A 52 year old woman presented with weakness of shoulder movements. Investigations revealed quadrangular space syndrome. Which of the following nerve is likely to be affected ?
First, I remember that the quadrangular space is an anatomical region in the shoulder. It's bounded by the four borders: the teres minor and subscapularis muscles, and the humerus. The axillary nerve passes through this space. The axillary nerve is responsible for innervating the deltoid and teres minor muscles, which are crucial for shoulder abduction and external rotation. If this nerve is compressed, the patient would have weakness in those movements.
Wait, the options aren't listed here, but the correct answer is supposed to be the axillary nerve. Let me think about other nerves in the area. The radial nerve runs in the spiral groove, not the quadrangular space. The musculocutaneous nerve goes through the coracobrachialis and is more involved in elbow flexion. The ulnar nerve is in the cubital tunnel. So, the axillary nerve is the right answer here.
The clinical pearl here is that quadrangular space syndrome specifically affects the axillary nerve, leading to deltoid weakness and sensory loss in the lateral shoulder. Students often confuse this with other nerve entrapments, like the radial nerve in the spiral groove. Remembering the boundaries of the quadrangular space and the structures passing through it is key. The correct answer should be the axillary nerve, which is option C in the original question.
**Core Concept**
Quadrangular space syndrome involves compression of the **axillary nerve** as it passes through the quadrangular space in the posterior shoulder. This nerve innervates the deltoid and teres minor muscles, critical for shoulder abduction and external rotation.
**Why the Correct Answer is Right**
The axillary nerve travels through the quadrangular space, bordered by the teres minor, teres major, humeral shaft, and long head of the triceps. Compression here impairs deltoid function, causing weakness in shoulder abduction and external rotation. Sensory loss may occur over the lateral shoulder due to the superior lateral cutaneous nerve branch.
**Why Each Wrong Option is Incorrect**
**Option A:** *Radial nerve* – Runs in the spiral groove, not the quadrangular space; affects wrist and finger extension.
**Option B:** *Musculocutaneous nerve* – Innervates the coracobrachialis and brachialis, involved in elbow flexion.
**Option D:** *Ulnar nerve* – Compressed in the cubital tunnel at the elbow, causing claw hand deformity.
**Clinical Pearl / High-Yield Fact**
Quadrangular space syndrome is a rare but important cause of shoulder weakness. Distinguish it from rotator cuff injuries by noting preserved passive shoulder range of motion but weak active abduction. Always consider axillary nerve injury in patients with "flat shoulder" deformity.
**Correct Answer: C. Axillary nerve**