How rare is cauda equina syndrome?

"A GP will see only one case of CES in their career..."

Thanks for reading the 42nd edition of my newsletter. This newsletter tracks my research as I write a book about lumbar radicular pain… and another one about cauda equina syndrome!

This is a bit of a rambling post that I hope will be diverting to interested readers. If you’re short on time, skip to the end where there is a ‘too long; didn’t read’ in bold.


Down the rabbit hole

"Cauda equina syndrome is so rare that a GP will see only one case in their career"

I have heard this factoid many times before. A quick twitter poll found that about 40% of respondents had heard it, too.

In the course of researching our book on CES I was surprised to find that it’s also repeated, mostly uncritically, numerous times in the CES literature.

Is the GP factoid accurate? Is it based on any actual research?

It’s important to know because, although no one disputes that CES is rare, the GP factoid implies that it’s really, really rare. If the base rate is as low as once in a career, it’s hard to argue that CES is something clincians should personally spend a lot of time worrying about.

On the other hand, if the GP factoid is underestimates the rate of CES, then it might be lulling clinicians into a false sense of security. “Kind of seems like this patient has CES, but it won’t be, right?”

To find out where the GP factoid came from, I had to follow the reference trail…

I started in the place I had seen the GP factoid most recently, which happened to be Greenhalgh and colleagues' paper Assessment and Management of Cauda Equina Syndrome. The authors repeat the claim that "a GP will see one CES patient in their entire career" and attribute it to an editorial by Underwood from 2009 (to be clear, Greenhalgh and colleagues aren't citing the factoid uncritically, and also cast some doubt on it).

I looked up Underwood's editorial. In it, I noticed a bit of Chinese whispers starting already (for US readers: Chinese whispers is what we Brits call ‘telephone’…). Underwood does not claim that a GP will see one case of CES in their career, but that they are likely to see no cases at all:

But as you can see, this is not the original source of the GP factoid. To make his claim, Underwood cites a 2009 article in the BMJ by Lavy and colleagues.

In that article, Lavy and colleagues claim that "most UK general practitioners are unlikely to see even one true case [of CES] caused by intervertebral disc herniation in their entire career". So again we have another bit of Chinese whispers here, as Lavy and colleagues are talking about CES caused specifically by a disc herniation (as opposed to by, say, metastases or trauma).

But this paper is still not the end of the reference trail. To make thier claim, Lavy and colleagues cite a Slovenian study which found that CES caused by a disc herniation occurs at a rate of 1.8 per million. According to Lavy and colleagues, "if these figures are even approximately correct", CES is so rare that a GP will likely never see it in their career.

So I read the Slovenian study, which was published by Podnar in 2006. I had a hard time working out exactly what it was saying. Here is my best understanding. 

Over an eight year period, Podnar observed two Slovenian rehab centers to identify people with CES. In that time, he found 67 (and a further 20 had conus medullaris syndrome).

Now Podnar makes quite a claim: he says that the 67 cases he observed at the two rehab centers represent every case of CES in Slovenia in that time period.

His logic is that in Slovenia, CES is diagnosed by lower-limb EMG and electrodiagnostics, and these tests are only available in two places in Slovenia, which were the two places he was observing for eight years. Therefore, everyone with CES in Slovenia must have come through these two places and been counted in this study.

Next, Podnar divides the cases of CES he saw across the whole population of Slovenia (about 2 million) to conclude that "the average annual incidence rate of cauda equina lesions was 3.4 per million". Of those, just 1.8 per million per year were caused by a disc herniation.

And that is the end of the reference trail for the factoid that "A GP will see one case of CES in their career". I did not think we would end up in Slovenia.

So how common is CES really?

The GP factoid is not really based on great data. I think the methods of the Slovenian study were liable to undercount the rate of CES. And indeed, 1.8 cases of discogenic CES per million per year does seem too low. At that rate, we would expect to see less than one discogenic CES every two years in the entire population of my home town of Newcastle upon Tyne.1

So are there any better, more useful ways to think about the incidence rate of CES? 

Let’s try to work it out.

For a start, and with all due respect to the people of Slovenia, I'm not sure that the population level incidence of Adriatic nations the best way to tell. Much more useful would be the incidence of CES amongst people seeking care. What you want to know isn’t how likely it is that someone in the general population will get CES, but how likely it is that the average back pain patient seeking care is going to turn out to have it.

The International Framework for Red Flags for Serious Spinal Pathologies has a number for this. The authors state that "Point prevalence of CES as a cause of low back pain is estimated at 0.04% in primary care". That is one in 2,500 people.

Unfortunately, this too is a zombie statistic. The citation is of a 1992 paper by Deyo and colleagues. That paper is a summary paper, not a research study that collected data. Instead, it includes some back-of-the-napkin arithmetic to say that if one to two per cent of all disc herniations cause CES (itself a dubious claim) then "its prevalence among all patients with low back pain is about 0.0004%.

I'm not sure how this becomes 0.04% in the IFOMPT document, maybe I'm missing something technical about how prevalence works (feel free to let me know). Either way, the statistic is not directly based on any data.

Fortunately, in a recent systematic review, Hoeritzauer and colleagues sought to better answer this question. They concluded that in primary care, "CES occurred in 0.08% of those with low-back pain". That is 1 in 1,250 people. 

Unfortunately, this conclusion is based on only one study, in which Henschke and colleagues found that of 1,172 people with acute low back pain who went to see a physio, chiropractor or GP in Australia, only one of them turned out to have CES.2

It's hard to know what to make of such a tiny number. At the risk of stating the obvious, had that one patient gone to the emergency department instead or primary care, there would have been zero CES patients in the whole study. On the other hand, had just one other CES patient attended primary care, the number of acute low back pain patients with CES would be 0.16%, or 1 in 625 people.

It's a shame that Hoeritzauer and colleagues’s review didn’t find more studies to help us get an idea of this number.

Hoeritzauer and colleagues did, however, find four studies on how common CES is amongst people with low back pain attending secondary, rather than primary care. Three of these studies looked at emergency departments, two in Canada, one in Finland, and one study looked at a spinal surgeon's caseload in the US. Meta-analysing these studies, Hoeritzauer and colleagues' concluded that the prevalence of CES amongst people with nontraumatic low back pain in these secondary care settings was .27. That is 1 in 370.

To summarise, we have two estimates for the prevalence of CES for people with back pain who seek care: 1 in 1,250 in primary care (specifically for acute low back pain, with a lot of uncertainty around that number) and 1 in 370 in secondary care (mostly in emergency departments).

What does that mean for a clinician’s career?

Let's map this on to the factoid that "GPs see one case of CES in their career". Is it accurate?

First of all, how long is a GP's career? Let's say it lasts 35 years at 5 days a week for 46 weeks a year. That's 8,050 days. Let's round that down to 7,000 days to account for admin, training, sickness, holidays etc.

For the sake of argument, let's take the 0.08% number from Henschke to be accurate. That means that at 7,000 days, in order for a GP to expect to see one case of CES in their career (as the factoid states), they would need to be seeing acute low back pains at a rate of 1 every 6 working days. That doesn’t seem like many to me, I think GPs see more.

More realistic is that GPs see acute low back pain at a rate of about 1 per day. In which case, per Henschke's stat, they can expect to see 6 cases of CES in their career.

Most people reading this are not GPs though! Most readers are clinicians who see MSK stuff exclusively. Let's work out how many they can expect to see in a career.

Let's assume an MSK clinician works a 40 year career (less training than GPs), 5 days a week for 46 weeks per year. That's 9,200 days. Let's round down to 8,000 to account for all the other stuff. How many cases of acute low back pain does the average MSK clinician see? Let's say two per day. That makes 16,000 in a career. That means, per Henschke's 0.08% figure, the MSK clinician can expect to see 13 cases of CES in a 40 year career (or about one every three years).

(Of course, this all goes back to the number from Henschke’s paper which could have been very different had one more or one less patient popped up in their study. So all my sums are a bit B.S. really. But the point is to get a rough idea of what to expect.)

Now, 13 cases of CES in an MSK clinician’s career might be an over-estimate because most MSK clinicians' careers don't consist of regular clinical work until they retire; they do other things. Also, many MSK clinicians don't work as a first point of contact exactly, but get referrals from GPs, orthopods etc., which would filter out some cases of CES. 

On the other hand, 13 cases in a career might be an under-estimate. As their career progresses, many clinicians move from primary care to see patients who are more complex, or have more severe back pain, in which case they are likely to see a higher rate of CES. Many clinicians also work some days on telephone or paper triage where they see many times more cases of acute back pain than they would in an ordinary rehab clinic. It's also worth noting that Henschke's paper tracked only acute low back pain, but as far as I know people with chronic low back pain are no less likely to get CES--how many of them were missed?

Either way, and again making the big assumption that Henschke's 0.08% statistic is accurate, it's easy for you personally to work out how likely you are to see a CES, say, in the next twelve months. Count how many acute low back pains you saw last week. Multiply by 46 weeks, or whatever you work a year. Take away a chunk for sickness, training etc. Now divide that number by 100 and multiply it by 0.08.

Tl;dr

So what have we learned from this ramble?

  1. The factoid that “A GP will see only one case of CES in their career" is, with many degrees of separation, based on a Slovenian paper that probably under-estimates the incidence of CES. 

  2. We don’t have great data on the incidence of CES in clinical settings but according to a recent review, it’s 1 in 1,250 people with acute low back pain in primary care and 1 in 370 people with low back pain in secondary care.

  3. Most GPs, if they work clinically until retirement and see a steady number of acute low back pain patients, can expect to see a handful of cases of CES (back of the napkin maths says 6 in 35 years of practice).

  4. Most MSK clinicians, of course, see more CES than GPs. Assuming they work clinically, in primary care, until retirement, they can expect to see about a dozen cases of CES (back of the napkin maths says 13 in 40 years of practice, or about one every three years).

  5. Clinicians who mostly see patients referred from e.g. GPs can expect to see less. Clinicians who work in specialist roles that see complex cases or lots of spinal cases, and clinicians that do a lot of telephone or paper triage, might see more.

  6. All this is based on not great data.

  7. It's important to think through what "rare" means, because rare can either mean "so rare you can practically forget about it" or "rare but there; and you will see it—more than once!". CES is the latter.

Do you have any better guesses about how to estimate the rate of CES in clinic? Any good studies or bright ideas? I’d love to hear them. Just reply to this email.

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Other bits and bobs

It’s been a while since I did a link roundup!

Til next time,

Tom

1

Arrigo and colleagues identified 397 cases per year of surgically-treated discogenic CES in the state of California, which is over 5x the rate identified by Podnar despite also almost certainly being an under-estimate.

2

I am not totally sure why Hoeritzauer represent this rate as 0.08% instead of 0.09%, which is what the original number of 1 in 1,172 people works out as. Maybe I’m missing something. The authors of the original paper, Henschke and colleagues, in fact round it up from 0.09% to 0.1%. Anyway, I will be conservative and just go with Hoeritzauer’s 0.08%.