Scalene Muscles

Anterior scalene (scalenus anterior)

• Origin: the anterior tubercles of the transverse processes of C3 to C6.
• Insertion: the scalene tubercle of Lisfranc on the inner border of the upper surface of the first rib, and a short ridge running forward from it.
• The tubercle separates two shallow grooves on the first rib: the groove for the subclavian vein anteriorly and the groove for the subclavian artery posteriorly. That single bony landmark encapsulates the whole relationship.

Middle scalene (scalenus medius)

• Origin: the posterior tubercles of the transverse processes of C2 to C7 (frequently described as C1 to C7).
• Insertion: the upper surface of the first rib, behind the groove for the subclavian artery, extending back to the tubercle for serratus anterior.
• It is the largest of the three and is pierced by the dorsal scapular nerve and by the C5 and C6 contributions to the long thoracic nerve.

Posterior scalene (scalenus posterior)

• Origin: the posterior tubercles of the transverse processes of C4 to C6.
• Insertion: the outer surface of the second rib.
• Often blended with middle scalene; occasionally absent.

Scalenus minimus (scalenus pleuralis, Sibson's muscle)

• A variable fourth muscle, present in a substantial minority of people, arising from the transverse process of C6 or C7 and inserting into the inner border of the first rib and into the suprapleural membrane (Sibson's fascia).
• Crucially, it passes between the subclavian artery and the T1 root, splitting the triangle. Where present, it is a recognised anatomical cause of neurogenic thoracic outlet syndrome and must be looked for and excised at scalenectomy.

Segmental muscular branches

The scalenes are supplied by direct muscular branches from the ventral rami of the cervical nerves as they emerge between them - there is no named nerve:

• Anterior scalene: C4 to C6.
• Middle scalene: C3 to C8.
• Posterior scalene: C6 to C8.

The nerves that matter are those passing THROUGH or ON them

• Phrenic nerve (C3, C4, C5) - descends on the anterior surface of the anterior scalene, deep to the prevertebral fascia, running from lateral to medial as it passes from the upper part of the muscle to its medial border, then entering the thorax between the subclavian artery and vein.
• Dorsal scapular nerve (C5) - pierces the middle scalene.
• Long thoracic nerve - the C5 and C6 contributions pierce the middle scalene; the C7 contribution passes anterior to it.
• Nerve to subclavius (C5, C6) - crosses in front of the plexus and the subclavian artery.
• Suprascapular nerve (C5, C6) - arises from the upper trunk at Erb's point, at the lateral edge of the triangle.
• Roots C5 to T1 - emerge between the anterior and middle scalene, C5 and C6 uniting to form the upper trunk within or just distal to the triangle.

Why this list is examinable

Every one of these nerves can be injured by a needle, a retractor or a scalpel in the same few square centimetres. In practice the four that get injured are the phrenic (block, scalenectomy), the dorsal scapular and long thoracic (block, middle scalene division, first rib resection), and the plexus itself.

Muscular supply

• Ascending cervical artery - a branch of the inferior thyroid artery from the thyrocervical trunk; the principal supply, ascending on the anterior surface of the anterior scalene alongside the phrenic nerve.
• Inferior thyroid artery itself, and the transverse cervical and suprascapular arteries crossing the anterior scalene.
• Deep cervical and vertebral artery muscular branches to the posterior and middle scalene.

The vessels that matter surgically are those crossing the field

• Subclavian artery - passes behind the anterior scalene in the interscalene triangle, grooving the first rib posterior to the scalene tubercle. Its third part lies lateral to the anterior scalene and is the vessel controlled in a supraclavicular exploration.
• Subclavian vein - passes anterior to the anterior scalene, grooving the first rib in front of the tubercle, in the costoclavicular space.
• Transverse cervical and suprascapular arteries - cross the anterior scalene transversely, superficial to the phrenic nerve, and must be ligated or retracted in the supraclavicular approach.
• Vertebral artery - ascends medially, in front of the transverse process of C7 and then through the foramina transversaria from C6 upward. It is the vessel accidentally punctured during a badly directed interscalene injection.
• Thoracic duct (left) and right lymphatic duct - arch over the subclavian artery and the dome of the pleura to enter the venous angle. Chyle leak is a specific complication of a left-sided supraclavicular dissection.

Venous drainage

To the vertebral, inferior thyroid and subclavian veins.

Anterior to the anterior scalene (superficial to deep from the skin)

• Skin, platysma, investing fascia.
• Sternocleidomastoid, omohyoid (inferior belly crossing obliquely), and the supraclavicular fat pad.
• Subclavian vein, internal jugular vein and the venous angle.
• Transverse cervical and suprascapular arteries.
• Phrenic nerve, on the muscle beneath the prevertebral fascia.
• Thoracic duct on the left, arching laterally to the venous angle.

Posterior to the anterior scalene (that is, in the triangle)

• Subclavian artery (second part).
• Roots and trunks of the brachial plexus.
• The dome of the pleura and Sibson's fascia below and medially.

Posterior to the middle scalene

• Levator scapulae and the posterior triangle contents, with the spinal accessory nerve running on the levator further posteriorly.
• The long thoracic and dorsal scapular nerves emerging on its lateral surface.

Medially

• The vertebral artery and vein, the sympathetic chain and stellate ganglion, the oesophagus and trachea, and the recurrent laryngeal nerve in the tracheo-oesophageal groove.

Structures at risk

• Phrenic nerve on the anterior scalene.
• Long thoracic and dorsal scapular nerves in the middle scalene.
• Brachial plexus in the triangle.
• Subclavian artery and vein, vertebral artery medially.
• Pleural dome, only a centimetre or two below the level of the clavicle at the medial end.
• Thoracic duct on the left.
• Sympathetic chain / stellate ganglion medially - Horner syndrome.

The three types

Anatomical causes

• Cervical rib - complete or incomplete, articulating with or attached by a fibrous band to the first rib.
• Fibrous bands - the most frequent finding at operation, often without any bony abnormality. Several distinct band types are described running from a rudimentary cervical rib, an elongated C7 transverse process, or the scalene fascia to the first rib.
• Elongated C7 transverse process without a true rib.
• Scalenus minimus muscle - passing between the subclavian artery and the T1 root.
• Scalene hypertrophy or fibrosis - post-traumatic, occupational or postural.
• First rib anomalies, clavicular malunion narrowing the costoclavicular space, and healed first rib fracture.
• Anomalous scalene insertions broadening the anterior scalene footprint on the first rib.

Investigation

• Radiographs of the cervical spine and chest - look for a cervical rib, an elongated C7 transverse process, and above all for an apical lung lesion.
• EMG and nerve conduction studies - to confirm a lower trunk lesion in true neurogenic disease and, more often, to exclude the mimics (ulnar neuropathy, C8 radiculopathy, carpal tunnel).
• Duplex ultrasound with provocative positioning for the vascular forms.
• CT or MR angiography and venography where a vascular cause is suspected, with the arm in the provocative position.
• MRI of the brachial plexus to demonstrate bands and to exclude a mass lesion.
• Anterior scalene block with local anaesthetic (or botulinum toxin) - relief predicts benefit from scalenectomy and is one of the more useful selection tools in a field short of them.

Management

1. Non-operative for at least 3 to 6 months in neurogenic disease: posture correction, scalene and pectoralis minor stretching, breathing retraining away from an upper-chest pattern, scapular stabilisation, ergonomic and load modification, and weight loss where relevant. The majority improve.
2. Surgery for failure of rehabilitation with a convincing clinical picture and a positive scalene block, and urgently for the vascular forms.
3. Venous disease - catheter-directed thrombolysis for acute effort thrombosis followed by decompression (usually first rib resection) and anticoagulation.
4. Arterial disease - decompression with arterial reconstruction where there is aneurysm or embolisation. This is limb-threatening disease and is not managed conservatively.

Prevalence and demographics

• Present in 2.0 per cent of 3404 consecutive adult cervical spine CT scans in the largest imaging series, which is higher than the 0.5 to 1 per cent classically quoted from plain radiographs - cervical ribs are subtle and easily missed, and radiologists commented on only about a quarter of those present.
• Commoner in women, and bilateral in a substantial proportion (often asymmetric in size).
• Symptomatic in only about 10 per cent of those who have one.

Classification of the extent

A cervical rib may be:

• A prominent, elongated C7 transverse process only.
• A short rib ending in a free tip.
• A rib with a fibrous band completing the connection to the first rib - the most dangerous variant, because the band is radiolucent and is easily missed on imaging while producing the tightest constriction.
• A complete rib articulating with or fused to the first rib.

Why it causes trouble

A cervical rib elevates and angulates the lower trunk and the subclavian artery as they cross it, converting a gentle curve into a sharp bend over a hard edge. The lower trunk, being the most inferior structure, takes the strain - hence the T1-dominant clinical picture. Arterial compression at the same point produces post-stenotic dilatation, mural thrombus and distal embolisation, which can present as digital ischaemia in a young patient with an otherwise normal vascular tree.

Association with other conditions

Cervical ribs are more common in some genetic and developmental conditions and are a recognised marker of a homeotic shift at the cervicothoracic junction. Their presence should prompt a look at the whole cervicothoracic segmentation on imaging.

Scalene myofascial pain

Trigger points in the scalenes refer pain to the chest, the medial scapular border and down the radial forearm and thumb, closely mimicking a C6 radiculopathy or a cardiac cause. It is common, benign, and responds to stretching and posture work.

Post-traumatic scalene fibrosis

After a whiplash or a direct blow, the muscle heals with fibrosis and loses compliance. Many patients with neurogenic thoracic outlet syndrome date their symptoms to a neck injury, and the histology of resected scalene muscle in these patients shows fibrous replacement.

Brachial plexus birth palsy

The scalene relationships are relevant to exposure in infant plexus surgery, where the phrenic nerve is proportionally even closer to C5 and the risk of hemidiaphragmatic paresis in a small infant is far more consequential.

Anaesthetic relevance - the interscalene block

• Target: the C5 and C6 roots and the upper trunk in the interscalene groove at the level of the cricoid cartilage (C6).
• Coverage: excellent for shoulder and proximal humeral surgery. It characteristically spares the inferior trunk (C8-T1) - so the ulnar border of the hand is often not blocked, and it is unsuitable as a sole technique for hand surgery.
• Hemidiaphragmatic paresis: occurs in the great majority of patients with conventional volumes because the phrenic nerve lies only millimetres from the C5 root at this level. Ultrasound-guided low-volume techniques and more distal or extrafascial injection reduce but do not abolish it.
• Clinical consequence: a 20 to 30 per cent reduction in forced vital capacity and forced expiratory volume is typical. This is well tolerated by a healthy patient and may be dangerous in severe chronic obstructive pulmonary disease, morbid obesity, contralateral phrenic palsy or contralateral pneumonectomy - all relative or absolute contraindications.
• Other complications: Horner syndrome from spread to the stellate ganglion; hoarseness from recurrent laryngeal nerve block; inadvertent vertebral artery injection causing immediate seizure; epidural, subdural or intrathecal spread through a dural cuff; pneumothorax; and direct injury to the dorsal scapular and long thoracic nerves in the middle scalene, producing post-block scapular winging.

Indication

Exploration and reconstruction of the supraclavicular brachial plexus (roots, trunks and divisions), scalenectomy and cervical rib excision for thoracic outlet syndrome, and access to the third part of the subclavian artery.

Position and incision

• Supine, with a sandbag between the scapulae, the head turned away, and the table in slight reverse Trendelenburg to reduce venous engorgement.
• Transverse incision approximately 2 cm above and parallel to the clavicle, from the posterior border of sternocleidomastoid extending laterally for 8 to 10 cm.

Layer by layer

1. Platysma divided in line with the incision.
2. External jugular vein ligated where it crosses.
3. Supraclavicular nerves identified and preserved where possible - dividing them gives numbness over the upper chest that patients notice.
4. Clavicular head of sternocleidomastoid retracted or partly divided medially; the omohyoid inferior belly divided or retracted.
5. Supraclavicular fat pad mobilised laterally to medially as a flap; the transverse cervical artery is encountered crossing the field and is ligated.
6. Prevertebral fascia opened. The phrenic nerve is now identified on the anterior surface of the anterior scalene and must be found before anything is divided.
7. Anterior scalene divided close to its first rib insertion, having slung and protected the phrenic nerve. Divide it under direct vision; do not simply cut down onto the rib.
8. The subclavian artery and the roots and trunks of the plexus are now exposed. The middle scalene may be divided for wider access, protecting the dorsal scapular and long thoracic nerves within it.

Orientation inside the wound

• C5 and C6 are found first, most superficially and laterally; the upper trunk and Erb's point, where the suprascapular nerve leaves, are the reliable landmarks.
• C7 lies deeper; C8 and T1 are deepest, behind and below the subclavian artery, and are the most difficult and most dangerous part of the exposure.
• The pleural dome is immediately below and medial.

Supraclavicular scalenectomy with plexus neurolysis

• Anterior scalenectomy (or complete anterior and middle scalenectomy) with excision of all fibrous bands, the scalenus minimus if present, and any cervical rib, followed by external neurolysis of the plexus.
• Preferred where the pathology is a cervical rib, an elongated C7 transverse process or fibrous bands - because the supraclavicular route gives direct vision of the plexus and of the bands.
• Advantage: the plexus is seen. Disadvantage: the first rib is harder to remove completely by this route.

Transaxillary first rib resection (Roos)

• Through a transverse axillary incision below the hairline, the first rib is exposed by retracting between the chest wall and the axillary contents, and resected subperiosteally from the costochondral junction to the transverse process.
• Advantage: excellent access to the first rib and the costoclavicular space, which makes it the operation of choice for venous thoracic outlet syndrome. Cosmetically favourable.
• Disadvantage: the plexus is not directly visualised, so bands are removed blind and traction on the arm during the procedure risks a traction plexopathy. The long thoracic nerve is at risk on the chest wall in the field.

Combined and supplementary procedures

• Combined supraclavicular scalenectomy with first rib resection for recurrent or complex disease.
• Pectoralis minor tenotomy where the retropectoralis minor space is the site (positive Wright test, tenderness at the coracoid).
• Venous reconstruction or angioplasty after thrombolysis in venous disease; arterial resection and interposition graft where an aneurysm has formed.

Selection and outcome

• Results are best in true neurogenic and vascular disease with an identified structural cause; they are least predictable in disputed (non-specific) neurogenic thoracic outlet syndrome without objective findings.
• A positive response to an anterior scalene block is one of the better predictors of benefit from scalenectomy.
• Recurrence after decompression is well recognised and is usually due to scarring around the plexus or an incompletely resected first rib; revision surgery has substantially worse results and higher complication rates.