October 04, 2010

USMLE 1 Question 3

A man came to the emergency room after a road traffic accident. He had swelling, pain and shortening of the left arm. On examination the man was found to have a fracture of the mid shaft of the humerus. On further examination the man was found to have a wrist drop, finger drop and thumb drop on the left side.

The cause for this is:

a) injury to the radial nerve
b) injury to the ulnar nerve
c) injury to the median nerve
d) injury to the axillary nerve
e) none of the above








ANSWER
The correct answer is A

Explanation
Radial nerve during its course along the arm passes through the radial groove present in the mid shaft area of the humerus. The nerve is commonly damaged due to a mid shaft fracture of the humerus.

The radial nerve is responsible for supplying the extensor muscles of the wrist, fingers and the thumb.

The damage to the nerve causes paralysis of the extensor muscles in the forearm. This results in unopposed action of the flexor muscles causing a wrist, finger and thumb drop.

The ulnar nerve is never injured in a fracture of the mid- shaft of the humerus. The injury to the ulnar nerve results in the paralysis of the flexor carpii ulnaris and medial half of the flexor digitorum profundus. The paralysis of these muscles causes flattening of the medial border of the forearm and claw hand deformity of the hand. It does not cause a wrist, thumb or finger drop.

Median nerve injury does not occur with the fracture of mid shaft of the humerus. Also the median nerve supplies the flexors of the wrist therefore injury to the nerve causes flexion weakness at the wrist and not a 'drop' deformity.

Axillary nerve is a short nerve which does not reach up to the level of mid shaft of humerus, thus cannot be injured in its fracture. Also it does not supply any muscles in the forearm therefore its injury does not cause a wrist, finger or thumb drop.

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