Our education manager Nathan Hers recently interviewed Dr. Tasha Stanton, a pain researcher from the Body in Mind research group. Dr. Stanton is an Associate Professor at the University of South Australia that conducts research into pain and perception and won the 2016 Rising Star Award from the Australian Pain Society. Dr. Stanton is keynoting our 2019 Physio Forum and will be discussing pain neuroscience and physiotherapy practice.
I'd like to start with a 10,000 foot view. For a clinician where this is their first exposure, could you explain what pain neuroscience is?
Sure. In a nutshell, pain neuroscience explores how pain works and importantly, why we hurt. Pain neuroscience aims to understand the complexity of pain – particularly understanding what sorts of things shape our experience of pain – and this is what intrigues me. This is important because then we might be able to modify or change our experience of pain. It’s interesting, most of us have actually experienced pain’s complexity ourselves. Think of a paper cut that hurts a ridiculous amount – this is a tiny injury but it can be so painful. Or if you have ever noticed a bruise on your leg and could not recall how you got it? You’ve actually experienced tissue damage but you’ve not experienced pain - otherwise you'd remember getting the bruise.
In the last decade or so, pain neuroscience has shown us that there are varied and vast contributors to our pain and it’s our brain’s job to consider everything that's going on in our body, mind, and lives, and use that information to decide whether or not we need to be protected at that given moment. And that understanding, that pain is there to protect us, is key because it then makes sense why we might experience more pain when we have other indicators of danger around us.
One area that you focus on is the field of multi-sensory integration. How would you describe that in the context of pain perception?
Basically, multisensory integration refers to how input from different sensory sources is combined together. Multisensory integration allows us to experience the feeling that our body is our own, and allows us to know what's going on with our body at any given moment. So, where it becomes really interesting is we're starting to understand that these sensory inputs can be paired together and coded together in the brain.
If you think about bending forward, you will have input from proprioceptors, input from tactile receptors in your skin, vestibular input from your head bending forward, and visual input of how far you moved. All these different sensory signals can become paired with nociceptive signals (i.e., the danger message that occurs with tissue damage, extreme temperatures, or mechanical changes) and thus, your resultant experience of pain. These pairings can become so strong that pain can be evoked even when only a few of those inputs are present (i.e., pain by slightly bending forward). But alternatively, changing those inputs – even a small change – can then have a large impact on the pain experience. That is, the way that movement has been coded and represented in the brain has included all those inputs (multisensory), so if you take away or alter one of them, it inherently changes the experience.
We also know that multisensory modulation – where one sense will alter another – is important to pain. An example of this is vision and nociception…if you merely see your own skin, a painful stimuli hurts less and brain imaging shows that visual input alters the nociceptive signal. I'm intrigued by the link between vision of our own body and pain, because there’s growing evidence to suggest that those two sensory sources are quite strongly linked in the brain and have the ability to change or modulate each other, even in pain conditions. For example, people with pathological limb pain often feel that their limb feels wrong, much bigger/more swollen than it is in reality. Merely changing the visual input of that body part – i.e., magnifying the size of their hand – makes their pain worse. I think it's so inherently relevant for us as physiotherapists to consider multisensory interactions. We use mirrors, we use vision, we use touch all the time, so it is critical that we understand how this sensory input might contribute to or modulate someone's experience of pain.
From your perspective, if someone is an orthopedic physical therapist or in that type of practice context, and they don't see CRPS or phantom limb pain, how important is the pain neuroscience field to that individuals’ practice?
I think it's essential. All pain, including pain from musculoskeletal injury, including back pain, including phantom limb pain, is an output of our brain that occurs because it is deemed that we need protecting. This decision is being made based on all the incoming information – sensory information, nociception, and other sources, from the injured body part, the emotions someone is experiencing, fear or anxiety, their beliefs such as movement will increase damage, etc. All this information is equally as relevant in acute pain as it is in chronic pain or as it is in a more rare pain condition.
We might just argue that the relative contribution of different inputs might be different in acute versus chronic situations. For example, let's say we are treating someone with a sprained knee ligament. If they are incredibly fearful of re-injury, pain science supports that they are likely to experience more pain than someone who is not fearful…pain is there to protect us and if factors are present (e.g., fear) that suggests we need further protecting, pain increases. But if a person understands that their fear can heighten pain, then it might make more sense to them why their knee hurts more when they return to train in the exact situation they injured it in originally – i.e., they understand that their system is being over-protective versus that they have re-injured themselves.
It is also important to remember that a large portion of the research that underpins pain neuroscience studies acute experimental pain. That is, we are providing a controlled nociceptive stimuli. For example, a burning hot painful probe on the skin, and we see that even in this highly controlled situation, the pain that is experienced can be modulated by numerous factors. Given this, I think that pain neuroscience is incredibly relevant for people who are treating people that have an acute injury.
You’ve recently performed a study on knee osteoarthritis. Do you mind giving a brief synopsis of the study design and some of the findings?
The background to this study is that there is evidence to suggest that people with osteoarthritis have altered perceptions of the size of their joints. Past work has shown that there is an interesting relationship between the size you perceive your body to be and the pain you experience. In healthy volunteers, merely making their hand look bigger results in analgesia. I was quite interested to see, what if we took people with painful knee osteoarthritis that have an altered perception of their knee size, and then we actually manipulated perceived knee size using a visual illusion? What would happen to their pain?
We used mediated reality, which allows us to manipulate video in real-time, to change what a person’s knee looked and felt like, and it changed right in front of their eyes. In this illusion, you look down at your own knee, and you see it stretching or tractioning. And at the very same time, we give a small tug on the calf muscle, pulling towards the foot. Thus, your brain is getting credible evidence from vision and touch that your limb is growing or tractioning. While people know that it isn’t actually growing, it really feels like it is! We measured pain levels before and after the illusion. But here’s what’s really important – we had people undergo various conditions, real illusion and numerous control conditions, all in a random order, and we did not tell them what the real illusion condition was. They were blinded to what we were testing. Despite this, we still found that there was significant pain relief, only when we did the real illusion. If we just pulled on the knee, touch only, or just visually changed the knee size, pain wasn’t affected.
I was also interested to see if there might be a cumulative effect on pain of these illusions. We found that sustaining the illusion didn't further reduce their pain, but it maintained the analgesia. However, repeating the illusion numerous times in a row resulted in larger pain relief, a clinically relevant reduction in pain.
I find this really interesting, because many people would strongly argue that the only driver of pain in people with knee osteoarthritis is nociception coming from the damaged joint. That is, there's nothing we can do about pain besides potentially strengthen the muscles to change the joint dynamics and if that doesn’t work, undergo a joint replacement.
What I think my work highlights is that even in these conditions, that we might largely assume only have pain due to joint/tissue damage, there still are other contributors to their pain experience. It highlights that multi-sensory input can alter pain. Critically, my work shows that the systems that underlie multisensory modulation of pain are intact in people with painful knee osteoarthritis. Thus, if we better understand these mechanisms, we can harness these effects in treatment.
In terms of mediated reality, do you see this as a feasible treatment tool in clinics? Or if not now, do you see it in two or five years?
It’s a good question and my answer for mediated reality is that I'm not sure. At the moment the technology is a bit too clunky to be used clinically. For example, the system that we're using is research-specific and not very user friendly. At the moment, I see the value of mediated reality primarily being to help us to investigate the mechanistic underpinnings of pain.
But there is some very cool stuff going on in virtual reality (VR) and augmented reality (AR) that is already starting to be used in clinics. I think virtual reality can be an excellent way to make exercise fun and engaging. A Masters student of mine from Norway has tested a program for people with back pain and not only did it help, but people loved it! Further, AR has some exciting applications. For example, you can watch a hologram person do an exercise and you can literally walk around this AR person so that you can see the movement from numerous angles. Then you can squat with the augmented person at the same time, potentially having an advanced feedback type system. I think this could have real impact for rehabilitation.
I think what is important here is that we make sure that we use AR/VR for specified reasons and in situations where they have been shown to provide benefit. We need to rigorously test these interventions, just like everything else, and ensure that our reasons for using them have a scientific basis. We want to avoid jumping on the bandwagon if there isn’t evidence to support it. Having said that, I am very excited for this space because of the immense improvements that have been made in the technology even in the past 5 years.
One quick question before I let you go. Do you have any burning clinical questions right now that are keeping you up at night?
I am intrigued by the people that have shockingly “bad” x-rays or MRIs, but have no pain. I would really like to systematically test how their systems are working. Is it that their inhibitory systems work incredibly well, and their facilitatory systems don’t, and this allows them to block any nociceptive signals? Or is it that their systems are completely normal and indistinguishable from someone with a completely clear MRI, thus supporting that for this person, there is unlikely to be a nociceptive signal being generated? And does this change if we show them the terrible MRI findings? It is well known that there is a disconnect between tissue damage and the pain experienced, but we don't always know exactly why that disconnect occurs. I'm really interested in digging into the “why”.