Scalene muscles

The scalene muscles are a group of paired muscles located in the lateral aspect of the neck. They play an important role in cervical movement, respiratory mechanics, and stabilization of the neck and upper thoracic structures. Because of their proximity to vital neurovascular structures, they hold significant clinical relevance in conditions like thoracic outlet syndrome and during surgical or anesthetic procedures involving the neck.

Definition and Overview

Meaning of Scalene Muscles

The scalene muscles are three paired muscles—anterior, middle, and posterior—situated on each side of the cervical spine. They extend from the transverse processes of the cervical vertebrae to the first and second ribs. Functionally, they assist in flexion and lateral bending of the neck and act as accessory muscles of inspiration by elevating the upper ribs during deep breathing.

General Characteristics

These muscles form part of the deep layer of the neck musculature and are enclosed within the prevertebral fascia. They are closely associated with the brachial plexus and subclavian vessels, which pass between or in front of them. The scalene group exhibits symmetrical arrangement on both sides of the neck and varies in size and strength depending on posture, respiratory activity, and individual anatomy.

• The scalene muscles consist of three distinct parts: anterior, middle, and posterior scalenes.
• They originate from the cervical vertebrae and insert onto the upper ribs.
• Each muscle receives innervation from the cervical spinal nerves (C3–C8).
• They function primarily in neck flexion, lateral flexion, and rib elevation during inspiration.

Functional Significance in the Neck Region

The scalene muscles play a dual role in the body: they assist in neck mobility and serve as accessory respiratory muscles. Their contraction stabilizes the cervical vertebrae, allowing coordinated head movement, and their action on the first and second ribs aids in the expansion of the thoracic cavity. Clinically, their location near major vessels and nerves makes them an important landmark for diagnostic and interventional procedures in the cervical region.

• Support cervical posture and balance.
• Assist in breathing, particularly during forced inspiration.
• Act as anatomical guides for identifying the brachial plexus and subclavian artery.

Anatomical Location and Structure

General Position in the Cervical Region

The scalene muscles lie in the lateral compartment of the neck, deep to the sternocleidomastoid muscle and superficial to the cervical vertebrae. They occupy the interval between the vertebral column and the upper ribs, forming an oblique muscular mass that extends from the cervical spine to the thoracic inlet. Collectively, they form the floor of the posterior triangle of the neck.

• Located deep to the prevertebral fascia and sternocleidomastoid.
• Bounded posteriorly by the levator scapulae and anteriorly by the carotid sheath.
• Extend from cervical transverse processes (C2–C7) to the first and second ribs.

Relationship to Surrounding Structures

The anatomical relationships of the scalene muscles are essential for understanding their clinical relevance. They are in close association with the brachial plexus, subclavian artery, subclavian vein, and phrenic nerve. These relationships form the basis for several clinical conditions and procedures in the neck and thoracic outlet region.

• Relation to the sternocleidomastoid and trapezius muscles: The sternocleidomastoid lies superficial and anterior to the scalenes, while the trapezius lies posteriorly, forming boundaries of the posterior triangle.
• Relation to the clavicle and first rib: The lower attachments of the scalene muscles lie near the first rib and behind the clavicle, contributing to the thoracic inlet structure.
• Relation to the brachial plexus and subclavian vessels: The brachial plexus and subclavian artery pass between the anterior and middle scalenes, whereas the subclavian vein passes anterior to the anterior scalene.

Because of these close neurovascular relationships, hypertrophy, spasm, or fibrosis of the scalene muscles can compress the underlying structures, resulting in conditions such as thoracic outlet syndrome or scalene muscle syndrome.

Classification and Individual Muscles

Anterior Scalene

The anterior scalene is the most anterior of the three scalene muscles and serves as a key anatomical landmark in the neck. It is situated deep to the sternocleidomastoid and anterior to the brachial plexus. Its oblique fiber direction extends from the cervical transverse processes downward and laterally to the first rib.

• Origin: Arises from the anterior tubercles of the transverse processes of the third to sixth cervical vertebrae (C3–C6).
• Insertion: Attaches to the scalene tubercle and the upper surface of the first rib, between the grooves for the subclavian artery and vein.
• Nerve supply: Receives innervation from the anterior rami of cervical spinal nerves C4–C6.
• Blood supply: Supplied mainly by branches of the ascending cervical artery and inferior thyroid artery.
• Functional role: Elevates the first rib during inspiration and flexes the neck laterally to the same side. It also aids in stabilization of the cervical spine during movement.

Middle Scalene

The middle scalene is the largest and longest of the scalene group. It lies posterior to the anterior scalene and extends from the cervical vertebrae to the first rib. The brachial plexus and subclavian artery emerge between the anterior and middle scalenes, making this muscle clinically important in nerve and vascular compression syndromes.

• Origin: Arises from the posterior tubercles of the transverse processes of the second to seventh cervical vertebrae (C2–C7).
• Insertion: Attaches to the upper surface of the first rib, posterior to the groove for the subclavian artery.
• Nerve supply: Innervated by the anterior rami of cervical nerves C3–C8.
• Blood supply: Supplied by branches of the ascending cervical and inferior thyroid arteries.
• Functional role: Assists in elevating the first rib during deep inspiration and contributes to lateral flexion of the neck. It also provides stability to the cervical vertebrae during head movement.

Posterior Scalene

The posterior scalene is the smallest and most posterior muscle of the scalene group. It lies behind the middle scalene and extends from the lower cervical transverse processes to the second rib. Although it is less involved in respiration than the other scalenes, it plays a role in neck motion and stabilization.

• Origin: Arises from the posterior tubercles of the transverse processes of the fifth to seventh cervical vertebrae (C5–C7).
• Insertion: Attaches to the outer surface of the second rib, posterior to the attachment of the serratus anterior.
• Nerve supply: Receives innervation from the anterior rami of cervical nerves C6–C8.
• Blood supply: Supplied by the ascending cervical artery and small branches of the superficial cervical artery.
• Functional role: Elevates the second rib during deep inspiration and assists in lateral flexion of the neck.

Attachments and Relations

Superior Attachments

All three scalene muscles originate from the transverse processes of the cervical vertebrae. The anterior scalene arises from C3–C6, the middle scalene from C2–C7, and the posterior scalene from C5–C7. These superior attachments anchor the muscles to the cervical spine, allowing them to exert force on the upper ribs and control neck movement.

• Anterior scalene: Anterior tubercles of C3–C6 transverse processes.
• Middle scalene: Posterior tubercles of C2–C7 transverse processes.
• Posterior scalene: Posterior tubercles of C5–C7 transverse processes.

Inferior Attachments

The scalene muscles insert onto the upper ribs, forming part of the boundary of the thoracic inlet. These inferior attachments provide leverage for rib elevation during inspiration and serve as important landmarks for vascular and neural structures entering the thorax.

• Anterior scalene: Scalene tubercle and upper surface of the first rib.
• Middle scalene: Upper surface of the first rib posterior to the subclavian artery groove.
• Posterior scalene: Outer surface of the second rib posterior to the serratus anterior attachment.

Neurovascular Relationships

The scalene muscles are closely related to several important neurovascular structures of the neck and upper thoracic region. These relationships are key in understanding various clinical syndromes, particularly those involving compression of nerves and vessels within the interscalene space.

• Brachial plexus: The roots and trunks of the brachial plexus pass between the anterior and middle scalene muscles, making this space a critical area for nerve compression and anesthesia administration.
• Subclavian artery: Passes between the anterior and middle scalene muscles along with the brachial plexus.
• Subclavian vein: Lies anterior to the anterior scalene muscle, separated from the artery by the muscle itself.
• Phrenic nerve: Descends on the anterior surface of the anterior scalene, passing between it and the subclavian vein.

Understanding these spatial relationships is essential for safe surgical and anesthetic procedures in the lower neck, such as interscalene nerve blocks and thoracic outlet decompression surgeries.

Functions of Scalene Muscles

The scalene muscles play a vital role in both neck movements and respiratory mechanics. Their oblique orientation between the cervical vertebrae and the upper ribs allows them to act as accessory respiratory muscles while also contributing to cervical flexion, lateral bending, and stabilization of the cervical spine.

• Elevation of the first and second ribs during inspiration: The anterior and middle scalenes elevate the first rib, while the posterior scalene elevates the second rib, aiding in deep inspiration by expanding the upper thoracic cavity.
• Flexion and lateral bending of the neck: When both sides contract simultaneously, they flex the cervical spine. When only one side contracts, they produce lateral flexion toward the same side.
• Stabilization of cervical vertebrae: They help stabilize the cervical spine during head and neck movements, maintaining posture and alignment.

Overall, the scalene muscles assist in respiratory effort during physical exertion or respiratory distress and provide fine control of head and neck positioning in coordination with other cervical muscles.

Applied Anatomy and Clinical Correlations

Scalene Triangle (Interscalene Triangle)

The scalene triangle, also known as the interscalene triangle, is an anatomical space bounded by the scalene muscles and the first rib. It serves as a passageway for major neurovascular structures that supply the upper limb. Understanding its boundaries and contents is essential for clinical diagnosis and surgical procedures in the lower neck and upper thoracic region.

• Boundaries:
  • Anteriorly – Anterior scalene muscle
  • Posteriorly – Middle scalene muscle
  • Inferiorly – First rib
• Contents: Brachial plexus trunks and subclavian artery pass through the scalene triangle.
• Structures anterior to the triangle: The subclavian vein and phrenic nerve lie anterior to the anterior scalene and do not pass through the triangle.

The clinical importance of the scalene triangle lies in its potential to compress neurovascular structures, resulting in thoracic outlet syndrome or scalene muscle syndrome.

Scalene Muscle Syndrome

Scalene muscle syndrome occurs when the brachial plexus or subclavian artery becomes compressed between the anterior and middle scalene muscles. It often results from hypertrophy, fibrosis, or spasm of the scalene muscles due to repetitive strain, trauma, or poor posture.

• Causes: Chronic muscle tension, cervical spine abnormalities, or anatomical variations such as an accessory scalene muscle.
• Symptoms: Pain radiating to the shoulder and arm, numbness, weakness, and tingling along the distribution of the brachial plexus.
• Diagnosis: Clinical examination, nerve conduction studies, and imaging such as MRI or ultrasound to visualize compression.
• Treatment: Includes physical therapy, posture correction, pain management, and, in severe cases, surgical decompression.

Thoracic Outlet Syndrome

Thoracic outlet syndrome (TOS) is a group of disorders caused by compression of neurovascular structures as they pass through the thoracic outlet, which includes the interscalene triangle, costoclavicular space, and subcoracoid space. The scalene muscles, particularly when hypertrophied or fibrotic, contribute to neurogenic, arterial, or venous forms of TOS.

• Compression mechanisms:
  • Neurogenic – Compression of brachial plexus roots between anterior and middle scalene muscles.
  • Arterial – Compression of the subclavian artery leading to reduced blood flow to the upper limb.
  • Venous – Compression of the subclavian vein anterior to the anterior scalene causing swelling and venous congestion.
• Symptoms: Pain, paresthesia, weakness, and circulatory disturbances in the affected limb.
• Management: Conservative treatment includes physiotherapy, muscle relaxation, and postural training. Surgical scalenectomy or first rib resection may be required in refractory cases.

Anesthetic and Surgical Relevance

The scalene muscles serve as key anatomical landmarks for anesthetic and surgical procedures involving the neck and upper thoracic region. Knowledge of their orientation and relationships minimizes the risk of injury to surrounding structures.

• Interscalene brachial plexus block: A regional anesthesia technique performed between the anterior and middle scalenes to provide pain relief or anesthesia to the shoulder and upper arm.
• Surgical relevance: During neck dissections or thoracic outlet decompression, precise identification of the scalenes helps protect the phrenic nerve and subclavian vessels.
• Precautions: Care must be taken to avoid inadvertent injury to the brachial plexus or puncture of the subclavian artery during procedures near the scalene region.

Development and Variation

Embryological Origin

The scalene muscles originate from the paraxial mesoderm during embryonic development. Specifically, they arise from the myotomes of the cervical somites, which give rise to the deep muscles of the neck. As development progresses, the myoblasts migrate and differentiate into individual scalene muscles, forming the anterior, middle, and posterior components. These muscles maintain their segmental innervation from the anterior rami of the cervical spinal nerves, reflecting their embryonic origin.

• Germ layer: Derived from the paraxial mesoderm.
• Somitic origin: Cervical myotomes of somites C3 to C7.
• Developmental differentiation: Myoblast migration and segmentation lead to distinct anterior, middle, and posterior scalene muscles.
• Innervation preservation: The segmental nerves from the cervical spinal region persist as their motor supply after development.

Any disturbance in the development or migration of these myogenic precursors can result in anatomical variations, including accessory muscles or abnormal attachments that may predispose individuals to compression syndromes.

Anatomical Variations

The scalene muscles display considerable anatomical variation among individuals. These variations may influence neck biomechanics and have clinical implications, particularly in relation to thoracic outlet syndrome and interscalene nerve blocks.

• Accessory scalene muscles: Additional slips of muscle may arise from adjacent cervical vertebrae or insert abnormally on the first rib, altering the size of the interscalene space.
• Fusion or absence: Partial or complete fusion between the anterior and middle scalene muscles may occur, or one muscle (usually the posterior scalene) may be absent.
• Variations in attachment: Some individuals may exhibit attachments extending to the second or third rib or to the scalene tubercle of the clavicle.
• Variations in innervation: Although primarily innervated by the cervical spinal nerves, accessory branches from neighboring nerves may occasionally supply the muscles.

Recognizing these variations is essential for clinicians, as they may influence the approach to regional anesthesia, diagnostic imaging, and surgical procedures involving the neck and thoracic outlet.

Physiological Importance in Respiration and Posture

The scalene muscles serve an important physiological role by contributing to both respiratory function and cervical spine stability. Their dual mechanical actions support the body during breathing and maintain posture of the head and neck, especially during dynamic activities.

• Role as accessory muscles of inspiration: During deep or labored breathing, contraction of the anterior and middle scalene muscles elevates the first rib, while the posterior scalene elevates the second rib, expanding the thoracic cavity to facilitate airflow into the lungs.
• Contribution to cervical spine stabilization: The scalenes act synergistically with other deep neck flexors to maintain spinal alignment and prevent collapse of the cervical curvature under head weight.
• Assistance in posture and movement: These muscles help counterbalance the forward pull of the head, particularly in upright posture, and provide fine motor control during head rotation and tilting.
• Effect of posture on function: Poor posture, such as forward head position or prolonged computer use, can lead to scalene shortening, resulting in neck stiffness, muscle fatigue, and nerve compression symptoms.

Thus, the scalene muscles play a vital biomechanical role, bridging respiratory dynamics and postural control. Their balanced function ensures efficient breathing and maintenance of proper cervical alignment, contributing to overall musculoskeletal health.

Clinical Examination and Diagnostic Imaging

Palpation and Surface Anatomy

Clinical examination of the scalene muscles is an essential part of evaluating neck pain, respiratory dysfunction, or suspected thoracic outlet compression. Proper understanding of their surface anatomy assists clinicians in identifying muscle tension, tenderness, or hypertrophy through palpation.

• Surface landmarks: The anterior scalene can be palpated deep to the posterior border of the sternocleidomastoid, extending toward the first rib. The middle scalene lies posterior to this, while the posterior scalene is usually difficult to distinguish on surface examination due to its depth.
• Palpation technique: The patient is positioned supine or seated with the head slightly turned to the opposite side. The examiner gently presses posterior to the sternocleidomastoid muscle, feeling for taut bands or tenderness.
• Findings in pathology: Tightness, hypertrophy, or spasm in the scalene muscles may produce referred pain to the shoulder and arm or reproduce neurological symptoms consistent with thoracic outlet syndrome.

Careful palpation also helps identify trigger points that contribute to myofascial pain, allowing clinicians to differentiate scalene involvement from other sources of cervical discomfort such as levator scapulae or trapezius muscle strain.

Imaging Techniques

Modern diagnostic imaging provides detailed visualization of the scalene muscles, their anatomical relationships, and any pathological changes affecting them. Imaging is particularly valuable in assessing nerve compression, muscle hypertrophy, or vascular anomalies within the interscalene region.

• Ultrasound: A dynamic and non-invasive method that allows real-time visualization of the scalene muscles, subclavian vessels, and brachial plexus. It is often used to guide interscalene nerve blocks.
• MRI (Magnetic Resonance Imaging): Offers high-resolution images of soft tissues, enabling detailed assessment of muscle size, fibrosis, or edema. It is useful in diagnosing thoracic outlet syndrome or myopathic conditions.
• CT (Computed Tomography): Provides detailed cross-sectional imaging of bone and soft tissue structures, helping identify bony abnormalities such as cervical ribs that contribute to neurovascular compression.
• Electromyography (EMG): Evaluates electrical activity of the scalene muscles, detecting abnormal muscle function or nerve involvement in cervical radiculopathy.

These diagnostic tools, when combined with clinical examination, provide a comprehensive evaluation of scalene muscle function and associated pathologies, ensuring accurate diagnosis and targeted treatment planning.

Rehabilitation and Management of Scalene Disorders

Effective management of scalene-related conditions involves a multidisciplinary approach combining physical therapy, postural correction, and, when necessary, medical or surgical interventions. Early diagnosis and conservative treatment can significantly improve outcomes and prevent chronic complications.

• Physical therapy and muscle stretching: Gentle stretching of the anterior, middle, and posterior scalenes helps relieve tension and restore normal muscle length. Strengthening of complementary muscles, such as the deep neck flexors, aids in balancing cervical biomechanics.
• Posture correction and ergonomic adjustments: Training patients to maintain proper head and shoulder alignment reduces strain on the scalene muscles, especially in those who spend long hours at desks or computers.
• Breathing exercises: Diaphragmatic breathing and controlled respiratory techniques can reduce overuse of the scalenes as accessory muscles of inspiration, preventing chronic fatigue.
• Manual therapy: Techniques such as myofascial release, trigger point therapy, and gentle mobilization of the cervical spine improve flexibility and blood flow.
• Interventional pain management: In cases of persistent muscle spasm or nerve entrapment, local anesthetic or corticosteroid injections around the scalene region may provide temporary relief.
• Surgical options: Reserved for severe cases of thoracic outlet syndrome or scalene fibrosis that fail to respond to conservative therapy. Procedures may include scalenectomy or decompression of the brachial plexus.

Long-term rehabilitation focuses on restoring balanced muscle function, improving posture, and preventing recurrence through lifestyle modifications and regular stretching exercises.

Comparative and Functional Anatomy

The scalene muscles, while distinct in humans due to upright posture and complex neck movements, share a common evolutionary origin with those found in quadrupedal mammals. Comparative anatomy provides insight into how these muscles have adapted across species to support locomotion, respiration, and head positioning.

• Scalene muscles in quadrupeds: In four-legged animals such as dogs and horses, the scalenes are more robust and function primarily in stabilizing the neck and aiding respiration during running. They assist in elevating the first ribs to facilitate rapid breathing.
• Evolutionary adaptation: As vertebrates transitioned from quadrupedal to bipedal posture, the scalene muscles became more involved in head balance, cervical spine stabilization, and refined respiratory control.
• Functional differences in humans: In humans, the scalenes provide fine control for neck flexion and lateral bending, and act as accessory muscles of inspiration during increased respiratory demand.
• Comparative morphology: The human scalene group shows greater separation into anterior, middle, and posterior portions compared to other mammals, reflecting increased specialization in movement and airway regulation.

This comparative analysis highlights the adaptive significance of the scalene muscles in maintaining efficient respiration and dynamic head control across different vertebrate species.

References

1. Standring S, editor. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. London: Elsevier; 2021.
2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. 9th ed. Philadelphia: Wolters Kluwer; 2022.
3. Drake RL, Vogl AW, Mitchell AWM. Gray’s Anatomy for Students. 5th ed. Philadelphia: Elsevier; 2023.
4. Netter FH. Atlas of Human Anatomy. 8th ed. Philadelphia: Elsevier; 2022.
5. Palastanga N, Soames R. Anatomy and Human Movement: Structure and Function. 7th ed. London: Elsevier; 2020.
6. Testut L, Latarjet A. Tratado de Anatomía Humana. 10th ed. Barcelona: Salvat Editores; 1984.
7. Peet RM, Henriksen JD, Anderson TP, Martin GM. Thoracic outlet syndrome: evaluation of a therapeutic exercise program. Proc Staff Meet Mayo Clin. 1956;31(9):281–287.
8. Sanders RJ, Hammond SL, Rao NM. Diagnosis of thoracic outlet syndrome. J Vasc Surg. 2007;46(3):601–604.
9. Simon NG, Kiernan MC. The role of imaging in the diagnosis of nerve disorders: cervical and brachial plexus pathology. Clin Neurophysiol Pract. 2015;1:78–89.
10. Wilke J, Krause F, Vogt L, Banzer W. What is evidence-based about myofascial chains: a systematic review. Arch Phys Med Rehabil. 2016;97(3):454–461.