Synovitis

What synovitis is

Synovitis is inflammation of the synovium, which is the thin lining on the inside of a joint. The word is the Latin name for that lining with the ending that means inflamed on the back of it, so there's nothing complicated hiding in the term itself. The lining is doing more work than most people realize.

That lining makes the fluid that lets the joint surfaces slide past each other without catching. When it inflames, it thickens, and it starts making more fluid than the joint has room for. The result is what you'd expect from a lining that's swollen and a joint that's overfull, which is a joint gone puffy, warm, and stiff. That swelling is the thing a doctor is feeling for when they press along your knuckles one at a time.

Quiet synovitis

Sometimes a scan shows inflammation while you feel perfectly fine, and the formal name for that is subclinical synovitis. Quiet is a fair enough word for what it is. It's common, and it's common even among people whose treatment is plainly working for them.

One study followed 126 people who were all in clinical remission, and it scanned every one of their joints. Among those in the deepest remission, power Doppler synovitis was visible in 25 of 76 people, which works out at 32.9 percent. Among those in a shallower remission it was 72.0 percent. Thickened lining showed in 44.7 percent against 78.0 percent, and both differences came in at P under 0.01.

So feeling well is one state and being clear on a scan is another. The second is considerably harder to reach than the first. That difference is the thing worth holding onto from this page, because it explains why a scan gets ordered for a hand that isn't bothering you.

What it predicts

What it predicts is a flare, and that's the reason anybody scans a joint that feels fine. Three of the studies here point in one direction. The fourth doesn't, and that one gets its own section further down rather than a footnote.

In the study of 126 people just described, 54 of them relapsed over follow-up, which works out at 42.9 percent. The average time to relapse was 6.8 months, with a standard deviation of 3.3. Split by how deep the remission was, relapse hit 30.3 percent of the deepest group against 62.0 percent of the rest, at P under 0.01. Time spent in remission before relapsing was 8.1 months against 5.9 months, at P under 0.05. Quiet power Doppler synovitis at the start was an independent risk factor for relapsing.

A separate study of 167 people in clinical remission found something very close to that. Of those, 51 recurred within a year, and four things independently predicted it. Two of the four were scan scores, the grayscale score and the power Doppler score, and the other two were high-titer anti-CCP antibodies and the starting disease activity index.

One more study followed 94 people, and quiet synovitis predicted a flare there too. The odds ratio was 3.67, at P equals 0.024. That's three separate groups of patients, followed by different teams, arriving at a finding that holds up.

What happens when treatment aims at it

This is where the evidence stops being interesting and starts being useful. One trial tested the obvious question, which is whether treating the quiet finding changes anything, and it's the only randomized study on this page. It took 108 people who had reached clinical remission or low disease activity but still had synovitis visible on ultrasound. Half stayed on unchanged treatment and half had their treatment stepped up.

Over a year, relapse hit 13 of 54 on unchanged treatment, which is 24.1 percent. On stepped-up treatment it was 5 of 54, which is 9.1 percent, at a P value of 0.039. Among those in full remission the difference was wider still, at 11 of 42 or 26.2 percent against 2 of 38 or 5.3 percent, with a P value of 0.026.

Scan scores fell in both groups and fell further in the stepped-up one. Clinical disease activity improved only in the group whose treatment went up. Treating the scan finding changed the year that followed, and no examination of those joints would have told you it was going to.

Aiming at the scan from the start

One study of 194 people took the idea further and compared two targets over a year. One target was clinical remission on its own. The other was clinical remission and scan remission together, which is a harder thing to reach and a different thing to aim at.

The average disease score across the year was 1.89, with a standard deviation of 0.51, against 2.33 with a standard deviation of 0.71 on the clinical target alone. That came in at P under 0.01, and flares hit 20.0 percent against 36.2 percent, at P under 0.05. The treatment side moved in step with that. More people in the scan-guided group managed to step their treatment down, at 66.0 percent against 44.7 percent, at P under 0.01, and fewer had to step it up, at 13.0 percent against 25.5 percent, at P under 0.05.

Within the scan-guided group, three things independently predicted a relapse. They were a starting disease score above 2.29, the presence of quiet synovitis, and having stepped treatment down. The third of those is worth noticing, because it says the thing that helped most people is also the thing that came with a risk.

The study that disagrees

One source here points the other way, and it's worth seeing. The disagreement is real and it isn't small. It followed 90 people in clinical remission for a year, and 26 of them flared.

At the start, 39 percent had synovitis on ultrasound, 23 percent had inflammation of the tendon sheaths, and 8 percent had both. Tendon sheath inflammation raised the risk of a flare, at a relative risk of 4.9 from 2.2 to 10.8. Joint lining inflammation didn't, at a relative risk of 1.3 from 0.76 to 2.2, and that second interval crosses 1, which is what makes it a null result rather than a small one.

On multivariate analysis, only two things stayed significantly associated with a flare. Quiet tendon sheath inflammation held up, at an odds ratio of 9.8 from 2.5 to 39.1, and so did the starting disease score, at 5.7 from 1.1 to 31.6. Both of those intervals are wide, which is what 90 people will give you.

That's 90 people set against several larger studies, and it looked at a structure the others didn't measure separately. It doesn't overturn them, and it isn't trying to. What it does mean is that a scan report mentioning your tendon sheaths is saying something worth asking about, and not a detail the sonographer threw in.

Common misconceptions.

Related terms.

• Rheumatoid arthritis, where nearly all of this evidence comes from.
• DMARDs, the drugs being stepped up or down in these studies.
• Rheumatoid arthritis blood tests, for what gets measured alongside a scan.