How Do Herniated Discs Cause Leg Pain?

Question: I have severe back and leg pain, and my doctor has diagnosed me with a herniated disc. He said something like, “You have a herniated disc at L4/L5 with a pain pattern descending on the right L5 nerve root.” What does that mean?
—Chaska, MN

Answer: Doctors use some convoluted language, don't we? I think, though, that I can explain what he meant and hopefully help you better understand your condition.

I'll explain the phrases he used one by one: herniated disc, L4/L5, pain pattern descending on the right L5 nerve root.

“Herniated disc”
Between your spine's bones (your vertebrae) are cushion-like structures called intervertebral discs. The best way to imagine your intervertebral discs is to think of a jelly donut. In a disc, there's the “jelly” on the inside, which is technically called the nucleus pulposus. It's the center of your disc that really helps cushion your movements and keep your spine mobile. Surrounding the nucleus pulposus is the annulus fibrosus; think of that like the donut part of the jelly donut. When you herniate a disc, it's essentially like the jelly squirting out of the donut. The inner nucleus pulposus pushes through the outer annulus fibrosus, which can cause the intervertebral disc to bulge or even rupture. The herniated or bulging disc can press on your spinal cord or nerve roots, creating pain.

“L4/L5”
Your spine is divided into regions: the cervical spine (neck), the thoracic spine (mid- to upper back), the lumbar spine (low back), and the sacrum and coccyx (commonly thought of as your tailbone). Within those regions, we assign a letter and number to each of the vertebrae; that's for identification purposes.

L4/L5 means that your herniated disc is in your lumbar spine—that's what the 'L' represents. There are five vertebrae in your low back, and your disc condition is between the fourth and fifth lumbar vertebrae.

Keep in mind that there's more than just discs and bones in your spine: you also have nerves travelling through that region. They branch out from the spinal cord and go to various parts of your body to help you move and feel. At the L4/L5 level, you have nerves exiting through the L4 vertebrae, plus there are nerves passing through to the L5 region and below. All this brings me to the final major component of your doctor's diagnosis.

“Pain pattern descending on the right L5 nerve root”
Your herniated disc is pressing the L5 nerve root that's travelling through. The nerve roots are paired, with one going to the right side of your body and one going to the left. The disc herniation is squeezing the right, so you'll probably feel symptoms on the right side of your body.

The nerve roots go to and control different parts of your body. The L5 nerve root, for example, goes down your leg, around the side of your calf, down your ankle, over the top of your foot, and into your toes. Your pain pattern most likely follows that path.

What you're experiencing is called “radiculopathy”, or pain that travels from the origin (in your case, where the herniation is pushing on a nerve structure) to other parts of your body. Since you're feeling the symptoms in your leg, you could also say that you're experiencing sciatica. The sciatic nerve, the longest and largest nerve in your body, starts in your low back and travels down your legs. It is made up of the nerves that exit from the lumbar spine. When something—like your disc—presses on a nerve root, many people call this “sciatica” although it is really is a radiculopathy.

I hope that helps you understand the herniated disc in your spine and how it's causing pain.