APLAR Blackboard (Spondyloarthritis)

Question 1

Sacroiliitis, which is inflammation of one or both sacroiliac joints, can often result in pain and may mimic symptoms of axial spondyloarthritis. However, many other conditions, including infection, may also contribute to sacroiliitis. Which of the following organisms is most commonly associated with infective sacroiliitis?
Question 1
- A. Streptococcus pneumoniae
- B. Staphylococcus aureus
- C. Escherichia coli
- D. Mycobacterium tuberculosis
- E. Pseudomonas aeruginosa
Answer with a detailed explanation
B. Staphylococcus aureus

Sacroiliitis caused by infection is less common than other types. However, it is primarily caused by Staphylococcus aureus bacteria. It is often responsible for septic arthritis in adults and children, and the infection usually occurs via the hematogenous route. Symptoms of Staphylococcus aureus infection can include pain, fever, and difficulty moving the affected joint, which can be similar to the symptoms of axial spondyloarthritis. Therefore, it is important to consider infection as a possible cause when evaluating patients with suspected sacroiliitis.

While other organisms can also cause septic arthritis, they are less commonly associated with sacroiliitis. Mycobacterium tuberculosis can cause skeletal infections such as Pott's disease, but it is not typically the main cause of infective sacroiliitis.

Reference
Dubost J, Soubrier M, De Champs C, et al. No changes in the distribution of organisms responsible for septic arthritis over a 20-year period. Ann Rheum Dis. 2002;61:267–269.

Hermet M, Minichiello E, Flipo RM, et al. Infectious sacroiliitis: a retrospective, multicentre study of 39 adults. BMC Infect Dis. 2012 Nov 15;12:305. DOI: 10.1186/1471-2334-12-305.
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Question 2

Which of the following is a feature visible on an MRI of the sacroiliac joints in diagnosing infectious sacroiliitis?
Question 2
- A. Hypointensity on T1-weighted images only
- B. Hypointensity on T2-weighted images only
- C. Hyperintensity on T1-weighted images only
- D. Hyperintensity on T2-weighted images and hypointensity on T1-weighted images
- E. Hypointensity on T2-weighted images and hyperintensity on T1-weighted images
Answer with a detailed explanation
D. Hyperintensity on T2-weighted images and hypointensity on T1-weighted images

Infective sacroiliitis, an inflammation of the sacroiliac joint due to infection, often presents on MRI as hypointense on T1-weighted images and hyperintense on T2-weighted images. This reflects the presence of oedema and inflammation in the bone marrow and, more importantly, the surrounding tissues in infective sacroiliitis.

The T1-weighted images on MRI depict the anatomy and fat content, while T2-weighted images highlight the fluid content. In infection, there is an increase in fluid content due to inflammation and oedema, which appears brighter or hyperintense on T2-weighted images. Conversely, the inflammatory process may replace the normal fatty marrow, leading to a decreased signal or hypointensity on T1-weighted images. It is important to note that while these findings are suggestive, they are not definitive as clinical correlation always remains essential.

Reference

Tsoi C, Griffith JF, Lee RKL, Wong PCH, Tam LS. Imaging of sacroiliitis: Current status, limitations and pitfalls. Quant Imaging Med Surg. 2019 Feb;9(2):318-335. DOI: 10.21037/qims.2018.11.10.

Kang Y, Hong SH, Kim JY, Yoo HJ, Choi JY, Yi M, Kang HS. Unilateral Sacroiliitis: Differential Diagnosis Between Infectious Sacroiliitis and Spondyloarthritis Based on MRI Findings. AJR Am J Roentgenol 2015; 205:1048-55. DOI:10.2214/AJR.14.14217

By Associate Professor Dr Fariz Yahya, University of Malaya, Kuala Lumpur, Malaysia
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APLAR Blackboard (Spondyloarthritis)

Step 1 of 2

Question 1

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Question 2

Which of the following is a feature visible on an MRI of the sacroiliac joints in diagnosing infectious sacroiliitis?

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