Thoracic outlet syndrome (TOS) develops when nerves or blood vessels get compressed in the narrow space between your collarbone and first rib. This compression can happen for several reasons: you might be born with an extra rib or abnormal tissue bands, suffer a neck injury, or spend years doing repetitive overhead work. About 95% of cases involve nerve compression, which causes pain, numbness, and tingling in the arm and hand. The remaining cases involve compression of veins or arteries, which can lead to blood clots or circulation problems.
The Space Where Compression Happens
The thoracic outlet is a passageway in your lower neck where a bundle of critical structures travels from your chest toward your arm. Three things share this tight corridor: the subclavian artery (supplying blood to the arm), the subclavian vein (draining blood back), and the brachial plexus (a network of nerves from the C5 through T1 spinal roots that control most shoulder and arm movement and sensation).
This passageway is divided into three zones, and compression can occur in any of them. The scalene triangle sits above the collarbone, formed by the front and middle scalene muscles of the neck and the first rib. The costoclavicular space lies between the collarbone and first rib. The subcoracoid space sits beneath the collarbone near the shoulder, where the pectoralis minor muscle attaches. The artery and nerves pass through all three zones, while the vein runs just in front of the scalene triangle before joining the others in the costoclavicular space. Anything that narrows these already-tight compartments, whether bone, muscle, scar tissue, or swelling, can squeeze the structures passing through them.
Congenital Causes: Extra Ribs and Fibrous Bands
Some people are born with structural variations that shrink the thoracic outlet before any injury or overuse enters the picture. The most well-known is a cervical rib, a small extra rib that extends from the lowest cervical vertebra in the neck. Cervical ribs occur in roughly 0.2% to 1% of the population, but about 90% of people who have one never develop symptoms. When a cervical rib does cause problems, it pushes into the scalene triangle and compresses the brachial plexus or subclavian artery against the surrounding muscles.
Less visible but possibly more common are anomalous fibromuscular bands. These are tough strips of connective tissue that form during fetal development in the scalene region. Ultrasound studies have identified these bands pressing into the lower trunk of the brachial plexus, causing the nerve to swell and become compressed. Because these bands don’t show up on standard X-rays, they’re often discovered only during surgery or specialized imaging. They represent one of the most frequent structural causes of neurogenic TOS.
Trauma and Whiplash Injuries
A sudden injury to the neck or upper body is one of the most recognized triggers. Car accidents are a classic example. Whiplash forces the neck through rapid flexion and extension, which can directly damage the scalene muscles. A prospective study of 110 patients found that traumatic neurogenic TOS typically involves two patterns of scalene muscle injury, both related to the physical forces of whiplash and associated cervical disc problems. The damaged muscles swell, spasm, and eventually develop scar tissue, all of which tighten the scalene triangle around the nerves.
Clavicle fractures work through a different mechanism. A broken collarbone can heal in a slightly displaced position, reducing the already-limited costoclavicular space. Even after a fracture heals properly, the callus (new bone that forms during healing) can be bulky enough to press on nearby structures. Dislocations of the shoulder or first rib can similarly shift the anatomy just enough to cause compression.
Repetitive Overhead Movements
You don’t need a single dramatic injury. Repetitive arm use, especially overhead, can gradually reshape the thoracic outlet over months or years. When you raise your arms, the collarbone drops slightly toward the first rib and the scalene muscles engage. Doing this thousands of times causes those muscles to thicken and the surrounding soft tissue to tighten.
High-risk activities and occupations include swimming, baseball, painting walls or ceilings, hairstyling, and auto mechanics. Johns Hopkins Medicine specifically identifies any job or sport that requires prolonged raised-arm positioning as a risk factor. Bodybuilders face an additional risk: heavily developed neck muscles can grow large enough to physically compress the nerves or blood vessels passing through the scalene triangle, even without repetitive overhead motion.
Effort Thrombosis in Athletes
Venous TOS has a distinct and dramatic cause. Known clinically as Paget-Schroetter syndrome, it occurs when repetitive arm motion compresses the subclavian vein so many times that the vein’s inner wall becomes damaged. The mechanism unfolds over time: chronic compression triggers inflammation in the tissue surrounding the vein, which generates fibrotic scar tissue. This scar tissue limits the vein’s ability to move freely. As the arm continues to rotate, the vein stretches and tears, each small injury layering more scar tissue and creating turbulent blood flow that eventually produces a clot.
This type affects 1 to 2 people per 100,000 each year, and 60% to 80% of cases follow vigorous exercise or physical activity. It has been documented in baseball, softball, swimming, wrestling, hockey, martial arts, and even backpacking. Baseball players are particularly vulnerable due to the extreme shoulder rotation involved in throwing. One study tracking a major league team and a collegiate team over 11 years confirmed four cases, and a broader study identified 14 baseball players among 32 high-level athletes with effort thrombosis over a decade. In overhead athletes, the symptoms often mimic common shoulder or elbow injuries: loss of throwing velocity, arm heaviness, or a “dead arm” feeling.
Who Gets TOS Most Often
Women develop thoracic outlet syndrome significantly more often than men. In one study of 96 patients with TOS, 70 were female and 26 were male, a ratio of nearly 3 to 1. Several anatomical factors likely contribute: women tend to have narrower thoracic outlets, less muscular support around the shoulder girdle, and more joint laxity, all of which may make the neurovascular bundle more vulnerable to compression.
While TOS is most commonly diagnosed in working-age adults, it also occurs in teenagers and young adults. The average age at symptom onset in a pediatric and young adult study was 15 years, with cases appearing as young as age 4. In younger patients, congenital factors like cervical ribs and fibrous bands are more likely to be the primary cause, while in adults, the picture more often involves trauma, repetitive use, or a combination of structural predisposition and an inciting event.
Multiple Factors Often Overlap
TOS rarely comes down to a single cause. The most common scenario is a person with a mildly narrow thoracic outlet, whether from a subtle cervical rib, fibrous band, or simply smaller anatomy, who then experiences a triggering event. That trigger might be a car accident, a new job requiring overhead work, or starting a sport that loads the shoulder. The structural predisposition alone wasn’t enough to cause symptoms, and the activity alone wouldn’t have caused problems in someone with a wider outlet. Together, they cross the threshold into compression.
Poor posture can compound any of these factors. A forward-head, rounded-shoulder position drops the collarbone closer to the first rib and tightens the scalene muscles, further reducing available space. For someone already on the edge, spending hours slumped over a desk may be the final variable that tips the balance toward symptoms. This layered nature is part of why TOS can be difficult to diagnose: the Society for Vascular Surgery requires at least three of four criteria to be present, including symptoms consistent with thoracic outlet compression, signs of nerve involvement that worsen with arms overhead or dangling, no better explanation for the symptoms, and a positive response to a targeted muscle injection test.