Surgical Anatomy of the Thoracic Outlet

Successful surgical treatment for all three types of TOS depends on a sound understanding of the relationships between musculoskeletal and neurovascular structures in this region, as well as the many anatomic variations likely to be encountered ( Fig. 15-1 ). One of the principal advantages of the supraclavicular approach is excellent exposure of the relevant anatomy, allowing more complete decompression compared with alternative approaches. To accomplish this with the greatest margin of safety, we have defined six “critical views” of the surgical anatomy that should be sequentially obtained during the course of supraclavicular decompression ( Box 15-2 ).

Anatomy of the thoracic outlet, focusing on the scalene triangle, the costoclavicular space, and the subcoracoid space. A, The scalene triangle is bounded by the anterior scalene muscle, the middle scalene muscle, and the first rib. The brachial plexus and subclavian artery pass through this space and over the first rib, whereas the subclavian vein passes over the first rib immediately in front of the scalene triangle. B, The costoclavicular space lies between the clavicle and the first rib and is bordered superiorly by the subclavius muscle, medially by the costoclavicular ligament, and posteriorly by the insertion of the anterior scalene muscle tendon on the first rib. The brachial plexus and subclavian artery pass over the first rib behind the costoclavicular space, whereas the subclavian vein passes over the first rib through the front part of the costoclavicular space. C, The subcoracoid space lies inferior to the clavicle and underneath the pectoralis minor muscle tendon, just below its insertion on the coracoid process. All of the structures of the neurovascular bundle pass through this space before reaching the axilla. D, Overview of the anatomy of the thoracic outlet.

Avoiding Inadequate Decompression and Recurrence

The potential for persistent or recurrent symptoms of brachial plexus compression remains one of the most challenging aspects of surgical treatment for neurogenic TOS. The supraclavicular approach is designed to avoid the most frequent causes of recurrence by addressing the following issues:

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  Extent of scalene muscle resection. Reattachment of the anterior scalene muscle is a well-documented cause of recurrent neurogenic TOS after simple scalenotomy, partial scalenectomy, or transaxillary first rib resection. In these circumstances the anterior scalene muscle may reattach to remaining portions of the first rib, to the bed of the resected first rib, to the extrapleural fascia, or directly to the brachial plexus nerve roots. Anomalous scalene muscles and fibrofascial bands may also persist as a source of brachial plexus compression if not removed. It is therefore recommended that both the anterior and the middle scalene muscles be resected, along with the anomalous scalene muscle and the fibrofascial bands that might be encountered, during supraclavicular thoracic outlet decompression.

  
  
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  Brachial plexus neurolysis. Most patients undergoing surgery for neurogenic TOS exhibit visual evidence of fibrous scar tissue surrounding the brachial plexus nerve roots, a reflection of previous injury and inflammatory tissue healing. This fibrous tissue may contribute to nerve fixation and irritation and when retained may be a cause for residual neurogenic symptoms. It is therefore recommended that perineural fibrous scar tissue be meticulously removed from around each of the brachial plexus nerve roots during the course of thoracic outlet decompression (external neurolysis).

  
  
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  First rib and cervical rib resection. There remains some room for debate regarding the necessity for first rib resection during supraclavicular thoracic outlet decompression, with some advocating routine first rib resection and others encouraging a more selective approach based on intraoperative findings after scalenectomy and brachial plexus neurolysis. It remains unclear whether there are distinct advantages attributable to retaining the first rib, and incomplete first rib resection is often cited as a factor contributing to recurrent neurogenic TOS. It is therefore recommended that first rib resection be included in supraclavicular decompression for neurogenic TOS, extending posteriorly as far as the level of the T1 nerve root and anteriorly to the costochondral junction (just medial to the scalene tubercle). The first rib is often abnormal in patients with a cervical rib and may serve as a source of persistent or recurrent nerve compression after isolated cervical rib resection. Thus first rib resection is also advocated in patients with cervical ribs, along with resection of the cervical rib, in order to ensure the most complete decompression feasible. In venous TOS, the subclavian vein is typically compressed at the point where it passes over the first rib and directly underneath the clavicle. Resection of the anteriormost portion of the first rib is therefore considered important in operations for venous TOS to prevent persistent or recurrent subclavian vein obstruction, but this cannot be achieved through the supraclavicular approach alone. For this reason, and to provide more complete access to the axillary-subclavian vein (in the event that direct venous reconstruction is warranted), a medial infraclavicular incision is added to the supraclavicular approach in operations for venous TOS.

  
  
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  Pectoralis minor tenotomy. Brachial plexus compression by the pectoralis minor muscle has become increasingly appreciated as a factor contributing to neurogenic TOS. In our experience up to 20% of patients with neurogenic TOS exhibit physical findings isolated to the subcoracoid space and another 30% to 40% have findings that colocalize to both the scalene triangle and the subcoracoid space. Even in patients with findings predominantly localized to the scalene triangle, residual nerve compression at the site of the pectoralis minor muscle may be a source of persistent or recurrent neurogenic TOS. Simple division of the pectoralis minor muscle tendon immediately below the coracoid process can provide substantial relief of brachial plexus compression while adding little to the operative procedure; thus pectoralis minor tenotomy should be included with supraclavicular decompression whenever suggested by preoperative clinical findings.

  
  
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  Hemostasis and fluid accumulations. Postoperative accumulation of blood and serum may enhance local wound healing responses that promote fibrosis, thereby contributing to the potential for late neural compression and recurrent symptoms after thoracic outlet decompression. Common local sources of bleeding include the ends of the divided first rib and the edges of the resected scalene or intercostal muscles. Although bleeding from these sites is typically minimal and self-limited, effort should be made to diminish fluid accumulation in the operative field. To this end, a topical hemostatic agent is placed along the edges of the resected muscles and the scalene fat pad is closely reapproximated over the brachial plexus to reduce potential space in the wound. The pleural apex is also purposefully opened to promote dependent drainage away from the operative field, and a closed-suction drain is placed within the supraclavicular space upon completion of the procedure.

  
  
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  Absorbable film barriers. Postoperative scarring around the brachial plexus nerve roots is inevitable after an operation for TOS, and dense perineural fibrosis is a potential cause of recurrent nerve entrapment and irritation. As in other operations involving direct nerve exposure, it is recommended that the brachial plexus nerve roots be covered with an absorbable antiadhesion film barrier, using one of several materials that have been developed for this purpose, to decrease the potential for later nerve encasement.

Avoiding Nerve Injury

The potential for nerve injury is a primary concern in operation for TOS and may occur because of transection, electrocautery, or excessive traction. The incidence of these complications should be negligible in experienced hands, but several factors may elevate the risk of nerve injury during supraclavicular thoracic outlet decompression, including unexpected anatomic variations, pathologic findings, intraoperative bleeding, and reoperative procedures.

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  Brachial plexus nerve roots. Meticulous operative technique is critical to avoid injury to the brachial plexus nerve roots by minimizing handling of the nerves, dissecting the perineural tissues under direct vision, and maintaining constant awareness of the extent of retraction being placed on individual nerve roots. The presence of a cervical rib or ligamentous band may displace the brachial plexus more forward than usually expected, and scalene muscle anomalies (e.g., a scalene minimus muscle) and fibrofascial bands may obscure the lower nerve roots. Division of the anterior scalene muscle from the first rib should be done with a finger placed between the muscle and the underlying brachial plexus and subclavian artery, using scissors rather than electrocautery. Before resection of the middle scalene muscle, the brachial plexus should be mobilized such that all five nerve roots are visible and gently retracted medially. Full visualization of the T1 nerve root should also be obtained, where it passes from underneath the first rib to join the C8 nerve root, before dividing the posterior neck of the first rib.

  
  
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  Phrenic nerve. The phrenic nerve lies on the surface of the anterior scalene muscle, passing from its lateral to its medial edge before descending behind the subclavian vein into the mediastinum. Even with the gentle mobilization of the phrenic nerve necessary to complete resection of the anterior scalene muscle, intraoperative traction or postoperative inflammation around the nerve may result in temporary neuropraxia and ipsilateral diaphragmatic paralysis. Because phrenic nerve palsy may be asymptomatic and compensated by the other side, it is important to verify normal phrenic nerve function on the side of the previous operation for TOS before undertaking a contralateral procedure. An accessory phrenic nerve often arises from the edge of the brachial plexus, where it passes medially to join the primary phrenic nerve near the lower aspect of the anterior scalene muscle. In some cases the primary or accessory phrenic nerve passes anterior to the subclavian vein, known as a “prevenous” phrenic nerve. This anomaly can serve as a potential cause of subclavian vein obstruction in venous TOS and is only observed with supraclavicular or infraclavicular exposure.

  
  
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  Long thoracic nerve. The long thoracic nerve forms within the body of the middle scalene muscle from three separate branches and then emerges from the muscle to pass over the lateral aspect of the first rib. Protection of the long thoracic nerve is achieved by direct visualization and gentle posterior retraction during middle scalenectomy.

  
  
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  First intercostal nerve. Injury to the first intercostal nerve branch during resection of the rib may result in a painful postoperative neuroma. This can be avoided by displacing the nerve away from the neck of the posterior first rib before dividing the bone. Blunt dissection of the posterior surface of the anterior rib, maintaining a plane immediately under the bone to further displace the intercostal nerve, is also helpful to avoid injury.

Avoiding Vascular and Lymphatic Injury

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  Subclavian artery. The subclavian artery is directly exposed during supraclavicular decompression and must be protected from injury throughout the procedure. Complete mobilization of the subclavian artery, with ligation and division of small arterial branches that commonly arise from its superior aspect (i.e., the thyrocervical trunk), is helpful to avoid traction or avulsion injuries that can extend onto the main vessel.

  
  
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  Subclavian vein. Although the subclavian vein is usually not exposed during supraclavicular decompression for neurogenic TOS, it is susceptible to injury underneath the medial clavicle and where it joins the internal jugular vein. In venous TOS an increased network of collateral veins is usually encountered throughout the supraclavicular space. Although larger collateral vessels are preserved, such as the external jugular vein, small venous collaterals should be ligated and divided to minimize intraoperative bleeding. Any localized bleeding that may occur during the course of supraclavicular decompression should be meticulously identified and controlled so that blood does not obscure the operative field and elevate the risk of nerve injury.

  
  
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  Thoracic duct. The thoracic duct joins the venous system on the left side near the junction of the internal jugular and subclavian veins. It is susceptible to injury during mobilization of the scalene fat pad and should therefore be sought and directly divided between silk ligatures during the early stages of the operation. When a lymphatic leak is observed during the course of the procedure, the site of the leak is identified and oversewn with a pledgeted polypropylene suture, and a topical hemostatic or fibrin tissue sealant is applied to the site before wound closure.