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The sternocleidomastoid muscle is a unique muscle that serves two purposes, depending on whether it is acting alone or in conjunction with its counterpart. It is supplied by the spinal accessory nerve, the eleventh nerve, which has a smaller cranial part arising from cells in the nucleus ambiguus and a main spinal portion. Testing the sternocleidomastoid muscle involves observing and palpating the muscle, as well as having the patient flex and turn their head to each side against resistance. This article will discuss the structure and function of the sternocleidomastoid muscle and provide an overview of testing methods.
| Characteristics | Values |
|---|---|
| Muscle function | Rotation of the head to the opposite side, tilts the head to the same side, flexes the neck and brings the head forward and down |
| Muscle structure | Five distinct topographical parts: superficial sternomastoid, profound sternomastoid, sterno-occipital, cleidomastoid, and cleido-occipital |
| Testing methods | Patient flexes and turns the head to each side against resistance, palpation of the muscle, electromyographic studies, neck/cranial CT/MRI scanning |
| Clinical presentation | Weakness, atrophy, or both, of the sternocleidomastoid muscle, and trapezius muscle |
| Innervation | Variable, with the "classical anastomotic pattern" observed in 50% of cases |
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What You'll Learn
Palpate the sternocleidomastoid muscle
Palpation of the sternocleidomastoid muscle is an important part of a neck examination. The sternocleidomastoid muscle is one of the most superficial muscles in the neck, and it is responsible for flexing the neck and bringing the head forward and down. It also plays a role in rotating the head and tilting it to the side. To begin the palpation, the clinician should ask the patient for their consent to touch the sternocleidomastoid muscle, which attaches to the mastoid process.
If the patient gives their consent, the clinician should position the patient's head in contralateral rotation to make the sternocleidomastoid muscle more prominent. This involves the patient turning their head to one side, which will cause the sternocleidomastoid muscle on the opposite side to stand out. Once the muscle is visible, the clinician can place one or two fingers on it. If needed, the clinician can ask the patient to hold this position while they provide resistance to the rotation by applying gentle pressure to the patient's head. It is important to provide resistance on the head and not on the mandible.
During the palpation, the clinician should follow the sternocleidomastoid muscle along its path to its attachment on the mastoid process, which is located behind the ear. They may also palpate the neck and thyroid gland to check for any masses or abnormalities. A nontender neck mass, or a mass that does not cause pain when touched, is often more suspicious for malignancy than a tender neck mass. However, the clinician should also be aware of normal anatomical structures that may be mistaken for a pathologic neck mass, such as the submandibular glands, the hyoid bone, the transverse process of C2, and the carotid bulb.
In addition to palpation, the clinician may also inspect the face and scalp for any swelling, edema, ulcerations, or pigmented lesions. They may also examine the oral cavity and oropharynx to look for ulcers, masses, or other suspicious signs. Overall, palpation of the sternocleidomastoid muscle is a crucial step in evaluating the neck and thyroid gland for any potential concerns.
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Test for rotation of the head
The sternocleidomastoid muscle is one of the largest and most superficial cervical muscles. It is thick and thus serves as a primary landmark of the neck, dividing it into anterior and posterior cervical triangles. The muscle has a cylindrical, strap-like shape that emerges from the side of the neck and tapers at the ends. It is thickest and narrowest in the centre, becoming broader and thinner at either end.
The sternocleidomastoid is responsible for rotating the head to the opposite side of the body and flexing the neck. It also tilts the head to the same side of the body. When both sides of the muscle act together, it flexes the neck and extends the head. When acting alone, it rotates the head to the opposite side and flexes laterally to the same side.
To test for rotation of the head, the subject is asked to maximally rotate the head by turning to one side, left or right, while in a sitting position. This is known as the neck rotation test. It is important to prevent unwanted cervical lateral flexion during cervical rotation, so clinicians and researchers emphasise cervical axial rotation to avoid complicating rotation with other neck motions, such as flexion, extension, and lateral flexion.
The neck rotation test is often performed to evaluate cervical spine dysfunction. Forward head posture (FHP) is a commonly noted abnormal neck posture in students who carry heavy loads or maintain a sitting posture for long periods. FHP can induce neck pain and increase electromyographic activity in the neck muscles.
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Assess the spinal accessory nerve
The spinal accessory nerve (Cranial Nerve XI) is a crucial player in controlling specific muscles involved in head and shoulder movements. Testing this nerve is particularly valuable in neurological examinations, helping to identify potential issues with the nerve and associated functions.
To assess the spinal accessory nerve, the patient should be comfortably seated or standing in an upright position. The examiner will then conduct a cranial nerve exam, which involves testing the strength and coordination of the sternocleidomastoid and trapezius muscles, both of which are innervated by the spinal accessory nerve.
One test involves asking the patient to pull their shoulders backward against resistance and then push them downward against the applied force. The examiner will observe the patient's movements and assess the strength, coordination, and overall performance of these muscles, documenting any abnormalities, weakness, or signs of dysfunction.
Another test to assess the spinal portion of the accessory nerve specifically is to apply resistance to shoulder shrugging. This helps isolate the spinal component's function and provides a more detailed understanding of its integrity.
The cranial nerve exam is especially relevant when there are suspicions of nerve damage or neurological disorders affecting the spinal accessory nerve. It can aid in diagnosing conditions that may compromise the proper functioning of the sternocleidomastoid and trapezius muscles, such as nerve injuries, compression at the jugular foramen, or vascular issues related to the internal carotid artery.
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Observe the muscle's origin and insertion
The sternocleidomastoid muscle is a two-headed neck muscle that is easily visible and palpable. It is an important landmark in the neck, dividing it into an anterior and a posterior triangle. This muscle binds the skull to the sternum and clavicle.
To observe the origin and insertion of the sternocleidomastoid muscle, begin by locating its two heads: the sternal head and the clavicular head. The sternal head originates from the manubrium of the sternum (breastbone), while the clavicular head originates from the medial third of the clavicle (collarbone). The clavicular head is typically the larger of the two heads, with the sternal head being smaller.
The two heads of the muscle join together to form a single muscle belly that travels up the neck in a superior and lateral direction. This belly inserts onto the mastoid process of the temporal bone of the skull and onto the superior nuchal line of the occipital bone. The insertion points are located on the lateral surface of the mastoid process and the anterior portion of the superior nuchal line.
To make the muscle more visible and palpable during examination, rotate the patient's head to the opposite side. This will help you observe the muscle's origin and insertion points more clearly.
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Compare the muscle's size and symmetry
To compare the muscle size and symmetry of the sternocleidomastoid, you can perform a variety of tests and observations. The sternocleidomastoid muscle is a two-headed neck muscle that is easily visible and palpable, making it accessible for examination. Here are some steps to follow to compare its size and symmetry:
- Visual Inspection: Begin by visually inspecting the neck area. Look for any visible asymmetries or abnormalities in the neck muscles, including the sternocleidomastoid. Observe the patient's posture and check for any differences in the level of the shoulders or positioning of the head.
- Palpation: Using your hands, feel the sternocleidomastoid muscles on both sides of the neck. Compare the muscle size, texture, and tension between the right and left sides. Note any differences in muscle tone, tenderness, or swelling.
- Range of Motion: Assess the patient's range of motion by asking them to perform various neck movements, such as flexion, extension, rotation, and lateral bending. Observe the symmetry of these movements and note any restrictions or pain experienced during the movements.
- Resistance Testing: Apply gentle resistance to the patient's head as they resist against your hand by pushing forward. This will help you assess the strength and symmetry of the sternocleidomastoid muscles. Observe any differences in the patient's ability to resist on each side.
- Manual Tests: Perform specific manual tests for the sternocleidomastoid muscle. For example, place your hand on the patient's forehead and push backward as the patient pushes their forehead against your hand. This isolates the sternocleidomastoid and helps you observe its function and symmetry.
- Surface Electromyography (sEMG): sEMG is a more advanced method to assess muscle activation and symmetry. It involves placing electrodes on the skin over the sternocleidomastoid muscles to record electrical activity during rest and various neck movements. This can provide quantitative data on muscle activation and help identify any asymmetries.
- Imaging: In some cases, medical imaging techniques such as ultrasound, MRI, or CT scans may be used to visualise the sternocleidomastoid muscles and assess their size and symmetry. This is particularly useful if there is a suspected injury or abnormality in the muscle structure.
- Comparison with Norms: Compare your observations with established norms or reference values for muscle size and symmetry. This can help identify significant deviations from what is considered typical or healthy.
- Repeat Examinations: For ongoing assessment or monitoring of a condition, repeat the above steps over time to track changes in muscle size and symmetry. This can help evaluate the effectiveness of treatments or interventions.
By following these steps and comparing the size, appearance, and function of the sternocleidomastoid muscles on both sides of the neck, you can gain valuable information about their symmetry and any potential imbalances or conditions affecting the patient.
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Frequently asked questions
The sternocleidomastoid muscle originates from the sternum and clavicle and inserts on the mastoid process. It rotates the head to the opposite side of the body and tilts the head to the same side.
To test the sternocleidomastoid muscle, have the patient flex and turn their head to each side against resistance. Contraction of the left sternocleidomastoid muscle turns the head to the right and vice versa.
The sternocleidomastoid muscle works with the trapezius muscle. Together, they flex the neck and bring the head forward and down.
To test the trapezius muscle, ask the patient to face away from you and observe the shoulder contour for hollowing, displacement, or winging of the scapula. You can also ask the patient to shrug their shoulders against resistance.
