Tuesday, December 29, 2009

The Scalene Muscle & Potential pain generating factors from scalene

The scalene muscles are a group of three pairs of muscles in the lateral neck, namely the scalenus anterior, scalenus medius, and scalenus posterior. They are innervated by the spinal nerves C3-C8.

Origin and insertion
Scalenus anterior & medius: Scalenus anterior muscle arises from anterior tubercles of transverse processes of C3-6 vertebrae. It gets inserted into scalene tubercle on the first rib. Scalenus medius arises from posterior tubercles of transverse processes of C2-7 vertebrae. It gets inserted on the superior surface of the first rib behind the groove for subclavian artery. (J.Anat.Soc. India 55 (2) 52-55 2006 52)

On neck: All 3 scalene muscles produce rotation of the cervical spine to the same side. Maximum stretching of the scalenes should include rotation to the opposite side. (J Orthop Sports Phys Ther. 2002 Oct;32(10):488-96.)

On ribcage & respiration: The action of the anterior and middle scalene muscles is to elevate the first rib and rotate the neck to the same side; the action of the posterior scalene is to elevate the second rib and tilt (side flexion) the neck to the same side. They also act as accessory muscles of inspiration, along with the sternocleidomastoids.

Anatomy of interscalene triangle (IST): (J.Anat.Soc. India 55 (2) 52-55 2006 52)

IST is bounded by scalenus anterior muscle anteriorly, scalenus medius posteriorly and first rib between their insertions inferiorly. Enclosed within this triangle are:
(a) In the upper part of the triangle, ventral rami of C3, 4, and 5 taking part in the formation of phrenic nerve.
(b) In the lower part are the upper, middle and lower trunks of brachial plexus along with the subclavian artery.

I dare to compare the anterior & the middle scalene to the piriformis muscle. The piriformis allows the sciatic nerve to pass through it where as the space between anterior & the middle scalene allows many more structures through it.

Summary of anatomical relationships:
The scalene muscles have an important relationship to other structures in the neck. The brachial plexus and subclavian artery pass between the anterior and middle scalenes. The subclavian vein and phrenic nerve pass anteriorly to the anterior scalene as it crosses over the first rib.
The passing of the brachial plexus and the subclavian artery through the space of the anterior and middle scalene muscles constitute the scalene hiatus (the term "scalene fissure" is also used). The region in which this lies is referred to as the scaleotracheal fossa. It is bound by the clavicle inferior anteriorly, the trachea medially, posteriorly by the trapezius, and anteriorly by the platysma muscle.
Anatomical variations: To know the anatomical variations of IST one is referred to J.Anat.Soc. India 55 (2) 52-55 2006 52

Pain from scalene muscle

1. Role of scalene muscles in TOS:

Scalene muscles cause TOS (both vascular & neural effects) in one of the following ways:
(i) Tough and unyielding tendinous insertions, against which the soft neurovascular structures may be forced during certain movements.
(ii) Aponeurotic insertion of single scalene with sharp sickle shaped margin or both the scalenes joining to form u-shaped sling cause subclavian artery to be highly arched, which might result in kinking of the artery or thrombus formation with distal embolization.
(iii) Triangle can be narrowed to form slit like inter-scalene space by approximation of insertions of the two scalenes or the insertions may overlap resulting in high arching and scissoring effect (Thomas 1983).
(iv) Either of the scalene muscles may be larger than normal thus narrowing the space.
(v) TOCS may also result from spasm of scalenus anterior resulting in ‘scalenus anticus syndrome’.

The TOS & it’s varieties in nutshell

Thoracic outlet syndromes (TOS): caused by compression of the brachial plexus or vascular structures due to anatomical anomalies of the cervico-axillary region.
Varieties of TOS: Two main types are seen i.e. the neurogenic TOS syndromes and the vascular TOS syndromes. The neurogenic TOS syndromes make up 90% of all TOS cases. Females are more affected than males in their 4th or 5th decade. Neurogenic thoracic outlet syndrome (NTOS): NTOS is attributed to compression of the brachial plexus at the scalene hiatus. There are 2 distinct clinical entities of the NTOS i.e. the true & nonspecific NTOS.
True NTOS: have typical clinical and electrophysiological changes.
Nonspecific NTOS: have predominantly sensory signs, not well-defined electrophysiological changes.

Common symptoms of neurogenic TOS:
a. A common initial symptom is pain along the medial side of the arm, which may be more diffuse and accompanied by paraesthesiae.
b. Most of these patients suffer from amyotrophy and gradually progressive weakness of the intrinsic muscles of the hand, particularly the thenar eminence.
c. The association of vascular signs and symptoms is rare.
X-ray: In almost all cases cervical ribs or elongated transverse aprophyses of C7 are found on plain X-ray.
EMG & NCV: Characteristic changes are seen on electrophysiological studies.

2. Trigger point pain scalene producing referred symptoms: Travell & Simmons have described trigger points in scalene. The referral pattern:

1. supra-clavicular area, anterolateral chest
2. antro & postero lateral area of arm
3. antro & postero lateral area of fore arm & hand

3. Mechanical abnormality produced by tight scalene: One should view the impact of the tightness on the cervical spine arthrokinematics as well as the neural mechanics. The neural mechanics must be viewed with reference to David Butler’s neural tissue mobility specifically ULTT3.


What effect of scalene on cervical spine mechanics?

Imbalance in the anterior & middle scalene length of left & right side might lead to laterally translated vertebra on the same side & rotation to the opposite side along with mild flexion. A tight scalene hinders rotation to the opposite side. Hence, forcing the movement to opposite side of the tightness might lead to lateral translation of the lower & middle cervical vertebra to wards the tight side & rotation to the opposite side.

The key assessment involves T1&2 rib palpation for mobility & symptoms & neck rotatory mobility assessment to assess tightness of anterior & middle scalene muscles.

Manual therapy prospective of Scalene

1. Scalene stretch: opposite side rotation & downward pressure to 1st rib during inhaling & attempting apical expansion.

2. MET of Scalene when the effect is on cervical spine- fixation of the 1st rib + isometric holds to same ride rotation (20% of MCV).

3. MET of Scalene when the effect is on 1st rib- fixation of the neck in full rotation + stabilization of the shoulder girdle from above- then take deep inspiration for apical expansion.


1. Molina-Martínez FJ, Thoracic outlet syndrome, Rev Neurol. 1998 Jul;27(155):103-7.
2. Colli BO, Neurogenic thoracic outlet syndromes: a comparison of true and nonspecific syndromes after surgical treatment. Surg Neurol. 2006 Mar; 65(3):262-71; discussion 271-2.
3. Mobilization of the nervous system; David butler, Churchill-Livingstone
4. J.Anat.Soc. India 55 (2) 52-55 2006 52
5. J Orthop Sports Phys Ther. 2002 Oct;32(10):488-96.

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