Ulnar nerve: sensory and motor distribution

What it is

The ulnar nerve holds one side of the upper limb innervation figure, and the muscles drawn on its side are the flexor digitorum profundus to the medial 4th and 5th digits, adductor pollicis, the palmar and dorsal interossei, palmaris brevis, the hypothenar muscles and the 4th and 5th lumbricals. Its sensory branches drawn there are a dorsal cutaneous branch and a palmar cutaneous branch. The median nerve holds the other side of the same figure, with pronator teres, flexor carpi radialis, flexor pollicis longus, flexor digitorum superficialis, pronator quadratus, flexor digitorum profundus to the lateral 2nd and 3rd digits, the thenar muscles and the 2nd and 3rd lumbricals.

It is compressed at two named places. The cubital tunnel at the elbow gives cubital tunnel syndrome, and Guyon's canal at the wrist gives Guyon's canal syndrome; Guyon's canal is a semi-rigid longitudinal canal in the wrist that allows passage of the ulnar artery and the ulnar nerve into the hand.

Causes and risk

Injuries around the elbow are the ones to know. A fracture of the medial condyle of the humerus goes with the ulnar nerve. A supracondylar fracture of the humerus goes with the median nerve, and the ulnar nerve can be affected too, so median or ulnar nerve injury is listed for it. The other nerve pairs are worth holding next to this one: an anterior dislocation of the shoulder goes with the axillary nerve, seen as loss of sensation over the deltoid in the regimental badge area; a humeral shaft fracture goes with the radial nerve, seen as wrist drop with loss of sensation over the dorsum of the hand; and median nerve injury is seen as loss of sensation in the palmar aspect of the third digit and the thumb, with decrease in thumb function.

The fractures and dislocations around the elbow are set out nerve by nerve on another page. Olecranon fractures are associated with injury to the ulnar nerve, and the patient may experience paresthesias and numbness in the ulnar nerve distribution or weakness of the interossei muscles. Posterior elbow dislocations are associated with injuries to the ulnar and median nerves, while anterior elbow dislocations, although uncommon, have a much higher incidence of vascular injuries, so the brachial artery must be evaluated. Humeral shaft fractures most commonly occur from a direct blow to the mid-upper arm, usually involve the middle third of the shaft, and their most common associated injury is damage to the radial nerve causing wrist drop and loss of sensation in the first dorsal web space; ulnar and median nerve injuries may also occur there but are much less common. Supracondylar fractures, common in young individuals, are associated with injuries to the brachial artery and the median nerve as the distal humeral fragment is displaced posteriorly. A Monteggia fracture, of the proximal one-third of the ulnar shaft with a radial head dislocation, is associated with an injury to the radial nerve, and a Colles fracture puts the median nerve at risk.

At the wrist the risk factors are different again: ganglion cyst, fracture of the hook of the hamate, ulnar artery aneurysm, lipoma and riding a bicycle.