INTRODUCTION TO THORACIC OUTLET SYDNROME

What is thoracic outlet syndrome?

Thoracic outlet syndrome or simply TOS is a collective term describing several conditions associated with 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 thoracic outlet. It is a heart shaped area bounded by the first thoracic vertebra, left and right first ribs and upper edge of sternum. It is called 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 that pass through the neck to 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 brachial plexus. 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 vessels' tone etc. Additionally, brachial plexus gives innervates skin and muscles in back of the neck,  the front, side and back portions of the chest as well and the shoulder girdle. 

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

The nerves and the vessels pass through a narrow space called 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 costo-clavicular space (the space between the collar bone and the first rib) and subpectoralis area (the space underneath the pectoralis muscle). 

See 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. Definite TOS diagnosis is made when the patient symptoms are proven to be due to brachial plexus compression. Disputable TOS is referred to cases when it is not possible to clearly attribute patient’s symptoms to brachial plexus compression.

Venous TOS – vTOS. Far less common (3-4%). Subclavian vein is affected and the symptoms are due to insufficient blood return from affected arm.

Arterial TOS – aTOS. The least common form (1-2%). 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. There might be several reasons for compression. Some people possess an additional or accessory cervical rib.  Normally, a developing fetus has cervical ribs which should completely disappear. In some people those accessory ribs fail to vanish and cause compression. Anomalous first ribs can also cause TOS. Although rib abnormalities are easy to identify on X-rays they contribute to small percentage of TOS patients.

In the vast majority of cases the cause of compression is a fibromuscular (combination of tough fibrous tissue with muscle) band or a thick ligament around nerves and vessels.  These sturdy bands of fibrotic tissue run from various portions of spine and first rib stretching and tethering the softer nerves, artery and vein. 

In some cases, hypertrophic muscles may cause compression. People extensively using their arms and hands for work and sports are especially prone to 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).

 

Types of TOS

There are three different clinical variations of TOS:

Neurogenic TOS – n-TOS. 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. Definite TOS diagnosis is made when the patient symptoms are proven to be due to brachial plexus compression. Disputable TOS is referred to cases when it is not possible to clearly attribute patient’s symptoms to brachial plexus compression.

Venous TOS – v-TOS. Far less common (3-4%). Subclavian vein is affected and the symptoms are due to insufficient blood return from affected arm.

Arterial TOS – a-TOS. The least common form (1-2%). 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 present in adults. It is estimated that approximately 0.5% of population harbor cervical ribs. In the vast majority of cases these ribs extend from the 7th cervical vertebra (which is the last one and stands on the top of 1st thoracic vertebra). Therefore, the accessory cervical rib is usually just above the 1st rib.  The shape and size of the rib may vary dramatically - from slighly elongated C7 transverse process (normal remnant of fetal acessory rib) to well-formed almost normally looking rib. If the accessory rib is quite big its tip usually touches the 1st rib either through bone or cartilageous fusion. 

2. Anomalous, fractured or subluxated  first ribs can also cause TOS by streching or compressing the neuro-vascular package. Although bone abnormalities are easy to identify on X-rays they contribute to small percentage of TOS patients.

3. Fibromuscular soft tissue bands constitute the majority of TOS cases. In this scenario cause of compression may be an abnormal muscle, a fibrous band or a combination of thereof. Scalenus anticus 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 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 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).