Correlations of Median Nerve Area, Strain, and Nerve Conduction in Carpal Tunnel Syndrome Patients

摘要

Carpal tunnel syndrome (CTS) is a compression neuropathy of the median nerve at the wrist. The diagnosis of CTS is usually based on a typical history, clinical examination, provocative test, and nerve conduction study. The most common symptom in idiopathic CTS is a tingling sensation along the median nerve distribution in the hands. The value of provocative physical tests, such as Tinel’s sign or Phalen’s test, is controversial, and results are often of doubtful clinical significance.4 As there is no clear definition of the severity of numbness, its clinical severity has sometimes been defined by the thumb opposition strength.11Nerve conduction studies have been reported to be highly specific in some studies.1,19 The median nerve shows a delay in the sensory and motor conduction velocities. Nerve conduction studies focus on defining whether there has been damage to the median nerve inside the carpal tunnel to quantify the severity of the nerve damage and to define the physiology of the disease as a conduction block, demyelination, or axonal degeneration. Because of the high specificity and sensitivity, nerve conduction studies are considered to be the gold standard for CTS diagnosis.In recent years, imaging techniques such as ultrasonography have been shown to be of value in the diagnosis of CTS. They have the potential advantages of lower cost, shorter examination time, and noninvasiveness. It is well established in the literature that ultrasound imaging can detect pathologies such as thickening and echogenicity alteration of the flexor tendons17 and flexor retinaculum,8 synovial proliferation, swelling of the median nerve in the proximal part of the carpal tunnel, and flattening of the median nerve in the carpal tunnel.2,7,14 In addition to the morphological changes, an imaging method for measuring tissue strain using a conventional ultrasound machine, so-called ultrasound elastography, has been introduced.3,21 Ultrasound elastography can estimate tissue stiffness either by strain analysis of tissue under compression (quasistatic methods) or by the imaging of shear waves.10 Recently, the application of ultrasound elastography for clinical diagnosis has been expanded.5,6,20,24,25 In previous studies, median nerve strain was measured and compared between CTS patients and healthy controls using ultrasound elastography.13,22 These studies suggested that the median nerves of CTS patients are harder (lower strain) than those of controls. In addition, it was suggested that the strain ratio (strain of the reference coupler / strain of the median nerve) was useful to standardize the results for the stiffness evaluation.27With compression of a normal nerve trunk, the structure is deformed when compression tissue is redistributed into noncompressed areas. It has been suggested that large fascicles in a small amount of epineurium are more vulnerable to compression than several small fascicles embedded in a large amount of epineurium.15 This is because the latter can transform larger than the former. This may be different in the pathological condition. However, the relations among the morphology, material property, and physiological function of the peripheral nerve are still unclear. To understand the pathological basis of nerve compression lesions, it is necessary to understand the effects of all individual characteristics of the nerve components. In this study, we wished to submit a new type of discussion for the progress of entrapment neuropathy by evaluating the relations of morphological, mechanical, and functional diagnostic tests. The objective of this study was to see if ultrasound-interpreted median nerve strain and cross-sectional area correlate with abnormal nerve conduction studies and thumb opposition strength in patients with CTS. We hypothesized that there are correlations between each evaluation; in addition, clinical staging of the disease can be expected with diagnostic tests.