Aetiology

The ulnar nerve may be damaged at a number of sites:

• at the elbow behind the medial epicondyle - commonly due to cubitus valgus, or bony thickening secondary to arthritis or the result of an old fracture
  
  
• in the cubital tunnel - due to entrapment of the nerve in the tunnel formed by the tendinous arch connecting the two heads of the flexor carpi ulnaris; the entrapment is aggravated by pressure on the fully flexed forearm which commonly occurs when sleeping in the prone position
  
  
• at the wrist - due to pressure in Guyon's canal from a deep ganglion or laceration
  
  
• in the hand - due to compression of the deep motor branch against the pisiform and hamate; this is usually seen in individuals whose occupation involves prolonged pressure upon the outer part of the palm, for example, motorcyclists and road workers using vibrating drills

Lesions at the wrist are characterised by hypothenar wasting and paralysis of the intrinsic muscles of the hand and consequent clawing of the hand. There is weakness of finger abduction and there is loss of adduction of the thumb. There is loss of sensation over the ulnar one and a half fingers.

The clawing of the hand is less marked with a high lesion because the ulnar half of flexor digitorum profundus is paralysed and so the terminal interphalangeal joints are not flexed. In other respects the motor and sensory losses are the same as a low lesion.