Chris - We've been mentioning this question of phantom pain for a while, but what actually is it and why does it happen?
Cathy - Well phantom limb pain is pain in a body part, in this case a limb, that is missing. Although there are some reports in pain in congenitally missing limbs, so people who've never had limbs, it's much more common in people who lose limbs in later life. Phantom pains do occur in other body parts, so there are phantom tooth pains and phantom bowel pains for people who've had bowel surgery, but by far the most common phantom pain we see id phantom limb pain.
Chris - How can we actually treat it? Because it's easy to understand if there's a part of your body that's damaged and you can put some drug on it to make it feel better. That's intuitive. But if the part of the body that you can feel hurting isn't there, what can you do about that?
Cathy - Well I think the interesting thing about phantom limb pain is that it reflects the complexity of all persisting pain syndromes, which is that a lot of the signals that give rise to the perceptual experience of pain actually arise within the nervous system itself. So obviously if you have a hand missing and you have a painful hand, the signals are obviously not coming from the hand. But you've got to remember that sensory information from the hand involves all sorts of processes up to the cortex and other parts of the brain, and when the hand is missing, the rest of that circuitry takes over to generate sensations. In answer to your question to how we treat it, the answer is that we're not very good at treating it. The condition was described about 500 years ago and we know masses about the neurobiology of the nervous system and the psychology of it, but we're not much better at treating it than ever we were.
Chris - Now that aside, that's obviously a kind of chronic pain, but we can think of pain in two sort of arms can't you. You've got acute pain, which is when you hurt yourself and you have pain now, and then you get chronic pain, which goes on and on forever. Why are the two different?
Cathy - They are different and the definition is really one based on time, so it's pain that's persisted after you'd expect healing to have taken place. But really they are two different types of phenomena. Acute pain, as you said, is expected pain or an everyday pain. So of you stub your toe against the door you'll get acute pain, or if you burn your hand. Commonly in hospitals we see acute pain following surgical procedures. The circuitry for how we process acute pain is fairly well known, fairly well mapped out and fairly predictable, and usually one or at most two treatment interventions are likely to get rid of acute pain, and it has a favourable natural history. It also has an important warning signal for people injuring themselves. With chronic pain it doesn't have that same warning signal function, and it's also much more complex in terms of the circuitry involved. It's very unusual for a single type of treatment to treat chronic pain and we usually have to use a raft of different therapies to treat it. Many chronic pains are resistant to therapy.
Chris - What are the consequences of living on pain killers, because doesn't your body become slowly less responsive to those agents? Do you become immune to those effects if you like, which means that you have to take bigger and bigger doses until in the end you can't take a bigger dose and so you get pain again?
Cathy - You're describing a phenomenon that we call tolerance and exactly as you say, it's your body getting used to a drug. Not very many drugs are associated with true tolerance. The most common ones are the opioid drugs morphine, where any normal person taking these drugs over a period of time will find that they need to take bigger doses to achieve the same analgesic effect. Other drugs aren't so much associated with tolerance, but I guess people sort of seem to get used to it and almost seem to reset their own thermostat if you like. Once they've been on drugs for a while they maybe forget how helpful they've been, and often we see in a clinic patients coming in who've been on drugs for many years and comment that they don't help at all. But when they stop them, the pain is indeed worse.
Chris - What about new fangled things that science has been able to throw at people with chronic pain in recent years? Electrical implants and things like that.
Cathy - That's a very interesting area that we call neuromodulation and the idea is that by electrically stimulating parts of the nervous system with very clever systems that can be completely internalised, one can modulate the sensory experience. The commonest of these in the UK is a treatment called spinal cord stimulation where we insert an electrode next to the spinal cord and elicit a pleasant tingling sensation in the painful part. This seems to override the pain message. That's quite difficult and one often can't elicit a tingling in a phantom limb, so spinal cord stimulation is maybe less useful for phantom limb pain than other techniques. There's some other research coming out of Oxford and we're starting to do some in Bristol looking at stimulating parts of the brain to treat phantom limb pain, and there are some promising early results. But of course these are quite invasive procedures that have not insignificant risks of their own.