Ankylosing spondylitis causes uveitis because the same genetic and immune factors that drive spinal inflammation also prime the eye for attack. About 26% of people with ankylosing spondylitis develop uveitis at some point, making it the most common complication outside the spine. The connection isn’t coincidental. It’s rooted in a shared genetic trigger, a shared inflammatory pathway, and immune cells that respond the same way in both locations.
The HLA-B27 Gene Links Both Conditions
The strongest connection between ankylosing spondylitis and uveitis is a gene called HLA-B27. This gene produces a protein that sits on the surface of your cells and helps your immune system distinguish your own tissue from foreign invaders. In most people, this system works smoothly. But in people who carry the HLA-B27 variant, the protein appears to malfunction in ways that trigger autoimmune inflammation in specific tissues, including the spine and the eye.
Researchers have identified several theories for how this happens. One is molecular mimicry: certain bacterial proteins look similar enough to proteins in your own joints and eye tissue that your immune system confuses the two. When your body fights an infection, HLA-B27 may accidentally teach immune cells to attack your own tissue as well. Another theory focuses on the HLA-B27 protein itself, which has an unusual tendency to misfold inside cells. When the protein doesn’t fold into its correct shape, cells activate a stress response that ramps up production of inflammatory signaling molecules. Up to 50% of patients with AS-related uveitis carry HLA-B27, and the misfolding of this protein directly increases production of the key inflammatory signal that connects both diseases.
A Shared Inflammatory Pathway
The specific inflammatory chain reaction that damages your spine also damages your eye. At the center of this chain is a signaling molecule called IL-23. When HLA-B27 misfolds inside cells, it triggers the unfolded protein response, which pumps out IL-23. This molecule activates a downstream cascade (the IL-23/IL-17 axis) that recruits aggressive immune cells to tissues throughout the body.
What makes the eye vulnerable is that certain anatomical sites, particularly the iris and the tissue just behind it (the ciliary body), already contain resident immune cells with receptors for IL-23. These cells are essentially primed to react. When IL-23 levels rise during a systemic flare, these resident cells respond rapidly, producing localized inflammation even though the original immune trigger started elsewhere. Elevated blood levels of IL-23 are directly associated with an increased risk of uveitis flares in people with ankylosing spondylitis. People with genetic variants in the IL-23 receptor gene face additional risk, suggesting this pathway is central rather than incidental.
Genetics Beyond HLA-B27
HLA-B27 is the dominant genetic risk factor, but it isn’t the only one. A gene called ERAP1, which helps trim proteins before HLA-B27 presents them to the immune system, has a larger effect on uveitis risk than on ankylosing spondylitis alone. This means the genetic profile that predisposes someone to eye inflammation is partially distinct from the profile that causes spinal disease. Other genetic variants associated with uveitis in AS include HLA-A*02:01, PSORS1C3, and TAP2, all of which play roles in how your immune system processes and presents proteins. These differences help explain why some people with AS never develop uveitis while others experience repeated flares.
Where Inflammation Strikes in the Eye
AS-related uveitis almost always affects the front of the eye. The iris and ciliary body bear the brunt of inflammation in 60 to 90% of uveitis cases linked to spondyloarthritis. This form, called acute anterior uveitis, typically hits one eye at a time, though it can alternate between eyes across separate flares. Only about 4% of patients develop inflammation in the back of the eye.
Symptoms come on suddenly. You’ll notice a red, painful eye, sensitivity to light, and blurry vision, often developing over hours rather than days. Flares tend to be self-limiting, meaning they resolve with treatment over weeks, but they frequently recur.
Uveitis Can Appear Before Back Pain
One detail that surprises many people: uveitis doesn’t always follow a diagnosis of ankylosing spondylitis. In 37% of cases, eye inflammation shows up before the first episode of inflammatory back pain. Another 18% of the time, both conditions appear simultaneously. This means more than half of patients with both conditions either had no spinal symptoms when uveitis struck or developed them at the same time. Ophthalmologists are increasingly aware that recurrent anterior uveitis, particularly in younger patients, can be the first visible sign of undiagnosed spondyloarthritis.
Long-Term Risks of Repeated Flares
A single episode of uveitis that’s treated promptly rarely causes lasting damage. The concern is with recurrence. In a large study of HLA-B27 uveitis patients, complications occurred in about 48% of affected eyes over time. The most common issues were scar tissue forming between the iris and the lens (about 40% of eyes), cataracts (22%), and elevated eye pressure (16%). Chronic, smoldering inflammation dramatically increased these risks: people with chronic inflammation were more than five times as likely to develop complications and more than eleven times as likely to develop cataracts compared to those with purely episodic flares.
Permanent vision loss, fortunately, remains uncommon. Moderate vision loss occurred in about 1.7% of eyes, and severe vision loss in under 1%. When permanent vision loss did happen, glaucoma caused by uveitis was the most common reason, responsible for roughly 40% of those cases. These numbers reinforce that while any single flare is manageable, keeping recurrences under control matters significantly for long-term eye health.
How Biologic Treatments Reduce Flares
Because the same TNF and IL-23 pathways drive both spinal and eye inflammation, biologic medications used for ankylosing spondylitis can also prevent uveitis flares. However, not all biologics work equally well for the eye. Monoclonal antibody TNF blockers (such as adalimumab and infliximab) significantly reduce uveitis recurrence. In one study, patients on adalimumab saw their flare rate drop from about 60 per 100 patient-years to roughly 8, a reduction of nearly 90%. Infliximab showed a similar pattern, dropping from about 40 flares per 100 patient-years to 9.
Etanercept, a different type of TNF blocker that works through a distinct mechanism, did not significantly reduce uveitis flares. Patients on etanercept still experienced roughly 72 flares per 100 patient-years, compared to 102 before treatment. Some patients even developed uveitis for the first time within the first year of starting etanercept, despite their spinal disease being well controlled. For people with AS who have a history of uveitis or are at high risk, switching from etanercept to a monoclonal antibody TNF blocker is a reasonable strategy that many rheumatologists already follow.