Cornerstone 02

What Causes Sciatica?

If you have pain, numbness, tingling or other symptoms travelling down your leg, one of the first questions you probably want answered is: what is causing this?

Unfortunately the answer isn’t always as simple as finding one structure that looks abnormal.

As we covered in What Is Sciatica?, the word sciatica describes symptoms. It does not tell us why those symptoms are occurring. Several different problems can affect the nerve roots that eventually contribute to the sciatic nerve — and sometimes symptoms that feel very much like sciatica aren’t coming from those nerve roots at all.

So rather than starting with “Which sciatica treatment should I try?”, I want to start one step earlier: what are the different ways someone can develop these symptoms?

First, where can the problem occur?

The sciatic nerve does not begin in your buttock.

Nerve fibres originating from several levels in the lower spine eventually come together to form the sciatic nerve. That means symptoms felt in the buttock, thigh, calf or foot can sometimes originate from a problem much further upstream.

This gives us a useful first distinction. A problem producing “sciatica-like” symptoms might involve:

a nerve root near the spine,
neural structures further along their course,
or another structure entirely that produces symptoms resembling sciatica.

Those are not interchangeable problems. And that is one reason two people who both say “I have sciatica” may need very different approaches.

Disc-related sciatica

One common source of sciatica is a problem involving an intervertebral disc. The discs sit between the vertebrae of your spine. They help distribute load and allow movement between the vertebrae. Changes to a disc can sometimes affect or irritate a nearby nerve root.

Diagram of three stacked lumbar vertebrae with discs between them, and a nerve root passing out beside a disc — A disc sits between two vertebrae, and a nerve root passes close by on its way out of the spine. Proximity is why a disc problem *can* matter — not proof that it does.

You may hear terms such as disc bulge, disc protrusion, disc extrusion or herniated disc. Those words describe structural findings. They do not, by themselves, tell us whether that disc is responsible for your symptoms.

Someone can have a disc abnormality on an MRI without having sciatica. Someone else may have symptoms that fit very well with irritation of a particular nerve root.

A common wrong turn

The useful question isn’t “Do I have a disc bulge?” It is: “Does the disc finding make sense in the context of my symptoms and examination?”

Spinal stenosis

Another common mechanism is spinal stenosis. Stenosis simply means narrowing.

There are several places within the spine where narrowing can occur, and depending on where that narrowing is located it can reduce the available space around neural structures. This can produce leg symptoms in some people.

Diagram comparing two spinal canal cross-sections: one with a large open central space, one with a visibly narrowed central space — Narrowing reduces the space available around neural structures. How much that matters depends on where it is, how much there is, and how your symptoms actually behave.

But again — the word stenosis on an imaging report doesn’t automatically explain someone’s symptoms. The location and degree of narrowing matter. The person’s symptoms matter. And perhaps most importantly, how those symptoms behave matters.

Someone whose leg symptoms predictably change with standing, walking, sitting or changes in spinal position may be giving us useful information about what is contributing to the problem. That pattern doesn’t diagnose stenosis by itself — but symptoms aren’t random noise. Their behaviour can provide clues.

What about arthritis and “degeneration”?

These words understandably scare people. An MRI or X-ray report may mention degenerative disc disease, facet arthritis, degenerative changes, bone spurs, or narrowing. It is very easy to read that report and think: there it is, that’s what’s causing my sciatica.

Maybe. But not necessarily.

Structural changes become increasingly common as we get older, and not every structural change produces symptoms. That doesn’t mean imaging findings are meaningless — it means they have to be interpreted in context.

The question isn’t whether your spine looks perfectly pristine. It is whether a particular finding provides a reasonable explanation for your particular clinical picture.

Can sciatica come from outside the spine?

This is where the terminology gets confusing. The sciatic nerve travels through the pelvis and buttock before continuing down the posterior thigh. Problems affecting neural tissue further along that pathway can sometimes produce symptoms people describe as sciatica.

Diagram of the pelvis from behind, with a muscle spanning the buttock region and the sciatic nerve passing beneath it into the thigh — The nerve passes through the buttock region on its way down the leg. That proximity is real — but proximity is not the same as compression.

We need to be careful here. Pain in the buttock or back of the leg does not automatically mean the sciatic nerve itself is being compressed. And pain located near the piriformis does not automatically mean someone has “piriformis syndrome.”

Location alone doesn’t establish the mechanism. The same principle we’ve been building from the beginning still applies: where you feel something and why you feel it are two different questions.

And sometimes it isn’t sciatica at all

This may be one of the most important things to understand. Not every pain that travels into the leg is caused by irritation of the nerve roots that contribute to the sciatic nerve.

Other structures can refer pain into the buttock or leg. Musculoskeletal problems can produce symptoms in similar locations. And some neurologic or medical conditions can also produce leg symptoms.

Why pain-location charts mislead

A diagram can tell you: “People with this problem sometimes experience symptoms here.”

It cannot tell you: “If you hurt here, you definitely have this problem.”

Those are very different statements — and this is why trying to diagnose yourself from a pain-location chart so often goes wrong.

So how do you work out which cause is more likely?

Usually we don’t answer that question from one piece of information. Instead we start putting pieces together:

Where are the symptoms?
What do they feel like?
When did they begin?
What movements or positions change them?
Do symptoms remain in one location, or travel further down the leg?
Is there numbness or tingling?
Is there weakness?
What does the physical examination show?
And if imaging is available: does what we see on the image actually fit the rest of the story?

Diagram comparing symptoms staying localised in the lower back with symptoms travelling all the way down the back of the leg — One of the more useful things to track: whether symptoms are pulling back toward your spine, or travelling further down the leg. That direction of change often tells you more than how intense things felt on the day.

Each piece adds information. None should automatically become the entire diagnosis.

Why this matters for treatment

This is where understanding the cause becomes practical.

If two people have similar pain travelling down the leg but the symptoms are being driven by different mechanisms, it doesn’t make sense to assume they should automatically perform the same exercises.

One person may benefit from working on a particular movement or position. Another may respond differently to that same movement. Another may need progressive strengthening or activity tolerance. And another may have neurologic findings that warrant medical evaluation rather than another exercise from the internet.

This is why Sciatica HQ isn’t going to organise treatment around “here are the five best exercises for sciatica.” The better question is: what are we trying to change, and why should this particular strategy help change it?

The useful takeaway

You do not need to become an expert in spinal anatomy to understand your sciatica. But you do need one important mental model:

Sciatica can have more than one cause.

A symptom tells us something. An MRI tells us something. A physical examination tells us something. How symptoms respond to movement tells us something.

The goal is to put those pieces together rather than allowing any one of them to tell the entire story.

Still worth repeating

New or rapidly progressing weakness, significant bowel or bladder changes, numbness in the saddle or groin region, or other significant new neurologic changes warrant prompt medical evaluation — not another exercise from the internet.

Next question

What can you actually do about it?

That page is being written now. It is the one that turns all of this into decisions — what you are trying to change, why a given strategy might help, and when it might not.

In the meantime, if you already have a sense of which direction your symptoms respond to: every tool we stock states the one limited problem it helps with, and says plainly when it is not the answer.

See the tools →

Haven’t read the foundations yet? Start with What Is Sciatica? — it covers why the location of a symptom doesn’t tell you where the problem is.

Sciatica HQ is educational and self-management oriented. It is not individualised physical therapy, and nothing here diagnoses your condition or replaces assessment by a qualified clinician.