A Herniated Nucleus Pulposus, the common medical description of a intervertebral disc protrusion, is HNP for short. When a HNP begins to compress a spinal nerve, the pain is felt down the back of the leg, along the sciatic, and this pain is also referred to as sciatica. Terms also used for a HNP are slipped disc, protruded disc, bulging disc, extruded disc (See Disc in glossary). At each segment a spinal nerve exits the spinal canal then joins with others (nerve roots) to form spinal nerves, the largest of which is the sciatic. Thus, if leg pain follows low back pain, the cause is usually a HNP.
A common non-medical term for this is a pinched nerve. The leg pain is more aching than burning and can be intermittent, depending on back and leg positions. A HNP will recede somewhat during periods of lying down, which often relieves pressure on the nerve root. Sciatic pain usually stops at the ankle, so if the pain is mostly burning and in the foot, is may be a neuropathy, a problem with the nerve and not a HNP.
Leg pain due to a HNP can be mild or very severe, often much worse than the back pain. Rarely, back pain goes away when severe leg pain begins because the HNP has extruded from its capsule and compresses the nerve root even more. The sensory nerves to the disc are in the capsule (annulus) and the stretching or disruption of the annulus causes back pain. It can also happen that leg pain goes away when weakness and numbness gets worse or a foot drop occurs. This is not progress but a severe worsening of the nerve root damage so that it is not even carrying pain signals any longer.