(USMLE topics) Thoracic outlet syndrome: anatomy, pathophysiology, types of TOS: neurogenic and vascular (venous and arterial), signs and symptoms, causes, diagnosis and treatments.

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The thoracic outlet is the space between the clavicle and the first rib, through which pass the nerves and blood vessels supplying the shoulder and arm. The nerves, altogether known as the brachial plexus, control muscles and provide sensations. The subclavian artery carries blood out to the arm, while the subclavian vein drains blood back.
Thoracic outlet syndrome, or TOS, occurs when this area is compressed, resulting in pain, numbness, among other symptoms.
TOS can be classified based on the structure that is affected:
Compression of nerves produces neurogenic TOS, the most common type. Symptoms of neurogenic TOS include numbness or tingling in the hand or fingers, pain and weakness in the shoulder or arm, and possibly hand muscle wasting.
Venous TOS happens when the subclavian vein is compressed, leading to formation of blood clots. The condition usually develops suddenly, often after excessive overhead motions of the arms. This is known as effort thrombosis. Impaired venous drainage causes swelling, heaviness, weakness and blueness of the arm, hand, and fingers. The veins in the shoulder and neck may also appear more prominent as they are dilated.
Compression of the subclavian artery results in arterial TOS. Reduced blood supply causes coldness, paleness, pain and weakness of hand and fingers. Arterial TOS is the least common but most serious type, as lack of blood supply can lead to death of tissue.
TOS may have different causes. These include:
Abnormal anatomical features, such as presence of an extra rib, extra or aberrant scalene muscle, or abnormal first rib or clavicle.
Traumatic injury to the neck or back, in particular whiplash injury.
Repetitive injuries from carrying heavy shoulder loads.
Repetitive activities, especially overhead motions.
Poor posture, tumors or large lymph nodes, weight gain, and excessive muscle building.
Diagnosis is based on symptoms, physical exam, provocation tests to reproduce symptoms, imaging tests to check for abnormal anatomy or obstruction of blood vessels, and nerve conduction studies for nerve functions.
Early diagnosis and treatment is essential to prevent permanent damage to body tissue. For neurogenic TOS, physical therapy is typically the first treatment and is usually effective if started early. Medications may include pain relievers, muscle relaxants; and for vascular TOS: anticoagulants and clotdissolving medicines. Surgery is rarely required for neurogenic TOS, but is a common treatment for vascular TOS. Surgery involves removing parts of the scalene muscles, cervical rib and/or first rib, to make room for the nerves or blood vessels.