Carpal Tunnel Syndrome Explained by Anatomy

“Why is my hand tingling?”

“Why do I have no strength in my hand?”

 These are the sort of questions that patients ask when they have carpal tunnel syndrome. But why does it happen? What is actually going on? The answer is easy when you understand the anatomy, so read on!

What is carpal tunnel syndrome?

Carpal tunnel syndrome is a clinical syndrome with signs and symptoms that includes pain, weakness and numbness of the hand.

What is the carpal tunnel?

The carpal tunnel is a region found near the wrist joint. The carpal tunnel lies on the palmar, or volar aspect of the proximal part of the hand, just distal to the wrist joint. The carpal tunnel is an anatomical space that permits the transit of several tendons and one nerve from the forearm into the wrist.

 Which tendons pass through the carpal tunnel?

Nine tendons pass through the carpal tunnel. These long slender flexor tendons attach to the thumb and each of the digits.

Flexor Pollicis Longus

The tendon of flexor pollicis longus arises from the radius and the interosseous membrane. It forms a slender tendon that attaches to the distal phalanx of the thumb. When the flexor pollicis longus muscle contracts, it shortens and causes the tendon to pull the distal phalanx of the thumb, which in turn flexes, or bends, the tip of the thumb.

Flexor Digitorum Superficialis

The four tendons of flexor digitorum superficialis are a continuation of the muscle, which has an extensive origin from the common flexor origin at the medial epicondyle on the distal humerus, the ulna and the radius.  After forming four tendons, the flexor digitorum superficialis passes through the carpal tunnel, and the tendons do something remarkable. Each tendon splits into two parts, flips 180 degrees and then inserts onto the middle phalanx of each finger. When the flexor digitorum superficialis muscle shortens, its tendons pull on the middle phalanx, and this causes flexion or bending at the proximal interphalangeal joint. When you look at it in a cadaver, it really is a fantastic piece of bioengineering. But you may be wondering why the tendon splits? Well, it allows the tendon of the flexor digitorum profundus to pass in between it.

Flexor Digitorum Profundus

The flexor digitorum profundus arises in the forearm from the ulna bone. It forms four tendons that pass through the carpal tunnel. The tendons of flexor digitorum profundus lie underneath the tendons of flexor digitorum superficialis and pass through the split to keep going distally to insert onto the distal phalanx. When the flexor digitorum profundus muscle shortens, its tendons pull on the distal phalanx and flexes or bends the tip of each finger.

Which nerve passes through the carpal tunnel?

The median nerve passes through the carpal tunnel at the wrist joint and goes on to supply the thenar muscles. There are three thenar muscles, and they all act on the thumb. The thenar muscles are intrinsic muscles of the hand, which means they arise and insert form within the hand.  The thenar muscles are flexor pollicis brevis, opponens pollicis and abductor pollicis brevis. Adductor pollicis is another intrinsic muscle of the thumb, but strictly speaking, it is not a thenar muscle as it is supplied by the deep branch of the ulnar nerve. The thenar muscles form the fleshy part of your hand. If you look at your palm with your thumb close to your hand, the thenar muscles form quite a mound at the bacs of your thumb. The thenar muscles work together to bring the thumb towards the palm, such as when making a fist. This action is essential if you are holding onto or gripping something. So if the muscle becomes weaker because the nerve supply has been disrupted, you will have a weaker grip and drop things.

Why do people with carpal tunnel syndrome have a weak grip?

Nerves are essentially a transport system. They allow information to travel from the brain to different body parts and vice versa. Information about pain, temperature and pressure goes from the body to the brain. Messages go from the brain to different muscles in the body, causing them to contract or relax. Electrical impulses and chemical molecules convey these messages. If the nerve becomes compressed or severed, the signal will be slowed down or completely cut off. This means that the messages will not get through, and the muscles powered by the nerve will no longer function. Muscles become weak and shrink if not used, which is called atrophy.  The area of skin that is supplied by that nerve will become wholly numb or may become tingly if the sensation is partly intact. This sensation is commonly known as pins and needles.

Do people with carpal tunnel syndrome have numbness?

Yes! This is one of the clinical tests you must do when examining someone with carpal tunnel syndrome. The skin on the palmar side of the hand of the thumb, index, middle and half of the ring finger becomes tingly or numb. The ulnar nerve supplies the ulnar side (little finger side) of the ring finger so that part of the finger retains its sensation.

 Why does the median nerve become constricted in the carpal tunnel?

The answer to this question is another marvel of bioengineering. For the flexor tendons to work most efficiently, they must be held together at the wrist joint to prevent bowstringing. The flexor retinaculum achieves this. The flexor retinaculum is a fibrous band of tissue that forms the roof of the carpal tunnel. The flexor retinaculum is attached to some of the carpal bones. On the radial side (thumb side), it attaches to the scaphoid and trapezium. On the ulnar side (little finger side), it attaches to the pisiform and hamate. If the flexor retinaculum becomes thickened, it will compress the carpal tunnel contents. This includes constricting the median nerve.

Why are the symptoms worse at night?

 At night time when you are sleeping, most people curl up in a fetal position. In this position, the wrist joint is flexed. This puts more pressure on the carpal tunnel, and patients often say that they wake up because their hand is tingling. Classically, they shake their hands and have to hang them over the side of the bed to get rid of the symptoms.

 Who gets carpal tunnel syndrome?

Carpal tunnel syndrome is more common in women and older people. It may be associated with certain medical conditions, including diabetes, rheumatoid arthritis and thyroid conditions. It is more common during pregnancy when hormonal changes cause increased water retention and swelling.

 How is carpal tunnel syndrome treated?

As with any treatment, start with easy, non-invasive things first. You can use wrist splints at night to prevent the wrist from flexing. Taking anti-inflammatory medication may reduce the symptoms. Physiotherapy may help to stretch out the carpal tunnel. Injection of steroids works for some people. If the carpal tunnel is caused by a change in physiology, such as during pregnancy, the symptoms often subside when the baby is born. A minor surgical procedure may be needed if conservative treatments do not work. This is usually done as a day case with infiltration of local anaesthetic. A small vertical incision is made in the middle of the wrist joint distal to the distal crease of the wrist. A sharp blade is used to cut the fibres of the flexor retinaculum, and the median nerve is released. The skin wound is closed with sutures and takes about ten days to heal. The function of the nerve usually recovers well. The sensation may return within weeks, but the strength may take months to recover fully.

 Summary

The carpal tunnel is an anatomical region found on the volar aspect of the wrist joint.

Boundaries of the carpal tunnel

Floor – Carpal Bones

Roof – Flexor Retinaculum

Contents of the carpal tunnel

• Median nerve
• Four tendons of Flexor Digitorum Superficialis
• Four tendons of Flexor Digitorum Profundus
• One tendon of Flexor Pollicis Longus

Symptoms of carpal tunnel syndrome

• Weakness and atrophy of the thenar muscles
• Weak grip strength, dropping things
• Numbness of the palmar surface of the thumb, index, middle and radial half of the ring finger
• Pain or tingling in the forearm and wrist

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Hope you found this helpful. I think that bringing in some clinical aspects brings anatomy to life and makes it much more relatable and relevant

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Until next time - Stay Funky!

Dr Susan xx