Anatomy, Head and Neck, Inter-scalene Triangle

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.[1]

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.[1] In some patients, the scalenes (specifically the anterior scalene) may become hypertrophied, contributing to future aneurysmal changes of the subclavian artery.[9]

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.[4]

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.[10] 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).[5] This syndrome is notorious for lacking a specific clinical presentation.[11] In general, the neurovascular structures in the triangle undergo compression.[5] 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.[11] Diagnostic testing for this condition includes maneuvers in the clinic and imaging (MRI, CT, X-ray).[5] 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.[12]

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.[1] 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.[12]