The middle scalene is the largest of the three scalenes, running from the C2 through C7 posterior tubercles to the first rib. The brachial plexus exits between the anterior and middle scalenes in the interscalene triangle, and the middle scalene forms the posterior boundary of this space. The dorsal scapular nerve, which supplies the rhomboids and levator scapulae, typically pierces through the middle scalene belly, making middle scalene hypertrophy a potential site of dorsal scapular nerve entrapment.
| Origin | Posterior tubercles of the transverse processes of C2 through C7 |
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| Insertion | Superior surface of the first rib posterior to the subclavian groove |
| Nerve Supply | Ventral rami of the cervical spinal nerves C3 through C8 |
| Blood Supply | Ascending cervical artery |
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As the primary rib elevator of the three scalenes, the middle scalene is the most important accessory inspiratory muscle during exercise, lifting the first rib to expand the upper thoracic aperture when the diaphragm alone cannot meet respiratory demand.
Interscalene brachial plexus block is performed by placing local anaesthetic between the anterior and middle scalenes at about the C6 level, around the roots and trunks of the plexus. The dorsal scapular nerve and the C5 and C6 roots of the long thoracic nerve usually pass through the middle scalene, so the muscle can entrap them, producing rhomboid and levator scapulae weakness with periscapular pain, or serratus anterior weakness with medial scapular winging. Both nerves must be identified and protected during scalenectomy for thoracic outlet syndrome.
The middle scalene is palpable in the lateral neck posterior to the anterior scalene and anterior to the posterior scalene, most accessible in the posterior triangle between the trapezius and sternocleidomastoid during lateral neck flexion against resistance.
Compression of the dorsal scapular nerve as it pierces the middle scalene causes interscapular pain and rhomboid and levator scapulae weakness, managed with scalene stretching, injection or release.
Entrapment of the long thoracic nerve roots within the middle scalene can weaken serratus anterior and cause medial scapular winging, sometimes alongside thoracic outlet symptoms.
