Conditions of the Shoulder and Clavicle Quiz

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Which pattern of motor deficit is most characteristic of an upper brachial plexus injury?

A Loss of shoulder abduction and external rotation with the arm held adducted and internally rotated ✓
B Isolated loss of wrist extension with preserved elbow function, causing the wrist to drop into flexion
C Clawing of the hand due to intrinsic muscle weakness, with hyperextension at the knuckles and flexion of the fingers
D Flaccid paralysis of the upper limb, with the arm and hand hanging uselessly
E Loss of finger flexion with preserved shoulder movements, leaving the hand unable to grip objects

The answer is A: Loss of shoulder abduction and external rotation with the arm held adducted and internally rotated

Explanation

Why A is correct

See Calcific supraspinatus tendinitis. Option A is correct. The brachial plexus is the network of nerves, formed from the spinal roots C5 to T1, that supplies the whole upper limb. An upper brachial plexus injury damages the C5 and C6 roots, most commonly through a stretching injury during a difficult vaginal delivery, and is called Erb's palsy. The nerves that carry C5 and C6 fibres supply the muscles that move the shoulder, so the deltoid, supraspinatus and infraspinatus (the muscles that lift the arm out to the side and turn it outwards) are weakened, along with the biceps and brachialis (the muscles that bend the elbow). Pectoralis major, the large chest muscle, is largely spared and, with little pulling against it, turns the arm inwards. The arm therefore hangs limply by the side, internally rotated, with the forearm turned palm-backwards (pronated) because the biceps is weak, and the wrist weakly flexed. This is the waiter's tip position, like a waiter discreetly holding out a hand for a tip. The functions lost are:

Shoulder abduction (lifting the arm out to the side) and flexion (lifting it forwards)
Lateral rotation of the arm (turning it outwards)
Supination of the forearm (turning the palm upwards) and elbow flexion
Sensation over the outer (lateral) side of the upper limb

Roots: An upper plexus injury damages the C5 and C6 roots.
Weakness: The deltoid, supraspinatus, infraspinatus, biceps and brachialis lose power.
Posture: The arm hangs by the side, internally rotated and pronated, in the waiter's tip position.

Why the other options are incorrect

B. Isolated loss of wrist extension with preserved elbow function, causing the wrist to drop into flexion: Isolated loss of wrist extension with preserved elbow function is the pattern of a radial nerve injury, which causes wrist drop through paralysis of the wrist extensors. An upper plexus injury also weakens shoulder abduction and elbow flexion, so it would not spare the elbow.
C. Clawing of the hand due to intrinsic muscle weakness, with hyperextension at the knuckles and flexion of the fingers: Clawing of the hand with hyperextension at the knuckles and flexion of the fingers occurs in a lower brachial plexus injury, where the C8 and T1 roots are damaged. An upper plexus injury affects the shoulder and elbow, so the intrinsic hand muscles are spared.
D. Flaccid paralysis of the upper limb, with the arm and hand hanging uselessly: Flaccid paralysis of the whole upper limb with no usable movement requires damage to all the roots of the brachial plexus, not just the upper trunk. An upper plexus injury damages C5 and C6, so the hand and much of the forearm are spared.
E. Loss of finger flexion with preserved shoulder movements, leaving the hand unable to grip objects: Loss of finger flexion with preserved shoulder movements points to a distal nerve problem such as a median nerve lesion, because the median nerve supplies the forearm muscles that flex the fingers. An upper plexus injury would weaken the shoulder, so shoulder movements would not be preserved.

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1
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