Sciatica describes pain that travels along the path of the sciatic nerve, the longest single nerve in the human body. This nerve originates in the lower back, passes through the hips and buttocks, and extends down the back of each leg. Sciatica occurs when the nerve roots that form the sciatic nerve become irritated or compressed, typically affecting only one side of the body. A “flare-up” refers to a sudden, acute episode of radiating pain, tingling, or numbness, resulting from increased pressure or inflammation on the compromised nerve.
Lumbar Disc Issues
The most common cause of a sciatic nerve flare-up originates from problems with the intervertebral discs, which cushion the bones of the spine. Each disc has a tough, fibrous outer layer (annulus fibrosus) surrounding a soft, gel-like center (nucleus pulposus). When the outer layer sustains damage, the inner material can push outward, leading to nerve compression.
A disc bulge occurs when the inner material presses against the outer layer, causing it to protrude without rupturing. A more severe herniation happens when the nucleus pulposus breaks through a tear, pushing directly into the spinal canal or the opening where the nerve exits. This mechanical pressure alone is enough to irritate the nerve root, causing the characteristic leg pain.
The material that escapes from a herniated disc also contains chemical irritants and inflammatory proteins. When this substance contacts the nerve root, it causes an inflammatory reaction that chemically sensitizes the nerve. This combination of physical compression and chemical inflammation is responsible for the intense pain experienced during an acute flare-up.
Structural Narrowing of the Spinal Canal
Sciatic nerve irritation can result from age-related changes that reduce the space available for nerves exiting the spine. This narrowing, distinct from a direct disc issue, creates chronic pressure that can trigger an acute flare-up. One condition is spinal stenosis, which is the narrowing of the spinal canal itself or the smaller bony openings (neural foramina) through which the nerve roots pass.
Stenosis usually develops over time due to degenerative changes like the thickening of ligaments, the formation of bone spurs (osteophytes), and the enlargement of the facet joints. These bony and soft tissue changes encroach upon the nerve space, increasing the likelihood of compression. The pressure is often exacerbated by standing or walking, which further reduces the available space.
Another structural cause is spondylolisthesis, where one vertebra slips forward over the one beneath it. This misalignment is caused by chronic instability or a fracture in the bone segment connecting the facet joints. When the upper vertebra shifts, it can pinch the nerve root as it exits the spinal column, leading to recurrent sciatic symptoms.
Muscular Compression
Not all sciatic flare-ups originate in the spine; sometimes, the nerve is compressed outside the spinal column by soft tissue. This is most frequently seen in Piriformis Syndrome, caused by the piriformis muscle located deep in the buttock. The sciatic nerve typically runs beneath this small muscle, which helps rotate the hip.
For a small percentage of the population, the sciatic nerve passes through the piriformis muscle, making them susceptible to compression. When the piriformis muscle becomes tight, inflamed, or goes into spasm—often due to overuse, injury, or prolonged sitting—it squeezes the nerve. This compression causes a deep ache in the buttock that radiates down the back of the leg, mimicking spinal-related sciatica.
Everyday Triggers and Exacerbating Factors
While the underlying cause of sciatica is anatomical—a disc issue, stenosis, or muscular compression—certain daily activities can irritate the nerve, leading to a painful flare-up. Poor lifting mechanics are a common trigger, particularly twisting the body while lifting a heavy object. This motion places uneven strain on the lower spine, which can suddenly push a bulging disc further onto a nerve root.
Prolonged static posture, such as sitting or standing for extended periods, is another frequent irritant. Sitting increases pressure on the intervertebral discs and lower spinal structures, which can inflame the nerve, especially if the posture is slouched or unsupported. Sudden, jarring movements, like those from a fall or a forceful sneeze or cough, can also momentarily increase spinal pressure enough to provoke an acute episode.