Progressive sacroiliitis due to accessory sacroiliac joint mimicking ankylosing spondylitis: A case report

Abstract

Rationale: An accessory sacroiliac (SI) joint usually has little clinical significance. However, severe arthritic changes can cause chronic buttock or low back pain and can be misdiagnosed as another disease presenting with sacroiliitis such as ankylosing spondylitis (AS).

Patient concerns: A 33-year-old woman was diagnosed with AS due to chronic buttock pain and progressive sacroiliitis on plain X-ray and magnetic resonance imaging (MRI). Her buttock and low back pain gradually worsened despite proper treatment for AS.

Diagnosis: Computed tomography revealed an accessory SI joint with arthritic changes.

Interventions: Nonsteroidal anti-inflammatory drugs (NSAIDs) and restricted movement were prescribed.

Outcomes: The symptoms were controlled with NSAIDs, rest, and restriction of excessive movement. The medication could be stopped after the pain subsided.

Lessons: An accessory SI joint can be a cause of chronic back pain and can be misdiagnosed as AS with sacroiliitis when progressive arthritic changes are observed. Therefore, additional imaging studies other than conventional X-ray or MRI may be required for accurate diagnosis.

Figure 1

Changes on plain radiography of the pelvis during 7 years of follow-up. A, Initial plain radiography showed a normal sacroiliac joint. B, Follow-up plain radiography showed progressive sacroiliitis with subcortical erosions and sclerosis at the right sacroiliac joint (black arrow).

Figure 2

Magnetic resonance imaging of the sacroiliac (SI) joint. Large subcortical erosions at both sacral and iliac sides of the right SI joint were observed. Subcortical erosions were more prominent at the sacral side. Bone marrow edema at the sacral side and minimal synovitis were seen in the right SI joint. Subcortical sclerosis with fat deposition was observed at the sacral side on both SI joints.

Figure 3

Computed tomography (CT) of the pelvic bone. CT showed an accessory sacroiliac joint with well-defined sclerosis and osteophytes and minimal bridging bone formation at the right SI joint. Sclerosis was also observed at the left SI joint.