The Spinal Accessory Nerve (CN XI) is primarily a motor nerve controlling movements in the neck and shoulder. Unique among cranial nerves, it originates from both the brainstem and the upper spinal cord segments. Testing CN XI function is a standard part of a neurological examination used to assess muscle strength and integrity related to specific head and shoulder movements. This evaluation helps identify potential damage or dysfunction along the nerve’s pathway.
Anatomical Targets of the Nerve
The Spinal Accessory Nerve supplies motor function to two muscles: the Sternocleidomastoid (SCM) and the Trapezius. The nerve’s spinal component arises from the upper cervical spinal cord (C1 through C5 or C6), travels upward to exit the skull, and then descends into the neck. This anatomical course makes the nerve vulnerable to injury as it passes superficially through the posterior triangle of the neck.
The Trapezius muscle is responsible for elevating the shoulder, commonly known as shrugging. The SCM muscle, located on the side of the neck, rotates the head to the opposite side. These specific actions form the basis of the clinical examination.
Testing the Trapezius Muscle Function
Testing the Trapezius muscle assesses the ability to elevate the shoulder (shrug) against resistance. The patient should be seated or standing, with the examiner positioned behind them. The examination begins with visual inspection, checking for muscle atrophy or asymmetrical shoulder drooping that suggests chronic weakness.
The examiner instructs the patient to raise their shoulders as high as possible. Once maximum elevation is achieved, the examiner applies firm, downward pressure to both shoulders simultaneously. This resistance is applied against the patient’s effort to maintain the shrug, testing the strength of the upper fibers of the Trapezius muscle.
The strength is measured by the patient’s ability to hold the shrug and resist the examiner’s pressure. This test is performed bilaterally, comparing both sides for symmetry in movement and strength.
Testing the Sternocleidomastoid Muscle Function
Testing the SCM muscle requires the patient to rotate the head against resistance, isolating the muscle on one side. The patient remains seated, and the examiner asks them to turn their head fully to one side, such as the right. This action contracts the contralateral SCM muscle (the left SCM), as the muscle turns the head to the opposite side.
The examiner places a hand against the side of the patient’s face, opposite the direction of the rotation, and instructs the patient to push their head back against the resistance. During this maneuver, the examiner observes and may palpate the contralateral SCM muscle to confirm its contraction and assess its bulk.
Strength is determined by the muscle’s ability to generate force and hold the rotation against the applied resistance. The process is then reversed to test the SCM muscle on the opposite side.
Interpreting Test Results
Test results are typically graded using the Medical Research Council (MRC) scale, which grades muscle strength from 0 to 5. A score of 5 indicates normal strength, meaning the muscle can contract fully and hold against strong resistance. Lower scores reflect varying degrees of weakness; for instance, a score of 3 means the muscle can move against gravity but cannot tolerate added resistance.
A common sign of Spinal Accessory Nerve dysfunction (palsy) is an asymmetrical presentation, such as noticeable shoulder drooping on the affected side. Trapezius weakness manifests as an inability to maintain the shoulder shrug against downward pressure. This weakness can also lead to “winging” of the scapula, where the shoulder blade protrudes from the back.
SCM weakness is identified by a diminished ability to turn the head forcefully against resistance compared to the unaffected side. Loss of motor function in CN XI may result from causes including surgical injury, blunt trauma, or neurological diseases. The degree of weakness observed helps localize the site and severity of the nerve damage.