A 19-year-old woman is evaluated in the clinic due to right hand clumsiness. The patient injured the right upper extremity after falling off her bicycle 6 months ago. She has since had a pins-and-needles sensation in her right hand that is associated with mild weakness. The patient is worried because these symptoms interfere with her piano practice and she has a recital in 3 weeks. Physical examination shows decreased sensation over the fifth digit and a flattened hypothenar eminence. Triceps reflexes are 2+ and symmetric bilaterally. The nerve affected in this patient is likely to be injured at which of the following locations?
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This patient's presentation is consistent with ulnar neuropathy. The ulnar nerve is a branch of the medial cord of the brachial plexus. It enters the forearm after passing behind the medial epicondyle of the humerus. Within the forearm, it innervates the flexor carpi ulnaris (FCU) and the medial portion of the flexor digitorum profundus (FDP) before entering the wrist between the hook of the hamate and the pisiform bone in a fibroosseous tunnel known as Guyon canal. Here, it divides into a superficial branch that provides sensation to the medial 1½ digits and a deep motor branch that supplies most of the intrinsic muscles of the hand.
The ulnar nerve is commonly injured by trauma or nerve compression at the elbow (funny bone) or at Guyon canal (eg, striking the ground during a fall).
Injury at Guyon canal causes sensory loss in the medial 1½ digits along with intrinsic hand weakness. The hypothenar eminence can appear flattened due to atrophy of denervated muscles. Weakness of the lumbricals in the fourth/fifth digits may also produce an ulnar claw appearance during finger extension.
Injury at the elbow will cause similar symptoms along with sensory loss in the hypothenar eminence (palmar cutaneous branch) and medial dorsum of the hand (dorsal cutaneous branch). Weakness on wrist flexion/adduction (FCU) and flexion of the fourth/fifth digits (medial portion of FDP) may also be present.
(Choice A) Carpal tunnel syndrome can result from any condition that reduces the size of the carpal tunnel (eg, pregnancy, hypothyroidism). The resulting median nerve compression typically causes pain/paresthesia affecting the first 3½ digits. Thenar atrophy with weakness on thumb abduction/opposition may also be seen.
(Choice B) The coracobrachialis muscle lies deep to the biceps brachii and is perforated and innervated by the musculocutaneous nerve. Entrapment of the musculocutaneous nerve within the coracobrachialis can lead to decreased strength on forearm flexion and sensory loss over the lateral forearm.
(Choice C) Subluxation of the radial head can injure the deep branch of the radial nerve (which becomes the posterior interosseous nerve) during its passage through the supinator canal. Injury to the nerve results in weakness during finger and thumb extension (finger drop).
(Choice E) Fracture of the midshaft of the humerus typically results in radial nerve injury leading to paralysis of the hand and finger extensor muscles (wrist drop) with sensory loss over the posterior forearm and dorsolateral hand.
(Choice F) Fracture of the surgical neck of the humerus may cause axillary nerve injury leading to paralysis of the deltoid and teres minor muscles with sensory loss over the lateral upper arm.
Educational objective:
The ulnar nerve is commonly injured at the elbow (funny bone) or wrist (Guyon canal). Injury at Guyon canal results in sensory loss over the medial 1½ digits and intrinsic hand weakness. The hypothenar eminence may flatten due to muscle atrophy.