Sciatic nerve pain is most often caused by a herniated disc in the lower back pressing on one of the nerve roots that form the sciatic nerve. But compression alone doesn’t tell the whole story. Inflammation from the disc material itself plays an equally important role, which is why some people with visible disc bulges on an MRI feel no pain at all, while others with smaller herniations are in agony. Up to 40% of people will experience sciatica at some point in their lives, and the likelihood increases with age.
How a Herniated Disc Triggers Sciatica
Your spine is stacked with vertebrae separated by rubbery discs that act as shock absorbers. Each disc has a tough outer shell and a soft, gel-like center. When that outer shell tears or weakens, the inner material can push outward and press against a nearby nerve root. In the lower back, those nerve roots feed into the sciatic nerve, which runs from the lower spine through each buttock and down the back of each leg.
The pain isn’t purely mechanical. When disc material leaks out, it releases inflammatory proteins that directly irritate nerve tissue. These chemicals increase pain signaling, damage nerve cells, and can slow the speed at which the nerve conducts signals. This is why sciatica symptoms result from a combination of mechanical compression and chemical inflammation, and why not all patients with mechanical compression develop symptoms. The inflammation can produce burning, shooting, or electric-shock sensations that travel down the leg, sometimes all the way to the foot.
Spinal Stenosis and Narrowing
Spinal stenosis is a gradual narrowing of the spaces within the spine. As those spaces shrink, typically from arthritis-related bone spur growth or thickened ligaments, the nerve roots get squeezed. This is the most common cause of sciatica in people over 60. The pain tends to worsen when you stand or walk and eases when you sit down or lean forward, because bending opens up the spinal canal slightly.
Vertebral Slippage
A condition called spondylolisthesis occurs when one vertebra slides forward over the one below it. This displacement narrows the openings where nerve roots exit the spine, putting direct pressure on the sciatic nerve. Most cases are low-grade, meaning the vertebra has shifted only a small amount, and can be managed without surgery. High-grade slippage, where the vertebra has moved significantly, is much more likely to require surgical intervention, especially when pain is severe or neurological symptoms develop.
Piriformis Syndrome
Not all sciatica originates in the spine. The piriformis is a small muscle deep in the buttock that sits directly on top of the sciatic nerve. When this muscle spasms, tightens, or swells from overuse or injury, it can compress the nerve and produce pain that feels identical to spine-related sciatica. Runners, people who sit for long periods, and those who suddenly increase physical activity are particularly prone. The key difference is that piriformis syndrome typically causes pain centered in the buttock without significant lower back pain.
The Role of Inflammation Without Compression
You can develop sciatica-like pain even when imaging shows no structural problem pressing on a nerve. Chemical irritation alone can trigger it. When the soft center of a disc leaks even small amounts of material near a nerve root, the inflammatory cascade that follows is enough to cause significant pain. The body treats this leaked material as a foreign substance and mounts an immune response that sensitizes the nerve, lowers its pain threshold, and can even cause nerve cell death over time. This helps explain cases where people have severe symptoms but “normal-looking” scans.
Risk Factors That Increase Your Odds
Several factors make sciatica more likely, and most of them relate to how much stress your lower spine absorbs over time.
Obesity is a modest but consistent risk factor. Extra body weight increases the load on your lumbar discs with every step, every bend, and every hour spent sitting. Over years, this accelerates disc wear.
Smoking raises sciatica risk more than many people realize. A large meta-analysis covering over 300,000 people found that current smokers had roughly 35 to 64% higher odds of developing sciatica compared to nonsmokers, depending on how sciatica was defined and measured. Smoking reduces blood flow to spinal discs, which depend on diffusion for their nutrient supply, accelerating degeneration.
Occupational demands matter too. Jobs that involve heavy lifting, prolonged sitting, whole-body vibration (like truck driving), or frequent bending and twisting place repeated stress on the lumbar spine. The combination of physical load and awkward posture is especially damaging over time.
Age is the factor you can’t control. Disc degeneration is a normal part of aging. The discs lose water content and become less flexible, making them more susceptible to tears and herniations. Most sciatica occurs between ages 30 and 50 for disc-related causes, while stenosis-related sciatica becomes more common after 60.
Pregnancy and Sciatica
Sciatica during pregnancy has its own set of causes. As the uterus grows, it shifts the center of gravity forward, increasing the curve of the lower back and putting more pressure on the lumbar discs and nerve roots. The growing baby can also press directly on the sciatic nerve from inside the pelvis.
Hormonal changes amplify the problem. During pregnancy, the body produces a hormone called relaxin that loosens muscles, joints, and ligaments to prepare for delivery. While this is essential for childbirth, it also destabilizes the pelvis and lower back. Relaxin can negatively affect posture because the back and pelvis become looser, and this instability can lead to pelvic pain, lower back pain, and sciatic nerve irritation. Most pregnancy-related sciatica resolves after delivery as hormone levels normalize and the mechanical load disappears.
Diabetes and Nerve Damage
High blood sugar from diabetes can damage nerves throughout the body, and the sciatic nerve is no exception. A type of diabetic nerve damage called proximal neuropathy specifically affects nerves in the thighs, hips, buttocks, and legs. Symptoms include severe pain in the hip, thigh, or buttock and difficulty standing up from a seated position. This can look and feel a lot like sciatica from a disc problem, but the underlying cause is metabolic rather than structural. Peripheral nerve damage from diabetes can also produce tingling, numbness, and increased pain sensitivity in the legs and feet, overlapping with sciatica symptoms and sometimes making them worse.
Tumors, Infections, and Other Rare Causes
In uncommon cases, a tumor growing near the spine or along the sciatic nerve can compress the nerve and produce sciatica symptoms. Spinal infections, though rare, can cause swelling that narrows the spinal canal or nerve root openings. Conditions like endometriosis have also been linked to sciatic pain when tissue grows near the nerve in the pelvis. These causes account for a small fraction of cases but are worth investigating when sciatica doesn’t respond to typical treatment or is accompanied by unexplained weight loss, fever, or progressively worsening neurological symptoms.
When Sciatica Becomes an Emergency
Most sciatica, while painful, resolves on its own or with conservative treatment. But a rare condition called cauda equina syndrome requires immediate medical attention. It occurs when the bundle of nerves at the base of the spinal cord becomes severely compressed, usually from a large disc herniation, and it can cause permanent damage if not treated within hours.
The warning signs to watch for are distinct from ordinary sciatica. Urinary retention, where your bladder fills but you don’t feel the urge to go, is the most common red flag. Loss of bowel or bladder control, numbness in the groin and inner thighs (sometimes called saddle numbness because it affects the areas that would touch a saddle), and sudden weakness in both legs all signal that compression has reached a critical level. If you experience any combination of these symptoms alongside back or leg pain, this is a situation that warrants an emergency room visit.