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technique SCIENCE OF NERVES Guyon's Canal Syndrome By Whitney Lowe The median nerve travels through a soft-tissue tunnel at the base of the hand. Nerve entrapment within the tunnel constitutes the most common upper extremity nerve entrapment, which we all know as carpal tunnel syndrome (CTS). People experiencing neurological symptoms in the hand often immediately suspect CTS as the cause. However, the median nerve is not the only one affected in the wrist and is also not the only nerve that travels through a soft- tissue tunnel at the base of the hand. The ulnar nerve also courses through a soft-tissue tunnel at the base of the hand, known as Guyon's canal, or Guyon's tunnel. Compression of the ulnar nerve within this tunnel is known as Guyon's canal (or tunnel) syndrome. It is far less common than CTS, but an important condition to be aware of when nerve pain in the upper extremity is present. ANATOMICAL BACKGROUND Guyon's canal is created by a series of stabilizing ligaments and connective tissues of the wrist and hand. The flexor retinaculum, also called the transverse carpal ligament, makes up the roof of the carpal tunnel (Image 1). This same connective tissue structure makes up the floor of Guyon's canal, so this tunnel is actually superficial to the carpal tunnel on the ulnar aspect of the wrist. The connective tissues of the wrist and base of the hand blend together in a complex webbing. As a result, it is difficult to separate and distinguish the exact boundaries of some of these connective tissues, which is the likely reason for the discrepancy in identifying the upper margin or roof of Guyon's canal. Some sources say the roof of the canal is created by the palmar carpal ligament (Image 2); 1 others say it is created by a connective tissue expansion of the flexor carpi ulnaris tendon. 2 It is important to note that the transverse carpal ligament is at the base of the tunnel, and there is a connective tissue band across the top that may extend from several tissues. Regardless of which tissue the tunnel roof originates from, it provides the upper boundary for the tunnel. A key difference between the carpal tunnel and Guyon's canal is the absence of tendons in Guyon's canal. There are nine tendons that run through the carpal tunnel, along with the median nerve. Because there are so many structures going through the tunnel, nerve compression can occur from inflammation and overuse of the tendons and their surrounding synovial sheaths. The situation is quite different in Guyon's canal. There are no tendons that run through Guyon's canal, so nerve compression in this condition does not occur from inflammatory tenosynovitis or pressure from aggravated tendons. The contents of Guyon's canal are the ulnar nerve, artery, and veins. The majority of problems that occur in Guyon's canal syndrome arise from external compressive forces at the base of the hand. That means nerve compression generally results from external pressure on the hand, instead of internal pressure generated inside the tunnel. There are specific aspects of neural anatomy that are important to understand for identifying the key symptom patterns in Guyon's canal syndrome. Prior to entering the canal, the ulnar nerve divides into the superficial and deep branches (Image 3). The superficial branch is primarily sensory, although it does carry a significant number of motor fibers as well. The deep branch carries primarily 1 This image depicts the flexor retinaculum as the roof of the carpal tunnel and the floor of Guyon's canal. Image is from 3D4Medical's Complete Anatomy application. Ulnar nerve Flexor retinaculum Median nerve 94 m a s s a g e & b o d y w o r k m a r c h / a p r i l 2 0 1 8