Introduction

The interscalene triangle (sometimes termed simply the scalene triangle) is a region of the neck with fundamental importance to the function of the upper limbs, as the roots of the brachial plexus and the third part of the subclavian artery pass through it. The borders of the triangle are the lateral border of the scalenus anterior muscle, the medial border of the scalenus medius muscle, and the first rib inferiorly. Clinical manifestations of complications relating to the interscalene triangle include thoracic outlet syndrome, which can result in vascular or neurological compromise of the upper limb following compression at the interscalene triangle.[1] This may require surgical decompression if the effects of the condition are sufficiently deleterious on the daily activities of life or cause compromise of the upper limb.

Structure and Function

The interscalene triangle is a paired structure situated at the root of the neck, with its apex pointed superiorly towards the base of the skull. The horizontal base of the triangle is formed by the superior border of the first rib. The 2 vertical sides of the triangle are formed by the scalenus anterior and scalenus medius muscles. The anterior covering of the scalene triangle (ie, the "roof" of the triangle) is the prevertebral fascia of the neck, which extends from the body of the T4 vertebra up to the base of the skull.[2] The angle of the aperture between the scalenus anterior and scalenus medius muscles is slight, such that only the roots of the brachial plexus and the corresponding part of the subclavian artery pass through (see Image. Superficial Neck Anatomy). 

The first rib is the broadest and flattest of all the ribs and the most curved. Its head articulates with the body of the T1 vertebra, and the medial part articulates with the manubrium sternum. Features along its upper surface include a groove for the subclavian artery and the scalene tubercle, to which the scalenus anterior is attached. The anterior scalene arises from the transverse processes of the anterior tubercles of C3 to C6, terminating as a narrow tendon at the first rib attached to the scalene tubercle on the upper surface of it. The middle scalene arises from the lateral parts of C1 and C2 (atlas and axis), as well as the transverse processes of the posterior tubercles of C3 to C7, and also inserts into the first rib, at the quadrangular area between its neck and the subclavian groove.[3] The function of these muscles is to allow postural stability, flexion, and rotation of the neck, as well as assist in the elevation of the first rib, thus aiding with respiratory effort.[3][4][3]

Embryology

Neck muscles, like the anterior and middle scalene muscles, are derived from the embryonic mesoderm (see Image. Anterior Muscles of the Neck).[5] Specifically, the paraxial mesoderm is split into differential cell masses called somites. These then produce the sclerotome and dermomyotome, the latter producing the body wall muscles.

Blood Supply and Lymphatics

The subclavian artery and vein are at the base of the interscalene triangle.[3] The subclavian artery passes directly through the triangle with the brachial plexus, originating on the left side from the aortic arch and the right side from the brachiocephalic artery. The subclavian arteries ascend superolaterally from their origin before curving laterally and passing posteriorly to the anterior scalene triangle, thus entering the scalene triangle.[6] The subclavian vein remains at the lower border of the triangle.[7] Compression of these vascular structures can lead to weakness, numbness, and vascular compromise.[8]

Nerves

Within the upper part of the interscalene triangle lie the anterior rami of the third through fifth cervical spinal nerves. The base of the triangle is the location of the brachial plexus.[3] The plexus is involved with the motor and sensory function of the shoulder down to the fingertips. Injuries to this plexus can lead to significant deficits in quality of life.[9] As noted previously, the roots of the brachial plexus pass through the interscalene triangle. Subsequently, the divisions of the brachial plexus pass behind the clavicle, and the cords spread out distally in relation to the axillary artery before terminating in distinct branches that supply the musculature and cutaneous innervation of the upper limb.

The phrenic nerve is oblique to the anterior scalene and passes anteriorly to the subclavian artery (C3, C4, C5). The phrenic nerve controls the diaphragmatic muscles for breathing, and although the phrenic nerve is not within the triangle, it is similar to the subclavian vein in that, because of its relative location to the triangle, it is easily prone to compromise from pathologies affecting the interscalene triangle.[10] Another nerve associated with the interscalene triangle, although not within it, is the long thoracic nerve. One of its anterior branches, specifically C7, crosses within the triangle. This nerve becomes subcutaneous near the first and second rib and innervates the serratus anterior muscle.[11]

Muscles

The anterior scalene arises from the transverse processes of the anterior tubercles of C3 to C6, terminating as a narrow tendon at the first rib attached to the scalene tubercle on the upper surface of it. The middle scalene arises from the lateral parts of C1 and C2 (atlas and axis), as well as the transverse processes of the posterior tubercles of C3 to C7, and also inserts into the first rib, at the quadrangular area between its neck and the subclavian groove.[3] The function of these muscles is to allow postural stability, flexion, and rotation of the neck, as well as assist in the elevation of the first rib, thus aiding in respiratory effort.[3][4][3]

Physiologic Variants

Variations of the anterior scalene muscle include differing origins from C2 to C6, C3 to C5, or insertion into the second or third rib. Variations of the middle scalene include an origin from C1 (the atlas) and, like the anterior scalene, inserting onto the second rib.[3] In some patients, the scalene (specifically the anterior scalene) may become hypertrophied, contributing to future aneurysmal changes in the subclavian artery.

The anterior and middle scalene insertions on the first rib can vary, creating a narrow space and leading to a higher-than-normal subclavian artery and brachial plexus. This is important to consider during a surgical intervention in this area. Also, in some patients, the middle scalene can be inserted across the full length of the first rib. In rare cases, the middle and anterior scalene have fused, so the artery and plexus have to pierce through the muscular sheath. The muscles are not the only variants that can occur. The first rib can have structural anomalies that narrow the space, including fusion of ribs, abnormal positioning, and a healed fracture. All variations can make the structures harder to visualize and dictate further surgical or nonsurgical treatment.[7][12]

Surgical Considerations

Treatment of thoracic outlet syndrome can be via scalenectomy for upper thoracic outlet syndrome, and a first rib resection is an option for lower thoracic outlet syndrome. Both procedures can also be done simultaneously for the best results. Regardless, these are done to give the vasculature and nerves wider access to the rest of the upper extremity.[3] This procedure can be performed via a trans-axillary or a supraclavicular approach, and current evidence does not demonstrate any significant safety or complication rate differences between the 2 techniques.[13]

During the procedure, the scalenus anterior and medius muscles are identified and divided before the first rib is excised; however, using an interscalene approach, it is also possible to complete this without the division of the scalenus anterior.[14] Alternatively, as a conservative measure, instead of surgical intervention, scalene muscle injections can help to reduce pain. This injection is more effective in patients with thoracic outlet syndrome due to whiplash or traction injuries.[15]

Clinical Significance

Interscalene Block

After surgery in the postoperative period, nerve blocks may be used to decrease the pain experienced by the patient (see Image. Interscalene Block). One widely used example of such a nerve block is the inter-scalene nerve block, used specifically in many upper extremity injuries.[16] This nerve block can be used before or after shoulder surgery, allowing anesthesia of the suprascapular, lateral pectoral, musculocutaneous, axillary nerves, and parts of the cervical plexus.[17] The needle is inserted in a mediolateral fashion to reduce the risk of damage to important associated nearby nervous structures, including the long thoracic and dorsal scapular nerves.[2] The risk of long-term complications following interscalene nerve blocks was found in 1 prospective trial to be 0.4%; however, given the critical nature of the upper limb to daily life, any dysfunction is highly impactful on a patient's quality of life (see Image. Interscalene Block).[18][19][20]

Thoracic Outlet Syndrome

Thoracic outlet syndrome (TOS) is a common syndrome due to abnormal regional anatomy or injuries to the associated structures, with an incidence of between 3 and 8 in 1000 people.[21] Importantly, the interscalene triangle is 1 of 3 sites where compression of the critical neurovascular structures traveling between the neck and the upper limb can be compressed. The 3 sites of compression causing thoracic outlet syndrome are the interscalene triangle, the costoclavicular space, and the sub-pectoral space. The causes for this syndrome are myriad and include supernumerary scalene muscles, congenital abnormalities of the first rib, cervical ribs, fibrous bands, aneurysms, and trauma.[22] Indeed, the clinical entity of thoracic outlet syndrome is a group of pathological entities depending both on the site of compression and on the specific structure that is being compressed and causing symptoms: neurogenic TOS (compression of the brachial plexus), arterial TOS (compression of the subclavian artery), venous TOS (compression of the subclavian vein), and disputed TOS (non-specific TOS with no clear origin.)[23]

For these reasons, thoracic outlet syndrome has long been considered difficult to diagnose, not least because there is no formal clinical diagnostic test.[24] The clinical history is important and may include neck, occiput, chest, shoulder, and upper limbs pain and paraesthesia, weakness, or heaviness in the affected upper limb. On examination of the patient, Raynaud phenomenon may be upper limb ischemia, upper limb swelling, cyanosis, muscle atrophy, or wasting, depending on the cause and site of compression.[24] Furthermore, palpation of the interscalene triangle may reproduce the symptoms if this is the level at which compression occurs. Thoracic outlet syndrome is notorious for lacking a specific clinical presentation.[25] 

In general, the neurovascular structures in the triangle undergo compression. The compression of the subclavian artery is more diagnostic of TOS than the compression of the subclavian vein because vein compression can occur in non-TOS patients.[25] Diagnostic testing for this condition includes clinic maneuvers and imaging. 

Diagnostic modalities for investigating suspected thoracic outlet syndrome include ultrasonography, chest radiography, CT angiography, magnetic resonance angiography, dedicated venography or arteriography, nerve conduction studies, or needle electromyography.[20] Different criteria can be used to help diagnose TOS. These criteria include weakness or pain in the upper body and face, tenderness of the anterior or middle scalene, positive Tinel sign, positive neck tilt test, and positive costoclavicular compression test.[15]