Damage to which spinal nerve root is most likely to cause weakness of elbow flexion and sensory loss over the lateral aspect of the thumb?
Solution
Correct Answer: Option B
Damage to the C6 spinal nerve root is most likely to cause weakness of elbow flexion and sensory loss over the lateral aspect of the thumb.
Here's why:
1. Motor Function: The C6 spinal nerve root contributes primarily to the musculocutaneous nerve, which innervates the biceps brachii muscle—a key muscle responsible for elbow flexion. Therefore, injury to C6 results in weakness when flexing the elbow.
2. Sensory Distribution: The C6 dermatome corresponds to the lateral aspect of the forearm and the thumb, meaning damage to this nerve root causes sensory deficits (numbness or paresthesia) in that area.
3. Differential Diagnosis:
- C5 mainly innervates the deltoid (shoulder abduction) and lateral arm sensation.
- C7 influences elbow extension (triceps) and middle finger sensation.
- C8 controls finger flexion and the medial hand sensation.
Hence, combining weakness in elbow flexion and sensory loss over the lateral thumb points directly to C6 nerve root involvement.
Reference: DeLisa’s Physical Medicine and Rehabilitation: Principles and Practice, Volume 1, Neuroanatomy and Muscle Testing, Chapter 3, Page 45
Here's why:
1. Motor Function: The C6 spinal nerve root contributes primarily to the musculocutaneous nerve, which innervates the biceps brachii muscle—a key muscle responsible for elbow flexion. Therefore, injury to C6 results in weakness when flexing the elbow.
2. Sensory Distribution: The C6 dermatome corresponds to the lateral aspect of the forearm and the thumb, meaning damage to this nerve root causes sensory deficits (numbness or paresthesia) in that area.
3. Differential Diagnosis:
- C5 mainly innervates the deltoid (shoulder abduction) and lateral arm sensation.
- C7 influences elbow extension (triceps) and middle finger sensation.
- C8 controls finger flexion and the medial hand sensation.
Hence, combining weakness in elbow flexion and sensory loss over the lateral thumb points directly to C6 nerve root involvement.
Reference: DeLisa’s Physical Medicine and Rehabilitation: Principles and Practice, Volume 1, Neuroanatomy and Muscle Testing, Chapter 3, Page 45
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