A 52-year-old woman undergoes surgical exploration for a spinal cord lesion at the C6 level. The surgeon needs to understand the myotomal innervation at this level. Which of the following muscles is NOT primarily innervated by the C6 myotome?

Explanation

## Myotome Organization and C6 Innervation ### Definition of Myotome **Key Point:** A myotome is a group of muscles supplied by a single spinal nerve root. Unlike dermatomes (which have significant overlap), myotomes have less overlap and are useful for assessing motor function in spinal cord and nerve root lesions. ### Upper Limb Myotome Map | Spinal Nerve | Primary Muscles | Key Actions | |--------------|-----------------|-------------| | **C5** | Deltoid, Infraspinatus, Supraspinatus | Shoulder abduction, external rotation | | **C6** | Biceps, Wrist extensors (ECRB, ECRL), Pronator teres, Brachioradialis | Elbow flexion, wrist extension, pronation | | **C7** | Triceps, Wrist flexors, Finger extensors | Elbow extension, wrist flexion, finger extension | | **C8** | Intrinsic hand muscles, Finger flexors | Fine hand movements, finger flexion | | **T1** | Intrinsic hand muscles, Interossei | Finger abduction/adduction | ### C6 Myotome in Detail **High-Yield:** The C6 myotome is clinically tested using: 1. **Biceps reflex** — tests C5–C6 (primarily C6) 2. **Wrist extension** — C6 dominant (extensor carpi radialis) 3. **Pronation** — C6 (pronator teres) 4. **Brachioradialis reflex** — tests C5–C6 ### Why Deltoid is NOT C6 **Clinical Pearl:** The deltoid is primarily innervated by the **C5 myotome** via the axillary nerve (branch of the posterior cord, C5–C6). Although C5 is the dominant innervation, C6 contributes minimally. In a pure C6 lesion, deltoid strength remains relatively preserved because C5 is intact. ### Muscles Correctly Assigned to C6 1. **Biceps brachii** — C5–C6 (C6 dominant) via musculocutaneous nerve 2. **Wrist extensors** — C6–C7 (C6 dominant) via posterior interosseous nerve 3. **Pronator teres** — C6–C7 (C6 dominant) via anterior interosseous nerve 4. **Brachioradialis** — C5–C6 (C6 dominant) via radial nerve **Mnemonic:** **"C5 Shoulders, C6 Elbows & Wrists"** — Remember that C5 controls shoulder movements (deltoid, rotator cuff), while C6 controls elbow flexion and wrist extension. ### Clinical Examination for C6 Lesion A patient with C6 radiculopathy or myelopathy would show: - **Preserved** deltoid strength (C5 intact) - **Weak** biceps (C6 affected) - **Weak** wrist extensors (C6 affected) - **Weak** pronation (C6 affected) - **Hypoesthesia** in C6 dermatome (thumb and lateral forearm) [cite:Clinically Oriented Anatomy Moore 8e Ch 5; Gray's Anatomy 42e Ch 48]