording were used for NCS. The severity of nerve conduction impairment was divided into 5 grades according to the PADUA classification: negative with normal findings; minimal with abnormal segmental tests; mild/moderate with abnormal digit sensory nerve conduction velocity and abnormal distal motor latency; severe with absence of sensory response and abnormal distal motor latency; and extreme with total absence of motor and sensory responses.12
Ultrasonography measurements were done with a linear array probe of 8 to 16 MHz. The median nerve was examined axially at the levels of the distal radioulnar joint (carpal tunnel entrance) and the pisiform bone (proximal carpal tunnel). No force was applied other than the weight of the probe to avoid causing any artificial nerve deformity. The angle of the ultrasonography beam was kept perpendicular to the surfaces of the nerve and the tendons.13
Initially, the presence of nerve edema was assessed. The normal median nerve is a bundle of hypoechoic nerve fascicles surrounded by hyperechoic epineural connective tissue, all of which is encased in the hyperechoic perineural sheath. Nerve edema alters the signal produced by the nerve components and results in increased hypoechoic signal of the nerve. Next, the presence of nerve swelling and nerve flattening was assessed. Nerve swelling is defined as enlargement of the CSA of the nerve within or proximal to the carpal tunnel. The mediolateral and anteroposterior diameters and CSA of the median nerve were measured. The circumference of the median nerve and its CSA were measured using a continuous boundary trace of the nerve including the surrounding echogenic rim. Nerve flattening is defined as a decrease in the minor axis combined with an increase in the major axis of the median nerve in the carpal tunnel, at the level of the pisiform or the hamate. Finally, color Doppler ultrasonography was used to recognize the possible presence of any intra-neural vascular structures not related to the presence of a persistent median artery.
To determine reference values for the ultrasonography measurements, a control group of the same number (n=96) of patients/hands of asymptomatic volunteers was recruited (Figure 1). None of them had any clinical symptoms or signs of CTS or other peripheral nerve disorder. Controls were recruited from the healthy subjects accompanying the patients at their visit to the hospital or from the hospital staff. In the control group, the current authors used only the values from the right side to avoid artificially lowering the variance.
Ultrasonography image showing the short axis view of a normal right median nerve in an asymptomatic female patient (mediolateral diameter, 0.56 cm; cross-sectional area, 8.9 mm2).
Eighty-seven (90%) of the 96 patients/hands with clinical symptoms of CTS showed positive findings in both ultrasonography and NCS. Six patients/hands showed positive findings only in NCS, and 3 patients/hands showed positive findings only in ultrasonography; the difference was not statistically significant (P<.05). A positive correlation between the ultrasonography CSA of the median nerve and the NCS severity grades was observed (Table 1); there was a proportional increase of the ultrasonography measurements compared to the increase of the NCS severity. In the patients/hands population (both hands of study patients) in this series, 10 patients/hands with clinical symptoms of unilateral CTS showed mild and moderate NCS findings at the clinically asymptomatic contralateral hand; the sensitivity and the specificity of NCS was 97% (95% confidence interval [CI], 91%–99%) and 89% (95% CI, 82%–95%), respectively (positive predictive value, 90%; negative predictive value, 97%). In contrast, 43 patients/hands with clinical symptoms of unilateral CTS showed similar ultrasonography findings at the clinically asymptomatic contralateral hand; the sensitivity and specificity of ultrasonography was 94% (95% CI, 87%–98%) and 55% (95% CI, 45%–65%), respectively (positive predictive value, 68%; negative predictive value, 90%).
US Measurements Compared With NCS Staging of the Patients/Hands Included in This Study
Compared with the control patients/hands, ultrasonography showed that swelling of the median nerve was the major finding, followed by longitudinal irregularities of the nerve in the carpal tunnel (Figure 2). The authors used the upper limit CI to set the cutoff point (95%) for the ultrasonography diagnosis of CTS; this was a mean CSA of 20.3 mm2 for the clinically severe CTS patients/hands (Figure 3), a mean CSA of 12.2 mm2 for the clinically mild CTS patients/hands, and a mean CSA of 15.9 mm2 for the clinically moderate CTS patients/hands.
Ultrasonography image showing short axis view of the left median nerve. Swelling of the nerve is seen as it enters the carpal tunnel in a female patient with a clinical and nerve conduction studies diagnosis of moderate carpal tunnel syndrome (mediolateral diameter, 0.71 cm; cross-sectional area, 16.6 mm2).
Ultrasonography image showing the tracing method to measure the cross-sectional area of a substantially enlarged median nerve in a female patient with a clinical and nerve conduction studies diagnosis of severe carpal tunnel syndrome (mediolateral diameter, 0.9 cm; cross-sectional area, 20.1 mm2).
There is no quantitative gold standard reference for the diagnosis of CTS.14–24 Nerve conduction studies have been widely used; however, their reported sensitivity ranges from 56% to 85%.15 In contrast, comparison between ultrasonography and NCS has shown that ultrasonography is associated with a better specificity and sensitivity ranging from 86% to 92%.16 Importantly, ultrasonography is noninvasive and cost-effective and allows for real-time imaging diagnosis.17 Therefore, ultrasonography can be used as an alternative method to NCS in clinical practice or as a first-line approach for CTS screening in the patients/hands with a clinical diagnosis of CTS. In the current study, the authors compared the findings of ultrasonography and NCS in patients/hands with clinical symptoms of CTS. They found no difference between the 2 diagnostic procedures, and a similar increase in disease severity with respect to the clinical, NCS, and ultrasonography. These findings support the role of ultrasonography for the diagnosis of CTS.
The authors aimed to enhance the sensitivity of ultrasonography for the diagnosis of CTS by measuring the swelling (CSA) and the flattening (mediolateral diameter) of the median nerve inside the canal. They believed that the automated ellipsoid area of calculation that was used is the most secure approach for CSA calculation of the median nerve and shows an excellent correlation with the calculated CSA in both symptomatic patients/hands and control groups.20 Ultrasonography has the ability to measure acute physiologic changes; therefore, it may help to evaluate patients/hands with clinical symptoms of CTS but with normal NCS.18 The authors also used the classic clinical classification of the CTS (mild, moderate, severe) to better correlate clinical symptoms with NCS21 and with the therapeutic implications of the traditional orthopedic approach. Considering the difficulties that many patients with CTS go through during NCS,21 the authors believed that ultrasonography would be more preferred by the patients because of the easier, faster, and noninvasive technique. However, the impact of this strategy has to be evaluated on its long-term accuracy by more controlled trials.
This study has 3 limitations. First, the study is retrospective; however, well-controlled retrospective studies are useful to draw conclusions for a technique or a treatment approach. Second, the authors performed NCS in patients/hands with a high clinical suspicion of CTS. Nevertheless, the results do not differentiate in comparison with the data of the prevalence of CTS in the common population.23 Although the group of patients/hands that underwent EMG was smaller than the group of patients/hands (study and control group) in which ultrasonography was performed, the cutoff values used for EMG are comparable to those cited in previously published literature.12,15,16,18,21 Third, quantitative analysis of the measurements was not based on a segmental basis; instead only the maximal alterations data were used for the analysis.
In the peripheral nervous system, ultrasonography evaluation of nerves' morphological characteristics may provide a better understanding of acute changes related to the development of peripheral nerve disorders. Moreover, identification and monitoring of physiological changes may lead to improved interventions to prevent chronic disorders or diseases of the peripheral nervous system. In this setting, additional use of gray scale, Doppler, dynamic, qualitative evaluation techniques, or elastosonography may be helpful (compression or shear-wave). More specifically, elastosonography has been rapidly used for the evaluation of median nerve elasticity and correlation with CTS.19 The future research goal will be if these methods provide significant improvement over conventional gray-scale ultrasonography.
Ultrasonography provides a diagnosis similar to that of NCS in patients with clinical symptoms of CTS but with a lower sensitivity and specificity. Swelling of the median nerve, a low CSA, and longitudinal irregularities of the nerve in the carpal tunnel are the most important ultrasonography findings. Therefore, ultrasonography may be used as a primary diagnostic method and/or it may increase the diagnostic accuracy of NCS in doubtful cases.
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- Sernik RA, Abicalaf CA, Pimentel BF, Braga-Baiak A, Braga L, Cerri GG. Ultrasound features of carpal tunnel syndrome: a prospective case-control study. Skeletal Radiol. 2008;37(1):49–53. doi:10.1007/s00256-007-0372-9 [CrossRef]
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US Measurements Compared With NCS Staging of the Patients/Hands Included in This Study
|NCS EMG Severity Staging (No. of Patients)||Mean±SD US Measurements|
|CSA, mm2||FR, mm|