A perineural cyst is a fluid-filled sac that forms on a nerve root in the spine, most commonly at the sacrum (the triangular bone at the base of your spine). Also called Tarlov cysts or meningeal cysts, they contain cerebrospinal fluid, the same liquid that normally cushions your brain and spinal cord. Most perineural cysts cause no problems at all. Only about 5% to 8% of people diagnosed with one ever experience symptoms.
Where They Form and What Makes Them Different
Perineural cysts grow along the posterior nerve roots, the bundles of nerves that branch off the spinal cord and exit through openings in the vertebrae. What sets them apart from other spinal cysts is a critical detail: nerve fibers are embedded in the cyst wall itself or sit inside the cyst cavity. Other fluid-filled cysts in the spine, like arachnoid cysts, contain cerebrospinal fluid too, but they don’t trap nerve tissue within their walls. Perineural cysts also occur almost exclusively in the sacral spine, while arachnoid cysts can appear at multiple levels.
In the Nabors classification system, which doctors use to categorize spinal cysts, perineural cysts fall under type 2: extradural meningeal cysts that contain neural tissue. Type 1 cysts lack neural tissue, and type 3 cysts sit inside the dural membrane rather than outside it. This distinction matters because the presence of nerve fibers in the cyst wall is what makes perineural cysts capable of producing neurological symptoms and what makes treating them more complicated.
How They Grow
The leading explanation for why perineural cysts expand is called the ball-valve mechanism. Cerebrospinal fluid flows into the cyst through a one-way flap in the tissue. Fluid gets in but can’t easily drain back out. Over time, this creates increasing hydrostatic pressure inside the cyst, which stretches the nerve lining and surrounding tissue, causing the cyst to enlarge.
Spinal trauma, physical exertion, or any event that temporarily raises cerebrospinal fluid pressure can accelerate this process. The pressure inside the cyst is thought to be the central driver of both cyst growth and nerve root compression. Some cysts remain stable for years, while others gradually expand until they press on adjacent nerves hard enough to cause symptoms.
Symptoms of a Perineural Cyst
The vast majority of perineural cysts are silent. They show up incidentally on MRI scans ordered for unrelated reasons, and the person never knew they were there. When cysts do become symptomatic, it’s typically the larger ones doing the damage. Symptoms reflect the nerves being compressed in the lower sacral spine, which control sensation and function in the legs, pelvis, bladder, and bowels.
Common symptoms include:
- Lower back pain that may feel deep and persistent
- Shooting or burning pain radiating from the lower back down through the legs
- Numbness, weakness, or loss of sensation in the legs
- Bladder and bowel problems, including difficulty urinating, increased frequency, or constipation
- Sexual dysfunction
- Headaches
The pain pattern often mimics other lower back conditions, which is one reason perineural cysts can go unrecognized. Sitting for long periods tends to worsen symptoms because of increased pressure on the sacral area. Some people notice their symptoms fluctuate, improving during rest and flaring with physical activity, which aligns with the pressure-driven mechanism behind cyst expansion.
How They’re Diagnosed
MRI is the primary tool for identifying perineural cysts. On imaging, they appear as well-defined, fluid-filled pockets along the sacral nerve roots. The challenge isn’t spotting the cyst. It’s determining whether the cyst is actually responsible for the symptoms, since so many people have them without any trouble. Doctors look at cyst size, location relative to nerve structures, and whether the symptom pattern matches the specific nerves affected.
Because perineural cysts can look similar to other fluid collections on imaging, the presence of nerve root fibers within the cyst wall or cavity is the key distinguishing feature. Arachnoid cysts, for example, are also cerebrospinal fluid collections, but they lack neural tissue and aren’t confined to the sacrum.
Treatment for Symptomatic Cysts
If a perineural cyst isn’t causing symptoms, it typically requires no treatment. Monitoring with periodic imaging is often sufficient. For cysts that do cause problems, treatment options range from conservative pain management to procedural interventions.
Non-surgical approaches focus on managing the pain and inflammation. Physical therapy, pain medication, and in some cases lumbar drainage of cerebrospinal fluid can provide temporary relief. These strategies don’t eliminate the cyst but can reduce the pressure and discomfort it causes.
When symptoms are significant and persistent, procedural options include CT-guided aspiration (draining the cyst with a needle) combined with injection of a fibrin sealant to prevent it from refilling, or surgical removal. A large study of 213 patients treated with the CT-guided aspiration and fibrin sealant approach, published in the American Journal of Neuroradiology, found that about 82% of patients were satisfied with their outcome at one year. At three to six years of follow-up, satisfaction remained at 74%, suggesting some cysts do recur or symptoms gradually return. About 3% of patients in that study developed a spinal fluid leak requiring a follow-up procedure to seal it.
Surgical treatment, typically involving opening the spinal canal to access and remove the cyst, showed stronger results in a smaller group from the same study. Of 34 patients who underwent surgery, over 91% achieved satisfactory symptom relief. Surgery carries more risk and a longer recovery period, but because perineural cysts contain nerve fibers in their walls, the procedure requires careful microsurgical technique to preserve nerve function.
Living With a Perineural Cyst
For many people, learning they have a perineural cyst is simply learning a name for something that will never affect their daily life. The cyst exists, it sits quietly at the base of the spine, and it stays that way. If you’ve been told you have one after an incidental finding on imaging, the odds strongly favor it remaining asymptomatic.
For the smaller percentage of people who develop symptoms, the trajectory varies. Some find that conservative management keeps symptoms tolerable. Others eventually pursue aspiration or surgery. Tracking your symptoms over time, particularly changes in bladder or bowel function, leg numbness, or worsening pain patterns, gives you and your doctor the clearest picture of whether the cyst is progressing or stable.