Medial Nerve Compression in the Carpal Tunnel: Carpal Tunnel Syndrome

Carpal tunnel syndrome (CTS) is the most common compression neuropathy in the upper limb. It occurs because the median nerve is compressed in a tunnel at the wrist joint and causes symptoms in the hand. We will first look at the anatomy of the median nerve, the signs and symptoms of CTS, and finally the surgical treatment in detailed steps.

Anatomy of the Median Nerve

The median nerve is also known as the laborer’s nerve. It arises from the medial and lateral cords of the brachial plexus. The cords meet each other in the anterior compartment of the arm. It runs down the front of the arm with the brachial artery. In the upper half of the arm, it is found lateral to the brachial artery, it then crosses the artery and is medial to it in the lower half of the arm. It does not give off branches proximally. Its first branch is to the pronator teres muscle. It enters the forearm between the 2 heads of the pronator teres muscle. In the forearm, it travels down to the wrist between the flexor digitorum superficial and profundus muscles; giving off branches along its path. In the forearm, the median nerve gives rise to the anterior interosseous nerve, which is a motor nerve and supplies the lateral 2 flexor digitorum profundus (FDP), flexor pollicus longus (FPL), and pronator quadratus (PQ) muscles. It also gives rise to the palmar cutaneous branch (sensory nerve, supplies the thenar eminence and central part of the palm).  As it comes closer to the wrist, the median nerve becomes more superficial laying between the flexor carpi radialis and flexor digitorum superficialis (FDS) tendons. It travels under the flexor retinaculum, a sheath covering the wrist joint, and enters the hand. It travels a tunnel known as the carpal tunnel along with the FDS, FDP, and FPL tendons. In the hand, it terminates into into the recurrent/motor branch to the thenar muscles (opponens pollicis, abd pollicis brevis, flexor pollicis brevis and the 1st + 2nd lumbricals), and digital cutaneous nerve (sensory nerve, supplies the palmar surface of the lateral 3.5 fingers).

In summary, we can divide the median nerve into its journey in the arm, forearm, and hand. In the arm, it gives off one muscular branch to the pronator teres near the elbow. In the forearm, it gives muscle branches to the superficial flexors (FCR, PL, FDS) except the FCU and the deep flexors (lateral 2/4 FDP, FPL, PQ). It gives a sensor branch in the forearm, the palmar cutaneous branch of MN. In the hand, it gives muscle branches to the thenars i.e. (1st and 2nd lumbricals, opponens pollicis, abductor pollicis brevis, and flexor pollicis brevis i.e. LOAF muscles) and gives sensory branch to the lateral 3.5 digits via the palmar cutaneous digital nerves.

Clinical features of compression at the Carpal Tunnel

Motor changes

The muscles supplied by the median nerve in the hand are the thenar muscles. So, there will be thenar wasting and APE thumb due to opponens pollicis weakness. When the patient is asked to make a fist, there will be a lag in the index and middle finger because the first two lumbricals are supplied by the median nerve.

Sensory changes

There will be sensory changes in the lateral 3.5 fingers, especially the palmar aspect.

Test for the tinnels sign, the “pen test” (ask the patient to place their hand flat on the table, palm up, place a pen above their hand, and ask the patient to touch the pen through abduction which tests the abductor pollicis brevis muscle), and finally the sensory scratch test (controversial).

Surgery

I have seen this surgery done many times since residency. Each surgeon does it a little different, but the bottom line is to make sure you have freed the median nerve in the carpal tunnel. Doing a partial release of the tunnel may cause recurrence of symptoms, or no symptomatic relief. It is important to explain to the patient that you are decompressing the median nerve, but in long standing cases, it is more difficult to provide a good result. Even when done under local anesthesia, we like to operate with a tourniquet on, and counsel the patient that there will be some compression in the arm for a while. The local anesthesia of choice is a lignocaine + adrenaline combination.

• Place the patient in supine position and with a tourniquet on. Because the surgery done is with a smaller incision than what most other surgeons use, and a little different from the conventional midline longer incisions over the palm and wrist, positioning is very important. For the right-handed surgeon, with the patient supine, when operating on the right hand, it is advised to be positioned on the radial side of the forearm, and when operating on the left hand, position yourself on the ulnar side. This helps in visualization and helps in the trajectory in which you slide the scissors down the flexor retinaculum, thus making the surgery easier.

• Markings and incisions. The incision is 6mm ulnar to the thenar crease. There are variations to the recurrent motor branch of the median nerve and the palmar cutaneous nerve. To avoid damage to these nerves, the incision is always made ulnar to a flexed ring finger. The palmar cutaneous nerve is located deep to the thenar crease ad radial to the palmaris longus

• After retracting the subcutaneous tissue and fat, incise the palmar fascia. Under the palmar fascia, we have the transverse fibers of the transverse carpal ligament. At the current institute, we use a retractor commonly used in rhinoplasties, and insert it in the carpal tunnel use it to lift the transverse carpal ligament away from the median nerve. The ligament is freed proximally to the antebrachial fascia using stout scissors. Volar wrist flexion also brings the median nerve away from the antebrachial fascia.
• Confirm the release is complete by gliding the scissors on the undersurface of the transverse carpal ligament. In this method, because we are not extending the incision over the carpal ligament, we rely heavily on the feel of the instruments as they are passed under the carpal ligament. In case of doubt, we can always extend the incision proximally.
• Closure is usually done in layers, using 4.0 RAPID VICRYL and 4.0 ETHILON. The initial dressing is with a 4” Gamgee roll. In case the incision had to be extended, we immobilize the patient in a slab till suture removal. Suture removal is usually done at post operative day 14.