Ok, we’ve made it through our discussion of various common neck problems and we will now move our discussion on to pathology of the lumbar spine (a.k.a. the low back.) The first lumbar spine problem we’ll discuss is lumbar stenosis which is just a fancy medical term for narrowing of the diameter of the spinal canal. Stenosis can occur at any level in the spine (i.e. in the neck stenosis can cause radicular pain or myelopathy) but in this post we’ll focus on stenosis of the lumbar spine where it is most common (especially in the retired patients who live here in the coastal community where I practice.) Finally, stenosis can be central or foraminal. In this post I will focus primarily on central stenosis and I will discuss foraminal stenosis in an upcoming post.
Several times a day in clinic a new patient will present saying that she can only walk a certain distance before she develops severe burning pain in her buttocks and legs. When she stops and sits down to rest the pain will subside and then she can start walking again. Down here in eastern North Carolina the patients usually rates themselves by which aisle they can reach in Walmart before they develop pain. This activity-related pain is called neurogenic claudication and any patient who’s ever had it will tell you that it can be crippling. I won’t bore you with the specifics of the microanatomy of the blood supply of the nerves of the cauda equina (the bundle of nerve roots at the terminus of the spine after the spinal cord ends at around T12/L1, see figure 1.) Basically what happens, though, is the patient develops narrowing of the spinal canal because of a variety of degenerative factors (see below.) This narrowing causes worsening compression of the nerve roots which in turn causes impaired circulation in the microscopic (read: very sensitive to compression) blood vessels that supply the nerve. This impaired circulation is more apparent during exertion as the starved nerves demand more oxygen and nutrients, hence the searing pain when the patient walks for any distance.
Figure 1: Normal sagittal (side view) MRI of lumbar spine on left and sagittal MRI indicating severe stenosis. The intervertebral disc (IVD), ligamentum flavum (LF), facet joint (FJ) and cauda equina (CE) are all marked on the normal MRI. The MRI on the right demonstrates the causes of lumbar stenosis: bulging of the IVD (pink arrow) and ligamentous and facet hypertrophy (yellow arrow). Clumping of the nerves of the cauda equina (blue arrow) indicates a particularly severe case.
Figure 2: Normal axial MRI of lumbar spine on left and MRI demonstrating severe stenosis on right. Intervertebral disc (IVD), facet joint (FJ) and ligamentum flavum (LF) are labeled in red on the normal MRI. The diameter of the spinal canal is indicated in yellow. Notice on the image on the left how the spinal canal is widely patent (the nerves of the cauda equina are visible within.) The image on the right demonstrates severe narrowing of the diameter of the spinal canal caused primarily by facet and ligamentous hypertrophy.
So why does the narrowing of the spinal canal occur? In order to understand this, it’s useful to think of the spinal canal as having four distinct sides (see figure 2, left): in the front of the canal is the intervertebral disc (IVD), the sides of the canal are formed by the facet joints and the back of the canal is formed by the ligamentum flavum. Stenosis can be caused by an acute change in one of these structures (like a large herniated disc which we’ll discuss in a future post.) More often, though, stenosis is causes by gradual degeneration of all of these structures in unison. First, as the IVD dries out and degenerates it collapses and bulges. As the disc loses its effectiveness as a shock absorber the facet joints then have to absorb more motion. The facet joint is just like any other joint in the body in that with enough wear and tear it will develop arthritis and become overgrown with bone spurs. The morphology of the facet joint becomes less compressive as the patient bends forward which is why patients with neurogenic claudication will tell me that they feel better when shopping for groceries when they can bend forward over the shopping cart (or buggy if you’re from The South.) Finally, we see ligamentous hypertrophy in which the ligamentum flavum thickens. This thickening is due to a combination of buckling from disc space height loss and fibrosis, or scarring. Slowly but surely as the patient ages, these factors contribute collectively to the development of lumbar stenosis.
The degenerative changes that cause severe stenosis such as that seen on the MRI in the figures take years to develop. Stenosis is a structural problem that requires a structural solution. While physical therapy and core strengthening are excellent for the overall health of your back, no amount of therapy is going to reverse stenosis. Epidural steroids may provide excellent relief of the pain associated with stenosis. Unfortunately this relief is usually only temporary as injections don’t address the underlying structural problem of stenosis. The best treatment for lumbar stenosis is surgery. This surgery is often done on an outpatient basis and has perhaps the best outcomes of any procedure that I perform. In the next post we’ll discuss the surgical treatment of lumbar stenosis: lumbar laminectomy.
Thanks for reading!
J. Alex Thomas, M.D.