An updated red flag for a rare type of back pain

Cauda equina syndrome (CES) is caused by compression of the horsetail-looking end of the spinal cord. It is a fairly rare kind of back problem, and sometimes very serious. A traditional CES signature symptom, a classic red flag for the condition, occurs just as often as it does in people who do not have that back problem … and that isn’t great look for a red flag. Ideally, red flags don’t represent quite so many false alarms.

Can we do better? Something a little more specific, more reliable? Probably, yes! Today I have a solid evidence-based update to CES red flags, plus interesting digressions about anus strength (seriously), and whether CES is so rare that it’s a “once in a career” spinal malfunction? (This is a re-run of a late 2021 post, with some modest improvements, and an audio version it didn’t have the first time.)

Red flags are obviously important for clinicians to know, but do patients really need to know this stuff? Mostly not — but reassurance is medicine for ordinary back pain. Just knowing that you don’t have a scary red flag symptom is helpful. This one or any of the others.

And, if you do have one of the red flags, it’s nice to know that they don’t always mean the scary thing — especially the old “classic” one, which turns out to be not so classic.

Pain + weakness in both legs is the new best red flag for cauda equina syndrome

Pain in both legs, especially combined with weakness, is now the closest thing we have to a reliable independent factor predicting cauda equina syndrome. CES isn’t always serious, but it can be, and it’s good to have good red flags for it.

The traditional red flag is not so reliable: numbness in the groin, buttock, and inner thighs (a “saddle” pattern). It certainly sounds like a weird, distinctive symptom that can only mean one thing … but it’s not. That one should probably be retired. Why? It is a CES symptom, but modern data (Angus 2021) shows that saddle numbness is very nearly as common in people without CES! Of course it’s a concern; it’s just no longer the best available red flag.

All CES symptoms are like this to some degree: they all occur in many people who have no MRI-findable compression. Every last one of them. Most are roughly equally common both with and without CES!

The only symptoms that are distinctly more likely with CES are pain and weakness in both legs. But even that symptom doesn’t always mean CES. It just means it more often the other symptoms.

And there is also one runner-up red flag: difficulty starting to urinate is also little more likely with actual CES.

Finally, one for the professionals (because it’s too tricky for self-assessment): dermatomal sensory loss is the only decent diagnostic sign.

If you’ve got all of the above — weak painful legs, slow-start pee, and dermatomal numbness — CES is getting very likely indeed. However, even a collection of the reddest flags can still be false alarms (see Fairbank 2011). The bottom line is that CES-ish symptoms have other causes.

From Tom Jesson’s superb primer on cauda equina syndrome: “So, uh, what is CES anyway?”

Another upgrade: testing anal grip strength isn’t helpful (yes, you read that right)

CES can cause plumbing malfunctions, so doctors concerned about CES may do a digital rectal exam, assessing anal grip strength — testing how hard the anal sphincter can clench. Fortunately, it turns out that this is probably not a reliable indicator of CES. Angus et al. on anuses:

“We found no relationship between digital rectal examination findings and the diagnosis of CES.”

Not that there’s anything wrong with a digital rectal exam when it’s called for! But I think we can all agree it’s nice to skip that when we can. Everybody wins.

How rare is cauda equina syndrome? “One case in a doctor’s career”?

As long as we’re on the topic of CES red flags, it would be nice to know how rare this condition truly is. Rarity puts red flags in context.

Tom Jesson went data-diving for the origins of the widespread claim that cauda equina syndrome is so rare that a family doctor will see only “one case in their career.” [Tom's post is no longer available.] Fact or fiction? Predictably, he found little to support the claim. It is, “with many degrees of separation, based on a Slovenian paper that probably under-estimates the incidence of CES.” So it’s a zombie stat, cited and repeated long after it should have died.

There is not much hard data, but what he found — mostly reviewed by Hoeritzauer 2020 — suggests that CES is indeed rare, but probably nowhere near that rare. A 2022 Scottish paper by Woodfield et al reported an incidence of 1.1 per 100,000, “at least four times higher than previous European estimates.”

And so physical medicine professionals can expect to see more like a dozen cases of CES in their working lives. Tom again:

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.

So the once-in-a-career thing appears to be a little myth we can bust. Good to know.

For more back pain red flag fun, see: When to Worry About Low Back Pain.