Thecal sac

Where you'll see this

This wording turns up on Lumbar Spine MRI reports — MRI imaging of the spine. A lumbar (low-back) MRI shows the vertebrae, the cushioning discs between them, the spinal canal, and the nerves that travel to the legs. Doctors order it for low-back pain, sciatica, leg numbness or weakness, or to plan care. Importantly, many of the changes it describes are extremely common and are seen in people with no pain at all — so the report is read together with how you actually feel.

Questions worth asking your doctor

• Do the findings on the MRI match where I actually feel pain or symptoms?
• Are these changes common for my age, or is anything unusual?
• What are my non-surgical options, such as physical therapy?
• Are there any signs that would mean I should seek care sooner?
• Do I need any follow-up imaging, and when?

How common is this?

The thecal sac is mentioned on close to every lumbar and cervical spine MRI report, because it is the structure a disc has to push against before it can trouble a nerve. It is the fluid-filled sheath running down the spinal canal that carries the spinal cord and, lower down, the nerve roots. Reports describe what is happening to its outline — indented, flattened, effaced — as a way of saying how much room is left.

When should I worry?

The wording sits on a scale, and the scale matters more than any single word. Indentation or contact means a disc is touching the sac, which is extremely common and frequently causes nothing at all. Effacement or compression means the fluid space is being flattened, which weighs more but still has to be matched to how you feel. The finding that changes everything is not about the sac at all: new weakness in the legs, numbness around the groin or inner thighs, or difficulty controlling the bladder or bowel needs emergency assessment the same day, whatever any report says.

What happens next

For the common picture — a disc indenting the sac, symptoms that fit, and no red flags — the first line is almost always conservative: time, movement, physiotherapy and pain control, because most of these settle without anything being done to the disc. Imaging is repeated only if the picture changes. Where symptoms are severe, persistent or accompanied by real weakness, the conversation moves on to injections or surgery, and the scan is then read alongside your examination rather than on its own.

Frequently asked questions

What does indentation of the thecal sac mean?

It means a disc or another structure is pressing on the outside of the sheath and denting its outline, without necessarily narrowing it much. It is one of the most common sentences in spine reporting and, on its own, describes anatomy rather than diagnosing anything.

Is the thecal sac the same as the spinal cord?

No. The sac is the sheath and the fluid inside it; the cord is the nerve tissue it protects. In the lower back the cord has already ended, so the sac there contains the loose bundle of nerve roots called the cauda equina rather than cord — which is why lumbar reports describe the sac and not the cord.

How much thecal sac compression is serious?

There is no threshold that answers this from the image alone, and that is the honest answer rather than a dodge. People with marked narrowing can be comfortable and people with mild narrowing can be in real pain. Severity is judged from your symptoms and examination, with the scan as one input.

Other words on the same report

• Disc bulge
• Disc herniation
• Disc desiccation
• Spinal stenosis
• Spondylolisthesis
• Nerve root compression
• Bone island (enostosis)
• Vertebra