Thanks for reading the 18th edition of my sciatica newsletter. This newsletter tracks my research into all things lumbar radicular pain!
Two newsletters ago we looked at the unusual role and location of the dorsal root ganglion. Continuing with the theme of "things that are pretty weird when you think about it", let's look this week at how your body has never seen the inside of your intervertebral discs, and kind of freaks out when it does.
Is disc herniation pain an autoimmune reaction?
The interior of the human intervertebral disc is cut off from the "outside world" of the rest of the body. Except for the outer few millimeters, the disc has no nerve supply and no blood supply. Nerves and blood vessels cannot infiltrate through the tightly bound, high-pressure fortress of the annulus and the cartilaginous end plates. And for added protection, this fortress is patrolled by cell-destroying molecules.
This makes the interior of the disc one of a select few sites in the body that have "immune privilege", along with the insides of eyeballs, the testes and the placenta and foetus.
If the annulus cracks or the disc herniates, the fortress maintaining this immune privilege is breached. This is the first time in a person's lifetime that their disc material is presented to their immune cells. Those immune cells (T cells), unable to differentiate this foreign substance from a pathogen or toxin, initiate an inflammatory reaction, recruiting other immune cells (lymphocytes, macrophages) to the area.
The autoimmune idea was first proposed in the 1960s. It was supported early on by an experiment in which nuclear material from rabbits was applied to the poor animals' ears, which caused inflammation and increased vascularity that looked like an autoimmune reaction. Other similar animal experiments followed. Marshall and colleagues, for example, implanted human disc material into the excised and preserved lung of a guinea pig, and observed bronchoconstriction and edema. Then they did the same to guinea pig intestines, which contracted repeatedly. Maybe I should have saved this topic for Halloween?
This autoimmune reaction probably explains some but not all of the clinical picture of radicular pain from a herniated disc.
It partly explains why this pain can go on for so much longer than normal tissue healing times. And, given that autoimmune reactions recruit lots of macrophages (eating-up cells), it partly explains why discs usually reduce in size over time. Extruded herniations that break out past the posterior longitudinal ligament contain more markers of an autoimmune reaction and also reduce in size more, so that fits the picture.
But there is an argument that the autoimmune reaction is not that important. Jonsson and Olmarker point out that an autoimmune reaction would take time to build up, but disc material seems to cause problems to neural tissue very quickly. And, they say, while it's true that the immune system might not have seen disc material before (so it doesn't have "peripheral tolerance"), it has seen the person's genetic signature ("central tolerance"), which should prevent an autoimmune reaction.
So what else might explain the pain of a herniation? As well as an autoimmune reaction, we know that pro-inflammatory enzymes and proteins lurk inside the disc itself, which probably cause pain and damage when they are released by a herniation... But that’s a topic for another time!
Other bits and bobs
You might remember that the American surgeons Mixter and Barr were the first people to confirm that disc herniation is a cause of radicular pain, in the 1930s. I found a paper by two Vermont doctors who happened to meet Mixter and Barr’s first ever patient with a herniated disc in 1985, fifty years later - and he was doing well! Mixter confirmed it was the right person, saying “He is the man who started all the damn trouble…”
My mate Adam Dobson in Boro has made a great sciatica leaflet. It’s open to share with anyone. (If you are in the NHS I think it is okay to rebrand it for your Trust, too - contact Adam.)
I am really focusing on the “what is radicular pain” part of the book at the moment so I think the newsletters will be on that topic for the next few weeks….
We were in Brazos State Park, about an hour outside of Houston, this week to celebrate our first wedding anniversary (that’s how we celebrate). Texas is a lot greener and more vibrant than I thought. We saw a beautiful murmuration of starlings (video). And I saw alligators in the wild for the first time... 🐊🐊
Til next time,
Tom