Identification of Aberrant Muscle Bellies in the Carpal Tunnel using Sonography

Abstract

Musculoskeletal sonography is being widely used for evaluation of structures within the carpal tunnel. While some anatomical variants, such as bifurcated median nerves and persistent median arteries, have been well documented, limited literature describes the sonographic appearance of aberrant muscle bellies within the carpal tunnel. Multiple examples of the sonographic appearance of flexor digitorum superficialis and lumbrical muscle bellies extending into the carpal tunnel are provided. Techniques for static image acquisition and analysis are discussed, and the use of dynamic imaging to confirm which specific muscle belly is involved is described. Knowledge of the potential presence of muscle bellies in these images and ability to identify these structures is vital to avoid misclassification or misdiagnosis as abnormal pathology. The case examples are situated among current published evidence regarding how such anomalies may be related to the development of pathologies, such as carpal tunnel syndrome.

Keywords: Carpal tunnel; aberrant muscle; musculoskeletal sonography.

Figure 1.

This image demonstrates the sonographic evaluation of the carpal tunnel at the level of the scaphoid (Sca) and Pisiform (Pis) in the transverse view. The median nerve (arrow-head) is positioned posterior to the transverse carpal ligament (curved arrow) and anterior to the flexor pollicis longus (FPL), flexor digitorum superficialis (S), and profundus (P) tendons. The flexor carpi radialis (FCR) tendon lies lateral and the ulnar artery is positioned medially in Guyon’s canal. (Image reprinted courtesy of the Journal of Diagnositc Medical Sonography from: Evans, Roll, Li and Sammet, 2009).

Figure 2.

A hand in the “lumbrical plus” position due to active engagement of the lumbrical muscles of the fingers, which lead to simultaneous flexion of the metacarpophalangeal joints (i.e., knuckles) and extension of the interphalangeal joints (i.e., fingers).

Figure 3.

Two long flexor muscle bellies (white arrows) are present in the right carpal tunnel on either side of the median nerve (asterisk) while the hand and fingers were relaxed (A). As the participant flexed his fingers to make a fist, the flexor muscle bellies retracted proximally out of the carpal tunnel, resulting in an image with no evidence of muscle bellies at full flexion (B).

Figure 4.

Serial images of the left carpal tunnel at the level of the pisiform. The participant was asked to open her hand with extended fingers (A), slowly flex her fingers to make a fist (B), hold the fist position for one second (C), and extend the fingers once again (D). As the participant makes a fist, the long flexor muscle belly (white arrow in A), retracts proximally into the forearm and is replaced by the flexor tendon (gray arrow in B). When the fingers are fully flexed, the lumbricals enter the carpal tunnel (blue arrows in C). Once the fingers return to extension, the long flexor muscle belly is seen in the tunnel once again (white arrow in D).

Figure 5.

A small portion of a lumbrical muscle (white arrow) is present in the carpal tunnel next to the median nerve (asterisk) when the participant’s fingers are relaxed in partial extension (A). Identification of this structure as a lumbrical muscle is confirmed as this structure becomes enlarged along with additional muscles (white arrows) as the participant makes a fist (B).