The earliest and most common symptom of ankylosing spondylitis (AS) is a deep, persistent pain in the lower back and hips that feels worse in the morning or after sitting still for a long time. Unlike a pulled muscle or a herniated disc, this pain actually improves with movement and exercise, and gets worse with rest. That pattern, called inflammatory back pain, is the hallmark that distinguishes AS from more common causes of back trouble.
About 80% of people with AS notice their first symptoms before age 30, and the condition is roughly twice as common in men as in women. Because early symptoms can look like ordinary back pain, many people go years before getting a correct diagnosis.
Inflammatory Back Pain
The back pain of AS behaves differently from mechanical back pain. It tends to come on gradually over weeks or months rather than after a specific injury. The stiffness is typically worst first thing in the morning and can last 30 minutes or longer before loosening up. Many people find the pain wakes them during the second half of the night, usually between 2 and 5 a.m. Getting up and moving around brings relief, while lying still makes it worse.
This pain usually starts in the sacroiliac joints, the two joints where your lower spine connects to your pelvis. Over time it can creep upward into the middle and upper back. Some people also feel it in the buttocks, sometimes alternating sides, which can be mistaken for sciatica or a hip problem.
Stiffness and Reduced Mobility
As the disease progresses, the inflammatory process can cause new bone to form along the spine. Vertebrae gradually fuse together, which reduces flexibility in measurable ways. You may notice it’s harder to twist your torso, bend forward, or take a deep breath. Doctors sometimes track this by measuring how far your lower back stretches when you bend forward (the modified Schober test) and how much your chest expands when you inhale deeply. Both measurements shrink as spinal and rib-joint mobility decreases.
In advanced cases, the spine can fuse into a rigid, curved position sometimes called “bamboo spine” on X-rays. People at this stage often develop a characteristic posture: the head juts forward, the upper back rounds into an exaggerated curve, and the natural inward curve of the lower back flattens out. Neck movement becomes limited too. Not everyone reaches this stage. Many people’s disease stabilizes well before significant fusion occurs, especially with modern treatment.
Pain Beyond the Spine
AS doesn’t always stay in the back. Inflammation at the points where tendons and ligaments attach to bone, called enthesitis, is common. This shows up most often in the lower body: the back of the heel where the Achilles tendon connects, and the bottom of the foot near the heel where the plantar fascia attaches. It can feel a lot like plantar fasciitis or Achilles tendinitis, and is sometimes treated as such before AS is recognized. Upper-body enthesitis is less common but can occur at the ribs, shoulders, or elbows.
Some people also develop pain and swelling in peripheral joints like the knees, ankles, or shoulders. This peripheral arthritis tends to be asymmetric, affecting one side more than the other, and often comes and goes in flares rather than being constant.
Eye Inflammation
One of the most important symptoms to watch for has nothing to do with your joints. Uveitis, an inflammation of the middle layer of the eye, is the most frequent non-joint complication of AS. In one large study tracking patients over two years, uveitis accounted for the majority of new non-joint diagnoses, with a cumulative incidence of about 3.1%.
An episode of uveitis typically comes on suddenly in one eye. You’ll notice redness, pain, sensitivity to light, and blurred vision. It can feel like something is in your eye. This is a medical issue that needs prompt treatment to prevent lasting damage to your vision. If you have AS and develop a painful red eye, get it evaluated the same day.
Fatigue and Sleep Problems
Fatigue is one of the most underappreciated symptoms of AS, and for many people it’s just as disabling as the pain. In the ASLEEP study, which looked at nearly 500 AS patients in Australian clinical settings, sleep disturbance was remarkably common. About 19% of participants reported moderate to severe clinical insomnia, and roughly 31.5% were identified as being at risk for sleep apnea. The biggest sleep complaints were difficulty staying asleep, dissatisfaction with sleep quality, and sleep interfering with daily function.
The fatigue goes beyond just being tired from poor sleep. People describe feeling drained of energy and having trouble keeping up with their usual activities. This is partly driven by the chronic inflammation itself, which places a metabolic burden on the body similar to fighting a low-grade infection around the clock.
Gut and Skin Involvement
AS shares genetic and immune pathways with inflammatory bowel disease (IBD) and psoriasis, and some people develop these conditions alongside their spinal symptoms. Gut inflammation is especially common. Many AS patients have subclinical inflammation in the intestines, meaning the gut lining is inflamed even without obvious digestive symptoms. When symptoms do appear, they can include chronic diarrhea, abdominal cramping, and blood in the stool.
Psoriasis, which causes raised, scaly patches on the skin, also overlaps with AS more than you’d expect by chance. If you notice persistent skin changes alongside your back symptoms, it’s worth mentioning to your doctor, as it can affect treatment choices.
How HLA-B27 Affects Symptoms
A gene called HLA-B27 is present in the majority of people with AS, and its presence appears to influence how the disease behaves. People who carry HLA-B27 tend to develop symptoms at a younger age, have more active disease, and score higher on standardized measures of disease severity and functional limitation. One study found that HLA-B27 positive patients had disease activity scores five times higher than those without the gene.
HLA-B27 positive patients also tend to experience more frequent and severe complications outside the joints, including eye, heart, and lung involvement. On the other hand, people who develop AS without HLA-B27 often face longer delays in diagnosis, partly because clinicians may not suspect the condition as quickly. Regardless of gene status, the core symptom pattern of inflammatory back pain, stiffness, and fatigue remains the same.
How Symptoms Typically Develop Over Time
AS usually follows a pattern of flares and remissions, especially early on. You might have weeks of significant pain and stiffness followed by periods where symptoms quiet down. This fluctuation can make the condition harder to pin down early, since a bad episode may resolve before your next doctor’s appointment.
Over years, the disease can progress through recognizable stages. Early on, inflammation may only be visible on MRI as swelling in the bone marrow around the sacroiliac joints. Later, structural changes show up on standard X-rays: erosion of the joint surfaces, followed by the growth of bony bridges (syndesmophytes) between vertebrae. Risk factors for faster progression include being male, smoking, having elevated inflammatory markers, and already having syndesmophytes at the time of diagnosis.
The timeline varies enormously from person to person. Some people have mild symptoms that barely change over decades. Others progress to significant spinal fusion within 10 to 15 years. Early recognition and consistent treatment are the strongest tools for slowing that trajectory and preserving mobility.