Arachnoiditis is a rare, chronic pain condition caused by inflammation and scarring of one of the protective membranes surrounding the spinal cord. The scarring traps and damages spinal nerve roots, producing persistent pain in the lower back, legs, and feet that can become severely disabling over time. It most commonly develops after spinal surgery, though infections and certain chemical exposures can also trigger it.
What Happens Inside the Spine
Your spinal cord is wrapped in three layers of protective tissue. The middle layer, called the arachnoid, is a thin, web-like membrane. In arachnoiditis, something irritates or injures this membrane, and the body’s immune response kicks in with inflammation. That inflammation is supposed to heal the area, but instead it triggers a buildup of collagen and scar tissue that keeps growing.
As the scar tissue thickens, it does two damaging things. First, it wraps around the nerve roots that branch off the spinal cord, squeezing and eventually injuring them. Second, it blocks the normal flow of cerebrospinal fluid, the liquid that cushions the spinal cord and delivers oxygen and nutrients to nerve tissue. With that flow disrupted, the trapped nerves lose their blood supply, can’t clear metabolic waste, and slowly deteriorate. This is why arachnoiditis tends to worsen rather than resolve on its own.
Common Causes and Triggers
Spinal surgery is the most common cause of arachnoiditis today. Lumbar (lower back) procedures carry the highest risk because the surgical site sits right where the spinal cord fans out into a bundle of individual nerve roots. Any disruption to the delicate arachnoid membrane during surgery can set off the inflammatory chain reaction.
Other established triggers include:
- Infections: Bacterial infections like tuberculosis and syphilis, as well as viral and fungal infections including HIV, can inflame the spinal membranes directly.
- Chemical exposure: Older oil-based dyes once used in spinal imaging studies (myelograms) are a well-documented cause. Certain preservatives in injected medications, epidural steroid injections, and even blood introduced into the spinal fluid during epidural blood patches have also been implicated.
- Direct trauma: Spinal injuries, disc herniations, and complications from epidural anesthesia can damage the arachnoid membrane.
- Autoimmune conditions: Disorders like ankylosing spondylitis and certain types of vasculitis occasionally trigger spinal membrane inflammation.
Because back surgeries have become increasingly common, the overall frequency of arachnoiditis has risen in parallel, even though it remains a rare condition on its own.
What Arachnoiditis Feels Like
Symptoms depend on where the scarring develops, but because lumbar surgery is the leading cause, most people experience pain concentrated in the lower back, buttocks, and legs. The pain is neuropathic, meaning it originates from damaged nerves rather than from a muscle or joint. People commonly describe burning, stinging, or electric shock-like sensations. Numbness and tingling in the legs and feet are also typical.
Some people notice unusual sensations on their skin, like the feeling of water dripping or insects crawling, even though nothing is there. Muscle cramps, spasms, and weakness in the legs can develop as the nerve roots progressively deteriorate. In more advanced cases, bladder and bowel function may be affected. The symptoms tend to be constant rather than coming and going, and they often intensify with physical activity or prolonged sitting.
Not everyone with evidence of arachnoiditis on imaging has symptoms. Some people show scarring on an MRI but experience little or no pain, while others with modest-looking changes have severe, debilitating symptoms. The disconnect between what imaging shows and what a person feels makes this condition particularly frustrating to diagnose and manage.
How It’s Diagnosed
MRI is the primary tool for identifying arachnoiditis. Doctors look for three characteristic patterns on imaging. In early stages, the nerve roots in the lower spine appear swollen and clumped together instead of fanning out freely. In more advanced disease, the nerve roots stick to the walls of the spinal canal, creating a ring-like appearance. In the most severe stage, sometimes called “empty thecal sac,” the nerve roots are permanently glued to the outer membrane and buried under layers of scar tissue, making the fluid-filled space around them appear hollow.
Neurosurgeon Charles Burton proposed a classification system based on these progressive stages: initial nerve inflammation, followed by adhesive arachnoiditis with visible clumping and early nerve shrinkage, and finally the end-stage pattern where nerves are completely encased. This staging helps communicate the severity of the disease, though treatment options remain limited at every stage.
Treatment and Pain Management
There is no cure for arachnoiditis, and the scar tissue that forms cannot be surgically removed without risking further damage and more scarring. Treatment focuses entirely on managing pain and preserving function.
Medications used for nerve pain are the first line of approach. These include drugs originally developed for seizures and depression that work by calming overactive nerve signals. Anti-inflammatory medications may help reduce ongoing inflammation, though they don’t reverse existing scar tissue. Opioid pain medications are sometimes used for severe cases, but their long-term effectiveness for neuropathic pain is limited and carries well-known risks.
For people who don’t get adequate relief from medications, spinal cord stimulation is one option. This involves implanting a small device that sends electrical signals to the spinal cord, interrupting pain signals before they reach the brain. Studies have reported success rates of 30 to 60 percent in arachnoiditis patients, meaning it helps some people meaningfully but is far from a guaranteed solution. The procedure itself also carries some risk, since introducing any device near already-scarred spinal tissue can potentially worsen inflammation.
Physical therapy plays a supporting role. Gentle, low-impact movement helps maintain flexibility, prevent muscle wasting, and manage pain levels. Water-based exercise is often better tolerated because it reduces stress on the spine. The goal isn’t to “fix” the underlying problem but to keep the body as functional and mobile as possible despite it.
Long-Term Outlook
Arachnoiditis is a progressive condition in many cases, meaning the scarring and nerve damage can worsen over time. The pace of progression varies widely. Some people stabilize after the initial injury and live with a manageable level of pain for years. Others experience a steady decline in nerve function, with increasing pain, weakness, and loss of mobility.
The condition can be profoundly isolating. Because it’s rare, many doctors are unfamiliar with it, and the invisible nature of nerve pain means that people with arachnoiditis often struggle to have their symptoms taken seriously. The combination of chronic pain, limited treatment options, and an uncertain trajectory makes psychological support and pain management programs especially important for quality of life.