First a huge and important disclaimer.  I am not in any way medically qualified, and do not pretend to be.  Everything I say is from a viewpoint of a person with 10 years-worth of sciatica suffering.    I have tried to write accurately but I cannot guarantee I have got everything right.  I’m very happy for physiotherapists, or other clinicians, to put me right on anything!

What causes sciatica, radicular pain and radiculopathy?”

In simple terms, sciatic pain and other sciatica symptoms are caused by one of the sciatic nerve roots becoming both compressed and inflamed.  Common causes for this are:

  • Herniated disc
  • Degenerative disc disease
  • Isthmic spondylolisthesis
  • Spinal stenosis.

My condition was primarily caused by a herniated disc and so that is the cause I will consider here.

A herniated disc is the most common cause of sciatica, radicular pain and radiculopathy.

Looking again at the diagram showing the spinal cord and the nerve roots in the thoracic part of the spine, you can see that the discs between the vertebrae are right next to the thecal sac (green column going down the middle of the picture housing the spinal cord) and the nerve roots.  This is also the case for the discs in the lumbar spine, with the exception that in the area of the lumbar spine we are interested in the thecal sac houses the cauda equina nerves and not the spinal cord.

The following series of diagrams will show how a disc prolapse (protrusion), disc herniation (extrusion) and disc sequestration affects the exiting nerve roots.   The diagrams are of a cross section across the vertebra and disc, and show only the exiting nerve root (this is for simplicity and understandability reasons).

It should be noted that at some levels of the lumbar spine, in particular at L5/S1, there may be another nerve root that has also left the thecal sac, and which therefore may also be affected by a disc prolapse, herniation or sequestration.  For example at L5/S1  the L5 nerve root will have left the thecal sac and be exiting at that level, but the S1 nerve root may also have left the thecal sac and be travelling down past the L5/S1 disc ready to exit out of S1/S2.  It is difficult to show this second nerve root on these cross sectional diagrams due to their direction of travel, and so I will attempt to illustrate this scenario later on.

The following diagram shows a cross section of a normal lumbar vertebra and spinal disc.  The spinal canal houses the thecal sac, which houses the cauda equina of nerves.  The parts that look a little like bull horns, are the exiting nerve roots.

In simple terms the disc is composed of two parts, a tough outer part and a softer inner part (called the nucleus).

Disc prolapse or protrusion

Unfortunately, sometimes a disc becomes degenerative or is injured and the nucleus of the disc starts to protrude through the outer layer and pushes on the nerve root, as shown below.  This compression of the nerve root may or may not be enough to cause sciatica symptoms.  This varies from person to person.

Unfortunately many people refer to a disc prolapse, or prolapsed disc, as a ‘slipped disc’, but this isn’t helpful terminology as the disc doesn’t move or slip, it actually changes shape as shown below.

Disc herniation or extrusion

If the nucleus breaks through the outer part and leaks out then this is called disc herniation, or extrusion.  The contents of the nucleus is extremely noxious and will inflame the nerve root it leaks onto.  Both the compression of the nerve and the noxious substance leaked onto it may cause sciatica symptoms.

Disc sequestration

Sometimes part of the nucleus breaks away and disc sequestration occurs. This happened in my case.

The following diagram looks in a little more detail at the processes involved.  You will see that the diagram is labelled showing the spinal cord, and so strictly speaking  this must be showing vertebra at the level of L1/L2 as that is where the spinal cord ends.  Lower down the lumbar spine the area marked in the diagram as spinal cord would be the thecal sac containing the nerves forming the cauda equina.

Finding accurate pictures I can use for this blog has been difficult, but perhaps the following diagram helps to illustrate how confusing the information in the public domain can be.   I’m sure I could be forgiven following looking at this diagram for thinking that the spinal cord runs through all the lumbar vertebrae.  Why wouldn’t I think that!  Nevertheless, the diagram does show a useful summary of the stages of disc herniation, and also serves the purpose of illustrating how confusing much of the information in the public domain can be.  I think it’s important to bear this in mind when seeking information, particularly from the Internet, about sciatica.

Whilst cleaning up this jigsaw piece I discovered I had another major misunderstanding.  I knew that the vertebral segment affected for me was my L5/S1 segment, and I also knew that it was my S1 nerve root that had been damaged.  As already explained the exiting nerve root takes the name of the vertebra above it in the lumbar region, and so the exiting nerve root for L5/S1 is the L5 nerve root, and not the S1 nerve root. This caused me great confusion, but it did lead me to discover that as well as the exiting nerve root being exposed to the disc (outside of the thecal sac) there could be another nerve root that has exited the thecal sac in advance of the lumbar segment that it will exit.

For example at L5/S1 it is common to find that as well as the L5 nerve root being exposed to a prolapsing/herniating disc, the S1 nerve root is as well.   The S1 root doesn’t exit at the L5/S1 segment, but is there ready to exit out of the sacrum (S1/S2).  I’m not sure people know why this happens, but I understand for many people it does.  This means that when the L5/S1 disc prolapses, herniates, sequestrates etc there are two nerve roots it might impact on, the L5 and S1 nerve root, not just the exiting nerve root (the L5 root).  There is the potential for both nerve roots to be affected by the prolapse/herniation, or just one.  This works in a similar way at other levels, although the possibility of having two nerve roots exposed to a disc is greater at the L5/S1 level.  In my case the disc herniated in such a way that my S1 nerve root was mainly affected.

I don’t think it is easy to illustrate the situation whereby a second nerve root is exposed to a prolapsing disc, but I hope the following diagram may give some idea.  In this diagram the lumbar segments are on top of one another, with the discs in between.

I had an MRI scan which revealed my herniated disc.  Interestingly I have discovered that it is not uncommon for an MRI to show a prolapsed, or even herniated disc, with the person concerned having no symptoms whatsoever.  MRI’s commonly show degenerative back changes.  This is entirely normal, especially for the slightly older population.  There is often nothing to worry about at all.

I’ve also discovered that sometimes a radiologists report for an MRI shows only a small disc protrusion, but the person concerned is suffering severe symptoms.  This was the case for me.

MRI results of a back are not always a good predictor of how a person may be affected.

I have had numerous MRI scans, and my experience is that it is important not to get over-concerned with the results of an MRI report on a back, but to discuss the results with your clinician.  A radiologist report can sometimes be a frightening thing for a patient to read, but as I learnt a lot of the degenerative changes in your back which are shown up on an MRI, and being written about in a radiologist’s report, are entirely normal.  This has been another important learning point for me.

So there we have it, this is my simple understanding of what causes sciatica, radicular pain and radiculopathy.

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