The inter-scalene triangle is a region of the neck that holds fundamental structures to the upper extremity function. The triangle is composed of two muscles and the first rib. The two neck muscles are the anterior and middle scalenes. The angle between the muscles can be as little as zero degrees, which can lead to complications. Regardless, this triangle is integral for upper extremity anesthesia because it identifies the location of the brachial plexus. The brachial plexus is the source of the nerves of the rest of the arm and is fundamental in the extremity’s functionality. Issues with this plexus, specifically at the level of the inter-scalene triangle, can be detrimental to the quality of life of a patient; these issues are otherwise known collectively as thoracic outlet syndrome. Other problems can occur with the vascular structures of the triangle, specifically the subclavian artery and the neighboring subclavian vein. Occlusions can lead to aneurysms and weakness of the arm.
The triangle is made up of two muscles and the first rib. The anterior side of the triangle is created by the anterior scalene, while the middle scalene forms the posterior side of the triangle. The anterior and middle scalene allow for flexion of the cervical spine. These muscles are also accessory muscles of respiration when the cervical spine is stationary; this is possible because of their insertion onto the first and second ribs. This triangle is not classically known for its respiratory functionality; regardless of this, its insertion onto the first two ribs allow for this important physiological mechanism of life.
Neck muscles, like the anterior and middle scalenes, are derived from the embryonic mesoderm.
The subclavian artery and vein are at the base of the inter-scalene triangle. The subclavian artery passes directly through the triangle with the brachial plexus, while the vein remains at the lower border. Compression of these vascular structures can lead to weakness, numbness, and vascular compromise.
Near the inter-scalene angle, located at the upper part of the triangle, lay the anterior rami of the third through fifth cervical spinal nerves. The base of the triangle is the location of the brachial plexus. 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. Oblique to the anterior scalene and passing anteriorly to the subclavian artery is the phrenic nerve. The phrenic nerve controls the diaphragmatic muscles for breathing. 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. Another associated nerve 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.
The two muscles that form the triangle include the anterior and middle scalenes, while the structure creating the base is the first rib (which is actually the insertion point for both muscles). The anterior scalene arises from the transverse processes of the anterior tubercles of C3 to C6. The middle scalene arises from the transverse processes of the posterior tubercles of C2 to C7.
Some variations of the anterior scalenes include origin from C2 to C6, or C3 to C5, or insertion into the second or third rib. And some variations of the middle scalene include an origin from C1 (the atlas) and, like the anterior scalene, inserting onto the second rib. In some patients, the scalenes (specifically the anterior scalene) may become hypertrophied, contributing to future aneurysmal changes of the subclavian artery.
The anterior and middle scalene insertions on the first rib can vary, which creates a narrow space. Because of this, the subclavian artery and brachial plexus can be higher than normal in position, which is important to consider during incisions. Also, in some patients, the middle scalene can insert across the full length of the first rib. In rare cases, the middle and anterior scalenes 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 treatment, whether surgical or non-surgical.
After surgeries in general, nerve blocks are often used to decrease the pain an individual may experience. An inter-scalene nerve block has been an option, specifically in many upper extremity injuries. Thoracic outlet syndrome (TOS) is a common complication due to abnormal anatomical locations and injuries. Some sites most often affected include the inter-scalene triangle, the costoclavicular space, and the retro pectoralis minor space). This syndrome is notorious for lacking a specific clinical presentation. In general, the neurovascular structures in the triangle undergo compression. The compression of the subclavian artery is more diagnostic of TOS than compressing the subclavian vein because vein compression can occur in non-TOS patients. Diagnostic testing for this condition includes maneuvers in the clinic and imaging (MRI, CT, X-ray). Different criteria can be used to help diagnose TOS. Some of 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.
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 best results. Regardless, these are done to give the vasculature and nerves a less narrow port of access to the rest of the upper extremity. Instead of surgical intervention, scalene muscle injections can help to reduce pain as a more conservative treatment. This injection is more effective in patients that have thoracic outlet syndrome due to whiplash or tractor injuries.
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