Anatomy and supply
The scalene muscles (Latin: “ladder”) are three paired cervical muscles running from the cervical vertebrae to the upper two ribs. Hereby they form a roof over the pleura and the upper thorax. The innervation is carried by direct branches of the cervical and brachial plexuses (C3-6). The scalene muscles are subdivided into:
- Scalenus anterior muscle: runs from the anterior tubercula of the transverse processes of the 3rd to 6th cervical vertebrae to the first rib.
- Scalenus medius muscle: originates at the posterior tubercula of the transverse processes of the 3rd to 7th cervical vertebrae and inserts at the first rib dorsally to the scalenus anterior muscle.
- Scalenus posterior muscle: has its origin at the posterior tubercula of the transverse processes of the 5th to 7th cervical vertebrae and its insertion at the second rib.
The triangle between the scalenus anterior muscle, the scalenus medius muscle and the first rib form the interscalene triangle. The subclavian artery and the brachial plexus pass through this gap. In contrary, the subclavian vein runs ventrally from the scalenus anterior muscle.
The scalene muscles elevate the ribs, and therefore the thorax. For that reason, they are also considered as accessory muscles of inspiration. A unilateral contraction bends the cervical spine to the side (lateral flexion). Furthermore, a bilateral contraction of the scalenus anterior muscle causes a bending of the cervical spine to the front (ventral flexion).
A too narrow interscalene triangle compresses the brachial plexus and subclavian artery (Scalene syndrome) causing paresthesia, more rarely circulatory disturbances (e.g. edema, ischemia) and pain. Typically the ulnar side and the little finger are affected. Causes for the Scalene syndrome may be, amongst others, a hypertrophy of the scalene muscles due to chronic overload and variations or deformities of the first rib.