Upper Extremity - Nerve Essentials
Upper Extremity - Nerve Essentials
Median, ulnar, and radial nerve essentials
Innervation
- Median nerve innervates the lateral flexors (below the elbow)
- Ulnar nerve innervates the medial flexors (below the elbow)
- Radial nerve innervates the extensors.
Major compression sites
- Median nerve can be compressed in the wrist at the carpal tunnel and in the forearm, much less commonly, between the heads of pronator teres.
- It manifests with lateral flexor weakness and wasting of the thenar eminence (the thumb pad).
- Clinical correlation: Median Neuropathy
- Ulnar nerve can be compressed in the wrist in Guyon's canal, more commonly in the elbow in the cubital tunnel.
- It manifests of the medial flexor weakness and clawing of the 4th and fifth digits, the so-called ulnar claw.
- Clinical correlation: Ulnar Neuropathy
- Radial nerve can be compressed in the supinator muscle in the forearm, or more commonly, at the mid-shaft of the humerus at the spiral groove, or more proximally, in the axilla.
- It manifests with extensor muscle weakness and wrist drop.
- Clinical correlation: Radial Neuropathy
Relevant anatomical landmarks
- Humerus, proximally.
- Radius in the lateral forearm.
- Ulna in the medial forearm.
- Brachial artery in the medial arm.
- Heads of the pronator teres muscle:
- Humeral head originates from the humerus.
- Ulnar head originates from the ulna.
median nerve
Proximal anatomy
- Passes along the medial arm, anterior to the brachial artery, then between the heads of pronator teres.
- Runs superficial to flexor digitorum profundus along a midline course into the hand.
Anterior interosseous nerve branch
- As an example, it supplies flexor digitorum profundus 2 and 3 (C7 – T1).
- When the anterior interosseous nerve is injured in isolation (anterior interosseous syndrome or neuropathy), patients exhibit the "OK" sign (or more aptly, the not "OK" sign); wherein, they are unable to form a circle with the thumb and index finger.
- Neither the thumb's interphalangeal joint nor the index finger's distal interphalangeal joints will flex.
The carpal tunnel (at the wrist)
- Here the median nerve passes beneath the flexor retinaculum through the carpal tunnel where it can be compressed.
- The median palmar cutaneous nerve enters the hand superficial to the carpal tunnel, so it's spared in carpal tunnel syndrome.
The recurrent motor branch to the thumb and other digital motor and sensory branches.
- The recurrent motor branch of the thumb innervates abductor pollicis brevis (C8, T1).
- A palm laceration can injure the recurrent branch of the thumb in isolation.
ulnar nerve
Proximal course
- It passes medial to the median nerve and brachial artery through the arm, then along the medial aspect of the ulna and into the hand.
- As an example, it innervates the flexor carpi ulnaris in the medial forearm
- And abductor digiti minimi in the medial hand.
Cubital tunnel syndrome
- The ulnar nerve passes:
- Posterior to the medial epicondyle of the humerus.
- Anterior to the olecranon of the ulna.
- If you flex and extend your arm, you may be able to feel it pop in and out of this groove: the ulnar groove.
- Through the cubital tunnel, which is the tendinous arch that joins the heads of flexor carpi ulnaris, which it runs deep to.
- The cubital tunnel is the most common ulnar nerve entrapment site.
- All of the motor and sensory components of the ulnar nerve lie distal to the cubital tunnel; therefore, in a cubital tunnel syndrome, all of the components of the ulnar nerve are affected.
Guyon's canal (aka Guyon's tunnel)
- Formed from the palmar carpal ligament, which runs transverse across the wrist.
- This is the other major ulnar nerve entrapment site.
- Variations of Guyon's canal entrapments exist affecting a variety of different distal ulnar components.
- Proximal to Guyon's canal, the ulnar nerve derives the dorsal ulnar cutaneous nerve, which passes posteriorly.
radial nerve
Axilla and spiral groove
- The radial nerve wraps around the posterior humerus, down the lateral forearm and into the hand.
- The radial nerve runs deep to the triceps muscle.
- Where it wraps around the posterior mid-humerus is the spiral groove, an important radial nerve compression site.
Distal humerus/proximal forearm
- It passes anterior to the lateral epicondyle of the humerus.
- Then a few centimeters distal to the lateral epicondyle, it divides into:
- The deep branch, which innervates the extensor carpi radialis (as an example), and transitions into the posterior interosseous nerve, which innervates the extensor digiti minimi (as an example).
- The posterior interosseous nerve passes through the arcade of Frohse, an arch within the supinator muscle, which is a key posterior interosseous nerve entrapment site.
- It also branches into the superficial radial sensory branch, which runs deep to brachioradialis into the lateral hand.
Sensory coverage of the hand
Median nerve
- Digital sensory branches cover the lateral palm, lateral half of the ring finger, middle and index fingers, and the palmar thumb and the dorsal tips of those same fingers, as well.
- Median palmar cutaneous nerve covers the palmar portion.
Ulnar nerve
- The ulnar nerve covers the medial one third of the hand and the medial half of the fourth digit and fifth digit.
Radial nerve
- The superficial sensory radial nerve covers the remainder of the dorsum of the hand (the lateral two-thirds of the dorsum of the hand, proximal thumb,