Tarlov cyst

What it means

A Tarlov cyst is a small pocket filled with cerebrospinal fluid — the same fluid that bathes the brain and spinal cord — that forms in the sheath surrounding a spinal nerve root, almost always at the sacrum near the very bottom of the spine. They form where the fluid-filled covering of a nerve root balloons outward, and they typically communicate with the surrounding spinal fluid space. On MRI, they show up as smooth, round or oval, fluid-bright pockets sitting right where a nerve root exits the spinal canal.

Why it appears on a CT or MRI report

Radiologists describe the size, the level it's found at (most often in the sacrum, labelled S1 through S4), and whether there's one or several. The report may also comment on whether the cyst appears to be pressing on or widening the bony canal the nerve travels through, since larger cysts can gradually reshape the surrounding bone. Most are noted briefly and described as incidental unless their size or the person's symptoms suggest otherwise.

What it usually means

Tarlov cysts are common, turning up in a meaningful share of people who have spine MRIs for unrelated reasons, and the great majority are entirely asymptomatic. They're generally considered benign developmental findings rather than a disease process. A small minority, usually the larger ones, can stretch or irritate the nerve root they surround, leading to lower back or tailbone pain, pain radiating into the buttock or leg, or, less commonly, changes in bladder, bowel, or sexual function if a cyst is large enough to affect several nerve roots. The size of a cyst on a scan doesn't reliably predict whether it will ever cause symptoms — most stay quiet for life.

When to follow up

A small, incidental Tarlov cyst found without matching symptoms usually needs no treatment and no routine follow-up imaging. If you have persistent lower back, tailbone, or radiating leg pain, or any new bladder, bowel, or sexual symptoms, mention the cyst to your doctor so they can judge whether it's contributing. Management for symptomatic cysts starts conservatively and only rarely progresses to specialist procedures, which are reserved for cysts clearly responsible for significant symptoms.

A plain-language way to picture it

Imagine a garden hose (the nerve) running through a slightly loose, fluid-filled sleeve (its protective sheath). In some people, a section of that sleeve balloons out a little more than usual, forming a small, soft pouch along the hose's path. Most of the time that pouch just sits there, filled with the same fluid as the rest of the sleeve, causing no trouble at all. Only occasionally, if the pouch grows large, does it start to crowd the hose it surrounds.