Arachnoiditis

What it means

Arachnoiditis is inflammation of the arachnoid, one of three thin membranes (the meninges) that wrap and protect the spinal cord and the nerve roots that branch off it. When this membrane becomes inflamed, it can thicken and become sticky, causing the nerve roots inside the spinal canal to clump together or stick to the membrane itself rather than floating freely in the cerebrospinal fluid that normally cushions them. Over time, this irritation can progress to scarring, sometimes called adhesive arachnoiditis, which is more permanent than the initial inflammation.

Why it appears on a CT or MRI report

MRI is the best way to see arachnoiditis, typically showing nerve roots that appear clumped together in the center of the spinal canal, stuck to the outer wall of the sac that holds the spinal fluid, or an empty-looking sac where fluid signal is replaced by scar tissue. The most common setting is the lower back after one or more prior spine surgeries, though it can also follow epidural injections, spinal anesthesia, an infection, bleeding around the spine, or in rare cases certain contrast agents used decades ago. The report may describe the extent, whether it affects a short segment or a longer stretch of the canal.

What it usually means

Not everyone with visible scarring on a scan has symptoms, and mild changes found incidentally after an old surgery are common and don't necessarily need treatment. When arachnoiditis does cause symptoms, people often describe chronic, burning or stinging pain in the low back and legs, along with numbness, tingling, muscle cramps, or in some cases weakness or bladder and bowel changes if the scarring is severe. Symptoms can be persistent and are sometimes difficult to fully resolve, which understandably makes this a worrying finding for many patients, but the severity varies enormously from person to person and many people manage well with the right care.

When to follow up

Discuss any new finding of arachnoiditis with your doctor, ideally a spine specialist or pain management physician experienced with the condition, especially if you're having pain, numbness, or weakness in your back or legs. Because the scarring itself generally cannot be surgically reversed, treatment focuses on managing symptoms through physical therapy, pain management strategies, nerve-targeted medications, and sometimes spinal cord stimulation for more severe or persistent pain. Seek prompt medical attention for new weakness, loss of bladder or bowel control, or rapidly worsening symptoms, as these need urgent evaluation.

A plain-language way to picture it

Imagine a bundle of loose electrical wires meant to run freely through a flexible tube, each wire able to move independently. Arachnoiditis is like glue seeping into that tube, gradually binding the wires together into a stiff clump stuck to the tube's inner wall. The wires can still carry signals, but they've lost their normal freedom of movement, and that binding can create irritation along their length. The glue, once set, is hard to fully dissolve, which is why treatment focuses on easing the irritation it causes rather than removing it.