A 38yr old man met with a motor vehicular accident, following which he was unable to abduct his right shoulder and flex at elbow. What is the most likely cause?
First, I need to think about the nerves involved in shoulder abduction and elbow flexion. Shoulder abduction primarily involves the deltoid and supraspinatus muscles. The deltoid is innervated by the axillary nerve, and the supraspinatus by the suprascapular nerve. Elbow flexion is mainly done by the biceps brachii, which is innervated by the musculocutaneous nerve. However, if there's a more proximal injury, like a brachial plexus injury, that could affect multiple nerves.
Wait, but the axillary nerve and musculocutaneous nerve both come from the C5 and C6 roots of the brachial plexus. If there's a C5-C6 root injury, both the axillary and musculocutaneous nerves would be affected. That would explain the loss of shoulder abduction (deltoid and supraspinatus) and elbow flexion (biceps). So the most likely cause here is a C5-C6 brachial plexus injury.
Looking at the options, the correct answer would be C5-C6 brachial plexus root avulsion. Let me check the other options. If the options included other nerves like radial or median, those are less likely here. For example, radial nerve injury affects wrist and finger extension, not elbow flexion. Median nerve affects hand movements. Axillary nerve alone would only affect shoulder abduction, not elbow flexion. So the key here is both shoulder and elbow movements are impaired, pointing to a higher-level injury affecting multiple nerves from C5-C6.
The clinical pearl is that combined loss of shoulder abduction and elbow flexion is classic for C5-C6 brachial plexus injury. Remember that the musculocutaneous nerve (from C5-C7) and axillary nerve (C5-C6) are both affected here. So the answer is C5-C6 root avulsion.
**Core Concept**
This question tests knowledge of brachial plexus anatomy and motor deficits following nerve injuries. Shoulder abduction and elbow flexion are controlled by nerves derived from the C5-C6 spinal roots. Injury to these roots disrupts multiple muscles, including the deltoid and biceps brachii.
**Why the Correct Answer is Right**
The inability to abduct the shoulder (deltoid/supraspinatus) and flex the elbow (biceps brachii) indicates damage to the C5-C6 brachial plexus roots. These roots give rise to the axillary nerve (C5-C6, innervating deltoid) and musculocutaneous nerve (C5-C7, innervating biceps). A root avulsion at C5-C6 would disrupt both, causing the described deficits. This is a classic presentation of upper trunk brachial plexus injury.
**Why Each Wrong Option is Incorrect**
**Option A:** Radial nerve injury causes wrist/finger extension loss, not elbow flexion.
**Option B:** Median nerve injury affects hand intrinsic muscles and forearm pronation, not shoulder abduction.
**Option D:** Axillary nerve alone