Thoracic Outlet Syndrome Causes | 3 Reasons Why It Happens And How To Treat It

Written by on June 25, 2022 — Medically reviewed by Mitch Torres (PT)

Thoracic outlet syndrome causes arm pain, numbness, and weakness. But why does this condition happen in the first place?

Well, TOS can be caused by congenital, acquired, or traumatic issues. We’ll explore each one as well as their treatments to know what to expect.

There’s a lot to unpack, so here’s what we’ll cover. Tap any of the sections below to quickly jump to its contents:

1) Congenital thoracic outlet syndrome

Congenital causes are physical abnormalities that are present at birth. They usually involve either an extra rib or muscle that has inconveniently developed near the base of your neck.

Inborn problems that can cause congenital TOS include:

Developing a cervical rib

A cervical rib is present in about 1 to 2% of the general population. (1) It comes from the overdevelopment of one of the neck bones, ultimately turning into the shape of a rib.

This can compress a neck artery, obstructing the blood flow to the arm. Or pinch the nerves in the area, causing numbness and other neurological disorders.

Research shows this abnormality is the reason behind (1, 2):

  • 20% of cases of neurogenic TOS.
  • 85% of cases of arterial TOS.

Related: Thoracic outlet syndrome – guide with types, treatments, and more.

Having extra scalene muscles

Normally, we have 3 scalene muscles on each side of the neck. They help us bend our heads to the side and assist with breathing motions.

The brachial plexus runs in the narrow area between them. But some people have a fourth scalene, reducing the space even further, and increasing the risk for TOS.

Treatment options for congenital TOS

Those with a cervical rib might benefit from physical therapy (PT). A physical therapist can help you improve your posture and movement, hopefully resulting in symptom relief.

If you have an extra scalene, an injection with painkillers or botox could potentially be beneficial. The former relieves symptoms while the latter reduces muscle contractions, preventing thoracic outlet compression. (3, 4)

But surgery is necessary when conservative options fail.

This might be the only way to get your thoracic outlet syndrome treated. Here, the surgeon removes the cervical rib or additional muscle to give more wiggle room for the nerves and blood vessels to pass through. (3)

2) Acquired thoracic outlet syndrome

These are habits or health issues that make you vulnerable to developing thoracic outlet syndrome.

The most common causes of acquired TOS include:

Poor posture

Keeping a forward head posture and rounded shoulders as a postural habit could shorten your muscles, narrowing the space within your thoracic outlet. (2)

Neck tumor

Tumors, either cancerous or not, could occupy some much-needed space within the thoracic outlet.

One example is a Pancoast tumor, which usually develops right on top of the lung. If left unchecked, it could grow and compress the brachial plexus. (3)

Repetitive overhead motions

Repetitive motions can cause minor strains after a long time. This trivial injury might cause damage and inflammation, making way for TOS to develop. (5)

This can increase the risk of developing effort thrombosis, a type of venous TOS. (3) Here, strenuous repetitive motions place the subclavian vein in harm’s way.

Over time, it could lead to dangerous blood clots to form that need immediate medical attention to fix.

Increase in muscle bulk

An increase in the size of the muscles around your collarbone can reduce the space within the thoracic outlet. Bodybuilding, repetitive overhead motions, or muscle spasms may cause this scenario. (1, 6)

Treatments for acquired TOS

Most of the acquired causes can benefit from PT to fix your posture and move better.

Losing weight and proper strength training is necessary for people with obesity and those with bulky muscles to alleviate their symptoms.

For those with a tumor, please consult an oncologist right away to know if it’s cancerous or not. Usually, the treatments here are chemotherapy, radiation therapy, or surgery.

3) Traumatic thoracic outlet syndrome

Traumatic causes came from a physical injury right onto or near the thoracic outlet.

These two injuries can make you prone to TOS:


Whiplash occurs when your head is rapidly forced to move backward then forward, or vice versa, much like using a whip. This could come from a car accident, sports, or falls. (7)

As much as 80% of TOS cases result from this traumatic injury, causing nerve compression and possibly leading to neurogenic TOS. (8)

Collarbone fracture/dislocation

A collarbone fracture could happen after falling and catching yourself on your outstretched hand, or in a car accident. For those with unaligned or dislocated collarbones, the risk of TOS is higher. (4)

Treatment options for traumatic TOS

Restricting movement via a neck brace or shoulder sling can help you heal initially. Afterward, PT can get you to move properly to avoid compressing the structures within your thoracic outlet.

If the fractured collarbone is not in a good position to heal, surgical treatment may be required.

Common thoracic outlet syndrome symptoms

The symptoms will depend on which structure is being affected – a nerve, artery, or vein. This means there are 3 types of thoracic outlet syndrome:

Neurogenic thoracic outlet syndrome

This involves compression of the brachial plexus, a network of nerves controlling arm movement and sensation.

It is the most common subtype of TOS, making up 95% of cases. Some of its symptoms include (2):

  • Numbness and tingling.
  • Arm weakness.
  • Sharp shooting pain along your affected arm.

Venous thoracic outlet syndrome

4% of TOS cases are considered to be venous in origin. (2) The subclavian vein is involved, which drains blood from your arms back to your heart to re-circulate.

Some of its symptoms are usually (2):

  • Bluish discoloration on your arm.
  • Arm swelling.
  • Arm pain.

Arterial thoracic outlet syndrome

Arterial TOS comprises 1% of TOS cases. (2) It involves the subclavian artery, an important vessel that supplies blood coming from your heart down to your arms.

Some of the symptoms of this blood vessel compression are (2):

  • Paleness of your involved arm.
  • Weakening of pulse with arm movements.
  • Easily fatigable arm.

How is thoracic outlet syndrome diagnosed?

Your medical doctor can diagnose thoracic outlet syndrome by going over a clinical assessment. This includes taking a thorough history, physical exam, and imaging.

History taking might involve your doctor asking you questions as to when and how your condition started. They might do this to check for patterns to what may have caused your TOS.

With a physical exam, the physician may look for any tell-tale signs of TOS. This includes your posture, any weakness among your shoulder muscles, and symptom-provoking maneuvers.

Imaging is helpful to get a good look at your bones, nerves, and blood vessels.

This may include an X-ray or MRI to check for structural abnormalities such as extra bone, muscle, tumors, or fractures.

Another option could be an electrodiagnostic test to check for slow nerve speeds.

You might also be required to undergo arterial or venous tests. They locate possible blockage or compression along a blood vessel’s pathway.

Learn more: The 3-step process to diagnose TOS.


Does thoracic outlet syndrome go away?

Yes, with the right treatment. This is key, as you run the risk of worsening your condition if you neglect it.

Can poor posture cause thoracic outlet syndrome?

Yes. Research shows that constantly being in a poor posture can increase the risk of thoracic outlet syndrome.

How long does physical therapy take for thoracic outlet syndrome?

For minor cases, it might take a few weeks. But it could take longer, months even, for severe types of thoracic outlet syndrome.

Conclusion: What causes thoracic outlet syndrome?

Different issues can lead to thoracic outlet syndrome. Such as inborn abnormalities, acquired conditions, or traumatic injuries near the neck region.

The best way to learn what caused your condition is to consult your doctor. They can assist you in working out your treatment plan via a thorough clinical assessment.

To recover, you may need a referral to physical therapy, injections to relieve your symptoms, or surgery to add more space within your thoracic outlet.


  1. Camporese, Giuseppe, et al. “Diagnostic and Therapeutic Management of the Thoracic Outlet Syndrome. Review of the Literature and Report of an Italian Experience.” Frontiers in cardiovascular medicine vol. 9 802183. 22 Mar. 2022,
  2. Li, Nathan, et al. “Thoracic Outlet Syndrome: A Narrative Review.” Journal of clinical medicine vol. 10,5 962. 1 Mar. 2021, DOI: 10.3390/jcm10050962
  3. Jones, Mark R et al. “Thoracic Outlet Syndrome: A Comprehensive Review of Pathophysiology, Diagnosis, and Treatment.” Pain and therapy vol. 8,1 (2019): 5-18. DOI: 10.1007/s40122-019-0124-2
  4. Hooper, Troy L et al. “Thoracic outlet syndrome: a controversial clinical condition. Part 1: anatomy, and clinical examination/diagnosis.” The Journal of manual & manipulative therapy vol. 18,2 (2010): 74-83. DOI: 10.1179/106698110X12640740712734
  5. Masocatto, Nilo Olímpio et al. “Thoracic outlet syndrome: a narrative review.” “Síndrome do desfiladeiro torácico: uma revisão narrativa.” Revista do Colegio Brasileiro de Cirurgioes vol. 46,5 e20192243. 20 Dec. 2019, DOI: 10.1590/0100-6991e-20192243
  6. Mallory, Anthony. “Chapter 9 – Thoracic Outlet Syndrome. “ Hand and Upper Extremity Rehabilitation (Third Edition): A PRACTICAL GUIDE. 2006, Pages 121-137.
  7. Bragg KJ, Varacallo M. Cervical Sprain. [Updated 2022 Feb 12]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from:
  8. Sanders, Richard. “Neurogenic Thoracic Outlet Syndrome and Pectoralis Minor Syndrome: A Common Sequela of Whiplash Injuries.” The Journal for Nurse Practitioners. Volume 4, Issue 8, September 2008, Pages 586-594.

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