The enigmatic dorsal root ganglion

"A very special organ that resides in a very dangerous place..."

Thanks for reading the 16th edition of my sciatica newsletter. This newsletter tracks my research into all things lumbar radicular pain!

Here's a mystery. Why is the dorsal root ganglion located in a foramen that narrows as we age, near a disc that is prone to proinflammatory prolapse? As Quinn Hogan puts it in this lecture, it's "a very dangerous place" to put such an important structure…

Here's another mystery. As we know, when a nerve root is compressed it won’t cause pain unless it has already been inflamed. But when a dorsal root ganglion is compressed it will fire off a lot of nociceptive action potentials, even in a perfectly healthy state. What good does that do?

"The dorsal root ganglion is an odder beast than most of us realize..." - Marshall Devor

The DRG is where the cell bodies of the primary sensory nerves are housed. The ganglion itself is about the size and shape of a the nail on your little finger, or maybe a flattened pea. Unlike the schematic drawings we usually see, it is infinitesimally tiny in relation to the rest of the neuron:

Inside the DRG, there are tens of thousands of cell bodies. The cell bodies are situated off to one side of the axon at the end of a "T junction". This means that action potential can pass by them uninterrupted:

The DRG is protected by the nerve root cuff. But under the cuff it has a relatively flimsy capsule. It is also, unusually, outside the protective blood nerve barrier. All this means it is exposed to noxious stimuli like inflammatory cytokines and circulating chemicals (which is one reason people who have had chemotherapy are more likely to get peripheral neuropathy).

In this paper, Marshall Devor argues that the exposed location and relatively weak protection of the dorsal root ganglion is a feature, not a bug. When the DRG is exposed to the environment outside the blood-nerve barrier it can sense what's going on out there.

Apparently there are very few parts of the nervous system that are outside the blood-nerve barrier. One is the part of your brainstem that elicits a vomit reflex, called the chemoreceptor trigger zone. It can detect instantly when noxious substances are in the blood stream and trigger a protective mechanism: puking.

Devor argues that the DRG has a similar role. For example, if you are sick it can react to circulating cytokines in your blood stream and trigger the aching limbs that make you want to rest and recuperate.

This theory also helps explain why the DRG is able to generate its own action potentials. Although its jumpiness contributes to the misery of many people with radicular pain, it is part of an evolutionary trade-off: we gain a chemoreceptor and in return accept a greater susceptibility to painful neuropathies.

If Devor's theory is true, maybe we can think of the DRG as being like a scout sent out from an army's camp to surveil the area: although it’s vulnerable, it can gather important information…

(If this has sparked your interest, I recommend the papers by Hogan and Devor, which are both very readable! I've simplified Devor's argument so it's worth reading the whole thing.)

Bits and bobs

• About one minute in to this video there is a great prosection of a spinal cord. A good accompaniment to A Visual Tour of the Lumbar Nerve Roots.

• Our literature review was published last week! 🎉 I will summarise it in a future newsletter. Taking a deep and relatively systematic dive into the evidence base was a really interesting experience. It definitely shifted my understanding of clinical practice and evidence based practice in ways I am finding quite hard to articulate... I guess I'll have to write about that too!

• Here is a nice observation on bilateral sciatica from a 1948 paper on sciatica by Falconer: 

  

• Can anyone suggest a surgeon who would be up for talking to me on the podcast? I've contacted a few with no luck.

• I will be on "Chewing it Over" with Jack Chew on Thursday at 12:30 UK time if you fancy listening to me talk to Jack about research and social media (that's all I got for a brief, no idea what Chew is going to ask me!). You can watch on the Physio Matters facebook page.

Book talk 

As most of you will know, this newsletter is sort-of-tracking my research as I write a book about lumbar radicular pain. The project is going pretty well. But, naive as it sounds, I had under-estimated the amount of work it takes to write a book compared to, say, putting up an online course. When I started I thought it would be about 50% more work but in fact it is probably about 150% more work. Writing has to be drafted and re-drafted; because of copyright, pictures have to be drawn instead of copy-pasted; formatting and self-publishing is almost a project in itself. As a consequence, the task feels, emotionally, like a weight. Not a ball and chain, more like a weighted vest! On good days I relish this, but on bad days, it can feel like the end is a long way off.

One solution I have considered is to release the book in installments. For example, first I would work on an ebook on the pathomechanisms and put that out, followed by an ebook on assessment, followed by an ebook on management, and finally the actual paper copy of the book. The ebooks would cost less to reflect the fact that they are installments. Putting out ebooks "as I go", essentially creating four smaller projects, would allow me to get some emotional and financial payoffs to maintain momentum.

I'm interested in people's first impressions of this idea, particularly whether it seems unusual or off putting?

Incidentally I'm also planning to make some materials for patients.

While I'm writing, I also want to say how rewarding the newsletter has been so far! It goes out to 1,185 people now. I really appreciate it each time someone shares it to recommend it or replies to ask a question or add their thoughts. It helps me to know what resonates with people. So as always, do get in touch!

Til next time,

Tom

A sunny day in Buffalo Bayou park last week