Findings explained
What a “Tarlov cyst” is on a spine MRI
Read Your Scan Editorial Team·Last updated ·5 min read
Written from published radiology references and reviewed for plain language. Not written or signed by a physician, and not a diagnosis. How we write these pages →
The short answer
A Tarlov cyst — also called a perineural or sacral meningeal cyst — is a small pouch of spinal fluid that has ballooned out along a nerve root, usually in the sacrum at the base of the spine. They are found incidentally on a meaningful share of lumbar MRIs and the large majority cause no symptoms at all. A minority are symptomatic, and how that minority should be treated is one of the more genuinely disputed questions in spine medicine.
- They are pouches of ordinary spinal fluid, not tumours and not cancer.
- Most are silent. They are found by accident on scans done for something else.
- They sit almost always in the sacrum — the fused bone at the bottom of the spine — at the S2 and S3 nerve roots.
- Size is the loose guide: larger ones, roughly above 1.5 cm, are the ones more often linked to symptoms.
- Whether and how to treat a symptomatic cyst is genuinely contested. That is not your doctor being evasive.
What they are
Each nerve leaving the spinal cord travels inside a sleeve. That sleeve is continuous with the sac holding the cord, and it contains the same cerebrospinal fluid. A Tarlov cyst is a place where that sleeve has ballooned out into a pouch, filled with the same fluid, sitting on the nerve root.
They are named after Isadore Tarlov, the neurosurgeon who described them in 1938. The name says who noticed them, nothing about severity — a pattern this blog keeps running into.
The word "cyst" is doing damage here. In everyday use it suggests a growth that might be something. These are not growths and not tumours: they are pouches of the fluid that is already there.
Where they sit, and how common
Almost always in the sacrum — the wedge of fused bone at the base of the spine — and typically at the second or third sacral nerve roots. That is at the very bottom of a lumbar spine MRI, which is why they so often turn up on a scan ordered for low back pain that turns out to have a different cause.
Imaging studies find them in a few percent of adults having lumbar MRI, and they are commoner in women. The great majority of those people have no symptoms attributable to the cyst.
The minority that cause symptoms
A cyst can become symptomatic when it grows large enough to press on the nerve root it sits on, or on neighbouring ones. Because the sacral roots serve the pelvis, the pattern is characteristic:
- Pain in the buttock, the back of the thigh, or deep in the pelvis.
- Pain that is worse on sitting and eased by lying down.
- Numbness or tingling in the same distribution.
- Bladder or bowel changes, or sexual dysfunction, in more affected people.
- Symptoms worsened by anything that raises pressure in the spinal fluid — coughing, straining, standing up quickly.
That last point is the most distinctive one, and it is worth mentioning to a clinician if it fits, because it argues for the cyst being involved rather than incidental.
Size is the loose guide, not a rule. Cysts above about 1.5 cm are the ones more often associated with symptoms, but large silent cysts and symptomatic smaller ones both exist.
Why this one is contested
Most findings on this blog have a settled answer. This one does not, and pretending otherwise would not help anybody.
The difficulty is that Tarlov cysts are common and mostly silent, so finding one in a person with pelvic or buttock pain does not establish that it is the cause. Meanwhile the surgical options — draining, sealing, wrapping or removing the cyst — operate on a structure that is part of a nerve root and connected to the spinal fluid, so they carry real risk, and cysts can recur after drainage.
The result is a genuine split. Some specialist centres operate on carefully selected patients and report good outcomes; other clinicians are much more conservative. There is no large randomised trial to settle it.
What follows practically: if a clinician says the cyst is probably incidental, that is the mainstream position and not dismissiveness. If your symptoms fit the pattern well and are disabling, asking for referral to a centre that sees these regularly is a reasonable request.
What usually happens next
For an incidental cyst — no symptoms, found on a scan for something else — the answer is nothing. No treatment, no routine repeat scan, and it does not need watching.
Where symptoms fit, the sequence is usually: confirm the cyst and its size on MRI, exclude the commoner causes of the same symptoms, try conservative management, and consider referral to a specialist if it remains disabling.
Frequently asked questions
- Is a Tarlov cyst cancer?
- No. It is a pouch of cerebrospinal fluid — the same fluid that already surrounds the spinal cord — that has ballooned out along a nerve root sheath. It is not a tumour and does not become one.
- Do Tarlov cysts grow?
- Some enlarge slowly over years; many stay the same. Growth is not routinely monitored in someone without symptoms, because finding a slightly bigger silent cyst would not change anything.
- Why did my doctor say it is not causing my pain?
- Because these cysts are common and usually silent, so their presence does not prove they are responsible. That is the mainstream position. If your symptoms match the classic pattern — pain worse on sitting, better lying down, worse with coughing or straining — it is worth saying so explicitly, since that pattern is what argues the cyst is involved.
- Can a Tarlov cyst be drained?
- It can, and the fluid often reaccumulates, which is why aspiration alone is not usually a durable answer. Surgical approaches that seal or reinforce the sac exist and are performed in specialist centres on selected patients. The evidence base is limited, and opinions among clinicians genuinely differ.
- Should I avoid exercise?
- There is no general restriction for an incidental cyst. If yours is symptomatic, activities that noticeably raise spinal fluid pressure — heavy straining in particular — may aggravate symptoms, and that is worth discussing with the clinician who knows your case.
When you want a second read of the images themselves
Understanding your radiologist’s report is free. A Second Opinion is the other thing: an independent AI reading of your images, done from the pictures rather than from the report, with the differences called out and a Q&A on the result. It is informational — not a diagnosis, and not a substitute for your doctor.
Words on this page
Each of these has its own page in the report glossary.
Report glossary, A–Z →Sources
- Patient-facing procedure descriptions (MRI, CT, X-ray, ultrasound)RadiologyInfo.org — Radiological Society of North America & American College of Radiology
- MRI scan — overview, how it is performed and resultsNHS (United Kingdom)
- ACR Appropriateness Criteria and practice parameters for reportingAmerican College of Radiology
Keep reading
Read Your Scan is informational only — not a medical diagnosis, and not a substitute for a licensed radiologist or your doctor. This page explains what a report says and what a radiologist looks at; it cannot tell you what your own images show. If you have urgent symptoms, seek care.