Anatomy clinical correlates: Median, ulnar and radial nerves

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Transcript

The median, ulnar and radial nerves course through the forearm and wrist, and they help coordinate the movement of our forearms and hands.

These nerves, however, are prone to injury because of various causes, and depending on which one of them is injured, that will result in characteristic symptoms that can help us recognize and identify it.

For the median nerve, the clinical manifestations depend on whether the lesion has occurred distally, as in carpal tunnel syndrome, or proximally, as in an anteriorly displaced portion of a medial supracondylar humerus fracture.

The most common cause of median nerve injury is carpal tunnel syndrome, which is when the tunnel in the wrist through which the median nerve passes becomes narrower and compresses the median nerve.

This can happen due to repetitive use, like typing on a keyboard, injuries like an anterior lunate dislocation, or associated with conditions such as hypothyroidism, diabetes, or in pregnancy.

Symptoms of median nerve injury would be pain and paraesthesia in the radial 3 and a half digits, weakness of the first and second lumbrical, thenar atrophy, and weakness of thumb abduction and opposition of the affected hand.

Specifically, the recurrent branch of the median nerve is what provides motor innervation to the thenar muscles of the hand, which are responsible for abduction, flexion and opposition, so with injuries, people may have issues opposing the thumb, and it may be difficult to perform actions like buttoning up a shirt.

Damage to the recurrent branch of the median nerve alone causes what is known as ‘ape hand’, which refers to atrophy of the thenar eminence and inability to oppose the thumb..

Damage to the entire median nerve at the level or the wrist, or distal median nerve, presents clinically as a “median claw”.

Let’s break this down quickly.

So, the first and second lumbricals are innervated by the median nerve, and the lumbricals normally flex the metacarpophalangeal joints and extend the distal and proximal interphalangeal joints.

If the median nerve is injured, the opposite occurs, so at rest, or when the patient tries to extend all of the fingers, the index and middle fingers stay extended at the MCP, and the DIP and PIP stay flexed, especially since the finger flexors are unopposed, resulting in the median claw.

Now, in a proximal lesion to the median nerve, as in a supracondylar fracture, we have all of the same deficits as in a distal lesion.

But now, all of the finger flexors for the 2nd and 3rd digit are also affected, such as the flexor digitorum superficialis and the lateral half of flexor digitorum profundus.

So, when an individual is asked to make a fist, they can only flex the 4th and 5th digit, and this presentation, or claw, is referred to as “Pope’s blessing”, or ‘hand of benediction’, and is indicative of a proximal median nerve injury.

Ok, next, there’s ulnar nerve injuries, which can occur anywhere between its origin from the brachial plexus and the ulnar canal, or Guyon canal, in the wrist.

Most commonly, it’s associated with elbow injuries, such as a fracture of the medial epicondyle of the humerus.

In a distal ulnar nerve injury, like a fracture to the hook of the hamate bone, or when cyclists compress the hook of the hamate bone when holding onto handlebars, there’s numbness and sensory loss to the medial 1 and a half digits, weakness in abduction and adduction in digits 2-5, weakness in adduction of the thumb, weakness in flexion of the fourth and fifth digits and opposition of the 5th digit, as well as hypothenar eminence atrophy.

It also manifests clinically as an “ulnar claw”.

This is the exact opposite of the “median claw”, where in the ulnar claw we have paralysis of the medial two lumbricals.

So at rest or when the patient tries to extend their hand, the ring and pinky fingers stay extended at the MCP, and the DIP and PIP stay flexed, as the finger flexors are once again unopposed.

Ok, so in a proximal lesion to the ulnar nerve, as in a medial epicondyle fracture, trauma, or prolonged leaning or sleeping on your elbow, we have all of the same deficits as in a distal lesion.