The accessory nerve, also known as Cranial Nerve XI, plays a significant role in controlling movements of the neck and shoulders, such as turning the head and shrugging. Understanding its function and how it is assessed provides insight into neurological health.
Understanding the Accessory Nerve
The accessory nerve is the eleventh of the twelve paired cranial nerves and is a purely motor nerve. It originates from both the brainstem and the upper spinal cord, specifically from cervical segments C1 through C6. Fibers from the spinal cord ascend into the skull through the foramen magnum and then exit the skull via the jugular foramen.
Upon exiting the skull, the accessory nerve travels down the neck to innervate two major muscles: the sternocleidomastoid and the trapezius. The sternocleidomastoid muscle, located in the front of the neck, is responsible for turning the head to the opposite side and tilting it to the same side. When both sternocleidomastoid muscles contract, they work together to flex the neck, bringing the head forward.
The trapezius muscle, a large, triangular muscle spanning the upper back and neck, performs several actions. These include elevating the shoulder blades (e.g., shrugging), as well as retracting and depressing them. The trapezius also contributes to head extension and rotation.
Indications for Testing
Healthcare professionals test the accessory nerve when a patient presents with symptoms suggesting nerve damage or dysfunction. These often involve unexplained weakness or difficulty with neck and shoulder movements, such as shrugging or turning the head. Other complaints that may prompt an examination include persistent neck pain, a noticeable drooping of one shoulder, or changes in head posture. Muscle atrophy, or wasting, in the neck or shoulder muscles can also be an indicator. Testing the accessory nerve is commonly integrated into a broader neurological examination to pinpoint the source of a patient’s symptoms.
The Physical Examination
Testing the accessory nerve involves a systematic physical examination focusing on the two muscles it controls: the sternocleidomastoid and the trapezius. This assessment helps determine the strength and function of these muscles, providing insights into the nerve’s integrity. Throughout the examination, the healthcare professional observes for any visible muscle wasting or asymmetry.
To test the sternocleidomastoid muscle, the patient turns their head strongly against the examiner’s hand placed on the opposite side of their chin. For example, if the patient turns their head to the right, the examiner applies resistance to the right side of the chin. The examiner observes the contraction and strength of the left sternocleidomastoid muscle.
For the trapezius muscle, the patient is instructed to shrug their shoulders upwards. While the patient maintains this position, the examiner applies downward pressure on the shoulders, resisting the shrugging motion. The examiner assesses the patient’s ability to maintain the shrug against this resistance, noting the strength and symmetry of both trapezius muscles.
Interpreting Findings
Normal findings during an accessory nerve examination involve strong and symmetrical resistance in both the sternocleidomastoid and trapezius muscles. A patient with normal nerve function will be able to turn their head and shrug their shoulders with full range of motion and strength against applied resistance.
Abnormal findings might include noticeable weakness, asymmetry in muscle strength, or an inability to perform the movements against resistance. The presence of muscle atrophy, or wasting, in the sternocleidomastoid or trapezius muscles also indicates potential nerve dysfunction. Such findings could suggest an injury to the accessory nerve itself.
Common causes of accessory nerve dysfunction include direct injury, compression, or damage sustained during surgical procedures in the neck, such as lymph node biopsies or neck dissections. Neurological conditions or blunt trauma can also affect the nerve. Only a qualified healthcare professional can accurately interpret these findings and establish a diagnosis.