A myotome is a group of muscles innervated by a single spinal nerve root, a bundle of nerve fibers exiting the spinal cord. Observing muscle weakness in a myotome allows clinicians to pinpoint the location of a nerve injury in the spine. The C6 myotome is associated with the C6 nerve root, which exits between the fifth and sixth cervical vertebrae (C5-C6). This level is a common site for degenerative changes and nerve compression in the neck, making assessment of the C6 myotome crucial for localizing potential spinal issues.
The Motor Function of C6
The C6 nerve root controls the primary movements of elbow flexion and wrist extension in the arm. While many muscles receive input from multiple nerve roots, C6 is the dominant source of motor signals for these actions. Elbow flexion, the bending of the arm, is primarily driven by the biceps brachii and the brachioradialis muscles. The brachioradialis assists the biceps and becomes more active when the forearm is in a neutral or thumb-up position.
The C6 myotome also powers wrist extension. This movement is executed mainly by the extensor carpi radialis longus and the extensor carpi radialis brevis muscles, which are situated on the back of the forearm. These muscles stabilize the wrist during gripping activities. The combined actions of elbow flexion and wrist extension define the motor territory of the C6 nerve root.
Clinical Testing and Grading
Clinicians assess the C6 myotome using Manual Muscle Testing (MMT) as part of a neurological examination. MMT focuses on isolating the two main C6 actions: elbow flexion and wrist extension. The patient performs the movement while the examiner applies resistance in the opposite direction.
To test elbow flexion, the patient attempts to bend their arm while the examiner pushes down on the forearm. For wrist extension, the patient cocks their hand back, and the examiner applies downward pressure to the back of the hand. Strength is quantified using the standardized 0-5 Medical Research Council (MRC) scale.
A score of 5 indicates normal muscle power against strong resistance, while a score of 0 signifies total paralysis. Intermediate scores, such as 3, mean the muscle can move the limb through the full range of motion only against gravity. The C6 nerve root also supplies the deep tendon reflex (DTR) of the biceps, tested by tapping the tendon with a reflex hammer. A diminished or absent biceps reflex suggests a problem at the C5 or C6 nerve root level.
Causes and Effects of C6 Impairment
C6 radiculopathy, or impairment of the C6 nerve root, typically results from compression or irritation as the nerve exits the spinal canal. The most frequent cause is a cervical disc herniation at the C5-C6 level, where the inner disc material presses directly on the nerve root. In older individuals, compression often stems from degenerative changes, such as osteophytes (bone spurs), or spinal stenosis (a narrowing of the spinal canal).
C6 radiculopathy is characterized by motor symptoms affecting the innervated muscle groups. Patients commonly experience weakness (paresis) during elbow flexion and wrist extension, making tasks like lifting objects or grasping difficult. This motor weakness can lead to arm fatigue and reduced endurance in the affected limb.
Compression of the C6 nerve root also produces sensory symptoms following a predictable pattern called the C6 dermatome. This involves numbness, tingling, or a “pins and needles” sensation that radiates down the arm, typically affecting the thumb and index finger. The dermatome identifies the specific area of skin sensation affected by the compromised nerve root. The severity of the motor deficit, as measured by MMT, helps determine the extent of the nerve root damage and guides treatment decisions.