Sciatic nerve pain, commonly called sciatica, is most often caused by a herniated disc in the lower back pressing on one of the nerve roots that form the sciatic nerve. The condition affects 10% to 40% of people at some point in their lives, with peak incidence in the 40s. While a bulging disc is the leading culprit, several other structural, muscular, and systemic conditions can irritate or compress this nerve.
The Sciatic Nerve’s Path and Why It’s Vulnerable
The sciatic nerve is the longest and thickest nerve in your body. It originates from nerve roots in the lower lumbar and upper sacral spine (L4 through S3), threads through the pelvis, passes deep to the piriformis muscle in the buttock, and runs down the back of each leg. Because it travels such a long distance and passes through several tight spaces, it has multiple points where compression or irritation can occur.
Herniated Discs: The Most Common Cause
The discs between your vertebrae act as cushions. Each one has a tough outer ring (the annulus) and a gel-like center. When the outer ring tears, the inner material can bulge or leak out, pressing directly on a nearby nerve root. This causes both mechanical compression and inflammation, cutting off blood flow to the nerve and triggering pain that radiates down the leg.
The outer ring is naturally thinner on the back and side of each disc, and it gets less structural reinforcement from surrounding ligaments in that area. That’s exactly where the nerve roots sit, which is why herniations so frequently cause sciatica. The specific nerve root affected depends on which disc herniates and in which direction. A herniation at the L4-L5 level, for instance, typically compresses the L5 nerve root, producing pain, tingling, or weakness along the outer leg and top of the foot.
Spinal Stenosis
As the spine ages, the canal that houses the spinal cord and nerve roots can narrow. This narrowing, called lumbar spinal stenosis, happens through a combination of thickened ligaments, bulging discs, and bony overgrowths (bone spurs) that form around arthritic joints. The result is a progressively tighter space that squeezes the nerves.
Stenosis can occur in three distinct zones. Central stenosis narrows the main spinal canal. Lateral recess stenosis pinches a nerve root just before it exits the spine. Foraminal stenosis compresses the nerve inside the small bony tunnel it passes through to leave the spinal column. Lateral and foraminal stenosis tend to produce one-sided sciatica symptoms, while central stenosis can affect both legs. Unlike disc herniations, which can happen suddenly, stenosis typically develops gradually over years and is most common after age 50.
Spondylolisthesis
Spondylolisthesis occurs when one vertebra slips forward over the one below it. The degree of slippage is graded on a scale: grade I means up to 25% displacement, grade II is 26% to 50%, and higher grades represent more severe slips. Even a modest slip can narrow the bony opening where a nerve root exits the spine. At the L5-S1 level, which is the most common location, the slippage encroaches on the foramen and compresses the exiting L5 nerve root. This produces sciatica symptoms that often worsen with standing and walking and ease with sitting.
Piriformis Syndrome
Not all sciatica originates in the spine. The piriformis is a small, deep muscle in the buttock that rotates the hip outward. In more than 80% of people, the sciatic nerve runs directly beneath this muscle. When the piriformis becomes tight, inflamed, or spasms, it can press on the sciatic nerve at the point where the nerve crosses just below the muscle belly.
Some people are anatomically predisposed to this problem. In a minority of the population, the sciatic nerve splits early into its two main branches, and one or both branches actually pass through the piriformis muscle itself rather than underneath it. This variant makes entrapment more likely. Piriformis syndrome is often triggered by prolonged sitting, overuse from running or cycling, or direct trauma to the buttock.
Risk Factors That Increase Your Odds
Certain lifestyle factors meaningfully raise the risk of developing sciatica. A large study tracking four prospective cohorts found that obesity (defined by BMI) increased the risk of hospitalization for sciatica by 36%, while current smoking raised the risk by 33%. Smoking likely contributes by reducing blood flow to spinal discs and accelerating their degeneration, while excess body weight increases mechanical load on the lower spine.
Occupations involving heavy lifting, prolonged sitting, or whole-body vibration (such as truck driving) are also well-established risk factors. Tall stature, physically demanding work, and psychological stress have all been linked to higher rates of sciatica as well, though the strongest modifiable risks remain weight and tobacco use.
Sciatica During Pregnancy
True sciatica during pregnancy is rarer than many people assume, occurring in only about 1% of pregnant women. When it does happen, it’s usually caused by a disc herniation or bulge compressing a nerve root, the same mechanism as in the general population.
What’s far more common is pregnancy-related lower back pain, which can mimic sciatica. As the abdomen grows, the body’s center of gravity shifts forward, increasing the curve of the lower back and placing additional stress on the lumbar spine. The hormone relaxin, which increases tenfold during pregnancy, loosens ligaments throughout the pelvis and spine to prepare for delivery. This ligamentous laxity can cause pelvic instability and spinal misalignment that contributes to pain, though research has not consistently confirmed a direct link between relaxin levels and the severity of back symptoms.
Diabetes and Nerve Damage
Diabetes can damage the sciatic nerve through a different mechanism than compression. In type 2 diabetes, chronically elevated blood sugar triggers changes in the tiny blood vessels that supply nerves. Perivascular fibrosis and endothelial damage reduce the permeability of these blood vessels, starving the nerve of oxygen and nutrients. Post-mortem studies of lower-extremity nerves in people with diabetic neuropathy have found that the pattern of damage at the distal end of the sciatic nerve closely resembles changes caused by ischemia (restricted blood flow). This process leads to demyelination, where the nerve’s insulating coating breaks down, and direct damage to the nerve fibers themselves. The result can be pain, numbness, and weakness in the legs that overlaps with or worsens sciatica from spinal causes.
How Sciatica Is Identified
Doctors often begin with a physical exam that includes the straight leg raise test. While you lie flat on your back, the examiner lifts your affected leg. If this reproduces your shooting leg pain between 30 and 70 degrees of elevation, it strongly suggests nerve root irritation. This test has a sensitivity of 72% to 97% for detecting disc herniation, meaning it catches most cases. Its specificity is lower (11% to 66%), so a positive result doesn’t confirm the diagnosis on its own but points the clinician toward imaging if needed.
A variation called the crossed straight leg raise, where lifting the unaffected leg reproduces pain in the symptomatic leg, is far more specific (85% to 100%). If that test is positive, a disc herniation is very likely.
Emergency Symptoms to Recognize
In rare cases, a large disc herniation or other spinal lesion compresses the bundle of nerves at the base of the spinal cord, a condition called cauda equina syndrome. This requires emergency treatment. The hallmark symptoms are loss of bladder or bowel control, numbness in the “saddle area” (the inner thighs and groin), and sexual dysfunction, sometimes accompanied by weakness in one or both legs. If you experience any combination of these alongside sciatica, it warrants an immediate trip to the emergency room, as delays in treatment can result in permanent nerve damage.