The thoracic outlet syndrome, or simply TOS, is a collective term describing several conditions associated with compression of nerves and/or blood vessels in the thoracic outlet area, which represents the upper exit (outlet) of the chest cavity.
The thoracic outlet is an area located at the top of the chest cavity arbitrarily representing its exit. The term thoracic inlet area was used in the past to refer to this region but had been abandoned. This area has heart-shaped contour bounded by the first thoracic vertebra, the left and right first ribs, and the upper edge of the sternum. The area is tightly packed with vital anatomical strucutes and has therefore great clinical significance.
The neurovascular bundle comprising brachial plexus, subclavian artery and vein is the main anatomical structure compromised during thoracic outlet syndrome. Five nerve roots emanating from the spinal cord (C5, C6, C7, C8, and T1) intermingle with each other to form a complex network called the brachial plexus. The brachial plexus itself branches to several nerves that eventually innervate the neck, upper chest area and the arm. These nerves carry electric neural impulses between the spinal cord and upper extremity. They control every aspect of arm function: skin sensation, muscle contraction, sweating, blood vessel tone, etc. Additionally, the brachial plexus innervates skin and muscles of the neck, upper chest, and the shoulder girdle.
The subclavian artery and subclavian vein are main vessels providing blood flow to and from the arm. They pass with the brachial plexus over the first rib at the thoracic outlet. Similar configurations can be found in many body areas and nerves frequently travel along with blood vessels, forming various neuro-vascular bundles.
Artistic drawing demonstrating the course of the brachial plexus, subclavian artery, and vein in the thoracic outlet area. Note that all structures pass above the first rib.
The neurovascular bundle travels directly above the first rib and components of neurovascular bundle are in contact with the first rib. In fact the first rib has two specific grooves to accomodate passing subclavian artery and vein. Additionally C8, T1 nerve roots and inferior trunk of the brachial plexus also are in direct contact with the first rib. The brachal plexus and subclavian artery pass through a narrow anatomical window called the scalene triangle. This triangle is located in between two scalene muscles (anterior and middle) and the first rib. The scalene triangle or space is densely packed and, thus, is prone to compression. The subclavian vein doesn't pass through this triangle and cannot be compressed in it. It passes through a separate triangle named costoclavicular or venous triangle. Subclavius muscle, anterior scalene musce and the first rib form this venous triangle which is just anterior and medial to scalene triangle. Thus, the vein can be compressed between the first rib and subclavius muslce. The other potential compression site is subpectoral area (the space underneath the pectoralis muscle).
There are three different clinical TOS variants:
The thoracic outlet syndrome is a collective term describing the site (location) of disease rather than the cause. This term is similar to other entrapment neuropathies like carpal tunnel syndrome. It is customary in peripheral nerve surgery to classify the conditions according to pathology site. However, the term itself doesn't pinpoint the cause of the problem. Rarely, the compressing factor can be identified (like cervical accessory rib or a tumor) on imaging studies. Unfortunately, this is not the case for the majority of patients even with advanced imaging modalities.
There are several conditions predisposing to thoracic outlet syndrome. Women are affected 3-4 more often than men. People working with repetitive arm and hand movements tend to develop TOS more frequently. Bad upper body posture, particularly shoulder slouching can contribute to compression. Various congenital and acquired conditions responsible for TOS development are summarized below:
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Qaja E, Honari S, Rhee R. Arterial thoracic outlet syndrome secondary to hypertrophy of the anterior scalene muscle. J Surg Case Rep. 2017;2017(8):rjx158. https://doi.org/10.1093/jscr/rjx158
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