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Does thoracic outlet syndrome exist in horses? Part 1

First of all, what is ‘thoracic outlet syndrome’? And secondly, how does this relate to horses?

“First of all, what is ‘thoracic outlet syndrome’? And secondly, how does this relate to horses?

Thoracic outlet syndrome is a condition in people where the nerves and/or blood vessels are compressed in the thoracic outlet. This can lead to pain in the shoulder, numbness or tingling in the affected hand, swelling, and more. The thoracic outlet is the space between your collarbone and your first rib. A horse doesn’t have a collarbone, so I guess strictly speaking he can’t have ‘thoracic outlet syndrome’. But could he still suffer from a syndrome where the nerves and blood vessels are compressed in this region? With bony structures in the area including the cervical spine, thoracic spine, rib cage, shoulder blade, forearm, shoulder, and elbow, there are plenty of possibilities, in my opinion.

So, what might the equine version of ‘thoracic outlet syndrome’ look like? How might we recognise it? To get an idea of how complicated it might be to answer these questions, let’s first look into some of the difficulties in finding the root cause of painful neuromusculoskeletal disorders in the horse, especially if they don’t create lameness. Since thoracic outlet syndrome is difficult to diagnose and treat in humans, it’s likely that an equine version would be equally difficult to recognise and resolve. I’ll start by explaining that ‘neuromusculoskeletal’ means relating to the bones, muscles, tendons, ligaments, fascia, and nerves. Pain related to internal organs, infection, and more, is not directly discussed here (although since everything in the body is in some way connected to everything else, and according to Newton’s third law, every action has an equal and opposite reaction, it’s likely that there are multiple links).

In Part 2, I’ll share some of my thoughts about how a horse might end up with the equine equivalent of thoracic outlet syndrome, and what we might be able to do about it.

“Diagnosing and treating neuromusculoskeletal disorders in the horse can be a tricky business.”

Diagnosing and treating neuromusculoskeletal disorders in the horse can be a tricky business. As an ACPAT and RAMP Registered Chartered Physiotherapist, I am often working without a definitive diagnosis (which means I’m working without knowing exactly what the root cause of the problem is). This is despite the fact that I work closely with the veterinarian wherever it’s appropriate to do so. There are several reasons for this lack of definitive diagnosis, and I’m sure I won’t cover them all here. But some of the important ones, in my opinion, are:

  1. The ‘diagnosis’ seen on imaging (for example, on an X-ray or ultrasound scan) does not always correlate with (match) the symptoms. So if you see arthritic changes on an X-ray, or degeneration in a tendon or ligament on a scan, that doesn’t mean for sure that there is pain related to those things. In the human field, you can explain (verbally) what’s hurting, and when. You can talk about what makes it worse, and what makes it better. The horse doesn’t have the option of using his voice. Therefore the findings from diagnostic imaging need to be understood in combination with the symptoms that the horse is showing, the history from the owner, and (wherever possible), the results of nerve blocks (diagnostic analgesia). For example, a big percentage of horses show kissing spine on an X-ray, perhaps up to 75%. But the vast majority of these horses do not display what we think of as symptoms of kissing spine. Equally, a horse’s sore back may be due to lameness. One study (1) concluded, “Fore- or hindlimb lameness and/or pain associated with the sacroiliac joints could mimic primary thoracolumbar pain. A combination of radiology and scintigraphy gives the most accurate prediction of thoracolumbar pain, but diagnostic analgesia is crucial for accurate diagnosis.” It’s a similar story in humans. A huge percentage of people have degenerative changes in their back, such as a slipped disc or osteoarthritis, but do not have back pain associated with these findings (here’s just one example (2) of numerous studies looking into this).

“… if you see arthritic changes on an X-ray, or degeneration in a tendon or ligament on a scan, that doesn’t mean for sure that there is pain related to those things.”

  1. A ‘diagnosis’ comes from the veterinarian, and the veterinarian is not always involved. For one thing, it can be costly to get a definitive diagnosis, and that’s not an option available to everyone. More importantly, in many cases, the dysfunction that I’m working with, or the performance problem that the owner has noticed, doesn’t appear to warrant veterinary investigation. It’s not like, as humans, we go to the doctor to get pain relief, an X-ray, or an MRI, for every ache and pain we experience. The vast majority of our minor discomforts resolve themselves in time. The difference with the horse, of course, is that he cannot verbalise those minor aches and pains, which means it’s extremely important that we do our best to understand his actions and to involve professionals in his care. If your horse is finding it a little more difficult to leg yield to the left than to the right, would you initially put in place a physio and strengthening program for him, or call the veterinarian? If he’s a bit stiff when he first starts his schooling session, would you first ensure he has regular physical therapy and a good warm-up program, or call the veterinarian? It is normal to have a level of soreness and stiffness at times, for example for a day or two after a longer ride than usual, or a heavier training session. It can be extremely difficult to know when to call the veterinarian or when to call the physiotherapist, give some time off, or try riding through it, and I’m not going to get into that in-depth here. Even if we do decide to opt for veterinary investigation and receive a diagnosis (we’re told which structure within the body is causing the problem), it’s not always the case that every single neuromusculoskeletal dysfunction has been recognised. We tend to aim to reach a treatment and rehabilitation plan that resolves things enough that the horse can do comfortably what is asked of him, whether that’s to hack, to compete, or to retire in the field. It’s generally not realistic as part of a veterinary investigation to pick up every little thing that isn’t quite right. Again, refer back to your own body, and you’ll see that this makes sense. It would be impossible to notice each and every small asymmetry, to find all areas of soft tissue damage (nerve, muscle, tendon, ligament, fascia), and to see every bony change within your body. For a start, each of us is different, and a level of variation in soft tissue and bone is normal. So, whilst it’s important to work with your veterinarian and get a diagnosis (find the root cause) where possible and where appropriate, that diagnosis doesn’t rule out other abnormalities that perhaps are not currently causing a problem, or that may be causing a problem too small to be seen at this time.
  2. We are limited in our diagnostic ability. It’s tempting to think of veterinarians as some kind of ‘god’ who knows everything and can find out exactly what’s wrong with our horse (at least, it’s tempting for me!). But we are limited to the tools, knowledge and experience that we currently have available to us. The tools that veterinarians use include diagnostics, such as MRI, ultrasound, CT scan, bone scan (scintigraphy), X-ray, and more. Those tools are developing and progressing all the time, but they are not infallible, and they cannot ‘find’ everything that is not functioning as it should within the body. Yet again, consider the field of human medicine. I suspect that we all know of someone who has struggled to get a definitive diagnosis from the doctor. Something is definitely wrong, but the doctors can’t find out what, often despite doing a vast array of tests. If we can’t always find the answer in the human field, then it’s no surprise that we can’t always find the answer in the equine field.

“… we are limited to the tools, knowledge and experience that we currently have available to us.”

  1. As Zig Ziglar said, “The first step in solving a problem is to recognise that it exists.” When I’m teaching my course ‘How To Keep Your Horse Sounder for Longer’, we talk about when physio could be helpful, and when you would refer to the vet. I teach several exercises to enable people to recognise sore spots, asymmetry, tightness, stiffness, and restriction in the range of movement through the spine or limbs. Only by first recognising that there is a problem can we assess whether or not we can make a change. It’s one thing being able to feel over your horse and recognise these things, though, and another thing altogether to relate your findings to how your horse actually feels in himself, or in his work. In my opinion, the behaviour of the horse (in hand, and especially ridden) is the most important factor in recognising whether poor performance is likely to be pain-related, or whether it’s perhaps down to a lack of understanding (on the part of either the horse, the rider, or both). In my book, ‘Brain, Pain or Training?’, I give lots of tips on how to tell the difference between a horse who is deliberately doing something different from what you are asking of him, a horse who is not understanding what you are asking of him, and a horse who is physically struggling to do what you are asking of him. In my book ‘Harmonious Horsemanship’, which I co-authored with Dr Sue Dyson, we explain how to use and understand the Ridden Horse Pain Ethogram (RHpE, also known as the Ridden Horse Performance Checklist) to recognise when a behaviour, performance problem, or lack of potential is due to neuromusculoskeletal pain. The RHpE is built on a solid foundation of scientific studies and is revolutionising our ability to improve our horse’s potential and performance, as well as our confidence in his level of comfort and well-being. This makes it dramatically easier to use diagnostic analgesia, as discussed in my first point above, to find the root cause of the problem. But there are some areas that we can’t get to with diagnostic analgesia, and we cannot ignore the signs of pain just because we can’t figure out what’s causing it (see point 3 above).

“The first step in solving a problem is to recognise that it does exist.” Zig Ziglar

Even in the human field of medicine, ‘thoracic outlet syndrome’ is a ‘diagnosis by exclusion’ in many cases. A 2019 review (3) describes neurogenic, venous and arterial thoracic outlet syndrome, with the neurogenic version counting for over 90% of cases. The authors state that “testing is often equivocal or negative in neurogenic thoracic outlet syndrome, making it a diagnosis of exclusion”. This means that despite all the tests they’ve done for this person, the doctors and physical therapists haven’t been able to find out for sure what structure (for example, bone, muscle, tendon, ligament, nerve, fascia) is creating the problem. They have ruled out lots of things, but they haven’t been able to rule anything in. But we like to label things, and so based on the symptoms and what the patient is saying, the doctor makes a diagnosis of thoracic outlet syndrome.

In Part 2 of this blog, we’ll look at how we might recognise what could be an equine version of thoracic outlet syndrome, what might cause it, and what we might do about it.

  1. Zimmerman M, Dyson S, Murray R. Close, impinging and overriding spinous processes in the thoracolumbar spine: the relationship between radiological and scintigraphic findings and clinical signs. Equine Vet J. 2012 Mar;44(2):178-84. doi: 10.1111/j.2042-3306.2011.00373.x. Epub 2011 Sep 1. PMID: 21880062.
  2. Kalichman L, Kim DH, Li L, Guermazi A, Hunter DJ. Computed tomography-evaluated features of spinal degeneration: prevalence, intercorrelation, and association with self-reported low back pain. Spine J. 2010 Mar;10(3):200-8. doi: 10.1016/j.spinee.2009.10.018. Epub 2009 Dec 16. PMID: 20006557; PMCID: PMC3686273.
  3. Jones MR, Prabhakar A, Viswanath O, Urits I, Green JB, Kendrick JB, Brunk AJ, Eng MR, Orhurhu V, Cornett EM, Kaye AD. Thoracic Outlet Syndrome: A Comprehensive Review of Pathophysiology, Diagnosis, and Treatment. Pain Ther. 2019 Jun;8(1):5-18. doi: 10.1007/s40122-019-0124-2. Epub 2019 Apr 29. PMID: 31037504; PMCID: PMC6514035.

About The Horse Physio

Sue Palmer and Belvedere

Sue Palmer, aka The Horse Physio, is an award-winning ACPAT and RAMP registered Chartered Physiotherapist, an Intelligent Horsemanship Recommended Trainerand holds an MSc. Formerly a competitive rider and BHSAI, she works full-time treating horses. Sue shares her passion for ethical and harmonious horsemanship through multiple courses, books, and articles. Sign up at to be the first to hear about new releases.


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Confidence from the ground: Exercises for the horse owner

Finding the sore spot: Exercises for the horse owner

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Warm-up: Exercises for the horse owner

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Easing stiffness: Exercises for the horse owner


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Horse Massage for Horse Owners, by Sue Palmer

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Brain, Pain or Training? by Sue Palmer

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Sue Dyson and Sue Palmer working together to write Harmonious Horsemanship

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