Question

In examining a patient with brachial plexopathy, the deltoid and triceps are weak, but the biceps and pronator tres are normal. The lesion is best localized to the:

a) Upper trunk
b) Lower trunk
c) Posterior cord
d) Medial cord

Answer 1

The muscle weakness in the patient's deltoid and triceps with normal biceps and pronator teres points to a lesion in the posterior cord of the brachial plexus.

Step-by-step explanation:

In diagnosing the muscle weakness described, where the deltoid and triceps are weak, while the biceps and pronator teres are normal, one can deduce the lesion is localized to the posterior cord of the brachial plexus (c). The weakness of the deltoid and triceps indicates an impairment in the nerves arising from the posterior cord, which include the axillary nerve that innervates the deltoid muscle and the radial nerve that innervates the triceps brachii. Since biceps and pronator teres are normal and these muscles are innervated by the musculocutaneous nerve and median nerve respectively, which come from other parts of the brachial plexus, the localization to the posterior cord is more accurate than the other options provided.

Additionally, knowing which muscles are responsible for certain actions can aid in localizing lesions when conducting a neurological examination. For example, muscle strength in the arms can be tested by having the patient contract their muscles against resistance, and observing any lateral differences in strength. The triceps brachii is the muscle that extends the forearm. Therefore, weakness in extending the forearm would suggest triceps brachii involvement, pointing to a posterior cord lesion.