The thoracic outlet comprises three triangular passages that travel from the upper neck, through the collarbone region and down into the arm. Bundles of nerves and blood vessels travel through this area to supply the arm with blood flow and feeling. However, if this area is compressed or injured, it can result in nerve damage or obstruction of blood flow. When this occurs, it is known as thoracic outlet syndrome (TOS). The most common symptoms of TOS include numbness, pain, tingling or weakness in the affected arm and hand.

There are a number of possible causes of TOS, including injury (e.g., whiplash), poor posture, carrying heavy backpacks, and aging and weakened musculature that results in drooping. This may be caused by obesity, repetitive stress injuries or lack of exercise. However, TOS may have other causes as well, including minor skeletal defects that are present at birth.

Symptoms may vary depending on the location of the pressure and whether nerves or blood vessels are being compressed. In most cases, there are symptoms related to both vascular obstruction and nerve compression. It is very unusual that somebody would be affected with only one or the other. In some cases, TOS can cause angina–like chest pain or arm pain. Other symptoms may include cold hands, headaches and dizziness.

There are five categories of TOS – true neurologic TOS, disputed TOS, arterial TOS, venous TOS and traumatic TOS. Medical professionals do not agree on how common TOS is in the general population. Because symptoms can vary so widely (and in some cases are absent) estimates run from 3 to 80 per 1,000 people. Physicians dispute whether some types of TOS actually exist.

Tests that can aid in diagnosis include physical examination, nerve and muscle studies, x-ray, MRI and ultrasound. Treatment may include physical therapy, exercise, anti–inflammatory pain medication or surgery. In general, many people benefit from physician–prescribed stretching and physical therapy. As with diagnosis, there is no standard treatment option.