I've just been diagnosed with a condition called neurogenic intermittent claudication (NIC). I understand the canal in my backbone that the spinal cord travels through is narrow or closed down but what actually causes the symptoms?

Pressure on any nerve tissue can result in numbness, tingling, pain, and/or weakness. This kind of phenomenon can occur at the site of any nerve. There are two main ways symptoms may be produced.

First, when compression or obstruction is placed on or around a nerve, signals through the spinal cord to the brain result in warning or red alert symptoms. The body is trying to let the brain (and person) know there's a problem. So pay attention and do something about it!

Second, this same compression or obstruction can cut off the blood supply to the nerve tissue. This is called neuroischemia. If the symptoms are present all the time, it's a static or chronic condition. In the case of NIC, certain positions improve the blood flow and reduce the symptoms.

For example, bending forward opens up the space around the spinal cord, taking pressure off the tissues in that area. Standing up straight or even arching the back into extension has the opposite effect. If the symptoms come and go based on whether your spine is flexed or extended, you may have what's called dynamic neuroischemia.

Reference: 

Paul A. Anderson, MD, et al. Treatment of Neurogenic Claudication By Interspinous Decompression: Application of the X STOP Device in Patients with Lumbar Degenerative Spondylolisthesis. In Journal of Neurosurgery: Spine. June 2006. Vol. 4. No. 6. Pp. 464-471.

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