How serious or dangerous is lumbar disk disease with myelopathy?

Need more info: Lumbar disease usually cannot cause myelopathy. Myelopathy is compression of the spinal cord or an intrinsic brain disease. The spinal cord ends at about L1 (the upper most part of the lumbar spine) in most patients so lumbar disease (meaning the lower back area) doesn't cause spinal cord compression. A more specific question may be more helpful.

Very serious: The spinal cord is dependent on good blood supply. If there is pressure on the cord and on the blood vessels that supply the cord, there can be permanent and irreversible damage. Very careful evaluation of blood supply to the cord must be performed, and then careful surgery to decompress the spinal canal can be performed. This should be in the hands of a competent and experienced neurosurgeon.