Background/Purpose: Carpal tunnel syndrome (CTS) is the most frequent neuropathy of all entrapment neuropathies, believed to be present in 3.8% of the general populatio. Idiopathic CTS is the most common diagnosis in patients with CTS. However, there are a lot of conditions that generate secondary CTS, it is considered that rheumatoid arthritis (RA) is one of the disease generate secondary CTS. The pathophysiology of CTS of RA might be a little bit different from idiopathic CTS. RA is a disease that has the characteristics to generate inflammatory synovial proliferation of the joint and tenosynovitis. Although inflammation of the wrist joint and synovial tissue of the flexor tendons can cause increased pressure in the carpal tunnel, there is a possibility that even RA patients without symptoms of CTS also have subclinical median nerve damage because of the synovial proliferation and inflammation. This aim of this study was to compare the elasticity of the median nerve between patients with RA without symptom of CTS and controls by quantitative elastosonography.

Methods: This study was performed with institutional review board approval and written informed consent from all participants. Four hundred two hands in 201 patients with RA and 222 hands in controls were included. All participants were examined both wrists. Patients were excluded in this study if they had conditions associated with an increased incidence of CTS (diabetes mellitus, acute trauma, pregnancy, hypothyroidism, hyperthyroidism or connective tissue disease except for RA), history of wrist or hand fracture, surgery, history of other systemic neurologic disorders and radiculopathy. Patients with bifid median nerve, or any mass lesion identified on US examination of the wrist were also excluded from this study. US was performed by using a 5- to18-MHz linear array transducer (HI VISION Ascendus; Hitachi-Aloka Medical,Tokyo, Japan). We attached an acoustic coupler (EZU-TECPL1; Hitachi-Aloka Medical) with a standardized elasticity to the transducer as a reference medium. The inlet of the carpal tunnel at the scaphoidpisiform level and the proximal portion of the carpal tunnel inlet were scanned in a transverse plane. The cross-sectional area (CSA) and the elasticity of the median nerve, which was measured as the acoustic coupler /median nerve strain ratio, were evaluated. The measurements were repeated two times, and the average strain ratio was used for analysis.

Results: Three hundred forty-two hands in 177 patients with RA (139 female, mean age was 63.8 years old) and 158 hands in 81 controls (68 female, mean age was 71.5 years old) were analyzed. The CSA of the median nerve at the level of inlet of the carpal tunnel and proximal portion of the carpal tunnel inlet were not significantly differenced in both groups. Strain ratio in the patients with RA were significantly higher than those in the controls (2.66 vs 2.20; p=0.003 in right hand, 2.59 vs 2.13; p=0.002 in left hand) at the inlet of the carpal tunnel level. However, strain ratio at the proximal portion of the carpal tunnel inlet level was not significantly differenced in both groups.

Conclusion: The median nerve stiffness measured by elastosonography in patients with RA without symptom of CTS is higher than controls. This results suggest that inflammation of flexor tendon and wrist joint may generate fibrotic change for median nerve.

Figure: The images of acoustic coupler /median nerve strain ratio.