Tarlov cysts are common, usually small, and rarely the cause of symptoms. Here is how to tell whether yours is the exception.
Have you been struggling with lower back pain, perhaps with some pain or numbness in the legs, and found that your MRI report mentions a Tarlov cyst? It is an unfamiliar term and an alarming thing to read. Let me explain what they are, how they form, when they cause symptoms, and whether they need treating.
The anatomy, briefly
Picture an MRI slice taken down the middle of the body, with the front of the spine on the left and the back on the right.
The spine is a stack of cylindrical blocks of bone, the vertebrae, each with an arch of bone attached behind it. Stacked together, these arches form a tunnel called the spinal canal, which contains and protects the spinal cord. The cord transmits messages between the brain and the body, terminating partway down, with the nerves continuing below it.
Within this canal runs a membrane, a sleeve, containing cerebrospinal fluid. That sleeve, the dura, forms a watertight seal, and where nerves leave the spine it continues around them.
How a Tarlov cyst forms
During development, and we suspect for partly genetic reasons, some people have weak points in that sleeve.
Over time, changes in the pressure of the fluid, which occur routinely with sneezing and coughing, cause the sleeve to pouch outwards at those weak points. On MRI you can see the lining pouching out and, in larger examples, eroding into the surrounding bone to form a substantial pouch.
Why they keep growing
There is a further layer worth knowing about. Inside the dura is another, thinner lining called the arachnoid. Textbooks describe it as web-like; in reality it is something between a spider’s web and cling film.
Sometimes additional membranes form within the fluid space that do not communicate properly with one another. Over time you end up with a flap or fold, pierced in a way that lets fluid pass from the spinal canal into the pouch. But pressure building inside the sac closes that opening from within.
The result behaves like a one-way valve: fluid enters but cannot escape back into the canal, so the cyst continues to expand.
And bone remains biological tissue even in adulthood. It responds to these pressure changes by remodelling around the sac.
Most of them mean nothing
Here is the reassuring part.
Most Tarlov cysts seen on MRI are small, incidental, and rarely the cause of symptoms.
They tend to form at the very bottom of the spine, by which point most of the nerves supplying the legs have already left. Some nerves at that level do supply the bladder and bowel sphincters, but they rarely cause trouble.
In the majority of cases, a Tarlov cyst is entirely incidental and requires no treatment whatsoever.
When they do cause problems
Occasionally these cysts become large.
What happens then is that nerves in the area, which run on the outside of the membrane rather than through the cyst, become compressed. This can cause:
- Pain in the legs
- Numbness in the saddle region
- Urinary retention, or sometimes incontinence
That is generally when treatment becomes necessary.
The difficulty with back pain alone
If your only symptom is back pain, establishing that the cyst is responsible is genuinely difficult. Back pain is complex, arising from a combination of muscles, joints, ligaments and other mechanisms, and a scan alone cannot tell us with certainty that a cyst is the culprit.
Where the pain is very localised to the sacral region, around the buttock area you sit on, there is a useful test. Steroid can be injected around the cyst via a caudal epidural to see whether it improves the pain. If it gives temporary relief and simpler measures such as painkillers subsequently fail, surgery becomes a reasonable consideration.
Where nerves are affected, causing leg pain or bladder or bowel problems, surgery is often required to preserve the function of those nerves by relieving the pressure.
What the operation involves
Through an incision in the middle of the lower back, we create a window at the back of the spine and work under the microscope, opening the dura and entering the cyst itself.
Some fluid is drained. But the important part is perforating any dividing membrane between the cyst and the spinal canal, so that fluid communicates freely and pressure is distributed evenly. No pressure gradient means no pressure on the surrounding nerves.
Why surgeons are cautious here
We are careful about operating on Tarlov cysts, because they carry specific risks.
Fluid leak. Once we open the lining of the cyst, which is also the lining of the spinal sleeve, fluid escapes. Achieving a good watertight closure is often difficult, and there is a small risk of continuous leakage requiring reoperation.
Recurrence. The arachnoid membrane can reform, becoming thicker and more scarred, sometimes necessitating further surgery.
Location. We are operating close to very sensitive nerves supplying the bowel and bladder sphincters, as well as nerves travelling to the legs.
The key takeaway
Tarlov cysts are common lesions. The majority are small, entirely incidental, and not the cause of your back pain.
Larger ones can compress nerves and cause genuine dysfunction, and that is when surgery is considered.
If one has turned up on your report, the question to ask is not “how do we get rid of it” but “is this actually causing my symptoms.”
