Ulnar nerve

The ulnar nerve is a major peripheral nerve of the upper limb, playing a critical role in both motor and sensory functions of the forearm and hand. Understanding its anatomy, course, and function is essential for diagnosing and managing related injuries and neuropathies.

Anatomy of the Ulnar Nerve

Origin

The ulnar nerve arises from the medial cord of the brachial plexus, receiving fibers from the C8 and T1 nerve roots. In some individuals, it may also receive a small contribution from the C7 root. This origin allows the nerve to carry both motor and sensory fibers.

Course

The course of the ulnar nerve can be divided into three main segments:

• Arm: The nerve travels medial to the brachial artery and passes posterior to the medial epicondyle of the humerus at the elbow.
• Forearm: It enters the forearm between the two heads of the flexor carpi ulnaris muscle, running along the medial aspect of the forearm, supplying motor branches to the flexor carpi ulnaris and the medial half of the flexor digitorum profundus.
• Hand: The nerve passes through the Guyon canal at the wrist, dividing into superficial sensory and deep motor branches that supply the intrinsic muscles of the hand and the ulnar side of the hand and fingers.

Branches

The ulnar nerve gives off several branches along its course:

• Muscular branches: Supply flexor carpi ulnaris, medial half of flexor digitorum profundus, and most intrinsic hand muscles.
• Cutaneous branches: Provide sensation to the medial one and a half fingers and the corresponding palm and dorsal hand regions.
• Articular branches: Supply the elbow and wrist joints.

Relations

In the arm, the ulnar nerve lies medial to the brachial artery and is initially covered by the biceps brachii and brachialis muscles. At the elbow, it passes behind the medial epicondyle in a subcutaneous position, making it susceptible to compression or trauma. In the forearm, it is situated between the flexor carpi ulnaris and flexor digitorum profundus muscles, while in the hand, it travels through the Guyon canal adjacent to the ulnar artery.

Function

Motor Function

The ulnar nerve innervates several muscles that contribute to movements of the forearm and hand:

• Forearm muscles: Flexor carpi ulnaris and the medial half of flexor digitorum profundus, which flex the wrist and fingers.
• Intrinsic hand muscles: Hypothenar muscles, interossei, adductor pollicis, and the medial two lumbricals, facilitating fine motor movements and grip strength.

Sensory Function

The sensory distribution of the ulnar nerve includes:

• The medial one and a half fingers, including the little finger and medial half of the ring finger.
• The corresponding palmar and dorsal regions of the hand.
• Conveyance of touch, pain, and proprioceptive sensations from these areas to the central nervous system.

Clinical Examination

Inspection

Visual examination of a patient suspected of ulnar nerve involvement includes:

• Muscle wasting in the hypothenar eminence and interosseous spaces.
• Presence of claw hand deformity characterized by hyperextension at the metacarpophalangeal joints and flexion at the interphalangeal joints of the fourth and fifth fingers.

Palpation

Palpation helps identify tenderness or nerve entrapment along its course:

• Medial epicondyle of the humerus for cubital tunnel tenderness.
• Flexor carpi ulnaris muscle for any points of compression.
• Guyon canal at the wrist for signs of local irritation or mass effect.

Motor Testing

Assessment of motor function includes:

• Testing finger abduction and adduction to evaluate interossei muscles.
• Assessing flexion of the fourth and fifth fingers at the distal interphalangeal joints to test flexor digitorum profundus.
• Evaluating grip and pinch strength to identify weakness in intrinsic hand muscles.

Sensory Testing

Sensory examination focuses on the areas supplied by the ulnar nerve:

• Light touch and pinprick testing on the medial one and a half fingers and associated palm and dorsum of the hand.
• Two-point discrimination testing to assess fine sensory perception.

Ulnar Nerve Injuries

Etiology

Injuries to the ulnar nerve can result from a variety of causes:

• Trauma: Fractures of the medial epicondyle, lacerations, or dislocations.
• Compression syndromes: Cubital tunnel syndrome at the elbow and Guyon canal syndrome at the wrist.
• Iatrogenic causes: Surgical procedures near the medial elbow or wrist causing accidental nerve injury.

Pathophysiology

The mechanism of ulnar nerve injury depends on the type and site of trauma:

• Neuropraxia: Temporary conduction block without axonal loss, usually from compression.
• Axonotmesis: Disruption of the axon with intact connective tissue, leading to Wallerian degeneration.
• Neurotmesis: Complete transection of the nerve, resulting in permanent loss unless surgically repaired.

Clinical Presentation

Patients with ulnar nerve injury typically exhibit:

• Motor deficits such as weakness in finger abduction and adduction, difficulty with grip, and clawing of the fourth and fifth fingers.
• Sensory deficits including numbness, tingling, or loss of sensation over the medial one and a half fingers and corresponding hand regions.
• Visible deformities like muscle wasting in the hypothenar eminence and interosseous spaces in chronic cases.

Diagnostic Investigations

Electrodiagnostic Studies

Electrodiagnostic testing is essential to evaluate the functional status of the ulnar nerve:

• Nerve conduction studies: Measure the speed and amplitude of electrical impulses along the nerve to detect conduction block or slowing at compression sites.
• Electromyography (EMG): Assesses the electrical activity of muscles innervated by the ulnar nerve, helping to identify denervation or reinnervation patterns.

Imaging

Imaging studies can assist in identifying structural causes of ulnar nerve pathology:

• Ultrasound: Visualizes the nerve along its course, detects swelling, entrapment, or masses compressing the nerve.
• MRI: Provides detailed images of soft tissue and nerve anatomy, useful for evaluating tumors, ganglions, or severe entrapments.

Management

Conservative Treatment

Initial management of ulnar nerve dysfunction often involves non-surgical approaches:

• Activity modification: Avoiding repetitive elbow flexion or pressure on the medial elbow or wrist.
• Splinting: Night splints to maintain elbow extension or wrist position to reduce nerve compression.
• Physical therapy: Stretching and strengthening exercises to relieve tension on the nerve and improve hand function.

Surgical Treatment

Surgery may be indicated when conservative measures fail or in cases of severe nerve injury:

• Nerve decompression: Releasing constrictive structures at the cubital tunnel or Guyon canal.
• Nerve repair or grafting: Repairing transected nerves or bridging nerve gaps using autologous grafts.
• Tendon transfers: In chronic cases with muscle loss, tendon transfers can restore functional hand movements.

Prognosis

The prognosis of ulnar nerve injuries depends on the severity, duration, and site of the lesion as well as timely intervention:

• Neuropraxia typically has an excellent prognosis, with full recovery expected within weeks to months.
• Axonotmesis may require several months for axonal regeneration, and functional recovery depends on the distance between the injury site and target muscles.
• Neurotmesis or severe chronic compression often has a guarded prognosis, requiring surgical repair and intensive rehabilitation for functional improvement.
• Early diagnosis and treatment significantly improve outcomes and reduce the risk of permanent muscle atrophy or sensory loss.

Prevention

Preventive strategies focus on reducing pressure and avoiding repetitive trauma along the course of the ulnar nerve:

• Maintain proper ergonomics during work or sports activities that involve prolonged elbow flexion or wrist pressure.
• Avoid prolonged leaning on the elbow and repetitive bending of the wrist and fingers.
• Use protective padding or braces during high-risk activities to minimize direct pressure over the cubital tunnel or Guyon canal.
• Early recognition and modification of activities in individuals with initial symptoms of numbness or tingling can prevent progression to severe neuropathy.

References

1. Standring S. Gray’s Anatomy. 42nd ed. London: Elsevier; 2020.
2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. 9th ed. Philadelphia: Wolters Kluwer; 2020.
3. Williams PL, Warwick R, Dyson M, Bannister LH. Gray’s Anatomy. 37th ed. Edinburgh: Churchill Livingstone; 1989.
4. Jabaley ME, Lanz U, Zawadski C. Surgical anatomy of the ulnar nerve. Clin Orthop Relat Res. 2001; (383):50-58.
5. Campbell WW. Evaluation and Management of Peripheral Nerve Injury. Clin Neurophysiol. 2008;119(9):1951-1965.
6. Rhee PC, Kubiak EN, Zlotolow DA. Ulnar nerve entrapment: Diagnosis and management. J Hand Surg Am. 2015;40(5): 965-972.
7. Beekman R, Visser LH. Sonography of the peripheral nerves. Muscle Nerve. 2004;29(6): 757-771.
8. Wilbourn AJ. Electrodiagnosis in Ulnar Neuropathy. Muscle Nerve. 2008;37(6): 689-704.