Anomalous origin of the thenar motor branch encountered during carpal tunnel release

Abstract

Background:

This illustrative case demonstrates the thenar motor branch (TMB) arising from the median nerve proper through the palmaris fascia, visualized during a carpal tunnel release procedure.

Case Description:

A 64-year-old with a history of hypertension and diabetes presents to the clinic with 1 year of neck pain, upper extremity numbness and tingling, wasting of hand muscles, and weakness of the left hand. Electromyography confirmed cubital tunnel syndrome bilaterally and right carpal tunnel syndrome. The patient underwent right cubital and carpal tunnel decompressions.

Conclusion:

Awareness of TMB anomalies and careful identification during surgery can prevent iatrogenic injury and further complications.

INTRODUCTION

Carpal tunnel release is a procedure performed across multiple surgical specialties for the treatment of carpal tunnel syndrome. These symptoms include numbness and tingling in the first three digits that can be triggered by flexion or extension of the wrist.[2] Patients will notice paresthesias that wake them from sleep and resolve with shaking of the wrist and hand. An electromyography/nerve conduction (EMG/NC) study is performed to confirm the clinical diagnosis and will demonstrate the slowing of signals across the carpal tunnel. The carpal tunnel is defined as the space below the transverse carpal ligament. Surgery to divide the transverse carpal ligament and release median nerve compression is a common outpatient procedure. This procedure has a low complication rate and a high success rate. There are, however, anatomical variants that can result in significant complications.

ILLUSTRATIVE CASE

A 64-year-old right-hand dominant male with a history of diabetes and hypertension presented with 1 year of neck pain and associated upper extremity numbness and tingling. Physical examination showed atrophy of his hand muscles, specifically wasting of the first dorsal interosseous muscle more prominently on the left than right. Strength examination was notable for hand weakness greater on the left than right and full strength in triceps, biceps, and deltoids. EMG/NC studies revealed severe bilateral ulnar neuropathy at the elbow with associated denervation as well as severe conduction slowing of the median nerve at the right wrist.

To prevent progressive muscle wasting, weakness, and numbness, the patient underwent right ulnar nerve and carpal tunnel release. The left ulnar nerve release was planned for a month later.

The incision for carpal tunnel release was planned from above the first wrist crease to Kaplan’s cardinal line. The skin was opened sharply, and subcutaneous fat was opened. The palmar aponeurosis was opened sharply. During the opening of the palmar aponeurosis, a nerve was observed perforating the flexor retinaculum (transverse carpal ligament) at its distal 2/3 portion on the ulnar side. This nerve coursed across the flexor retinaculum to the thenar musculature [Figure 1]. The nerve branch was accompanied by small vessels. Upon recognition of the aberrant nerve branch, a checkpoint disposable nerve stimulator was opened and used to assess the function of this nerve. 0.2 mA stimulation of the branch produced contraction of the abductor pollicis, opponens pollicis, and flexor pollicis brevis [Video 1]. The flexor retinaculum was sharply incised around the aberrant thenar motor branch (TMB) carefully bisecting the ligament proximal and distal to the perforating nerve branch. The accompanying video demonstrates stimulation of the aberrant motor branch following the complete opening of the flexor retinaculum. The surgery was completed in the usual fashion, and the patient experienced excellent symptomatic relief from the nerve entrapments.

Figure 1:

Illustration of intraoperative findings with thenar motor branch traversing the preligamentous space in a ulnar to radial direction.

DISCUSSION

Typically, the median nerve divides into medial and lateral branches at the distal aspect of the transverse carpal ligament. The medial branch divides into two common palmar digital nerves supplying motor innervation to the second lumbrical and sensory innervation to the palm and fingers.[1] The lateral branch divides into the TMB before giving rise to proper palmar digital nerves supplying motor innervation to the first lumbrical and sensory innervation to the lateral side of the hand.[1] The TMB supplies motor innervation to the opponens pollicis, the abductor pollicis brevis, and the superficial part of the flexor pollicis brevis.[1] Because of the importance of thumb motor function, this nerve is known as the “million-dollar nerve” due to high legal fees after injury to it during surgery.[3]

In 1977, Lanz[5] described four categories of variations found in the median nerve at the carpal tunnel and suggested that variation was highly prevalent. These include variations in the course of the thenar branch, accessory branches at the distal carpal tunnel, high divisions of the median nerve, and accessory branches proximal to the carpal tunnel.[1]

A more recent prospective analysis of 890 carpal tunnel release surgery found that TMB variations were observed much less frequently with the current surgical techniques. In this series, 4 instances of anomalous TMB were encountered, or approximately 0.5%. In two cases, the TMB arose from the volar aspect of the median nerve and penetrated the midportion of the transverse carpal ligament. One TMB originated from the volar and ulnar aspect of the median nerve. One TMB originated from the ulnar aspect of the median nerve proximal to the carpal tunnel.[6] This case report describes a rare variation of an anomalous TMB arising in the carpal tunnel from the volar and ulnar side of the median nerve and piercing the flexor retinaculum on the ulnar side coursing to the thenar muscles above the flexor retinaculum. Mullin et al.,[6] described a similar variant with the TMB coursing above the flexor retinaculum, which they termed a “preligamentous variation.”[4] Awareness and recognition of these variants are essential in preventing iatrogenic injury. Endoscopic recognition of this preligamentous variant may be particularly difficult with typical techniques.

Lessons

Awareness of median nerve anomalies and careful identification during surgery can prevent injury to the TMB of the median nerve. It is important to recognize that there are many variations in the anatomy of the median nerve at the carpal tunnel, some not described.

CONCLUSION

Carpal tunnel release is considered a straightforward operation. Multiple surgical subspecialties routinely perform it with a low complication rate and high success rate. Infrequently, however, there are anatomical variants that can result in surgical injury with significant functional impairment of the hand. This illustrative case demonstrates the TMB arising in the carpal tunnel, piercing the flexor retinaculum, and coursing in the preligamentous space. Early recognition and nerve stimulation allowed for the diagnosis of this variation and successful median nerve release.

Ethical approval:

Institutional Review Board approval is not required.

Declaration of patient consent:

The authors certify that they have obtained all appropriate patient consent.

Financial support and sponsorship:

Nil.

Conflicts of interest:

There are no conflicts of interest.

Use of artificial intelligence (AI)-assisted technology for manuscript preparation:

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

Video available on:

https://dx.doi.org/10.25259/SNI_3_2025

Disclaimer

The views and opinions expressed in this article are those of the authors and do not necessarily reflect the official policy or position of the Journal or its management. The information contained in this article should not be considered to be medical advice; patients should consult their own physicians for advice as to their specific medical needs.