A 52-year-old man from Delhi presents to the emergency department with acute onset severe pain in the right shoulder and upper arm, radiating down the lateral aspect of the arm to the thumb and index finger. On examination, he has weakness of shoulder abduction and external rotation. Sensory testing reveals diminished sensation over the lateral shoulder and dorsal aspect of the first web space. His reflexes are intact. What is the most likely nerve root involved?

Explanation

## Clinical Presentation Analysis **Key Point:** The C6 nerve root is responsible for shoulder abduction and external rotation (shared with C5), elbow flexion, and wrist extension. Its dermatome covers the lateral forearm, thumb, and index finger — including the **dorsal first web space**. ### Dermatome Mapping The patient's sensory loss over the **lateral shoulder AND dorsal first web space** is the critical localizing clue. The C6 dermatome encompasses: - Lateral forearm - Thumb and index finger (dorsal and palmar surfaces) - Dorsal first web space - Lateral shoulder (overlap with C5) The C5 dermatome, by contrast, is limited to the **lateral shoulder and upper arm (deltoid patch)** — it does NOT extend to the thumb or first web space. ### Motor Deficit Correlation | Nerve Root | Motor Deficit | Key Muscles Affected | |---|---|---| | C5 | Shoulder abduction, external rotation | Deltoid, supraspinatus, infraspinatus | | **C6** | **Shoulder abduction, elbow flexion, wrist extension** | **Biceps, brachialis, ECRL, deltoid** | | C7 | Elbow extension, wrist flexion | Triceps, FCR, FDS | | C8 | Intrinsic hand muscles | Lumbricals, interossei | **Clinical Pearl:** C6 radiculopathy classically presents with pain radiating from the neck to the lateral arm, forearm, and into the **thumb and index finger**, with sensory loss over the **first web space** — the hallmark distinguishing it from C5 radiculopathy. The biceps reflex (C5–C6) may be diminished in C6 lesions. ### Why This Patient Has C6 Involvement 1. **Pain radiation**: Down the lateral arm to the **thumb and index finger** = C6 dermatomal distribution 2. **Motor**: Weakness of shoulder abduction and external rotation — C6 contributes to these movements alongside C5 3. **Sensory**: Diminished sensation over lateral shoulder AND **dorsal first web space** = C6 dermatome (first web space is NOT part of C5 territory) 4. **Reflex**: Intact reflexes noted; biceps reflex (C5–C6) may be subtly affected in C6 lesions ### Why NOT C5? C5 radiculopathy produces sensory loss confined to the **lateral shoulder/deltoid patch** and does NOT extend to the thumb or first web space. The involvement of the dorsal first web space in this patient definitively points to C6, not C5. [cite: Clinically Oriented Anatomy, Moore 8e, Ch 6; Gray's Anatomy for Students 3e; Snell's Clinical Neuroanatomy 8e]