spinal nerve root query

Dear everyone,

I am sorry to bother you all with what may seem like a perfectly obvious answer but I am getting a little confused!!! it is regarding an anatomical query...... I don't understand why a compression (caused lets say by a disc herniation) of a nerve root between L3/L4 would cause compression to L4 (and not L3) given that after C7 all the spinal nerves exit beneath their respective vertebra? Similarly I am perplexed regarding the nerve root for L4/L5 being L5 (and not L4) and L5/S1 being S1 (and not L5). Does anyone know the answer why this is the case and maybe have any diagrams to clearly show which nerve roots are being impinged and why?

many thanks!

Emily

Hi Emily

This isn't a silly or obvious question at all. The key is the concept of the exiting nerve root vs the traversing nerve root. If you have access to Gray's Anatomy 39th edition this is explained and illustrated very well (page 758).

Failing that have a look at this article:

Lumbar and Cervical Radiculopathy

Lets take your example of a disc herniation at L3/L4. the exiting nerve root is the L3 nerve root. It exits the spinal canal just cranially and laterally to the L3/L4 disc so often escapes trespass. The traversing nerve root is the next one down, in this case the L4 nerve root and it passes immediately posterior to the the the L3/4 intervertebral disc where it has little room to move in a prolapse, and it is the one that is most often impinged. So in the lumbar spine it usually (but not always) is the traversing nerve root which exits in the next vertebrae caudal to the intervertebral disc and is in the direct firing line of a posterolateral prolapse

Hi emily and gcoe!
I came across the same information about a week ago, during reading Hoppenfeld's Orthopaedic Neurology, chapter 2, page 66. It's about the same explanation as gcoe gave but I'll add my comments.
Remember that in the lumbar spine, most of the nerve roots travel a lot down across the canal and do not exit it at the same level of origin from the cauda equina (the last reaching the L1-L2 level and then it's ONLY the roots themselves in the canal, not like at the rest of the spine where the roots travel down just a little ALONG WITH THE CORD and then exit it through the intervertebral foraminae). As they travel down, the roots pass through different spinal levels, with the lower roots passing more than the higher ones (e.g. L5 root passes through L1-L2, L2-L3, L3-L4 and L4-L5 while L3 only through the first 2). So, in the case of a herniated disc at the lumbar spine, it is obvious that many roots should be irritated and the higher the level of hernia the more roots would be. Anatomically, according to Hoppenfeld's book, the site of greater pressure is at the root just one level down. For example, in an L4-L5 herniated disc, the L4, L5, S1 etc roots pass posterior to the disc and one could guess that all of them should be irritated. In fact, due to anatomical structure, it's the second root (L5 in this example) that is most irritated because that's where the disc will find less pressure and move towards, impinging the root that is located there. I would add that because of limited space in the canal and that the roots are close to each other (we’re not talking about cm but about mm), the FIRST root to be irritated would be L5 in our example. If the problem gets worst, though, then other structures in the area, that is the other roots too, will be irritated. That’s why symptoms do not always depict malfunction at one spine level, in spite the fact that an MRI would demonstrate a hernia only at, let’s say, L2-L3. This must be because not only L3 root is irritated but possibly L2 or L4 or even L5, in different degrees of course.
To conclude, try reading this page from Hoppenfeld’s book together with gcoe’s article and it will become obvious.
Looking forward to other comments too!
ilias

Dear Gcoe and Iastolos,

thank you so much for your replies - everything makes a lot more sense now!!!
many thanks

Emily