Traction or inflammation of the brachial plexus can account for a variety of cervical spine, periscapular and upper extremity symptoms.
Myofascial causes of this has been largely attributed to thoracic outlet syndrome. A review of the fascia anatomy and physiology of thoracic outlet syndrome can throw new light on this condition in osteopathic practice. Specific considerations in this field are:
- Anatomy and mechanical strains of the prevertebral fascia that forms a sheath for the brachial plexus (axillary sheath).
- Hypertrophy of the scalene (prevertebral) fascia in thoracic outlet syndrome.
- Role of the omohyoid muscle in tensing the prevertebral fascia.
- The continuity of the prevertebral fascia with the costocoracoid membrane and intern the continuity of the costocoracoid membrane with the sheath of the brachial plexus (axillary sheath).
- Vascular changes in response to traction and the resultant fibrosis in the fascia of the brachial plexus this produces.
The article is split into:
- Anatomy of the brachial plexus.
- Anatomy of the prevertebral fascia.
- Anatomy of the clavipectoral fascia.
- Anatomy of the thoracic outlet.
- Pathological findings in the thoracic outlet.
- Anatomical variations in the infraclavicular space.
- Anatomy of the brachial plexus sheath.
- Biomechanics of the brachial plexus and resultant inflammatory changes.
- Traction forces causing inflammatory changes and fibrosis of the brachial plexus fascia.
Anatomy of the brachial plexus (Revankar et al, 2013)
- Nerve roots of the brachial plexus (ventral rami):
- The brachial plexus is formed by the union of the ventral rami from C5 to T1.
- After exiting the intervertebral foramen these ventral rami run in the interval between the scalene anterior and scalene medius (scalene hiatus). This lies between the anterior tubercles (scalene anterior attachment) and posterior tubercles (scalene medius attachment) of the corresponding transverse processes of the cervical vertebrae.
- As well as the prevertebral fascia splitting between the scalene anterior and medius to create the scalene hiatus it also ensheaths the intervening C5 to T1 nerve roots. This prevertebral fascial sheath continues inferolaterally as the axillary sheath encompassing the nerve roots, trunks, cord and terminal branches.
- After leaving the scalene hiatus these nerve roots of the brachial plexus descend in front of the scalene medius.
2. Ventral rami to the nerve trunks of the brachial plexus:
- The nerve trunks lie the posterior triangle of the neck (borders: SCM, trap and middle 1/3 of clavicle).
- Upper trunk: formed where the C5 & C6 ventral rami unite at the lateral border of scalenus medius.
- Middle trunk: continuation of the C7 ventral rami.
- Lower trunk: formed where the C8 & T1 ventral rami unite behind the scalene anterior.
3. Nerve trunks to nerve cords of the brachial plexus:
Each nerve trunk (upper, middle and lower) has anterior and posterior divisions. These divisions, behind the clavicle and through the costocoracoid space, form the posterior, lateral and medial cords:
- Posterior cord: formed from the posterior divisions of all three trunks (C5-C8, T1).
- Lateral cord: formed from the anterior divisions of the upper and middle trunks (C5-C7).
- Medial cord: continuation of the anterior division of the lower trunk (C8 & T1).
4. Nerve cords to terminal branches:
Between the lateral aspect of the Pectoralis Minor and axilla the nerve cords turn into the terminal nerve branches:
- Musculocutaneous nerve: formed from the lateral cord of the brachial plexus (C5-C7).
- Axillary nerve: formed from the posterior cord of the brachial plexus (C5 & C6).
- Radial nerve: formed from the posterior lateral and medial cords of the brachial plexus (C5-T1).
- Median nerve: formed from the lateral (C6 & C7) and medial cords (C8 & T1) of the brachial plexus.
- Ulna nerve: formed from the lateral (C7) and medial cords (C8 & T1) of the brachial plexus.
Anatomy of the Prevertebral fascia (Feigl, 2015 and Natale et al, 2015)
- Can attach to the cervical vertebral bodies.
- Attaches to the Alar fascia.
- Covers the deep dorsal muscles of the neck.
- Attaches to the basilar part of the occiput.
- Envelopes the rectus capitis anterior and the vagus nerve.
- Passes anteriorly to the prevertebral muscles and envelopes the scalene anterior.
- At C6 the fascia splits. Laterally along the scalene anterior and medially along the longus colli. This creates a gap that forms the triangle of the vertebral artery.
- Attaches to the transverse processes of the cervical vertebrae.
- Blends with the superficial cervical fascia at the Trapezius.
- Attaches to the upper border of the scapula.
- Continues inferolaterally as the axillary sheath.
- With the scalene anterior and medius to rib 1 and scalene posterior to rib 2 (Syed et al 1953)
- Inserts into the fascia of the subclavis and from there to the costocoracoid membrane that runs to the axillary sheath.
- Runs continuous with the endothoracic fascia.
- Descends to the thoracic cavity within the adipose tissue of posterior mediastinum giving off fibrous bundles to the pericardium and the lung hilum proceeding up to the tendinous centre of the diaphragm.
- Vagus nerve.
- Phrenic nerve.
- Sympathetic trunk.
- Rectus capitis anterior.
- Central tendon of the omohyoid.
Anatomy of the clavicopectoral fascia
The clavipectoral fascia is a deep layer of fascia in the pectoral regions. It acts to suspend the floor of the axilla and protect the axillary nerve, artery and vein. Its boundaries are:
- Coracoid process of scapula.
- Coracoclavicular ligament.
- Short head of bicep.
- Suspensory ligament of the axilla.
- First costal cartilage.
- External intercostal membrane of the first two intercostal spaces.
The portion extending between the first rib and coracoid process is often thicker and called the costocoracoid membrane.
- The fascia splits anteriorly and posteriorly to enclose the subclavis and attach to the clavicle. The posterior part of the fascia that encloses the subclavis fuses with the prevertebral fascia and the axillary sheath.
- Invests the pectoralis minor.
- Extends in continuity with the axillary sheath.
- Blends with the deep fascia of the pectoralis major.
Anatomy of the thoracic outlet (Demondion et al 2006)
The thoracic outlet is comprised of three confined anatomical spaces:
- Interscalene triangle.
- Costoclavicular space.
- Retropectoralis minor space.
- Interscalene triangle
- Anterior: scalene anterior.
- Posterior: scalene medius and posterior.
- Inferior: rib 1.
Inferior: sublclavian artery.
Middle: Upper (C5 & C6 ventral rami) and middle (C7 ventral rami) trunks of the brachial plexus.
Superior: Omohyoid muscle.
Inferior and posterior: Lower (C8 & T1 ventral rami) trunk of the brachial plexus.
Leonhard et al (2017) found anatomical variations in 62.1% of cadavers where the scalene anterior was pierced by the C5 ventral ramus, superior trunk or superior and middle trunks of the brachial plexus.
Chen et al (1998) found in 88.3% of cadavers the T1 nerve root or the lower trunk of the brachial plexus crossed rib 1 over the proximal scalene minimus tendon.
2. Costoclavicular space
- Superior: clavicle.
- Anterior: subclavis.
- Posterior: rib 1 and scalene medius.
- Anterior: subclavian vein
- Posterior: subclavian artery.
- Superior (above subclavian vessels): brachial plexus (lateral, medial and posterior cords).
3. Retropectoralis minor space
- Anterior: posterior part of the pectoralis minor.
- Postero-superior: subscapularis.
- Postero-inferior: anterior chest wall.
Contents: as costoclavicular space.
Just lateral to the Pectoralis Minor the cords of the brachial plexus divide into the terminal branches (median, ulna, musculocutaneous, axillary and radial nerves).
Pathological findings in the thoracic outlet
Demiondion et al (2006) found:
- Nerve compression: costoclavicular space = interscalene triangle.
- Arterial compression: costoclavicular space > interscalene triangle.
The retropectoralis minor space was rarely implicated in thoracic outlet syndrome.
Fiske (1952) found that hypertrophy of the omohyoid can irritate the brachial plexus. Anatomical variations of the omohyoid can find it attaching to the C6 transverse process anterior to the scalene medius (Tubbs et al 2004). This can directly contribute to thoracic outlet syndrome.
There is a possibility that the Omohyoid could mechanically predispose to thoracic outlet syndrome:
- Williams & Warwick (1980) describes the Omohyoid as tensing the lower part of the prevertebral fascia. The prevertebral fascia forms the sheath of the brachial plexus. Could contraction of the Omohyoid predispose to tension in the brachial plexus sheath and the vascular and hypertrophic changes that ensue? (Refer below, 'biomechanics of the brachial plexus and resultant inflammatory changes').
- The intermediate tendon of the omohyoid is attached to the clavice and rib 1 via a fascial sling from the prevertebral fascia. Could contraction of the omohyoid affect the mechanics of rib 1 and the clavicle creating a dynamic compression of the brachial plexus?
Pathological findings in thoracic outlet syndrome have largely been attributed to the scalene anterior and medius and its fascia (prevertebral fascia). It should be remembered that this fascia forms the sheath of the brachial plexus.
Anatomical variations in the scalenes include:
- Scalene Minimus hypertrophy: compression of the T1 nerve root or the lower trunk of the brachial plexus (Chen et al 1998). Chuang et al (2016) found the scalenus minimus associated with 30-50% of patients with neurological thoracic outlet syndrome.
- Scalene anterior and scalene medius: crossed tendinous origins from the anterior and posterior tubercle of C4 and C5 transverse process can compress the C5 and C6 nerve root or the upper trunk of the brachial plexus (Chen et al 1998).
- Roos’ bands: compression from these fibrous bands can compress the brachial plexus rami. These bands extend from the cervical rib, rib 1, elongated C7 transverse process, scalene anterior and medius to rib 1 or the cupola of the lung (Demondion et al 2006 and Spears et al 2011).
Chuang et al (2016) found in patients having thoracic outlet surgery:
- Tight or hypertrophic scalene anterior.
- Bifid insertion of the scalene anterior into rib 1.
- Thickened posterior fascia of the scalene medius with palpable intramuscular fibrosis.
- The scalenus minimus muscle was found to be associated with 30-50% of patients with neurological thoracic outlet syndrome.
Tanna (1947) even found the thickened lateral edge of the cervical fascia posterior and lateral to the scalene anterior can become an organised fibrous cord. The author claimed this had pressed on the third portion of the subclavian artery resulting in a thrombosis.
In addition to the scalene and its (prevertebral) fascia causing thoracic outlet syndrome by hypertrophic changes could it also cause a pathology of the triangle of the vertebral artery. The triange of the verterbral artery is formed where the prevertenral fascia splits between the longus colli and scalene anterior.
Triangle of the vertebral artery (Tubbs et al 2005)
Inferior: subclavian artery.
Posterior: scalene medius.
Medial: Longus Colli.
Lateral: scalene anterior.
- Vertebral artery.
- Ganglionated sympathetic chain.
- C7 and C8 spinal nerve.
- Phrenic nerve.
Stecco et al (2014) found at least some of these pathological findings in the scalene fascia can be reversed with osteopathic treatment. The authors found significant increased thickness of the scalene fascia in patients with neck pain. With manipualtion a significant decrease in pain and thickness of the fascia was found.
Anatomical variations in the infraclavicular space
Even though the subclavis, in standard anatomical form, is intimately related to the prevertebral and clavipectoral fascia it has not been directly related to thoracic outlet syndrome.
Variations in the subclavis have been associated with compression of the subclavian artery and the brachial plexus in the costoclavicular space. Variations in this muscle is termed the subclavius posticus muscle or chondroscapularis. These anatomical variations include (Grigorita et al 2018):
- Originates from the postero-superior side of the costocoracoid membrane and costoclavicular ligament to insert onto the superior border of the scapula and on the superior transverse scapular ligament. This shares a common insertion with the inferior belly of the omohyoid muscle.
This muscle runs under the subclavian artery, subclavian vein, and trunks of the brachial plexus. The suprascapular nerve as it runs above the superior transverse scapular ligament perforates and innervates this muscle. This muscle can cause an entrapment neuropathy of the brachial plexus and suprascapular nerve.
2. Originates on the first costal cartilage or on the superior surface of the sternal end of the first rib. Variable insertion include:
a. Superior boarder of the scapula mediocaudal to the inferior belly of the omohyoid.
b. Superior boarder of the scapula and superior transverse scapular ligament.
c. Transverse scapular ligament and coracoid process.
d. Superior angle of the scapula.
e. Medial margin of the suprascapular notch.
f. Capsule of the acromioclavicular joint.
g. Axillary sheath.
h. Fascia covering the subscapularis muscle.
Coracobrachialis (Maiti and Bhattacharya 2018)
Origin: coracoid process with a conjoint origin of the short head of biceps. It is formed of two fused heads.
Insertion: antebrachial fascia and the medial epicondyle of the humerus.
Variations include the coracobrachialis brevis. This muscle can insert proximally to the capsule of shoulder joint, root of coracoid process or conoid ligament of clavicle. It can insert distally into the medial intermuscular septum, medial supracondylar ridge, medial epicondyle and ligament of Struthers.
Entrapment neuropathies of the coracobrachialis:
(1) Musculocutaneous nerve.
In human beings, the upper two heads of the coracobrachialis are usually fused while taking origin from the coracoid process. They enclose the musculocutaneous nerve in between the two fused heads. This gives the impression that the musculocutaneous nerve pierces the coracobrachialis muscle.
(2) Median nerve
The lower head of the coracobrachialis which is usually suppressed in human beings is sometimes present as the ligament of Struthers. The Median nerve and brachial artery passes deep to this ligament and are vulnerable to compression by being entrapped between the ligament and the bony surface. Interestingly the Pronator Teres (the other site of entrapment for the median nerve) can attach onto this ligament.
Anatomy of the brachial plexus sheath
Thompson & Rorie (1983) described the connective tissue forming the sheath of the brachial plexus as terminating via the axillary sheath at the anterior and posterior laminae of the medial intermuscular septum.
Being organised more densely as it leaves the prevertebral fascia the sheath becomes more loosely organised distally as it ended by joining the medial intermuscular septum.
The connective tissue of the sheath extends internally forming septa. These septa are similar in thickness and density as the surrounding fascia and compartmentalise the brachial plexus sheath. Brenner et al (2018) found fascial layers in the neurovascular sheath in the infraclavicular space.
These sheaths run continuous proximally and distally. At the level of the axilla the compartment of the terminal nerve runs continuous with the cords that form it. Thus the compartment of the median nerve runs continuous with that of the medial and lateral cords and the compartment of the ulna nerve runs continuous with that of the medial cord.
However Cornish and Leaper (2006) argued against a sheath for the brachial plexus. They found the brachial plexus runs between different tissues that closely surround the clavicle, scapula, chest wall, and humerus. So whilst the brachial plexus maybe surrounded by connective tissue, this tissue is not homogenous, but is derived from the surrounding structures.
Biomechanics of the brachial plexus and resultant inflammatory changes
Mihara et al (2018) analysed strain on the brachial plexus during different movements they found:
- C5 nerve root: strained during cervical spine extension and contralateral sidebending.
- Lower trunk of the brachial plexus < C7 and C8 nerve roots: strained with upper limb abduction. However Pan et al (2008) found the lateral cord (C5 to C7) is still strained during abduction as it moves closer to the coracoid process at 60 degrees than at 30 degrees of abduction.
- Kitamura et al (1995) found a reduction of traction of the brachial plexus by shoulder elevation therefore presumably an increase in traction by shoulder depression.
Traction forces causing inflammatory changes and fibriosis of the brachial plexus
Kitamura et al (1995) noted vascular and inflammatory changes to the brachial plexus accompany fibrotic changes to its surrounding connective tissue.
They found the peripheral nerves had an extrinsic and intrinsic vascular supply:
- Extrinsic vascular system: includes segmental regional vessels approaching the nerve trunk at various levels. These regional vessels run in the loose "adventitia" or along the paraneural tissue surrounding the nerve. The looseness of this “adventitia” allows the nerve trunk considerable mobility in its bed.
- Intrinsic vascular system: includes numerous vascular plexa in the epineurium, perineurium, and endoneurium.
When the peripheral nerves are stretched, the surrounding blood vessels are extended, resulting in decreased blood flow within the nerve bundle. How this effects the extrinsic and intrinsic blood flow varies:
- Extrinsic system: blood flow decreases sharply in the presence of relatively small amount of traction and then decreases in a slow linear fashion with further increases in traction.
- Intrinsic system: on the other hand, the blood flow through the intrinsic system showes a slower linear decrease in response to traction.
The difference in blood flow between the two systems can be explained by the biomechanics of the nerve in response to stretch. If stretching the nerve stretches the blood vessels. Stretching the blood vessels results in less circulation. Then the area stretched the least and has the better circulation should redirect blood to the area that is stretched the most and has the worst circulation.
When a nerve is stretched blood is supplied from the less elongated portion i.e. the extrinsic system as it flows through the loose “adventitia” to the more elongated portion i.e. the intrinsic system that gets pulled tighter.
The sharp decrease in the blood flow through the extrinsic system can lead to local elevation of vascular permeability and oedema. With repeated stretching over a two week span this has been shown to lead to hypertrophic connective tissue and vascularization of the epineurium and the surrounding tissue. Could this cause a cascade that results in a further decrease movement and vascular changes? Could this also be the cause of fibrosis seen around the brachial plexus during operations on patients with traumatic thoracic outlet syndrome? (Chuang et al 2016).
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