Time to forget Thoracic outflow syndrome

The Problem

Thoracic outlet syndrome (TOS) is regarded as a diagnosis of exclusion, affecting females more than males and with an incidence between 1 and 3% reported in the literature (1). When we talk about Thoracic outlet syndrome, what exactly do we mean by the thoracic outlet? Anatomists regard this as the region of the diaphragm, where the thorax and abdominal cavity communicate (2). Clinicians typically regard this as either the space between both of the first ribs, manubrium and body of T1 or the entire region between the base of the neck, the axilla and the space between the first ribs (3).

As one can quickly observe, there is great confusion as to exactly what the term means. The term TOS is an anatomically ambiguous umbrella term which incorrectly defines the site of compression and also does not indicate the structures being compressed. As a clinical term its covers a broad range of symptoms with different etiologies and is therefore unhelpful as a diagnostic term (3). As an anatomical term it is functionally incorrect.

The main confusion arises from how the term thoracic outflow is defined differently by Anatomists and Clinicians. The problem starts in undergraduate education where students of anatomy are taught that the thoracic outlet is essentially the location of the diaphragm and the thoracic inlet is the region that connects the thorax, neck and axilla. While some clinicians insist the latter is the thoracic outlet. The terms outlet and inlet are also functionally redundant, implying neurovascular structures run in only one direction.

The importance of precise scientific nomenclature cannot be understated. It can help patients, students and clinicians understand the nature of the pathology and helps convey information in a consistent manner between different professionals involved in the patient’s care and allow patients to have accurate information to make informed decisions, regarding their care.

The following article aims to discuss the current usage of the term, define the relevant anatomy and highlight how the structures involved in TOS, are compressed in anatomical regions external to the thoracic outlet. It will be suggested that we consider using alternative terminology, which is more anatomically and functionally accurate.

Current practice

Clinicians use TOS as an umbrella term, then refine this to VTOS, ATOS and NTOS based on clinical assessment and investigation. As discussed, above, TOS is a potentially confusing term, the prefix (V-vascular, A-arterial, N-neurogenic) is useful in identifying the structure being compressed, but we still lack anatomically accurate terms to define the sites of compression.

Patients often present with range of symptoms, those with purely neurogenic compression typically present with symptoms of pain, paraesthesia and or clumsiness. Those with arterial compression may have symptoms of claudication, cool peripheries and reduced peripheral pulses. Venous compression often produces symptoms of cyanosis, swelling and pronounced collateral supply. Patients may also present with a mixture of symptoms. Typical symptoms can help suggest the structure being compressed but do not necessarily indicate the specific site and nature of the compression (3).

The problem with the current terminology is that a patient with compression of the subclavian artery between the anterior and middle scalene muscles and another patient with compression of the axillary artery between the coracoid process and tendon of pectoralis minor will both be referred to as ATOS; this is despite the compression being at two different locations and neither being the thoracic outlet.

Anatomy

The first step to this problem is to discard the terms outlet and inlet and instead have functional definitions of the thoracic apertures, as suggested by Ranney in his 1996 paper. The term thoracic inlet is a functionally inaccurate term, the subclavian vein enters, but the artery leaves, so is it an inlet or outlet? Ranney argues a functionally correct term would be the superior thoracic aperture, thus defining the aperture in relation to its relative location, leaving no ambiguity. Next the boundaries of the superior thoracic aperture. The Superior thoracic aperture boundaries are the posterior upper border of the manubrium, the medial borders of both 1st ribs, costal cartilages and the anterior surface of the body of the first thoracic vertebrae. An important point to consider, is that the subclavian vessels when they are compressed in this syndrome, are not compressed within these boundaries, making the use of the term TOS inaccurate.

The brachial plexus is often involved in “TOS”, a fundamental flaw of using the term TOS, is that it implies the brachial plexus trunks are travelling through the thoracic outlet (AKA thoracic inlet or superior thoracic aperture), they obviously do not. The roots originate in the neck and then the trunks travels through between scalene anterior and the middle scalene muscles enclosed in a fascial sleeve, a continuation of the prevertebral fascia (2). This sleeve travels between the clavicle and first rib then underneath pectoralis minor in the axilla. At no point do the brachial plexus roots, trunks or cords go through the thorax, thus the term thoracic outlet is misleading. As mentioned earlier, some clinicians refer to the Interscalene triangle as the thoracic outlet, I will address this later.

The next step, is to understand in TOS where are the sites of compression? To appreciate this we need to understand the structures being compressed and their paths. Even the term superior thoracic aperture syndrome would be inaccurate, as in reality there is no compression in this area. In fact the right subclavian branches off the brachiocephalic behind the right sternoclavicular joint. The left subclavian artery originates off the arch of the aorta, passes through the superior thoracic aperture, but compression does not occur within the boundaries of this aperture. So where does compression occur? For the subclavian arteries, veins and brachial plexus, compression occurs in potentially any of the three following regions:

These are:

  1. Interscalene triangle region
  2. Costoclavicular space
  3. Retropectoral minor space

TOS

Figure showing the three main sites of compression in the Interscalene, Costoclavicular and Retropectoralis minor spaces (3).

Interscalene triangle

The scalene triangle is formed between the anterior and middle scalene and the first rib as the base. Therefore the structures which potentially may become compressed are the trunks of the brachial plexus or the subclavian artery. The subclavian vein is anterior to the scalenus anterior, therefore will not be compressed in the scalene triangle. Ranney in his 1996 (4) paper suggests the interscalene space should be subdivided into a superior component known as the cervical outlet and an inferior component, the thoracic outlet. His rationale for using the term thoracic outlet, stems from the fact that the subclavian artery is leaving the thorax through the superior thoracic aperture and then through this inferior component of the interscalene triangle. Accompanying the subclavian artery are also the C8 and T1 roots or lower trunk. Therefore once again, using the term thoracic outlet would be misleading, neither C8/T1 or the lower trunk exit the thorax. The author believes retaining the term TO only continues to confuse the issue. One has to remember that the sites of compression (even for the subclavian vessels) involve the junction between the cervical and axillary regions, not the thorax.

Costoclavicular space

Observed from an oblique anterior angle, this region is the space between clavicle superiorly and 1st rib inferiorly. It includes the subclavian vein, with scalene anterior in the middle of the space, the subclavian artery and brachial plexus sandwiched between the scalene anterior and middle. All three of these neurovascular structures may potentially be compressed here. The structures, as they pass through this region have now entered into the axilla by its roof, the boundaries of which are the clavicle anteriorly, first rib medially and the scapula posteriorly.

Retropectoralis minor space

This region is essentially the anterior wall of the axillary space. The anterior wall of the axilla is pectoralis minor, both the subclavian artery, vein and cords of the brachial plexus may become compressed in this space between the tendon of pectoralis minor and its attachment to the coracoid process of the scapula.

Cervicoaxillary syndrome

Hence, TOS or any other term that infers compression occurs at the superior thoracic aperture or using the term TO to describe the interscalene space are anatomically inaccurate. Ranney suggests the term Cervicoaxillary syndrome, as at some point all three compressible structures are part of these regions when compression occurs. He advocates using this term until investigations identify the specific site of compression. The author agrees with this, the term Cervicoaxillary syndrome or its abbreviation CAS should be used. If there is enough evidence from clinical assessment alone, as to which structure is being compressed, then prefixing with V, A or N can be considered. Once investigation or surgery has established the specific site of compression, then the terms Interscalene, Costoclavicular or Retropectoral minor space should be used to identify the site of compression and also adding which structure or structures are being compressed.

References

1. Huang JH, Zager EL. Thoracic Outlet Syndrome. Neurosurgery. 2004 Oct;55(4):897–903.

2. Standring S, editor. Gray’s anatomy: the anatomical basis of clinical practice. Forty-first edition. New York: Elsevier Limited; 2016. 1562

3. Kuhn JE, Lebus GF, Bible JE. Thoracic Outlet Syndrome: J Am Acad Orthop Surg. 2015 Apr;23(4):222–32.

4. Ranney D. Thoracic outlet: An anatomical redefinition that makes clinical sense. Clin Anat. 1996;9(1):50–2.