People with neurogenic thoracic outlet syndrome (nTOS) usual have a history of neck injury (e.g. whiplash etc.) which can result in injury or tears to the scalene muscles in the neck and thoracic outlet. As the injury heals, the damaged muscle can form scar tissue either within or adjacent to it. This muscle fibrosis process is often the cause for compression of the nerves of the brachial plexus which results in the neurogenic TOS.
There are other predisposing factors to nTOS, including anomalous cervical ribs, congenital ligaments or bands compressing or impinging on the nerves or nerve roots, narrow scalene triangle or muscle anomalies (variations of the scalene minimus), all of which can cause nTOS. While the first rib (not cervical rib) is in itself not directly responsible for nTOS, it can contribute to the problem by allowing scar tissue to compress the nerves against it. A cervical (complete or incomplete) rib occurs in up to 5% of patients with thoracic outlet syndrome and can form an attachment to the first rib, either through a band of cartilage, tendinous or bony fusion.
Primary Symptoms of NTOS
The main symptoms of nTOS occur predominantly in the affected arm or hand and can cause pain, discomfort, numbness or weakness. The symptoms are usually not localised to one particular nerve or nerve distribution and can involve any part of the entire arm, neck or even part of the upper back. Activities that involve sustained or repetitive actions, especially those where there is elevation of the arm or repeated turning of the neck (e.g. prolonged typing on a keyboard, driving a car, brushing hair, hanging up the washing etc.), can cause strain and scarring of the scalene muscles which may potentiate potential compression of the underlying nerve roots by narrowing the scalene triangle.
Neurogenic thoracic outlet syndrome can also manifest as headaches usually occurring as a result of referred pain to the back of the head because of secondary spasm within the trapezius and paraspinous muscle. Prolonged, severe extrinsic compression of peripheral nerves can result in muscle weakness and atrophy, but such findings are actually rare in patients with nTOS – due to intermittent nature of nerve compression in neurogenic thoracic outlet syndrome.
Examination involves a thorough musculoskeletal and neurological assessment of the neck and upper limb. Occasionally, “trigger points” may be present within the scalene muscle that re-create the patient’s symptoms of pain and numbness. If identified, this is a useful manoeuvre as it serves to reinforce the diagnosis of nTOS. The EAST test may also be performed where the arms are held up above the head and the patient opens and closes their fists repeatedly. In those patients where the symptoms occur soon after starting this test, the diagnosis of nTOS is reinforced.
Diagnosis of neurogenic thoracic outlet syndrome can be quite difficult and rests mainly on pattern recognition of clinical symptoms. There is no single imaging or diagnostic test that accurately diagnoses or excludes nTOS however, there is a strong correlation between a positive scalene block and good response to surgery. Non-invasive cross-sectional imaging (e.g. MRI) is useful to exclude any other obvious causes of nerve compression in and around the thoracic outlet. Nerve conduction studies are often negative because nerve compression happens very proximally. Plain X-rays of the neck are useful to exclude a cervical or bony rib abnormality. Scalene muscle block is perhaps the most useful diagnostic test, where local anaesthetic is injected into the anterior scalene muscle around the brachial plexus. If the patient experiences a relief in their symptoms following this procedure, the chances of success of thoracic outlet decompression surgery are much higher.
VENOUS THORACIC OUTLET SYNDROME
Venous thoracic outlet syndrome is a condition that results in compression of the axillary/subclavian vein as it exits the arm and enters the thoracic cavity resulting in arm swelling and discomfort. The condition is most commonly diagnosed when the a patient presents with a significantly swollen arm and is found to have a clot in the subclavian vein (DVT or deep venous thrombosis).
Venous thoracic outlet syndrome usually occurs due to problems with the subclavian vein being compressed in the costo-clavicular space, the small area where the first rib, clavicle collarbone) and the sternum (breastbone) all connect. The costo-clavicular space is also home to the costo-clavicular ligament and the subclavius muscles. Repeated compression and pinching of the subclavian vein due to these structures in the costoclavicular space results in narrowing and scarring of the vein, eventually leading to an occlusion or thrombosis (clotting of the vein). This condition is often referred to as “effort thrombosis”.
Primary Symptoms of VTOS
Common repetitive activities of the arm (e.g swimming, weightlifting, throwing etc) can all contribute to the vein being compressed. vTOS occurs equally in men and women and usually manifests in the early stages with some mild swelling, discomfort and possibly some numbness of the arm with repeated use of the arm. As the condition worsens and the vein becomes more scarred, the patient may develop cyanosis (bluish discolouration) of the arm and develop small collateral veins around the neck, chest and shoulder region whereby smaller veins are recruited to help drain the blood from the arm as the main vein (axillary or subclavian vein) is compressed.
Ultrasound, MRI or CT-scans are usually the scans of choice when trying to prove/disprove the diagnosis of venous thoracic outlet syndrome, however these scans cannot always accurately predict when the vein is being compressed given that most of the compression occurs when the arm is moving while the scans are done with the patient still. Occasionally a venogram is required, whereby contrast is injected into the veins of the arm, with the arm in different position to check whether there is compression of the vein when the arm is moving.
ARTERIAL THORACIC OUTLET SYNDROME
Arterial thoracic Outlet Syndrome is the least common type of TOS comprising of approximately 1% of all thoracic outlet syndrome cases. It is however the type which has the most serious consequences and the strongest indication for surgical repair.
Almost all cases of aTOS are associated with a bony abnormality, usually an accessory cervical rib. These extra ribs originate one level higher than that usual 1st rib and form a bony fusion with the first rib. The cervical rib often pushes the subclavian artery forward where it is compressed against the scalenus anterior muscle. Over time, this compression may result in a decreased blood supply to the arm, as well as the potential to develop a dilatation or aneurysm of the artery just after where it is compressed. Any dilatation of aneurysm may be a source of clot which may dislodge and travel into the arm, thereby potentially blocking off one of the arteries.
Arterial thoracic outlet syndrome usually happens in young fit patients and most often comes to be diagnosed when the patient presents with either a pulsatile swelling above their collarbone or with evidence of a small clot lodging in one of the arteries in the hand. A simple neck X-ray is all it takes to definitely diagnose the presence of an accessory or cervical rib. A contrast-CT is then usually organised to assess the arterial problem and degree of compression.