Most Common Causes of Sciatica

The Root of Sciatica Symptoms

The sciatic nerve is the body’s longest and largest nerve. There is one on each side of the body. It controls muscle movement and sensation in the lower back, buttocks, thighs, calves, and soles of the feet.

Symptoms of sciatica develop when some part of the nerve becomes irritated, compressed (or "pinched"), or inflamed. Symptoms develop depending on where and why the nerve is affected.

Sciatica typically involves pain that radiates down one leg from the lower back to below the knee. But it can also cause achiness, numbness, a burning sensation, tingling, or jolts of pain anywhere along the nerve’s path. Symptoms vary in intensity from one episode to another and can range from mild to severe.

Anything that affects the sciatic nerve, such as trauma or illness, can trigger symptoms of sciatica. But by far the two most common causes are: 

• A bulging or herniated disc in the lower back
• Spinal stenosis

Risk Factors For Sciatica

Several things can increase the risk of sciatica. These include older age and its attendant age-related changes in the spine. Excess weight increases pressure on the spine, which can compress the sciatic nerve.

You could be at risk if you have a job that requires you to sit for long periods (either at a desk or behind the wheel) or if you do a lot of heavy lifting with frequent twisting and bending. If you have diabetes, you are more vulnerable. Cigarette smoking can also contribute to degenerative changes in the spine that heighten sciatica risk.

Herniated Disk

A “slipped” or herniated disk is the most common cause of sciatica.

Disks are shock-absorbing, round “cushions” that lie between each of the bones in the spine (the vertebrae). When the jelly-like tissue inside a disk bulges through its protective covering or pushes its way out between two vertebrae, it can put pressure on surrounding spinal nerves and trigger symptoms of sciatica.

Slipped disks in the lower back usually affect the vertebrae in the lower part of the spine. This area, called the lumbar region, is in close proximity to the sciatic nerve.

Spinal Stenosis

Progressive narrowing of the bony column and space between the lumbar vertebrae—a condition called spinal stenosis—can “squeeze” the sciatic nerve, triggering sciatica symptoms. 

The same processes that cause bony damage over time can also affect the spinal ligaments that hold vertebrae in place. These ligaments can eventually swell, leading to a smaller spinal canal and nerve impingement.

Other Causes

Other far less common conditions can affect the sciatic nerve. These include:

• Traumatic injury, such as a pelvic fracture
• Vertebrae that are misaligned (spondylolisthesis)
• Excess weight or weight gain during pregnancy, which puts pressure on the sciatic nerve
• Cancers that compress the sciatic nerve if they spread to the spine or pelvis
• Cysts or abscesses on or near the spine
• Muscle spasm or inflammation
• Paget’s disease (which causes bony overgrowth) 
• Piriformis syndrome, in which a too-tight piriformis muscle in the buttocks constricts the underlying sciatic nerve 

The Prognosis is Good

The outlook for most people with sciatica is good. Symptoms usually resolve in about a month, with or without treatment. If symptoms are especially severe, recovery can be prolonged, but the prognosis is still generally excellent.