Sacroiliitis

There are several causes. The most important group is axial spondyloarthritis — inflammatory conditions in which the immune system drives inflammation of the sacroiliac joints and spine, often beginning in younger adults. Other causes include mechanical wear-and-tear (degenerative) change in the joint, the increased strain and hormonal changes of pregnancy, other forms of inflammatory arthritis (such as those linked to psoriasis or inflammatory bowel disease), and, rarely, infection of the joint. Identifying the cause is central, because treatment differs substantially between them.

The typical symptom is pain in the buttock and lower back, sometimes spreading into the back of the thigh, which may be felt on one or both sides. A particularly important pattern is "inflammatory back pain": pain that begins in younger adulthood, comes on gradually, is associated with prolonged morning stiffness, wakes people in the second half of the night, and — tellingly — improves with movement and exercise rather than with rest. This pattern raises the suspicion of axial spondyloarthritis. Mechanical or pregnancy-related sacroiliitis, by contrast, tends to be more clearly linked to activity and position.

Certain features point towards an inflammatory cause and prompt referral to a rheumatologist: inflammatory back pain as described above, a good response to anti-inflammatory medication, a family history of related conditions, and associated features such as psoriasis, inflammatory bowel disease, or episodes of a painful red eye (uveitis). Separately, sacroiliitis accompanied by fever and severe, rapidly worsening pain may indicate the rare but important possibility of joint infection, which needs urgent assessment.

Diagnosis combines the pattern of symptoms, a clinical examination (including specific tests that stress the sacroiliac joints), blood tests, and imaging. Blood tests may show raised inflammatory markers and, in axial spondyloarthritis, a genetic marker called HLA-B27 is often present. MRI of the sacroiliac joints is especially valuable because it can show active inflammation (bone marrow swelling) before any damage is visible on ordinary X-rays, allowing earlier diagnosis; X-rays show established structural change. Where infection is suspected, additional tests and sometimes joint sampling are required.

Because sacroiliitis has different causes, treatment is directed at the underlying condition. For axial spondyloarthritis, anti-inflammatory medication and a structured exercise and physiotherapy programme are first-line, and where the condition remains active, modern targeted medicines that suppress specific parts of the immune system can be highly effective; this care is led by a rheumatologist. Mechanical or pregnancy-related sacroiliitis is usually managed with pain relief, physiotherapy, activity modification and, for pregnancy, supportive measures that settle after delivery. A targeted injection into the sacroiliac joint can relieve pain in selected cases. Infection is treated urgently with antibiotics and, sometimes, drainage.

The outlook depends on the cause. Mechanical and pregnancy-related sacroiliitis often settle with conservative treatment, the latter typically improving after delivery. Axial spondyloarthritis is a long-term condition, but with early diagnosis and modern treatment many people control their symptoms well, stay active, and limit long-term joint damage — which is precisely why recognising the inflammatory pattern early is so important. Infective sacroiliitis usually resolves with prompt, appropriate treatment.

Myth: "Back pain in a young adult is just a strain." Fact: Inflammatory back pain — gradual onset in younger adults, worse at night, better with exercise — can signal sacroiliitis from an inflammatory condition and deserves assessment.

Myth: "A normal X-ray rules out sacroiliitis." Fact: MRI can show active inflammation before X-rays show any change, so a normal X-ray does not exclude early inflammatory sacroiliitis.

Myth: "Sacroiliitis always means a serious lifelong disease." Fact: It has several causes; mechanical and pregnancy-related cases often settle, and even inflammatory disease is well controlled in many people with modern treatment.

This page is for general education and does not replace an individual assessment. Treatment depends on a person's specific symptoms, examination findings, imaging findings and overall health, and should always be discussed with a suitably qualified healthcare professional.

The sacroiliac joints are two strong joints, one on each side, where the triangular bone at the base of the spine (the sacrum) meets the pelvis. Sacroiliitis means inflammation of these joints, and it is a common source of pain in the buttock and lower back. Understanding sacroiliitis matters because it can be the first clue to an underlying inflammatory condition — particularly axial spondyloarthritis, a family of conditions that includes ankylosing spondylitis — which benefits from early recognition and treatment. It can also arise from wear-and-tear, the strain of pregnancy, and, uncommonly, from infection. This page explains the causes of sacroiliitis, how it is diagnosed, and how it is treated depending on what is behind it.

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• —National Institute for Health and Care Excellence (NICE). Spondyloarthritis in over 16s: diagnosis and management. NICE guideline NG65. 2017. https://www.nice.org.uk/guidance/ng65
• —Guideline note (place near references): NICE guideline NG65 (spondyloarthritis) and the ACR/SAA/SPARTAN recommendations informed the management of inflammatory sacroiliitis. Treatment of the underlying cause is led by the relevant specialty (for example rheumatology for inflammatory disease).

This information is for general patient education and is not a substitute for an individual consultation. Always seek personalised advice from a qualified spinal specialist.