AS Glossary

Plain-English explanations of terms you'll encounter with ankylosing spondylitis.

A

ASAS(Assessment of SpondyloArthritis International Society): Assessment of SpondyloArthritis International Society. The international body that sets the classification criteria and guidelines for spondyloarthritis conditions including AS. Their 2009 criteria established the current axSpA classification that includes non-radiographic disease.
ASDAS(Ankylosing Spondylitis Disease Activity Score): Ankylosing Spondylitis Disease Activity Score. A tool used by rheumatologists to measure how active your AS is. It combines answers to a short questionnaire with a blood marker (CRP). Scores above 2.1 indicate high disease activity.
Adalimumab(Adalimumab (anti-TNF biologic)): A biologic medication that blocks TNF-alpha, used to treat AS when NSAIDs are not sufficient. Sold under the brand name Humira. Taken as a self-injection every two weeks.
Ankylosing(Ankylosis): Fusing or stiffening. In ankylosing spondylitis, it refers to the way the spinal vertebrae can gradually fuse together over time as inflammation triggers bone formation.
Ankylosing spondylitis (AS)(Radiographic axial spondyloarthritis (r-axSpA)): A type of inflammatory arthritis that primarily affects the spine and sacroiliac joints. The immune system attacks these joints, causing pain, stiffness, and over time potentially structural changes. AS is the original name; it is now often classified under the broader term axial spondyloarthritis.
Anti-CCP antibodies(Anti-cyclic citrullinated peptide antibodies): Anti-cyclic citrullinated peptide antibodies. A blood marker associated with rheumatoid arthritis. Usually negative in AS, which helps distinguish AS from RA when the diagnosis is unclear.
Anti-TNF therapy(TNF inhibitor therapy): A class of biologic medications that block tumour necrosis factor-alpha (TNF-alpha), a protein that drives inflammation in AS. Includes adalimumab (Humira), etanercept (Enbrel), certolizumab (Cimzia), golimumab (Simponi), and infliximab (Remicade).
Autoimmune condition(Autoimmune-mediated inflammatory disease): A condition where the immune system mistakenly attacks the body's own tissues. AS is an autoimmune condition — the immune system attacks the sacroiliac joints and spine, causing inflammation.
Axial spondyloarthritis (axSpA)(Axial spondyloarthritis): The umbrella term for inflammatory arthritis that primarily affects the spine and sacroiliac joints. It includes ankylosing spondylitis (also called radiographic axSpA) and non-radiographic axSpA. Both have the same underlying disease process.

B

BASDAI(Bath Ankylosing Spondylitis Disease Activity Index): Bath Ankylosing Spondylitis Disease Activity Index. A six-question questionnaire that measures how active your AS is over the past week, covering pain, stiffness, fatigue, and swelling. Scored 0-10; a score above 4 is considered high disease activity.
BASFI(Bath Ankylosing Spondylitis Functional Index): Bath Ankylosing Spondylitis Functional Index. A ten-question questionnaire measuring how AS affects your ability to do everyday activities — bending, reaching, putting on socks, doing physical tasks. Scored 0-10; higher scores mean more difficulty.
Bamboo spine(Vertebral fusion with syndesmophyte formation): The appearance of a heavily affected AS spine on X-ray. When inflammation over many years drives bone formation that bridges the vertebrae, the resulting image looks like a bamboo stalk. It represents significant structural change. Not everyone with AS develops this.
Biologics(Biological disease-modifying antirheumatic drugs (bDMARDs)): A class of medications made from biological (living) sources that target specific parts of the immune system driving AS inflammation. Unlike NSAIDs, which broadly reduce inflammation, biologics work on precise molecular targets like TNF-alpha or IL-17.
Brain fog: Cognitive difficulties — difficulty concentrating, forgetting words, struggling to follow complex thoughts — that many people with AS experience. It is partly driven by systemic inflammation directly affecting brain function, and partly by the cognitive load of managing chronic pain.

C

CRP(C-reactive protein): C-reactive protein. A general inflammation marker measured by blood test. Elevated CRP is common in active AS. However, CRP is normal in up to 40% of AS patients even with active disease, so normal CRP does not mean AS is not active.
Certolizumab(Certolizumab pegol (anti-TNF biologic)): A biologic TNF inhibitor used to treat AS. Sold as Cimzia. Taken as a monthly self-injection. Notable for being the biologic with the best evidence for safety during pregnancy and breastfeeding, because it does not cross the placenta significantly.
Chest expansion: The increase in chest circumference from breathing in fully versus out fully. AS can restrict chest expansion as the costovertebral joints (where ribs meet the spine) stiffen. It is measured in rheumatology assessments and used as a functional indicator of AS progression.
Costochondritis(Costal chondritis / costosternal enthesitis): Inflammation of the cartilage that connects the ribs to the sternum (breastbone). In AS, this is a form of enthesitis. It causes chest pain that can feel alarming because it resembles cardiac pain. It is worsened by pressing on the chest wall.

D

Dactylitis: Swelling of an entire finger or toe, giving a sausage-like appearance. It occurs in some forms of spondyloarthritis (particularly psoriatic arthritis) when inflammation affects the whole digit. Sometimes called "sausage finger" or "sausage toe."
Disease-modifying antirheumatic drugs (DMARDs): Medications that modify the underlying disease process rather than just managing symptoms. In AS, this category includes conventional DMARDs (like methotrexate and sulfasalazine, used mainly for peripheral joint involvement) and biological DMARDs (biologics). NSAIDs and painkillers are not DMARDs.

E

ESR(Erythrocyte sedimentation rate): Erythrocyte sedimentation rate. A general blood test for inflammation — it measures how quickly red blood cells settle in a tube. Like CRP, ESR can be elevated in active AS but is normal in a significant proportion of patients. Used alongside CRP as a general indicator.
Enthesitis(Enthesopathy / enthesitis): Inflammation at the point where a tendon or ligament attaches to bone. A characteristic feature of AS and related spondyloarthritides. Common sites: Achilles tendon (heel), plantar fascia (sole of foot), patellar tendon (knee), ribs (chest wall), and the base of the spine.
Etanercept(Etanercept (anti-TNF biologic)): A biologic TNF inhibitor used to treat AS. Sold as Enbrel. Taken as a weekly self-injection. One of the most long-studied biologics for AS with good long-term safety data.
Extra-articular manifestations(Extra-articular features of spondyloarthritis): Disease features that occur outside the joints. In AS, these include: eye inflammation (uveitis/iritis), gut inflammation (colitis), and skin manifestations (psoriasis). About 25-40% of AS patients experience uveitis at some point. These manifestations may appear before, alongside, or after the joint symptoms.

F

Fatigue: In AS, fatigue is not ordinary tiredness — it is a direct symptom of systemic inflammation. The inflammatory proteins (particularly IL-6 and TNF-alpha) active in AS trigger a 'sickness behaviour' response that causes profound exhaustion. It often does not improve with rest alone and can persist even on low-pain days.
Flare: A period of significantly increased disease activity — usually lasting a week or more — with noticeably worsened pain, stiffness, and fatigue. Different from a bad day, which is shorter-lived. A flare may be triggered by illness, stress, overdoing physical activity, or may occur without obvious cause.

G

Golimumab(Golimumab (anti-TNF biologic)): A biologic TNF inhibitor used to treat AS. Sold as Simponi. Taken as a monthly self-injection.

H

HLA-B27(Human Leukocyte Antigen B27): A gene carried by about 8% of the general population. Roughly 90% of AS patients in Caucasian populations have it. HLA-B27 is a useful diagnostic clue — a positive result combined with inflammatory symptoms warrants investigation — but it is neither necessary nor sufficient for an AS diagnosis. You can have AS without it, and most people who have it never develop AS.
Hydroxychloroquine(Hydroxychloroquine (conventional DMARD)): An antimalarial drug sometimes used in rheumatic conditions like lupus and rheumatoid arthritis. Not typically recommended for axial AS but sometimes used for peripheral joint involvement. Less common in AS management than methotrexate or sulfasalazine.

I

IBD (inflammatory bowel disease)(Inflammatory bowel disease): Crohn's disease and ulcerative colitis. AS and IBD share a genetic and immunological link — about 5-10% of AS patients develop full IBD, and up to 60% have subclinical gut inflammation. This is why gastroenterology is sometimes involved in AS care.
IL-17 inhibitors(Interleukin-17 inhibitors): A class of biologic medications that block interleukin-17 (IL-17), a protein involved in AS inflammation. Includes secukinumab (Cosentyx) and ixekizumab (Taltz). An alternative to TNF inhibitors and equally effective for many people. Not recommended for people with IBD.
Inflammatory back pain(Inflammatory back pain (IBP)): The specific pattern of back pain associated with AS and related spondyloarthritides. Key features: worse at night and in the morning, improves with movement (but not completely with rest), morning stiffness lasting over 30 minutes, onset before age 45, gradual onset. Responds better to NSAIDs than to rest.
Infliximab(Infliximab (anti-TNF biologic, IV)): A biologic TNF inhibitor used to treat AS. Sold as Remicade. Unlike most other biologics, infliximab is given as an intravenous infusion in a clinical setting every 6-8 weeks, rather than self-injected at home.
Ixekizumab(Ixekizumab (IL-17 inhibitor biologic)): A biologic IL-17 inhibitor used to treat AS. Sold as Taltz. Taken as a self-injection (loading doses, then monthly). An alternative to TNF inhibitors for people who have not responded or who cannot tolerate them.

J

JAK inhibitors(Janus kinase inhibitors (JAKi)): The newest class of medications for AS — oral tablets rather than injections. Tofacitinib (Xeljanz) and upadacitinib (Rinvoq) are the main ones. They block JAK signalling pathways involved in the inflammatory process. Used when biologics have not worked adequately. Higher side effect profile than biologics, so used later in the treatment ladder.

K

Kyphosis(Kyphosis / hyperkyphosis): An exaggerated forward rounding of the upper back. In AS, progressive spinal fusion can cause the spine to fuse in a forward-flexed position, resulting in kyphosis. Good posture habits and physiotherapy from early in the disease can help minimise this. It does not happen to everyone.

L

Leaky gut(Increased intestinal permeability): A colloquial term for increased intestinal permeability — the gut wall becomes more permeable, allowing bacterial products to cross into the bloodstream and trigger immune responses. Research suggests this may be part of the mechanism connecting gut microbiome changes to AS inflammation.
Lumbar spine(Lumbar vertebrae (L1-L5)): The lower portion of the spine — the five vertebrae in the lower back. Along with the sacroiliac joints, the lumbar spine is one of the primary sites of AS inflammation.

M

MRI (Magnetic Resonance Imaging)(Magnetic resonance imaging): A imaging technique that uses magnetic fields rather than X-rays. For AS, MRI is more sensitive than X-ray because it can detect active bone marrow oedema (inflammation) in the sacroiliac joints years before structural damage is visible on X-ray. An MRI of the sacroiliac joints is the key investigation for early AS diagnosis.
Methotrexate(Methotrexate (conventional DMARD)): A conventional DMARD sometimes used for peripheral joint involvement in AS (arthritis in joints outside the spine). It is not effective for the spinal/axial component of AS. It is absolutely contraindicated in pregnancy and for men who are trying to conceive. Requires regular blood monitoring for liver function.
Morning stiffness: Stiffness of the spine and joints that is worst immediately after waking and improves with movement. In AS, morning stiffness lasting more than 30-45 minutes is a diagnostic feature and one of the most characteristic symptoms. Unlike mechanical back pain where stiffness resolves quickly, AS-related stiffness typically takes 1-2 hours to ease.

N

NSAIDs(Non-steroidal anti-inflammatory drugs): Non-steroidal anti-inflammatory drugs. First-line treatment for AS. Includes naproxen, ibuprofen, diclofenac, and celecoxib. They reduce inflammation and pain. In AS specifically, continuous use may also slow the bone-forming process that causes spinal fusion. Long-term use requires monitoring for GI and kidney effects.
Non-radiographic axSpA(Non-radiographic axial spondyloarthritis (nr-axSpA)): Axial spondyloarthritis without visible structural damage on X-ray. The same inflammatory disease as AS (ankylosing spondylitis), but caught earlier — the sacroiliac joint inflammation shows on MRI but has not yet caused X-ray changes. Often incorrectly interpreted as "not a real diagnosis." The symptoms and treatment are essentially the same as AS.

O

Opioids(Opioid analgesics): Strong pain-relieving medications (morphine, oxycodone, tramadol, codeine). Not recommended for AS by most guidelines — they do not address the underlying inflammation, carry significant side effect and dependency risks, and may worsen long-term outcomes. Most rheumatologists avoid them for AS management.

P

PPI(Proton pump inhibitor): Proton pump inhibitor. Medications like omeprazole or pantoprazole that reduce stomach acid. Commonly prescribed alongside NSAIDs to protect the stomach lining from the GI side effects of long-term NSAID use.
Pacing: An energy management strategy for people with chronic illness. Pacing means managing activity as a finite resource — doing less than you feel capable of on good days to build reserves, and avoiding the boom-bust cycle of overexerting on good days and crashing on bad ones. It is a skill, not a passive approach.
Peripheral arthritis(Peripheral spondyloarthritis): Arthritis in joints outside the spine — hips, knees, ankles, shoulders, elbows. Affects about 30% of AS patients. Hip involvement is particularly common and can be severe. Peripheral arthritis in AS responds to biologics but conventional DMARDs (methotrexate, sulfasalazine) can also be useful for peripheral joint involvement specifically.
Physiotherapy(Physiotherapy / physical therapy): Physical therapy. A core component of AS management — not optional. A physiotherapist designs an exercise programme suited to AS, teaches posture maintenance, and may use hands-on techniques to reduce stiffness. The evidence for physiotherapy combined with medical treatment is better than for either alone.
Plantar fasciitis(Plantar fasciitis / plantar enthesitis): Inflammation of the plantar fascia — the band of tissue running along the sole of the foot, from heel to toes. In AS, plantar fasciitis is often a form of enthesitis (inflammation at the tendon/ligament insertion point). It causes heel and arch pain, especially first thing in the morning.
Postpartum flare: A worsening of AS disease activity in the weeks and months after childbirth. Occurs in 50-75% of women with AS. The immune system, which shifts toward tolerance during pregnancy, rebounds after delivery — and that rebound can trigger significant inflammation. It typically hits during a period of severe sleep deprivation and physical recovery, making it particularly challenging.
Psoriasis(Psoriasis vulgaris): A skin condition characterised by red, scaly plaques. Psoriasis is associated with spondyloarthritis — psoriatic arthritis is a related condition, and about 10-25% of AS patients have or will develop psoriasis. The skin and joint manifestations often track together in terms of disease activity.

R

Radiographic axSpA(Radiographic axial spondyloarthritis (r-axSpA) / Ankylosing spondylitis): The form of axial spondyloarthritis where visible structural damage (sacroiliitis) is present on X-ray. This is what has historically been called ankylosing spondylitis. The name change reflects the broader axSpA classification introduced in 2009.
Remission: In AS, remission means the disease is not causing active symptoms — inflammation is controlled, pain is minimal, and function is near-normal. It is achievable with treatment for some people, particularly with biologics. Remission in AS is measured by ASDAS or BASDAI scores falling below specific thresholds.
Rheumatoid factor (RF)(Rheumatoid factor): An antibody found in the blood. Elevated in most people with rheumatoid arthritis, but typically negative in AS. Testing for RF helps distinguish between these two forms of inflammatory arthritis when the diagnosis is unclear.
Rheumatologist: A specialist doctor in inflammatory arthritis and autoimmune conditions. The rheumatologist is the key specialist for AS diagnosis and management. GPs can manage acute issues and order initial tests, but long-term AS management belongs with rheumatology.

S

Sacroiliac joints(Sacroiliac joints (SI joints)): The joints where the base of the spine (the sacrum) connects to the pelvis (the ilium). There are two — one on each side. These joints are the hallmark site of AS inflammation. Sacroiliitis (inflammation of the sacroiliac joints) is the defining feature of AS on imaging.
Sacroiliitis: Inflammation of the sacroiliac joints. This is the hallmark finding of AS on MRI and X-ray. On MRI, it shows as bone marrow oedema (swelling) in the joint area. On X-ray (in later disease), it shows as erosion, sclerosis (hardening), or fusion of the joint.
Secukinumab(Secukinumab (IL-17 inhibitor biologic)): A biologic IL-17 inhibitor used to treat AS. Sold as Cosentyx. Taken as monthly self-injections after an initial loading period. An effective alternative to TNF inhibitors. Not recommended for people with inflammatory bowel disease, as IL-17 inhibition can worsen IBD.
Spondylitis(Spondylitis / vertebral inflammation): Inflammation of the vertebrae. In ankylosing spondylitis, spondylitis refers to the inflammatory process affecting the spinal vertebrae alongside the sacroiliac joints.
Spondyloarthritis(Spondyloarthropathies / spondyloarthritis (SpA)): A family of related inflammatory arthritis conditions that includes ankylosing spondylitis, psoriatic arthritis, reactive arthritis, arthritis associated with inflammatory bowel disease, and undifferentiated spondyloarthritis. They share common genetic features (particularly HLA-B27) and overlap in their inflammation patterns.
Spoon theory: A framework for explaining limited energy resources in chronic illness, originally developed by Christine Miserandino. Each activity costs "spoons" (units of energy); once the spoons are gone for the day, they are gone. It helps explain to people without chronic illness why someone can do something on Tuesday but not Wednesday, and why trade-offs are real.
Sulfasalazine(Sulfasalazine (conventional DMARD)): A conventional DMARD sometimes used for peripheral joint involvement in AS. It is not effective for spinal/axial disease. Important note: it causes reduced sperm production (oligospermia) in up to 80% of men who take it — this is reversible on stopping, but men planning to conceive should switch medications in advance.
Syndesmophyte: A bony outgrowth that bridges the edges of adjacent vertebrae. When inflammation drives bone formation along the outer edges of the spinal column, syndesmophytes form. They are the building blocks of the "bamboo spine" appearance that occurs in advanced, long-standing AS.

T

TNF-alpha(Tumour necrosis factor-alpha (TNF-α)): Tumour necrosis factor-alpha. A protein produced by the immune system that drives inflammation in AS. It is the primary target of the first generation of biologic medications for AS. Blocking TNF-alpha reduces the inflammatory process and can significantly improve symptoms.
Thoracic spine(Thoracic vertebrae (T1-T12)): The middle portion of the spine — 12 vertebrae in the mid-back, each connecting to ribs. AS affects the thoracic spine less commonly than the lumbar spine and sacroiliac joints, but thoracic involvement can cause mid-back pain, restricted chest expansion, and costochondritis.

U

Uveitis(Anterior uveitis / iritis): Inflammation of the uvea — the middle layer of the eye. Anterior uveitis (iritis) is the most common form in AS, affecting 25-40% of patients at some point. It presents as sudden eye pain, redness, and light sensitivity — usually in one eye. It requires urgent treatment (steroid eye drops) to prevent complications including vision loss.