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Painful Flexor Carpi Radialis Brevis Muscle: An Ultrasound and Magnetic Resonance Imaging Assessment

With the advent of modern cross-sectional imaging techniques, such as ultrasound (US) imaging, computed tomography, and magnetic resonance imaging (MRI), accessory muscles are frequently encountered and can be accurately assessed noninvasively. Accessory muscles are frequently asymptomatic and are commonly overlooked during routine imaging evaluations performed for other purposes. In a minority of patients, they present as painless masses or can cause local symptoms related to compression of adjacent structures such as nerves, vessels, and tendons. Awareness of the existence of accessory muscles as well as their location, resulting in possible nerve compression, is necessary for establishing an accurate diagnosis and appropriate treatment for compressive neuropathies. Accessory muscles can present with symptoms of median or ulnar nerve compression. The flexor carpi radialis brevis is a rare anomalous muscle of the wrist first described by Fano in 1851 and subsequently by Woods in 1867 (Figure 1). The incidence of the flexor carpi radialis brevis has been reported to be 2.6% to 7.5%. We report the US and MRI findings of a flexor carpi radialis brevis causing pain in the distal radial forearm, irradiating into the volar wrist. A 62-year-old professional piano player presented with overuse pain in the radial volar aspect of the distal right forearm and wrist. The patient had a nearly unremarkable clinical history. The clinical examination showed a normal appearance of the forearm and wrist. The skin was normal. No tenderness was noted even at deep palpation. A US examination was performed with a broadband high-resolution transducer working at 5–18 MHz to rule out tendinopathy. Ultrasound showed the presence of an accessory muscle arising from the lateral part of the volar aspect of the distal radius, distally to the origin of the flexor pollicis longus (Figures 2 and 3). The muscle ran anterior to the pronator quadratus and presented a thin tendon located deep to the flexor carpi radialis tendon. Distally, the flexor carpi radialis brevis tendon inserted into the capitate bone and the base of the third and fourth metacarpals. The muscle showed a regular echo structure, and the tendon was normal, with no sign of tendinopathy. On US imaging, the clinically painful site corresponded to a normal-appearing proximal aspect of the muscle. Contralateral US images showed absence of accessory muscles. To confirm the clinical suspicion of overuse edema of the accessory muscles, we performed an MRI examination after asking the patient to play piano for 3 hours. The scan was performed about 30 minutes after the patient played the piano. The MRI scan was performed with a 1.5-T scanner using an extremity coil. Axial proton density– weighted fat-saturated and T1-weighted, sagittal T1weighted, and postcontrast axial T1-weighted fat-saturated images were obtained. Contrast images were performed because of the edema noted in the muscle to increase the detection of pathologic findings. The MRI confirmed the presence of the flexor carpi radialis brevis and had an excellent imaging correlation with US (Figures 2 and 3). In addition, MRI showed local edema and gadolinium uptake in the postcontrast T1-weighted, fatsaturated images of the proximal flexor carpi radialis brevis and at its attachment into the radius (Figure 4), confirming that the local swelling and inflammation were responsible for the clinical symptoms. The patient