Terminology time

In the last blog post - So, Uh, What is CES, Anyway? - we looked at bilateral sciatica, a red flag for CES. I also mentioned the words radicular pain and radiculopathy. But the terminology doesn't end there! When considering bilateral sciatica, you need to also hold in mind that your patient might actually have bilateral referred pain.

So there's four words - sciatica, radicular pain, radiculopathy and referred pain - that are all easily confused. In this blog post, I'll explain what each one means.

Referred pain

Referred pain is felt in a part of the body remote to that of the original injury. The original definition, from the International Association for the Study of Pain, is "pain perceived as arising or occurring in a region of the body innervated by nerves or branches of nerves other than those that innervate the actual source of pain".

Why does this happen? The standard theory is that when danger messages (nociceptive signals) from an injury arrive in the spinal cord, they are mixed up with normal messages from other parts of the body. Those danger messages and normal messages are passed up the spinal cord to the brain together. The brain is unable to tell the two apart and creates a pain experience for both.

That's the standard theory, and it's perhaps a little over-simplified, but it gets at the basic idea of referred pain: the brain is 'confused' about the exact location of the problem.

There are two types of referred pain.

First, visceral referred pain is caused by danger messages from internal organs like lungs, intestines, and kidneys. An example you'll be familiar with: danger messages from the spleen can be felt in the shoulder, and danger messages from the heart can be felt in the left arm.

Second, somatic referred pain is caused by danger messages from somatic tissues like bones, cartilage and muscles. For example, danger messages from an intervertebral disc or a facet joint can be felt in the buttock and down the leg:

Image: Adapted from a 1941 experiment, in this picture you can see some examples of patterns of referred pain from the spine (Kellgren, 1941). 

All illustrations are from our forthcoming book on sciatica.

Referred pain is dull, aching, gnawing, often deep and difficult to localise. 

Radicular pain

Radicular pain is a kind of nerve pain. It's caused by action potentials that emanate from the nerve root and/or the dorsal root ganglion (Bogduk, 2009). This, of course, is not where action potentials are supposed to come from. Ordinarily, they should start in nerve endings in target tissues such as skin, bone, muscle and so on. 

We call action potentials that emanate from the nerve root ‘ectopic impulses’ (ectopic means ‘in the wrong place’). 

Image: some of the nerve roots and dorsal root ganglia of the lumbar spine. Action potentials are supposed to be passed up and down these roots; they're not supposed to spark from them. When they do, this can cause radicular pain.

How does radicular pain feel? Classic radicular pain roughly tracks the territory of the affected nerve root. The pain is sharp, shooting, stabbing and usually severe. There's often an accompanying dull or burning background ache, as well as ‘nervey’ sensations like pins and needles and tingling. 

Image: classic radicular pain is usually described as a dermatomal band of pain (Left) or as a 'line' of pain fown the back of the leg (Right; pic adapted from Bogduk, 2009.)

Radiculopathy

Radiculopathy is another nerve problem. However, it is not, technically, a pain condition. The term describes loss of nerve function. This just means that fewer action potentials are conducted up and down the affected nerve because of an injury to the nerve root or its ganglion. In other words, the nerve can't do its job.

A loss of nerve function is a pretty common everyday experience. If you sit too long and your leg goes numb, that's a loss of nerve function. A radiculopathy is not too different, although of course it involves the nerve root in the spine and not the nerve trunk in your bum, and often involves more lasting damage to the nerve too. 

Radiculopathy manifests as a dulled or absent reflex response, a loss of sensation to different sensory stimuli (e.g., touch, sharp prick, warm/cold), and/or a loss of muscle strength.

All three can all overlap

Of course, referred pain, radicular pain and radiculopathy can all occur together. 

First, although radicular pain and radiculopathy can occur separately, they often co-exist as a 'painful radiculopathy'. This makes sense, of course - they both involve a problem with a nerve root, so it's not surprising that such a problem can cause both pain and loss of function. Sometimes the radicular pain is serious and the radiculopathy is mild, and sometimes it's the other way round - and everything in the middle. 

Image: Schematic representation of a painful radiculopathy. The green line shows normal impulses from the periphery being blocked or slowed at an injured nerve root. They will not (all) get to the brain (i.e. a radiculopathy). The red line shows aberrant impulses emanating from the injured nerve root, and these do get to the brain (i.e. radicular pain).

And referred pain can also join the party. If you think about it, this makes sense. Imagine a big disc herniation that injures a nerve root and causes radicular pain. That disc herniation and all the associated inflammation might easily also trigger nociceptive signals from the disc. That nociception might cause referred pain in the buttock or down the leg.

So where there is radicular pain there can also be some somatic referred pain.

These mixed pain presentations partly explain why so many cases of radicular pain do not look like they are 'supposed' to look: it’s often not only radicular pain, but radicular and referred pain.

Radicular pain doesn't always look how it does in textbooks

To add to this jumble, radicular pain itself very often deviates from the ‘classic’ picture (shown above) of a band or line of pain that roughly tracks the territory of the affected nerve root. Numerous studies show that radicular pain very often doesn't obey the textbooks' dermatomes; in fact, it’s near-impossible to tell from the pain pattern what nerve root is causing pain (Albert et al., 2019; Furman and Johnson, 2019; Taylor et al., 2013). And, radicular pain can expand beyond the narrow band or line we might expect to find it in (Murphy, 2019), possibly because of sensitisation at the ganglion and spinal cord.

It's still good to have the classic idea of radicular pain in your head - whether it's a straight line or a dermatomal strip. Yes, radicular pain can present atypically (even showing up in patches sometimes, in the buttock or shin or calf...) but the classic picture is a good reference point for building some pattern recognition.  

Image: some various kinds of nerve root pain. Don't get too caught up in the weirdness - the classic picture is a classic picture for a reason. But it's good to know about the variation around it.

Sciatica

Finally, what about “sciatica”?

The word "sciatica" is less a diagnosis and more of a vague gesture -  "there's pain in the back of the leg... for some reason". It doesn't really have any official definition and different people mean different things by it. It's a throwback to a time when we had much less medical knowledge. The eminent spinal surgeon Jeremy Fairbank called sciatica “an archaic term”.

That said, sciatica is a word that patients and other laypeople recognise, which is important. And it's a useful word for clinicians who want to refer to everything in this article without having to say 'referred pain or radicular pain or radiculopathy or some mixture of the three'! It's kind of a catch-all term, in that respect. I use it all the time!

So, whereas the scientific community sometimes discourages the use of the term ‘sciatica’, it has its uses. It's just probably best left out of clinical notes. 

I think that's a good place to leave it for today. Soon, I'll write about how we directly apply this knowledge when we're in clinic and thinking about CES.

Key points:

• Referred pain is when pain from tissues like muscles, joints and discs is felt in the wrong place. It’s usually a diffuse ache. 

• Radicular pain is when pain from the nerve root in the spine is felt in the territory of that root. In the case of lumbar radicular pain, that’s down the leg. Radicular pain is usually sharp and severe.

• Radiculopathy is when an injury to the nerve root stops it from conducting impulses to and from the brain. This makes muscles weaker and sensation duller

• Because all of these things can exist together, in different amounts, the clinical picture is often far from clear! Additionally, radicular pain itself has a varied presentation, not always appearing in the expected dermatome. 

• Sciatica is an old-fashioned term without any specific meaning. Despite this, it is a useful and easy way to refer to pain down the back of the leg that seems to be related to a nerve.

All illustrations are from our book, Understanding Sciatica.