In our last post, we introduced the concept that there are three main types of pain. We covered nociceptive pain and presented this as the most common type of pain; the one you’re most likely to have experienced.

In this post, we move on to neuropathic pain, which is an entirely different beast. Simply put, neuropathic pain is the product of an injured nerve. The most common example of this is sciatica, an injury to the sciatic nerve. But this is just one example and is typically poorly understood. We need to cover a few topics to develop our understanding. Here goes.

The nerve of it

Nerves run throughout our body, from our brains down to our fingertips and toes. Signals can pass up and down.

From the top, we send signals down to our body to give instructions to muscles. This produces movement.

In return, sensory endings throughout our body collect information and pass it back up to our brains. Our brain then figures out what these signals mean. In some cases, they are painful, in others, not – this is nociception.

The rule of three

An important model to consider is that every signal upwards from your body to your brain passes through three nerves. Only three, and always three.

The first nerve runs from your fingers or toes all the way to your spinal cord. A second nerve relays the signal up your spinal cord to your thalamus, a relay station in the centre of your brain. The third nerve takes the signal from your thalamus to the relevant part of your sensory cortex so your body can recognise where the signal came from.

Just as an aside, these are three individual cells. Yes, cells are tiny things, invisible to the naked eye, but nerve cells are bizarre in that they have a tiny body and extremely long tails. These tails reach as far as from our low back to our toes. Mind-boggling.

For the purposes of understanding neuropathic pain, we are thinking about those first nerves, collecting information from your body and relaying it to your spinal cord. These nerves are known as peripheral, to contrast the central structures of the spinal cord and brain.

So, what happens when a peripheral nerve is damaged?

When nerves go wrong

Nerves get damaged in a mixture of ways. Most often, they are compressed, impeding the flow of signals. Think of an electrical cable that gets crushed or a hosepipe that is stood on. They can also be stretched, like two workers trying to pull a cable through a pipe from both ends at the same time. Last, they can also be irritated by chemicals, perhaps from our immune system – an autoimmune attack.

The most common site of nerve damage is around our spinal column, between our vertebrae. Our spinal column does a great job of protecting our spinal cord, but signals still need to pass out of the column to the rest of our body, and information needs to flow back in. Peripheral nerves start connected to the "second" nerves in the spinal cord but have to pass out of the spinal column to get to the rest of the body.

Therefore, between every pair of vertebrae, a “spinal nerve” branches off from the spinal cord and heads off to supply a part of the body. The point at which these spinal nerves exit the spinal column is the place where they are most likely to suffer an injury. The small apertures between the vertebrae where each nerve escapes are liable to become narrowed (stenotic) if our spinal joints overgrow, or if an intervertebral disc bulges or prolapses. There’s more to this process that we could cover here, but for now, let’s just satisfy ourselves knowing that this is the area where nerves are most often injured.

Besides injuries around the spinal column, there are other locations where nerves can get compressed or are prone to damage, and these are collectively known as peripheral injuries. Examples include carpal tunnel syndrome, where the median nerve is compressed in the congestion of our wrist, or trigeminal neuralgia, where a nerve that receives sensory information from our face has been damaged.

Two products of a damaged nerve – part 1 - PAIN

First, we should talk about pain since that’s what brought us here.

Pain from a damaged nerve is neuropathic pain. This type of pain is just a different beast from nociceptive pain. There is an element of damage that triggers most cases of neuropathic pain, similar to nociceptive pain, but with neuropathic pain, the damaged nerve keeps sending noxious signals, regardless of many of the factors that can amplify or ameliorate nociceptive pain. Simply put, neuropathic pain tends to be constant and unremitting. The damaged nerve just can’t stop producing signals that we interpret as pain.

It hurts when you wake up, it hurts an hour later, and it hurts throughout the day. It hurts when you sit, it hurts when you walk. It hurts when you bend forwards, and it hurts when you bend backwards. It hurts when you run, and it hurts when you stand still. It hurts in the evening, and it hurts when you go to bed. It hurts through the night, and into the next morning. Get the idea?

Can medications help?

Typically, this type of pain does not respond to painkillers. Why? Because painkillers work on the mechanism of nociceptive pain, so have very little impact on neuropathic pain. Anti-inflammatories also have little impact, since they work against inflammation, which is not typically the dominant issue with a damaged nerve.

The drugs that are designed to work on nerve pain are pregabalin and gabapentin, but they’re not reliably successful – some patients do well, but many don’t. For this reason, patients with persistent nerve pain often end up also taking medications like amitriptyline, and perhaps some very strong painkillers in the hope that they eventually “breakthrough” and offer some relief.

It all sounds pretty horrible, and that’s because it is. Established nerve pain, also known as neuralgia, is known as one of the worst patient experiences on offer. When it relates to a nerve as it exits the spine (where we said the nerves are most frequently damaged) it gets called a radiculopathy (radish = a root vegetable, radicular = from the root) and in these cases, the whole distribution of that nerve root is affected. Remember, it’s one nerve all the way from the spinal cord to the periphery. These pain distributions are recognised by manual therapists since we’re made to learn them.

For some clinical context, a patient with some back pain, that they occasionally feel as a dull ache into their leg when they sit still for too long does not have a likely picture of sciatica. In contrast, a patient who describes their leg as feeling like it’s permanently on fire from their backside down to their foot, who has had no relief from the pain for three weeks and barely slept since it began, is perhaps living with sciatica.

Is there any good news?

Contrary to what you might expect, one of the positives that patients can take from nerve pain is that exercising and being active, while painful, doesn’t necessarily make things worse. It’s not true in 100% of cases, but there is enough evidence to support patients in giving their preferred activities a go when they are suffering nerve injuries.

Two products of a damaged nerve – part 2 – dysfunction

We’re drifting a little away from our topic of pain here, but we can’t talk about nerves without discussing neuropathy – when a nerve is not functioning properly.

Nerve dysfunction can be positive or negative. By this, we don’t mean good or bad, but instead, we’re describing a nerve that’s either doing more than it should or less.

Let’s start with the motor side. If a nerve is mildly damaged, it often produces extra signals that trigger muscle activity. This is a positive fault – extra movement besides what we want or need. This makes muscles twitch or shake – the technical term is “fasciculation” since muscle units are called “fascicles”. Certain brain conditions also cause tremors, eg Parkinsonism or cerebellar dysfunction. These are a similar "positive" fault but start from further up the wiring diagram.

Negative problems with motor signals are simpler. Weakness and then paralysis. These can be serious, so in clinic, we test power if there is any suspicion of a nerve injury. Any presence of motor weakness is often a trigger for escalating a case to imaging, and perhaps a surgical opinion. If weakness is allowed to progress to paralysis, it is rarely reversed, hence the need for swift action in some cases. Again, the brain can be responsible for weakness and paralysis, for example with a stroke.

Let’s now look at sensory problems. The "positive" problem is excessive signalling from the periphery inwards. The technical term is “paraesthesia”, and this most commonly produces a tingling sensation, or pins and needles. You’ll remember from our article about nociception that there are also nerve fibres that report temperature and vibration, so we can sometimes also get incorrect sensations of heat and cold, or even of vibration. This is more to do with the spinal cord since there is some reorganisation of the different signals as the spinal nerves merge into the spinal cord. Too much for this discussion.

Last, we can be left with a "negative" sensory problem, which simply put, is numbness. A damaged nerve can begin by producing positive faults, pins and needles etc, but if the damage progresses, this leads to numbness. So again, it’s something to watch out for but perhaps takes a seat slightly behind motor weakness in the hierarchy of undesirables.

One last point about nerve problems

This is an interesting point, or at least to me and evident in clinic.

Patients can display a mixture of nerve pain and nerve dysfunction. There isn’t always a correlation. At the extreme ends we have:

• Patients with extreme nerve pain, but no dysfunction. They are in agony but have full power, full sensation, no pins and needles, and no twitching muscles.
• Other patients with nerve dysfunction, but no pain. They have numbness or weakness, but no pain. Or they might have twitching muscles and pins and needles, but no pain.

It is this spectrum of possibilities that serves to illustrate the function of nerves, and how they can also be pain producers.

To Recap

• Nociceptive pain is triggered by a noxious sensation, with an important contribution from our current and past experiences that can dampen the pain sensation, or even amplify it.
• Neuropathic pain is specifically the product of a damaged nerve, the apparatus that normally delivers nociceptive information. When such a nerve is damaged it can produce a constant pain that does not respond to any of the tools that can help with nociceptive pain. There is also a grey area of intermittent neuropathic pain, and there are also considerations to be made about whether the nerve is able to perform its functions.

Next time?

In our last lesson about pain, we will look at nociplastic pain. Yet another creature, but easier to understand once we have these basics under our belts.