Ulnar nerve entrapment

Ulnar nerve entrapment occurs when the ulnar nerve is compressed, typically at the elbow or the wrist. Compression at the elbow is called cubital tunnel syndrome; compression at the wrist it is referred to as Guyon's canal syndrome or ulnar tunnel syndrome. The compression causes paresthesias, numbness, and/or pain in the ulnar nerve distribution. Depending on the site of compression, the patient may experience weakness in certain hand muscles. Ulnar entrapment neuropathy may be suspected based on clinical symptoms and signs, but it must be confirmed by electromyography (EMG). Conservative treatment involves NSAIDs, behavior modification, and bracing. Severe, persistent, or worsening symptoms require surgical decompression.

Overview

• Derived from nerve roots C8–T1, which form the medial cord of the brachial plexus
• The ulnar nerve arises from the medial cord.
  • Runs through the medial bicipital sulcus up to the extending side at the level of the mid-upper arm
  • Passes the medial epicondylar groove at the elbow
  • Enters the palm through Guyon's canal

Sensory innervation

• Palmar and dorsal aspects of the medial side of the hand
• Palmar and dorsal aspects of the entire little finger and the ulnar aspect of the ring finger
• 

Motor innervation

• Third and fourth lumbricals
  • Flex at metacarpal phalangeal (MCP) joint
  • Extend at proximal interphalangeal (PIP) joint
• Adductor pollicis: adducts the thumb
• Abductor digiti minimi: abducts the little finger
• Flexor carpi ulnaris: helps flex the wrist
• Dorsal and palmar interossei: finger abduction and adduction respectively
• Flexor digiti minimi brevis: flexes the MCP joint

The ulnar nerve is most commonly compressed at or near the cubital tunnel of the elbow and Guyon canal of the wrist.

Cubital tunnel syndrome

• Leaning on the elbow or prolonged elbow flexion during occupational activities (e.g., leaning on a desk), athletic activities, or surgical procedures (e.g., during general anesthesia)
• Blunt trauma
• Masses (e.g., tumors, hematomas)
• Metabolic abnormalities (e.g., diabetes)

Guyon canal syndrome

• Often associated with cycling, likely caused by direct pressure from the handlebars
• Blunt trauma (e.g., hook of hamate fracture)
• Masses (especially ganglion cysts)

Muscle weakness and atrophy

• Atrophy of the hypothenar muscles
  • Inability to flex the ring finger and little finger when asked to make a fist
  • Results in a hand position that is similar to the “OK sign” (proximal nerve lesions)
• Claw hand deformity (ulnar claw) ; ^[1]
  • Mainly in distal nerve injuries
  • Palsy of the 3^rd and 4^th lumbricals with preserved function of extrinsic flexors
  • Hyperextension at MCP joints and flexion at PIP and DIP joints of the ring finger and little finger
  • Present at rest, increases when the patient is asked to extend the fingers
• Wartenberg sign: little finger in persistent abduction due to weak third palmar interosseous muscle
• Froment sign: The thumb flexes at the interphalangeal joint while pinching a piece of paper to compensate for a weak adductor pollicis muscle.
• Jeanne sign
  • Caused by the loss of function of the adductor pollicis and the predominance of the extensor pollicis longus and the abductor pollicis brevis
  • Leads to hyperextension of the thumb's metacarpophalangeal joint

Sensory loss and pain

• Lesions at the elbow
  • Positive Tinel test: marked paresthesias can be reproduced in the ulnar portion of the hand by tapping on the medial epicondyle of the humerus.
  • Elbow lesions typically present with referred pain in the forearm.
• Lesions at the ulnar canal (located in the wrist)
  • The ulnar canal is divided into three zones
    • Zone I: proximal to the bifurcation of the ulnar nerve
      • Motor and sensory symptoms (i.e., sensory loss in the palmar surface of the small finger and medial aspect of the ring finger) possible
      • Sensory features similar to those caused by proximal lesions at the elbow
    • Zone II: lesions at the deep motor branch cause motor symptoms only
    • Zone III: lesions at the distal sensory branch cause sensory symptoms only

Proximal and distal lesions of the ulnar nerve lead to claw hand deformity.

• Electrodiagnostic studies
  • Typically involves nerve conduction studies and EMG
  • Identifies the level of nerve compression
• Ultrasound and MRI: used to support the EMG findings and to detect possible causes of compression (e.g., space-occupying lesions)
• X-ray: Consider cervical spine, chest, elbow, and wrist films to rule out other possible causes of symptoms.

• Conservative therapy
  • Modify behavior (e.g., avoid prolonged resting on elbow, repeated flexion of elbow)
  • Bracing at night
  • Analgesia (e.g., NSAIDs)
• Surgical decompression: if clinical features are severe, persistent (i.e., lasting more than 6 to 12 weeks), or progressively worsen despite conservative therapy.