A 43-year-old right-handed man presents to A&E after an accident at work. He was carrying a large glass window when he tripped and fell onto his left side. You note extensive bruising and swelling around his left elbow. An X-ray demonstrates a supracondylar fracture of the elbow. You assess his range of movement and find he is unable to abduct his fingers.

Which nerve has been damaged?

Options:
explanation: The radial nerve supplies the BEST muscles: brachioradialis, extensors of the forearm, supinator and triceps. It also supplies sensation to the dorsal arm and forearm, and the dorsal aspect of the radial 3 1/2 fingers. Finger abduction/adduction will not be affected since the interossei are innervated by the ulnar nerve. The radial nerve can be damaged at multiple points, including the wrist (eg. tight handcuffs), which causes finger drop (loss of finger extension), proximal forearm or in the spiral groove (eg. a fracture of the humerus or prolonged tourniquet), which presents with wrist drop, or at the level of the brachial plexus (crutches or a honeymoon/Saturday night palsy), which causes loss of triceps as well wrist drop.
name: Radial nerve palsy
label: c
answer: no
explanation: The axillary nerve supplies the deltoid and teres minor muscles. This manifests with inability to abduct the arm. It also supplies sensation to the regimental badge area. Finger abduction is conducted by the interossei muscles, which are innervated by the ulnar nerve. The axillary nerve does not supply any intrinsic hand muscles.
name: Axillary nerve palsy
label: d
answer: no
explanation: The musculocutaneous nerve supplies the coracobrachialis (which flexes and adducts the shoulder), biceps brachii (which flexes and supinates the forearm) and brachialis muscles (flexes the forearm). Injury to this nerve manifests with inability to flex and adduct the shoulder and flex the elbow. Finger abduction is conducted by the interossei muscles, which are innervated by the ulnar nerve. The musculocutaneous nerve does not supply any intrinsic hand muscles.
name: Musculocutaneous nerve
label: e
answer: no
explanation: In the hand, the ulnar nerve provides motor innervation to the interossei muscles, which are responsible for finger abduction and adduction. It also supplies the third and fourth lumbricals, the adductor pollicis and sensation over the hypothenar eminence and ulnar 1 and 1/2 fingers. The ulnar nerve runs medially around the elbow, through the cubital tunnel and along the medial aspect of the forearm. The most likely area of impingement is around the elbow, where there is notable swelling on examination. Damage can either occur due to bony impingement in the context of a fracture or oedema compressing the ulnar nerve through its course in the cubital tunnel.
name: Ulnar nerve
label: a
answer: yes
explanation: In the hand, the median nerve supplies the muscles of the thenar eminence (opponens pollicis, abductor pollicis brevis and flexor pollicis brevis), as well as the radial 2 lumbricals. Its motor supply can be remembered using the helpful mnemonic LOAF. It also supplies sensation to the thenar eminence and the radial 3 1/2 fingers on their palmar aspect. Finger abduction/adduction will not be affected since the interossei are innervated by the ulnar nerve. The most common site of compression of the median nerve is at the carpal tunnel.
name: Median nerve palsy
label: b
answer: no

Correct answer: a


Learning point: The ulnar nerve innervates the interossei muscles, crucial for finger abduction and adduction, and can be injured by supracondylar fractures.