Ultrasound (US) is an excellent modality for evaluating the carpal tunnel. Its advantages include being patient friendly, readily available, and offering diagnostic performance comparable to electrodiagnostic studies. Importantly, US provides direct anatomic assessment, enabling detection of anatomic variations that may increase the risk of iatrogenic injury during carpal tunnel release (CTR). It can also identify many causes of secondary carpal tunnel syndrome (CTS) and distinguish CTS from several “CTS look-alikes.”
In this individual, referred for CTS evaluation and consideration for an US-guided CTR, US identified a transligamentous branch (TLB) of the median nerve (MN). This first video shows a short-axis (SAX) view of the mid-carpal tunnel at the level of the hook of the hamate (HH). As I sweep slowly back and forth, a hypoechoic structure is seen exiting the MN and coursing through the transverse carpal ligament complex (TCL-C) into the palmar subcutaneous tissue – findings consistent with a TLB.
I find it important to confirm a TLB in orthogonal views. In the long-axis (LAX) video, the TLB is clearly visualized branching from the MN, passing through the TCL-C, and entering the palmar subcutaneous tissue (blue arrow). In this case, the TLB functions as a secondary palmar cutaneous branch, given its termination within the palmar subcutaneous tissue. However, a TLB may also function as an accessory thenar motor branch in other individuals.
This final image is a SAX view obtained during the US-guided CTR. The device is positioned along the ulnar aspect of the MN, with two balloons deployed to create a safe working corridor between the nerve, the cutting blade, and the ulnar artery. The blade is visualized ulnar to the TLB during transection of the TCL-C. Recognition of this anatomic variation prior to CTR is important and can reduce the risk of iatrogenic nerve injury – as demonstrated in this case.
