Scalenus Anterior: Origin, Insertion, Nerve, Action & Thoracic Outlet Syndrome

The Scalenus Anterior (Anterior Scalene) is a key muscle of the lateral neck, located deep to the Sternocleidomastoid. It serves as a critical anatomical landmark in the neck, defining the boundaries for nerves and vessels leaving the thorax. It is notoriously involved in nerve compression syndromes affecting the arm.

[Image of Musculocutaneous nerve piercing coracobrachialis]

Quick Anatomy Snapshot

Group	Lateral Vertebral Muscles (The Scalenes).
Origin (Proximal)	Anterior tubercles of the Transverse Processes of C3, C4, C5, and C6.
Insertion (Distal)	Scalene Tubercle on the inner border and upper surface of the 1st Rib.
Nerve Supply	Anterior Rami of Cervical Spinal Nerves (C4, C5, C6).
Primary Actions	Unilateral: Lateral flexion of the neck to the same side; Rotation to the same side (ipsilateral). Bilateral: Flexion of the neck. Inspiration: Elevates the 1st Rib (Accessory muscle of breathing).

Deep Dive: The Interscalene Triangle

The Anterior Scalene is clinically famous because of the structures that pass around it.

1. The Triangle

The Scalene Interval (Interscalene Triangle) is a triangular space formed by:
• Anterior Border: Scalenus Anterior
• Posterior Border: Scalenus Medius
• Inferior Border: 1st Rib
The Brachial Plexus (nerves to the arm) and the Subclavian Artery pass through this triangle. If the Anterior Scalene is tight or hypertrophied, it strangles these structures.

2. The Phrenic Nerve

The Phrenic Nerve (C3, C4, C5) which controls the diaphragm, runs vertically on top of the Anterior Scalene muscle belly, deep to the prevertebral fascia. This is a critical danger zone during neck surgery or interscalene nerve blocks.

3. The Vein Separation

The Subclavian Vein does not pass through the triangle. It passes anterior to the Scalenus Anterior (between the muscle and the clavicle). This separation is a key way to identify the muscle in dissection.

Physio Corner: Clinical Relevance

💪 Functional Fact: As an accessory muscle of inspiration, the Scalenes are often overactive in patients with COPD, asthma, or stress breathing ("Upper Chest Breathers"). This chronic tension can lead to neck pain and nerve compression.

Palpation

1. Have the patient sit. Locate the clavicular head of the Sternocleidomastoid (SCM).
2. Move your fingers just posterior to the clavicular head of the SCM, into the supraclavicular fossa.
3. Ask the patient to sniff quickly (inspiration). You will feel the Scalenus Anterior tightening as it lifts the 1st rib.

⚠️ Clinical Pathology: Thoracic Outlet Syndrome (TOS)
Scalenus Anticus Syndrome is a type of TOS where the Anterior Scalene compresses the Brachial Plexus or Subclavian Artery.
Adson's Test: Locate the radial pulse. Extend and rotate the patient's arm. Ask them to take a deep breath, extend their neck, and rotate the head toward the affected side. Disappearance of the pulse or reproduction of symptoms suggests Anterior Scalene tightness.

Manual Muscle Testing (MMT)

We usually test the scalenes as a group (Anterior, Middle, Posterior) via lateral flexion.

Testing Tip: To bias the Anterior Scalene, combine Flexion with Rotation to the same side, contrasting it with the SCM (which rotates to the opposite side).

Step-by-Step Procedure (Lateral Flexion)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Side-lying or Sitting. Action: Patient brings the ear toward the shoulder (Lateral Flexion) without shrugging. Resistance: Applied to the temporal region of the head, pushing the head back to neutral. Grade 3: Full range against gravity (side-lying). Grade 4/5: Holds against resistance.
Grade 0, 1 (Palpation)	Action: Palpate behind the SCM, just above the clavicle. Cue: "Try to tilt your ear to your shoulder." Grade 1: Muscle tension felt. Grade 0: No activity.

Frequently Asked Questions

What passes in front of the Anterior Scalene?

The Subclavian Vein, the Phrenic Nerve, and the Transverse Cervical/Suprascapular arteries pass anterior to the muscle.

Does it rotate the head to the opposite side?

Generally, no. Most anatomical texts agree it rotates the cervical spine to the same side (ipsilateral), unlike the SCM which rotates to the opposite side.

How do I stretch the Anterior Scalene?

Side-bend the neck to the opposite side and extend the neck slightly (look up and away). Anchoring the clavicle/1st rib with your hand enhances the stretch.

Test Your Knowledge: Scalenus Anterior Quiz

1. Where does the Scalenus Anterior insert?

2. Which major nerve runs vertically on the anterior surface of this muscle?

3. The Interscalene Triangle contains the Brachial Plexus and the:

4. What is the origin of Scalenus Anterior?

5. Adson's Test is used to diagnose:

6. What is the role of Scalenus Anterior in breathing?

7. Which muscle forms the posterior border of the Interscalene Triangle?

8. Unilateral contraction of the Anterior Scalene causes:

9. Which vessel runs ANTERIOR to the Anterior Scalene insertion?

10. The Scalenes are innervated by:

References

• Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2018). Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer.
• Standring, S. (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. Elsevier.
• Magee, D. J. (2014). Orthopedic Physical Assessment. 6th ed. Elsevier.