Most Common Causes of Sciatica

Sciatica is a term often used to describe referred buttock and leg symptoms that go down one extremity only (as opposed to both), and that result from one or more irritated nerve roots in the lumbar spine. 

One common misconception about sciatica is that it is always caused by a herniated disc.  Here's another one:  Contrary to what many people believe, sciatica does not involve issues with the sciatic nerve.

Read on to learn about the most common causes of sciatica and the role each plays in creating the pain in your butt or down your leg.

A herniated disc is the most common — though not the only - cause of sciatica.  Technically speaking, the type of sciatica resulting from a herniated disc is called radiculopathy.  

The "rad" in radiculopathy means "root," and refers to the spinal nerve root, which is tissue that emerges from the spinal cord (at every level) before branching into nerves that subsequently extend out all over the body.  The nerve roots exit out holes on either side of the spinal column known as the intervertebral foramen.

Radiculopathy can occur when a herniated nucleus pulposus (the soft material normally located on the inside of the disc) escapes out of the disc and presses on the spinal nerve root.  Generally, the nucleus pulposus moves through tears that occur in the outer fibers of the disc.  Such tears are known as annular tears. When the nucleus contacts the spinal nerve root, it may irritate it, causing pain and/or other symptoms that go down one leg or buttock — i.e., sciatica.

Often related to age and/or arthritis, spinal stenosis is a narrowing of the spinal canal or intervertebral foramen.   (The word stenosis is a general term that refers to a narrowing.)  

Types of Spinal Stenosis

Spinal stenosis comes in 2 main types: central canal stenosis, where the narrowing occurs in the spinal canal, and neuroforaminal stenosis, where narrowing occurs in the holes located on the sides of the spinal column.

Let's talk central canal stenosis first; the spinal canal is the centrally located passageway down which the spinal cord passes. When it narrows, it may irritate the spinal cord which can cause sciatica and other symptoms (such as neurogenic claudication.)

With neuroforaminal stenosis, the narrowing occurs in the holes located on the sides of the spinal column.  Tissue that branches out from the spinal cord en-route to the rest of your body first becomes a nerve root and, upon further branching, bundles of fibers we commonly call nerves.  The nerves pass through these foramen (i.e., the holes) as they exit the spinal column.   As with central cord stenosis, when the foraminal opening becomes blocked (even partially), the nerve tissue may come into contact with the bone spurs responsible for the narrowing.  This, in turn, may irritate the spinal nerve root, leading to sciatica-like symptoms.

Piriformis syndrome is a rare condition that can result in sciatica-like symptoms.  Unlike the other two causes (i.e. stenosis and herniated disc as described earlier) of one-sided leg and/or buttock pain, piriformis syndrome-related sciatica does not originate in the spine. For this reason, it is sometimes referred to as "pseudo-sciatica."

Instead, a buttock muscle — aptly named the piriformis, and located over the sciatic nerve in the back of the hip — compresses this nerve, causing pain and electrical symptoms in that buttock and/or down that same leg.