Piriformis syndrome

What it means

The piriformis is a small, flat muscle that runs from the base of the spine to the top of the thigh bone, deep underneath the larger buttock muscles. It helps rotate and stabilize the hip. The sciatic nerve, the thickest nerve in the body, passes just beneath this muscle in most people — and directly through it in a notable minority. When the piriformis becomes tight, spasms, or swells, it can squeeze the nerve against nearby bone, producing pain that travels from the buttock down the back of the leg.

Why it appears on a CT or MRI report

Piriformis syndrome is usually a clinical diagnosis made from a physical exam, but imaging is often ordered to rule out other, more serious causes of the same symptoms — a herniated disc, spinal stenosis, or a hip joint problem. On MRI, a radiologist may note an enlarged or asymmetric piriformis muscle, signal changes suggesting inflammation or spasm, or an anatomical variant where the sciatic nerve splits and runs through rather than under the muscle. Often, though, the muscle and nerve look essentially normal, and the report simply notes that no other cause for the pain was found.

What it usually means

In most cases, piriformis syndrome develops from overuse, prolonged sitting, a change in exercise habits, or a minor injury that leaves the muscle chronically tight or in spasm. It is more common in runners, cyclists, and people whose jobs involve sitting on hard surfaces for long stretches, such as driving. The pain typically sits deep in one buttock, sometimes radiating down the thigh and calf, and often worsens with sitting, climbing stairs, or hip rotation. It is rarely dangerous and does not usually damage the nerve permanently, but it can be persistent and frustrating without the right treatment.

When to follow up

Mention buttock pain that radiates down the leg to your doctor, especially if it has lasted more than a couple of weeks or interferes with sitting, walking, or sleep. Physical therapy focused on stretching and releasing the piriformis, activity changes, and anti-inflammatory medication resolve most cases. Seek prompt evaluation if you develop numbness, weakness in the leg or foot, or any loss of bladder or bowel control, since these point toward a spinal cause that needs a different work-up rather than a hip-muscle problem.

A plain-language way to picture it

Picture a garden hose (the sciatic nerve) running underneath a heavy wooden beam (the piriformis muscle) laid across a walkway. Most of the time water flows through freely. But if the beam swells with moisture or someone leans extra weight onto it, it presses down and kinks the hose, slowing the flow and causing pressure to build up downstream. Loosening and lifting the beam — the goal of piriformis stretches and physical therapy — takes the pressure off the hose and lets things flow normally again.