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If lumbar spinal stenosis is caused by a narrowing of the spinal canal, how does a steroid injection help with the pain? Does it open up that space somehow? Before I consider this treatment, I'd kinda like to understand how it works.

Lumbar spinal stenosis refers to a narrowing of the spinal canal, a long tube through which the spinal cord travels from the brain down to the tip of the spine. Many degenerative changes that come with aging reduce the space in this area. Bone spurs, thickening of the spinal ligaments, and decreased disc height are just a few of the factors that play a role in the development of lumbar spinal stenosis. The spinal cord is covered by a material called dura. The space between the dura and the spinal column is called the epidural space. It is thought that injecting steroid medication into this space fights inflammation around the nerves, the discs, and the facet joints. This can reduce swelling and give the nerves more room inside the spinal canal. There is some evidence that the steroid portion of the injection inhibits pro-inflammatory cells. The net effect is to reduce inflammation and thereby reduce pain. It's also possible that the local anesthetic that's part of a steroid injection puts a stop (or blocks) signals from the receptors for pain up the spinal cord to the brain. It is believed that pressure on the spinal nerve roots sets off nociceptors (pain receptors) that are self-sustaining. In other words, once they get turned on, the pain messages have a way of continuing without turning off.

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