Fibromyalgia vs. Ankylosing Spondylitis: What's the Difference?
Fibromyalgia and ankylosing spondylitis (AS) both cause chronic, widespread pain and fatigue, and it's common for people to wonder which one, if either, fits what they're experiencing. They're different conditions with different causes, different tests, and different treatments, but the overlap in symptoms is real enough that misdiagnosis happens in both directions.
The core difference: inflammatory vs. non-inflammatory
Ankylosing spondylitis is an inflammatory autoimmune condition, primarily affecting the spine and sacroiliac joints. It shows up on bloodwork (elevated CRP/ESR in many cases), on imaging (sacroiliitis on MRI or X-ray), and often correlates with a genetic marker, HLA-B27.
Fibromyalgia is a disorder of pain processing in the central nervous system. It doesn't cause inflammation, doesn't show up on imaging, and doesn't correlate with HLA-B27. Standard bloodwork is typically normal, which is part of why fibromyalgia is a diagnosis of exclusion: it's reached after ruling out other explanations.
How the pain patterns differ
AS pain classically improves with movement and worsens with rest. Stiffness is often at its worst first thing in the morning and eases over the day. It tends to concentrate in the lower back, hips, and sacroiliac joints, though peripheral joints and entheses (where tendons attach to bone) can be involved too.
Fibromyalgia pain is widespread rather than localised to the spine, doesn't have the same clear morning-stiffness-then-improvement pattern, and is frequently accompanied by "fibro fog" (cognitive symptoms like difficulty concentrating and short-term memory issues) alongside profound fatigue and unrefreshing sleep.
Can you have both?
Yes. Fibromyalgia is more common in people who already have an inflammatory condition like AS, possibly because chronic pain signalling can sensitise the nervous system over time. This is one of the reasons the two get confused: someone with AS who develops overlapping fibromyalgia may find that anti-inflammatory treatment controls the joint symptoms but leaves a layer of widespread pain untouched.
Why the distinction matters for treatment
AS is typically managed with NSAIDs first, then biologics (TNF or IL-17 inhibitors) if inflammation isn't controlled. These treatments aim to suppress the underlying immune response. Fibromyalgia doesn't respond to anti-inflammatories in the same way. Management usually centers on medications that affect pain processing (like duloxetine or pregabalin), sleep, gentle graded exercise, and pacing.
Getting the diagnosis right changes what actually helps, which is why tracking your specific symptom pattern over time (not just "pain," but where it is, when it's worse, and what else is happening alongside it) is so useful heading into a specialist appointment.
Track fibromyalgia-specific symptoms with Fibro
Flares, brain fog, FIQ score, and barometric pressure.
See FibroThis article is for general information and isn't a substitute for medical advice. Talk to a rheumatologist or your doctor about your own symptoms and diagnosis.