Introduction

The scalene muscles are deep and positioned laterally in the neck (see Image. Neck Muscles). Innervation involves both the brachial plexus and the cervical plexus, contributing to a complex functional role. These multiarticular muscles facilitate accessory breathing and contribute to neck and head movement, as well as postural stability between the neck and head. Dysfunction can lead to pathological conditions such as thoracic outlet syndrome (TOS), reduced ventilatory capacity, and cervical pain. Medical interventions often aim to decrease excessive muscle tone, while rehabilitation focuses on restoring physiological length or enhancing contractile strength.

These muscles likewise have significant surgical importance. The anterior scalene serves as a key landmark for identifying the subclavian artery, the brachial plexus, and the phrenic nerve, which courses along its anterior surface. Partial or complete anterior scalene resection may be performed to relieve neurovascular compression during TOS decompression surgery. Additionally, the scalene muscles are relevant in central venous catheterization via the subclavian approach, as their proximity to major vessels increases the risk of vascular or nerve injury. Understanding the anatomy, function, and clinical significance of the scalene muscles helps clinicians manage various conditions in the cervicothoracic area.

Structure and Function

The scalene muscles elevate the 1st and 2nd ribs during forced inspiration, functioning as secondary respiratory muscles.[1] Electrical activity persists even during nonforced breathing. When not engaged in respiration, these muscles remain active in the supine position and with the arm raised. Additionally, they contribute to cervical posture and play a role in neck movements, including lateral inclination and the initial degrees of head rotation. Bilateral contraction facilitates neck flexion.

Intercostal muscle afferents, particularly between the 8th and 10th ribs anteriorly, are closely linked to the scalene muscles, which respond bilaterally to unilateral stimulation. The exact mechanism remains unclear, but inspiration involving the lower ribs likely recruits the scalene muscles to support breathing.

The scalene muscles lie deep to the sternocleidomastoid muscle and lateral to the cervical spine, extending from the vertebrae to the 1st and 2nd ribs. These muscles are enclosed by the deep fascia, also known as the prevertebral fascia.

The anterior scalene originates from the anterior tubercles of the transverse processes of the 3rd to 6th cervical vertebrae and inserts on the scalene tubercle of the superior surface of the 1st rib (see Image. Anterior Muscles of the Neck). Some fibers may contact the pleural dome. The omohyoid muscle crosses over both the anterior and middle scalene muscles. The anterior scalene is thickest at the level of the cricoid cartilage and thinner at its origin and insertion. The phrenic nerve runs along its surface, sharing the muscle's fascia from the lateral to the medial border, passing behind the subclavian vein. The anterior and middle scalene muscles define the posterior interscalene gap (fissura scalenorum), through which the brachial plexus passes.[2]

The middle scalene originates from the transverse processes of the last 6 cervical vertebrae, between the anterior and posterior tubercles, and inserts on the superior surface of the 1st rib, posterior to the subclavian artery sulcus. The upper portion of the long thoracic nerve may pass between the middle and posterior scalene, through the middle scalene, or above it.[3] The posterior scalene originates from the posterior tubercles of the transverse processes of the last 3 or 4 cervical vertebrae and inserts on the anterior surface of the 2nd rib.

A 4th scalene muscle, the scalenus minimus, is present in approximately 30% to 71% of individuals.[4] This muscle arises from the anterior tubercle of the transverse process of the 7th cervical vertebra and inserts on the 1st rib and the Sibson fascia posteriorly, which covers the pleural dome. The scalenus minimus can be mistaken for the transverse pleural ligament.

Embryology

The scalene muscles develop from myoblasts originating in the hypaxial portion of the cervical myotomes, along with the prevertebral and geniohyoid muscles, during the 7th week of embryonic development.[5] The formation of these muscles is regulated by the surrounding connective tissue, which guides myoblast migration. The cervical connective tissue differs from the somitic mesoderm.

Blood Supply and Lymphatics

The scalene muscles receive blood supply from the 1st portion of the subclavian artery, referred to as the "prescalene area." The thyrocervical trunk passes medial to the anterior scalene and ascends before dividing into 2 terminal branches.[6] The inferior thyroid artery, the 1st branch, supplies the anterior scalene muscle. The ascending or superficial cervical artery, the 2nd branch, supplies the middle and posterior scalene muscles.[7][8][9][8] Venous drainage from the scalene muscles is directed into the brachiocephalic venous trunk.

The cervical region contains approximately 300 lymph nodes. Those draining the scalene muscles are consistently located within the posterior triangle of the neck. The drained lymph flows into the right and left jugular lymphatic trunks. Lymph from the right side enters the right lymphatic duct, while lymph from the left side drains into the thoracic duct.[10]

Nerves

The scalene muscles receive innervation from the anterior branches of the cervical spinal nerves spanning C3 to C8. Both the cervical and brachial plexuses contribute to this innervation, and their interaction is essential for the diverse functions of these muscles. Neuroanatomical variations in this region exist but are rare.[11]

Physiologic Variants

The scalene muscles, like any other anatomical region, exhibit various anatomical variations. The anterior scalene muscle may originate from C2 or exclude C6. This muscle's insertion may extend beyond the 1st rib to the 2nd or 3rd thoracic rib.[12] The muscle may also pass behind the subclavian artery, either unilaterally or bilaterally, and, in some cases, lie posterior to the 1st thoracic nerve. Literature reports rare instances of complete absence of the anterior scalene muscle, either unilaterally or bilaterally.[13] The middle scalene muscle may insert into the transverse processes of the atlas and extend toward the 2nd thoracic rib. Some reports describe an accessory middle scalene muscle forming a bridge with the anterior scalene, a variation potentially linked to symptoms resembling TOS.

The posterior scalene muscle may originate from C3 to C7, with some fibers merging with the middle scalene or the 1st intercostal muscle. The insertion of this muscle may extend to the 3rd thoracic rib, and in some individuals, the muscle may be absent. The middle scalene may be perforated by brachial plexus branches, such as C7 and C8, which terminate at the posterior scalene. Some studies describe a double-layered posterior scalene with distinct ventral and dorsal portions. The scalenus minimus may present as a flattened structure occupying a broad region of the thoracic outlet. The origin of this muscle may involve C6 and C7.

Surgical Considerations

Thoracic Outlet Syndrome

TOS arises from direct or indirect mechanical compression of the subclavian vessels or the brachial plexus between the clavicle, scalene muscles, and 1st rib. The most common clinical symptoms include fatigue, pain, and paresthesias affecting the upper extremity.[14] Diagnosis can be challenging due to dynamic, posture-dependent symptoms, which result from neurovascular compression when the upper extremity is elevated overhead. TOS should be considered in patients presenting with these symptoms, particularly if a cervical rib is present, which occurs in approximately 0.5% of the population.[15]

Case reports describe various forms of TOS associated with a hypertrophic anterior scalene muscle.[16] In some instances, resection of the anterior or middle scalene muscle (scalenectomy) is necessary. When abnormal muscle bundles compress the brachial neurovascular structures, ultrasound imaging may reveal the “wedge-sickle sign,” indicating the need for surgical intervention. In cases where TOS is directly linked to the scalene muscle, symptoms may be managed temporarily with type A botulinum toxin (Botox) injections, particularly in older patients requiring surgical delay.[17] Botox injections may also alleviate symptoms in individuals with spasmodic torticollis or cervical dystonia.

Neck Mass Dissections

Tumors can develop within the scalene muscles, including rare cases of myopericytoma.[18] Surgical removal is often required. Dissection for tumor or mass removal involves intramuscular dissection to achieve either marginal or radical resection of the tumor bed. Another rare tumor affecting the head and neck is myxoma, a mesenchymal neoplasm.[19] More commonly, benign tumors such as fibromatosis or hemangioma are found within the muscle and frequently require surgical excision.[20] Lymph node biopsies of the scalene region can help determine tumor type. In some cases, a scalenectomy may be performed to create access for the removal of deeper tumors, such as cervical schwannomas. Additionally, mass effects can result from direct trauma, leading to intramuscular hemangiomas.

Nerve Blocks and Catheter Considerations

Nerve blocks and catheters are increasingly utilized as healthcare shifts toward outpatient-based procedures.[21] Indications for nerve blocks include neurological pain management and certain depressive syndromes. Literature supports continuous interscalene blocks over single-injection blocks due to lower toxicity rates and reduced symptomatic rebound once the anesthetic wears off.[22][23] Another invasive technique is the stellate ganglion block, which involves passing a needle through multiple tissues, including the anterior and middle scalene muscles.[24]

Clinical Significance

Electromyography can assess scalene muscle function and detect contractile anomalies, aiding in the diagnosis of conditions like TOS.[25] Patients with temporomandibular joint dysfunction exhibit increased anterior scalene electrical activity compared to those without mandibular issues.[26] This finding may be valuable for physiotherapists and osteopaths, as scalene dysfunction can reduce ventilatory capacity, with mandibular disorders potentially being a symptom rather than the cause.

Hypertrophy or increased tone of the anterior scalene muscle may produce distal symptoms. Vascular compression of the subclavian artery can lead to pallor, cold sensations, cyanosis, and impaired arm function. If C5-C6 crosses the muscle in an abnormal manner, such as through hypertrophy or shortening, paresthesia, anesthesia, or weakness in the corresponding muscle groups may occur. Compression of the C8 root between 2 muscular attachments of the anterior scalene can cause tingling, pain, and weakness in the hand and interosseous muscles. Fusion of the anterior scalene tendon with the longus capitis muscle may contribute to headaches.[27]

If the suprascapular nerve traverses a hypertonic, hypertrophic, or shortened middle scalene muscle, weakness of the supraspinatus and infraspinatus muscles may result, along with paresthesia and shoulder pain, leading to scapulohumeral arthrokinematic dysfunction. Compression of the deep branch of the transverse cervical artery within the middle scalene could present with subtle symptoms, complicating diagnosis.

Increased posterior scalene thickness may alter the course of the long thoracic and dorsal scapular nerves, causing pain or weakness in the serratus anterior, levator scapulae, and rhomboid muscles. Additionally, the presence of the minimum scalene muscle may reduce space for the brachial plexus, subclavian artery, and subclavian vein, contributing to symptoms associated with TOS.[28]

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Disclosure: Bruno Bordoni declares no relevant financial relationships with ineligible companies.

Disclosure: Felix Jozsa declares no relevant financial relationships with ineligible companies.

Disclosure: Matthew Varacallo declares no relevant financial relationships with ineligible companies.