In general, compression of a single nerve anywhere in the body may cause symptoms such as pain, numbness, tingling and weakness. Doctors describe this process as a nerve entrapment syndrome. Neurogenic TOS occurs through the same process, but is much more complex than entrapment of a single peripheral nerve.

In detail, five nerve roots arise from the spinal cord on each side of the neck. After leaving the spinal cord, these nerve roots enter the thoracic outlet. In each thoracic outlet, the nerve roots form a complex branching network, called the brachial plexus. Compression or tension on any part of the brachial plexus may cause symptoms, resulting in neurogenic TOS. Given the complexity of the structure of the brachial plexus, patients can suffer a broad range of symptoms.

Thus, doctors see a complex clinical presentation, since compression may involve any number of the branches of the brachial plexus. Symptoms may include pain, numbness, tingling, coldness and weakness of the affected upper extremity. The neurogenic type of TOS may cause symptoms in one or both arms or hands, as well as the neck, chest, back or shoulders.

A single, large subclavian vein drains the blood from each arm. Compression of this vein can prevent normal venous drainage of the arm. When compression is repetitive, severe, or prolonged, it can cause damage to the inner wall of the vein. As a result of this damage, blood clot can form within the vein. In this event, swelling, heaviness and cyanosis (abnormal blue color) develops in the affected arm. Some patients may develop new superficial veins of the chest and shoulder, as blood must return to the heart through new pathways.

When a doctor sees a patient with this clinical picture, they should immediately consider the diagnosis of venous TOS. This type of TOS accounts for about 2 to 5% of all TOS cases. Quick and accurate diagnosis is vital to prevent complications such as blood clot fragments traveling to the lungs or, rarely, to the brain, causing a stroke.

Treatment of this type of TOS should begin urgently, using methods to dissolve or remove the blood clot in the subclavian vein. After this, imaging studies can show whether the vein is damaged or compressed by external structures. If these structures are not removed or reduced, the same damage and blood clot may recur, resulting in the same type of TOS.

Arterial TOS develops when compression of the subclavian artery in the thoracic outlet results in symptoms. A single, large subclavian artery provides blood flow to each arm. Repetitive and severe compression of this artery in the thoracic outlet causes damage to the arterial wall. Damage limited to the inner arterial wall can cause a fixed scar and stenosis (narrowing of the artery). In contrast, damage to the full thickness of the wall can result in an aneurysm (focal ballooning of the artery). Arterial blood clots may form in the damaged arterial segment. These blood clots can break off and travel down the arm. These clots lodge in distal arteries, causing sudden and severe loss of blood flow. Doctors call this process acute arterial insufficiency.

Arterial TOS accounts for about 1% of all TOS cases. Most patients with this type of TOS present with an acute onset of symptoms, including pain and loss of pulses. It is important to realize that if not treated emergently, loss of tissue (gangrene) may occur. Therefore, doctors need to recognize and treat this arterial TOS emergently. Initial treatment requires emergent removal or dissolving of the arterial blood clot. Imaging tests can then show the damaged arterial segment and the external structures that caused the damage. Later, doctors would repair the damaged arterial segment and remove or release the abnormal external structures.