Scalene muscles

Musculi scaleni

Also known as: scalenes, anterior scalene, middle scalene, posterior scalene, scalenus anterior, interscalene triangle

Three paired lateral neck muscles running from the cervical transverse processes to the 1st and 2nd ribs. They raise the upper ribs in inspiration and flex the neck to the side.

Overview

Anterior scalene runs from the anterior tubercles of the C3–C6 transverse processes to the scalene tubercle on the 1st rib. Middle scalene, the largest, runs from the posterior tubercles of C2–C7 to the 1st rib behind the groove for the subclavian artery. Posterior scalene, the smallest, runs from the posterior tubercles of C4–C6 to the outer surface of the 2nd rib.

The anterior scalene is a crucial landmark. The phrenic nerve descends on its anterior surface, and the subclavian vein passes in front of it. The roots and trunks of the brachial plexus and the subclavian artery pass behind it, emerging through the interscalene triangle between the anterior and middle scalenes above the 1st rib. The scalenes are active in every inspiration, even at rest, lifting and fixing the 1st and 2nd ribs. Acting alone, each flexes the neck laterally to the same side.

Location

Lateral neck, deep to sternocleidomastoid, in the floor of the posterior triangle.

Parts & subdivisions

Anterior scalene

C3–C6 anterior tubercles → scalene tubercle of the 1st rib. Phrenic nerve on its front; subclavian vein in front of it; subclavian artery and brachial plexus behind it.

Middle scalene

C2–C7 posterior tubercles → superior surface of the 1st rib behind the subclavian groove. Pierced by the dorsal scapular nerve and the long thoracic nerve roots (C5–C6).

Posterior scalene

C4–C6 posterior tubercles → external surface of the 2nd rib.

Interscalene triangle

Gap between the anterior and middle scalenes and the 1st rib. Transmits the brachial plexus roots and trunks and the subclavian artery.

Anatomy

Origin

Transverse processes of C2–C7 (anterior scalene C3–C6; middle C2–C7; posterior C4–C6).

Insertion

1st rib (anterior and middle scalenes) and 2nd rib (posterior scalene).

Action

Elevate the 1st and 2nd ribs (inspiration); laterally flex the neck to the same side.

Innervation

Anterior rami of the cervical spinal nerves.

Nerve roots

C3–C8 (anterior C4–C6; middle C3–C8; posterior C6–C8)

Arterial supply

Ascending cervical artery (inferior thyroid artery, thyrocervical trunk) and the superficial cervical artery.

Relations

The phrenic nerve lies on the anterior surface of the anterior scalene, deep to the prevertebral fascia. The brachial plexus and subclavian artery pass between the anterior and middle scalenes. The stellate ganglion lies medially, near the neck of the 1st rib.

Clinical significance

Thoracic outlet syndrome

Compression of the lower brachial plexus (C8–T1) and/or the subclavian artery in the interscalene triangle causes paraesthesia along the ulnar border, wasting of the small hand muscles and arm claudication. Causes include a cervical rib or a fibrous band, or a hypertrophied anterior scalene. Adson's test may reproduce the symptoms.

Interscalene nerve block

Local anaesthetic injected between the anterior and middle scalenes blocks the upper brachial plexus for shoulder surgery. It almost always blocks the phrenic nerve as well, causing ipsilateral diaphragmatic paresis.