What is the sternocleidomastoid (sternomastoid) muscle?

The Sternocleidomastoid Muscle: Anatomy and Clinical Significance

The sternocleidomastoid (sternomastoid) muscle is a paired neck muscle that connects the sternum and clavicle to the mastoid process of the temporal bone, functioning primarily to rotate and flex the head while serving as an important anatomical landmark that divides the neck into anterior and posterior triangles. 1, 2

Anatomical Structure

• Origin: The muscle has two distinct heads:

  • Sternal head: Arises from the anterior surface of the manubrium of the sternum
  • Clavicular head: Arises from the medial third of the clavicle

• Insertion: The two heads merge as they ascend obliquely across the neck to insert on the mastoid process of the temporal bone and the lateral portion of the superior nuchal line 2

• Innervation: Supplied by the spinal accessory nerve (Cranial Nerve XI) 3

• Blood Supply: Primarily from branches of the occipital artery and the superior thyroid artery

Functional Role

The sternocleidomastoid muscle performs several important functions:

• Unilateral contraction: Rotates the head to the opposite side while slightly flexing and tilting it toward the ipsilateral shoulder 4
• Bilateral contraction: Flexes the neck, bringing the chin toward the chest
• Accessory muscle of respiration: Assists in forced inspiration by elevating the sternum

Clinical Significance

1. Anatomical Landmark:

   • Divides the neck into anterior and posterior triangles
   • Serves as a surgical landmark during neck dissections for head and neck cancer 1

2. Pathological Conditions:

   • Congenital muscular torticollis: A condition in infants characterized by shortening of the sternocleidomastoid muscle, causing head tilt and rotation limitation 5
   • Sternocleidomastoid syndrome: Can result in head and face pain, nausea, dizziness, coryza, and lacrimation due to dysfunction of the muscle 6

3. Surgical Considerations:

   • Important structure to identify and potentially preserve during neck dissections
   • The National Comprehensive Cancer Network recommends preservation when possible during selective neck dissections 1
   • Comprehensive neck dissections may or may not preserve this muscle depending on the extent of disease 1

4. Diagnostic Imaging:

   • MRI and CT are complementary in evaluating the sternocleidomastoid muscle, especially when assessing for atrophy or denervation changes 3
   • Loss of volume and fatty infiltration may be noted on imaging in cases of accessory nerve palsy 3

5. Anatomical Variations:

   • Variations in attachment have been reported in approximately 27.8% of specimens in cadaveric studies 2
   • Rare variations such as sternopharyngeal branches have been documented 7

Assessment of Function

• Clinical examination includes testing head rotation against resistance
• Electromyography can be used to assess muscle function during various head movements 4
• Weakness or atrophy may indicate accessory nerve (CN XI) palsy 3

The sternocleidomastoid muscle is not only important for head and neck movements but also serves as a critical anatomical landmark for surgeons and a structure that must be carefully evaluated in patients with neck pain, limited range of motion, or suspected accessory nerve pathology.