Stenosis: Foraminal vs Central

What Are the Two Kinds of Spinal Stenosis?

Stenosis is a general term that means, “narrowing”. There is no suggestion from the word about what is causing the narrowing, it just means there is narrowing and it’s up to us to figure out why.

There are two sets of tunnels that occur throughout the spine to provide safe passage for the spinal cord and spinal nerves that branch off from the spinal cord. The first tunnel is a hollow tube of bone extending from the base of the skull to the tailbone. This hollow tube is referred to as the “central canal” or “spinal canal”. It is the largest tube or tunnel and provides safe passage for the spinal cord. Of course there are certain things that define the structure and borders of the spinal canal and it is these structures that can cause stenosis or narrowing. We now know that the spinal cord travels in the central spinal canal and narrowing occurring here is called “central stenosis”.

The big question is, how do the spinal nerves get out of the spinal canal after they branch off of the spinal cord? Is there a transitional canal to pass through before going to the arms and legs? The answer is yes. There is a second smaller set of tunnels, the “foraminal canals”, at each level that provide safe passage of the spinal nerves out of the larger spinal canal. After passing through the foraminal canal, they go to the appropriate muscle or organs. The foraminal canals represent a second set of canals that can have a stenosis issue, which is referred to as “foraminal stenosis”.

“Spinal stenosis” is a more general term that could mean narrowing of the central or foraminal canals. The MRI or CT scan report is the most likely place to find out, more specifically, where the stenosis is occurring.

Probably the most important thing to know about stenosis is whether it is really causing a problem or not. The facts are that

  1. Stenosis is a very common finding on the MRI report
  2. That most of the time it is asymptomatic (doesn’t cause any symptoms). 

The reason that stenosis doesn’t always cause symptoms is that it usually occurs slowly over a number of years and this gives the nerves a chance to “adjust” to the pressure. One source of amazement for me is when I see moderate to severe stenosis that isn’t causing any symptoms.

This is why a good history is so important. The history of where the pain, numbness or weakness occurs tells which nerve is being compressed. When you know which nerve is being compressed, then you know where to look on the MRI for signs of compression. This is possible because of the consistent anatomy of the spine and the nerves that pass through its tunnels. Each nerve has a chance to be compressed in the central canal or as it passes out of the foraminal canal. Sometimes the nerve can be compressed in both locations, but usually it is one or the other. This is important because the surgical approach to each area is different when using more minimally invasive techniques. This is why it is essential to match the symptoms and the findings on the MRI, I want to reduce the amount of surgical exposure for decompression.

When someone with a history of spinal stenosis comes to me with complaints of shoulder, arm, leg, or foot pain. The first question I ask myself is, “where is the stenosis (narrowing) occurring?” Does the stenosis occur in the central canal, the foraminal canals, or both? These questions are important to focus on the exact area of narrowing that is causing the symptoms.

The takeaways here are that:

  1. Stenosis is commonly found in the spine but usually doesn’t cause symptoms.
  2. The history really helps to identify which nerve is being compressed.
  3. Stenosis can be present in the central or the foraminal canals.
  4. The understanding of the location of nerve compression can help limit the surgery needed to decompress a symptomatic nerve and often eliminate the need for a fusion.