An updated red flag for a rare type of back pain
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A signature symptom of a rare, serious kind of back problem occurs just as often as it does in people who do not have that back problem. Which isn’t great for a red flag.
Can we do better? Something a little more specific? Probably, yes. Today I have a solid evidence-based update to a classic red flag.
Red flags are obviously useful for clinicians to know, but do patients really need to know about the symptoms of a rare back disaster? Probably not — but reassurance is actually good medicine for ordinary back pain, and just knowing that you don’t have a scary red flag symptom is helpful.
And, even if you do have that symptom, it’s also reassuring to know how often it doesn’t mean the scary thing.
Pain + weakness in both legs is the new best red flag for cauda equina syndrome
Pain in both legs, especially combined with weakness, is the new closest thing we have to a reliable independent factor predicting cauda equina syndrome (CES): insult to the horsetail (“equina”) of fine nerves at the bottom of the spinal cord, usually compression.
A not-so-reliable red flag is the main traditional one: numbness in the groin, buttock, and inner thighs (a “saddle” pattern). This red flag should probably be retired. Why? It is a CES symptom, but modern data (Angus 2021) shows that saddle numbness is 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 detectable compression (with MRI). Every last one of them. Most are equally common both with and without CES.
The only symptoms that are distinctly more likely with CES are pain and weakness in both legs. And one runner-up: difficulty starting to pee is also little more likely with actual CES. (And pros take note, dermatomal sensory loss is the only solid diagnostic sign.)
But in any case, many cases with CES-ish symptoms, if not most, turn out to be false alarms (see Fairbank 2011).
Another upgrade: testing anal grip strength isn’t helpful (yes, you read that right)
Doctors concerned about CES may do a digital rectal exam, assessing anal “grip strength,” because CES can cause plumbing malfunctions. But 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 really is. Rarity puts red flags in context. It is rare, but not crazy rare — most doctors will probably see a few cases in their careers. Not just one case. That appears to be a little myth we can bust.
Tom Jesson is writing a book about sciatica, and is writing a newsletter as he goes. In a recent post, Tom 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.” 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. Clinicians that help people with musculoskeletal health regularly “can expect to see about a dozen cases of CES.”
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.
Good to know. Big thanks to Tom — his post got me started on this, and he also gave me a heads up about Agnus et al., which became the bigger story.
For more information about red flags for back pain, see: When to Worry About Low Back Pain.