The Thoracic Outlet Syndrome #1 - Causes

The Thoracic Outlet is a space or a passageway that lies just above the first rib and behind the collar bone, the clavicle. There are some muscles that form the boundaries of the Thoracic Outlet. The Thoracic Outlet runs from the base of the neck to the armpit. The Thoracic Outlet is present on both sides of the body. A number of important anatomical structures viz. the brachial plexus, the subclavian artery and the subclavian vein pass through the Thoracic Outlet. The brachial plexus is a group of nerves that pass from the neck to the arm. The Thoracic Outlet Syndrome occurs when any one or more of these important anatomical structures are squashed or compressed when it passes through the Thoracic Outlet. In most of the cases it is the brachial plexus that is compressed. It is estimated that more than 90% of all Thoracic Outlet Syndromes are neurogenic i.e. due to compression on the brachial plexus. In some cases the Subclavian Artery or the Subclavian Vein is compressed. It is estimated that about 3-5% of all Thoracic Outlet Syndromes are venous and less than 1% are arterial in nature. Hence the Thoracic Outlet Syndromes are manifested by two types of symptoms viz. Neurogenic and Vascular.

The onset of the Thoracic Outlet Syndromes is from the 2^nd– 8^th decades with the peak in the 4^th decade. It is more common in women than in men. The Thoracic Outlet Syndrome usually affects one side of the body and hence the symptoms may be unilateral. But the compression may occur bilaterally and in such cases the symptoms may be bilateral. Various reasons can be attributed to the cause of compression of the brachial plexus or the blood vessels or both in the Thoracic Outlet. The symptoms may vary on the basis of which anatomical structure is compressed.

1. The Cervical Rib- Causes compression. Cervical rib is a congenital anomaly. About 1 in about 200 people are born with an extra rib. About 1 in about 10 people who have a Cervical Rib develop Thoracic Outlet Syndrome i.e. most of the people who have Cervical Rib do not develop Thoracic Outlet Syndrome. Posteriorly (at the back) the Cervical Rib originates from the 7^th Cervical Vertebra. Anteriorly (in the front) the Cervical Rib may be ‘floating’ as it is not attached to any structures. In some cases it is attached anteriorly to the first rib by a band of tough fibrous tissue. Rarely, it makes an articulation with the first rib. The Cervical Rib may be unilateral or bilateral. A Cervical Rib causes the narrowing of the Thoracic Outlet resulting in the compression of the Brachial Plexus or the Subclavian Blood Vessels or both.
2. Anomalous Tissue Growth – This is a congenital abnormality. In some people an extra band of tissue may be present underneath the skin around the Thoracic Outlet area. This band is called fibrous band. The fibrous band acts more or less like a Cervical Rib and cause narrowing of the Thoracic Outlet resulting in the compression of the anatomical structures passing through the outlet.
3. Poor Posture – Poor posture can cause mechanical problems. A forward head posture incurred from an increased dorsal spinal khyphosis places the head ahead of the centre of gravity. The head of an average adult usually weighs approximately 8-12 pounds. If the head is considered to weigh 10 pounds and is held 3 inches ahead of the centre of gravity, the head now apparently ‘weighs’ 30 pounds. This causes an increase in cervical lordosis and closes the posterior foramina, thus entrapping the nerve roots. Moreover, in the case of forward head posture, not only the head protrudes forward but also the shoulders droop and the scapulae rotates forwards and downwards. This results in the narrowing of the Thoracic Outlet and compression of the neuro vascular structures passing through the outlet.
4. Large breasts and Breast Implants-When the shoulders droop, the scapulae rotate forwards and downwards resulting in the sagging of the large breasts. Breasts with implants will also sag. In such cases the vital capacity of the thoracic cavity is diminished. The person apparently looks shorter in height. The large breasts and breasts with the implants, when sag, pull the chest wall forward and causes the symptoms of the Thoracic Outlet Syndrome.
5. Trauma-Usually the patients presenting with the symptoms of Thoracic 0utllet Syndrome may have the history of recent trauma to the neck e.g. trauma due to whiplash injuries. Trauma during an accident can cause structures in the neck and the chest wall mal aligned and may get displaced away from the normal anatomical position causing narrowing of the Thoracic Outlet. In the case of fracture of the collar bone (the clavicle) or the 1^st rib, the broken fragments may encroach into the Thoracic Outlet and compress the anatomical structures passing through the Outlet. During the process of bone healing, the callous formed may also exert compression on the structures presenting symptoms. The bleeding may lead to haematoma formation which in turn narrows the Outlet. Pseudo aneurysms may also cause narrowing of the Outlet.
6. Repetitive Occupational Movements –Thoracic Outlet Syndrome is usually common in persons whose job demands repetitive movements of the cervical spine and the shoulders. Repetitive occupational movements of the neck may lead to wear and tear of cervical spine. Such movements exert excessive strain and stretch to the brachial plexus. Thus these factors also lead to the manifestation of symptoms of the Thoracic Outlet Syndrome. It is quiet common in sports personal whose sports activity demands excessive arm movement e.g. Swimmers, Javelin Throwers and Shot Putters.
7. Bony Overgrowth in the Clavicle or First Rib- narrows the Thoracic Outlet causing compressive pressure on the anatomical structures passing through the Outlet, presenting the symptoms of Thoracic Outlet Syndrome.
8. Elongated Transverse Process of the 7^th Cervical Vertebra-
9. Anterior Scalene Syndrome- the Scalene Muscle originates from the Cervical Spine and divide to contain brachial plexus and the Subclavian Artery. The Middle Scalene Muscle is posterior and the Anterior Scalene Muscle is anterior to the Subclavian Artery. The Subclavian Vein is anterior to the Anterior Scalene Muscle. After passing over the first rib the neurovascular bundle passes under the Pectoral Minor Muscle. The Clavicle covers the neurovascular bundle and lies parallel to the first rib. Hypertrophy of the Scalene Muscle due to excessive athletic activities and the congenital abnormalities of the Scalene Muscle development also contribute to the evolvement of the Thoracic Outlet Syndrome.
10. Improper sitting and standing postures - may cause Thoracic Outlet Syndromes.
11. Congenital Postural Abnormalities – such as Scoliosis and exaggerated Thoracic khyphosis may lead to Thoracic Outlet Syndrome.
12. Precursors of Myofascial pain – can cause the Thoracic Outlet Syndrome. Conditions such as Sleep Disorders, Oestrogen Deficiency, Hypothyroidism, Myalgia and inflammatory conditions like Rheumatoid Arthritis.
13. Claviculocostal Syndrome- Symptomatic Thoracic Outlet, Neurovascular Bundle Compression, or Thoracic Outlet Syndrome can be attributed to the compression of the bundle between the collarbone and the first rib. Since both the bones are responsible for neurovascular compression, this syndrome is termed as Claviculocostal Syndrome.
14. Pectoralis Minor Syndrome- The neurovascular bundle may be compressed between the Pectoralis Minor Muscle and the rib cage. The symptoms may get aggravated when the arms are elevated in a position of abduction and moving the hand behind the head. Hence this syndrome is also termed as Hyper Abduction Syndrome.
15. Subclavian Muscle Hypertrophy-
16. Pathological Lesions in Subclavian Blood Vessels-Narrowing of the Subclavian Artery and Vein is another cause for Thoracic Outlet Syndrome. Some people can have a congenital narrowing of one of these blood vessels. Such people are prone to get a blood clot, if the upper limb is over used. Weight Lifters or people who does the exercise for a long period with the upper limb held above the head are likely to get the symptoms of Thoracic Outlet Syndrome.
17. Paget-Schroetter Syndrome- Thrombosis of the Subclavian Vein is formed following heavy exercises of the upper limbs, in young fit persons. Such conditions manifest the symptoms of Thoracic Outlet Syndrome.
18. 'Pancoast’s Syndrome- Lung Cancer infiltrates the brachial plexus and manifests the symptoms of Thoracic Outlet Syndrome.

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