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INTRODUCTION TO THORACIC OUTLET SYNDROME

What is thoracic outlet syndrome?

Thoracic outlet syndrome, or simply TOS, is a collective term describing several conditions associated with the compression of nerves and/or blood vessels at the thoracic outlet area, which represents the upper exit (outlet) of the chest cavity.

What structures are affected by TOS?

The nerves and vessels supplying the arm pass at the top of the lung in the region called the thoracic outlet. It is a heart-shaped area bounded by the first thoracic vertebra, the left and right first ribs, and the upper edge of the sternum. It is called a thoracic outlet because it forms the exit from the thoracic (chest) cavity.

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 on their course.

At the central portion of this outlet, several important anatomical structures pass through the neck to the chest and vice versa. At the sides portion, it is occupied by the lungs as well as nerves and blood vessels passing to and from the arm.

Five nerve roots emanating from the spinal cord (C5, C6, C7, C8, and T1) form a complex network called the brachial plexus. The brachial plexus itself branches off to several nerves that eventually reach the arm. These nerves carry electric impulses between the spinal cord and the arm. They control every aspect of arm function: skin sensation, muscle contraction, sweating, blood vessel tone, etc. Additionally, the brachial plexus innervates skin and muscles in the back of the neck, the front, side, and back portions of the chest, as well as the shoulder girdle.

The subclavian artery and subclavian vein are the major vessels providing blood flow to and from the arm. They pass together with the brachial plexus over the first rib at the thoracic outlet. Together with the brachial plexus, they form a neuro-vascular package of the arm. It should be noted that in many body areas, nerves travel along with the vessels, forming neuro-vascular packages.

The nerves and the vessels pass through a narrow space called a scalene triangle. This triangle is located just above the first rib in between two scalene muscles (anterior and middle scalene). The scalene triangle or space is densely packed and, therefore, is prone to compression. The other potential compression sites are the costoclavicular space (the space between the collar bone and the first rib) and the subpectoral area (the space underneath the pectoralis muscle).

See the picture below – circles demonstrate potential compression sites.

Types of TOS

There are three different clinical variations of TOS:

  • Neurogenic TOS – nTOS. The most common form (approximately 95-98%). Brachial plexus is involved, and symptoms are due to nerve compression. Some physicians further divide this group into disputable and definite groups. A definite TOS diagnosis is made when the patient's symptoms are proven to be due to brachial plexus compression. Disputable TOS refers to cases when it is not possible to clearly attribute the patient's symptoms to brachial plexus compression.
  • Venous TOS – vTOS. Far less common (3-4%). The subclavian vein is affected, and the symptoms are due to insufficient blood return from the affected arm.
  • Arterial TOS – aTOS.The least common form (1-2%). The subclavian artery is compressed, and the symptoms are due to insufficient flow to the affected arm. Arterial and venous cases are sometimes collectively called vascular TOS.

Causes of TOS

Women are affected more often than men for unknown reasons. There might be several compressive factors causing TOS.

  1. Cervical accessory rib. Some people possess an accessory cervical rib. Normally, a developing fetus has cervical ribs, which should completely disappear. In some people, those accessory ribs fail to vanish and are present in adults. It is estimated that approximately 0.5% of the population harbor cervical ribs. In the vast majority of cases, these ribs extend from the seventh cervical vertebra (which is the last one and stands on the top of the first thoracic vertebra). Therefore, the accessory cervical rib is usually just above the first rib. The shape and size of the rib may vary dramatically - from a slightly elongated C7 transverse process (normal remnant of fetal accessory rib) to a well-formed, almost normally looking rib. If the accessory rib is quite big, its tip usually touches the first rib either through bone or cartilaginous fusion.
  2. Anomalous, fractured, or subluxated first ribs can also cause TOS by stretching or compressing the neuro-vascular package. Although bone abnormalities are easy to identify on X-rays, they contribute to a small percentage of TOS patients.
  3. Fibromuscular soft tissue bands constitute the majority of TOS cases. In this scenario, the cause of compression may be an abnormal muscle, a fibrous band, or a combination of them. Scalenus antics or scalenus minimus are examples of such muscles. However, in the majority of cases, muscular bands are combined with fibrous tissue. These sturdy bands of fibro-muscular tissue run from various portions of the spine (usually C7 transverse process) to the first rib stretching and tethering the neurovascular package.
  4. Hypertrophic or fibrotic muscles may cause compression. People extensively using their arms and hands for work and sports are especially prone to this type of TOS. This condition is frequently seen in athletes. Scalene muscle hypertrophy usually results in n- and a-TOS (since both the artery and brachial plexus run inside the scalene triangle). Sublclavius muscle hypertrophy, on the other hand, may cause compression and even thrombosis of the subclavian vein (Paget–Schroetter disease).

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