A 17-year-old female, previously healthy, was admitted with symptoms of sudden hip pain and fever.
Blood count showed microcytic anaemia, and increased leukocytes. C-reactive protein and erythrocyte sedimentation rates were elevated. Blood and throat cultures, covid PCR tests were negative.
A diagnostic work-up for possible arthropathies and other chronic infectious diseases were done and all resulted unremarkable. Tuberculosis spot test and QuantiFERON tests were negative. Brucella serologic tests were negative.
Initial plain pelvic radiography was not remarkable. An abdominopelvic CT was done to rule out appendicitis which showed minimal sclerosis on the right sacroiliac joint (Figure 1). The patient was discharged with anti-inflammatory analgesics, and 2 weeks after she had complaints of increased joint pain, fever, and diarrhoea.
Initial ultrasound imaging showed a loculated collection at the level of the right sacroiliac joint. MRI showed decreased T1, increased T2 signal intensity of ilium and sacrum adjacent to right sacroiliac joint. And two T2 hyper, T1 hypointense nodular lesions which showed peripheric contrast enhancement in the iliacus muscle and posterior to sacroiliac joint were seen. Imaging was compatible with right sacroiliitis complicated with abscess formation (Figure 2). Patient was admitted to the hospital and recieved an antibiotic treatment regimen. A control imaging of the pelvis was obtained and showed no radiologic remission of the disease (Figure 3).
Acute sacroiliitis is an uncommon clinical entity that presents with fever, and lower back pain radiating to the buttocks. [1] Acute sacroiliitis is mostly seen in the young adult population owing to the high blood supply to the sacroiliac joint. [2]
In pyogenic sacroiliitis, the leukocyte count and c-reactive protein level are significantly higher than spondyloarthropathies. [3] Blood culture helps isolate the microorganism in pyogenic cases. Empirical treatment with gram-positive coverage should be started after the cultures are taken since mostly s. aureus and streptococcus species are isolated in the pediatric population. [4]
Three imaging features of infective aetiology on MRI are defined as intense bone marrow oedema with intra-articular fluid, inflammation involving peri-articular soft tissues, and fluid collection or abscess formation. [5]
The case raised two questions. One of which was despite the negative cultures, and imaging favourable of infective aetiology, should the next decision be made on the interventional joint aspirate. In this case, due to clinical remission of the disease during hospital admission, joint aspiration was not considered.
And the second one is if there is an ongoing inflammatory disease of the sacroiliac joint in cases with acute pyogenic sacroiliitis since it is a rare clinical entity. In some cases; recent trauma, skin infections, genitourinary or gastrointestinal infection was defined. In this case, there was minimal sclerosis of the sacroiliac joint at the beginning. Despite the resolution of clinical symptoms, some inflammatory markers remained elevated. Most of the possible etiologies of arthropathies have been searched with a diagnostic workup but yet were unremarkable.
Take-Home Message / Teaching Points:
Acute sacroiliitis is a rare condition mostly seen in young adults due to the high blood supply to the sacroiliac joint.
Diagnosis of sacroiliitis is challenging due to its rarity and clinical mimickers.
Infectious sacroiliitis could be diagnosed by imaging and laboratory findings, but the suspicion should be raised if it is solely an infection or infection superposed on other clinical conditions.
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Acute pyogenic sacroiliitis
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Based on the provided CT and MRI images, the following main features can be observed:
Combined with the patient’s acute hip pain and fever, the imaging findings are consistent with acute sacroiliitis, particularly suggestive of an infectious (pyogenic) etiology.
Based on the patient's symptoms, laboratory results, and imaging findings, the main considerations for diagnosis or differential diagnosis are as follows:
Taking into account the patient’s age, acute onset, local imaging changes (pronounced bone marrow edema, soft tissue inflammation), and laboratory results indicating acute inflammatory response (elevated WBC, CRP), the most likely diagnosis is:
“Acute Pyogenic Sacroiliitis (Acute Infectious Sacroiliitis)”
Although blood cultures and certain etiological tests are negative, this is still relatively common in clinical practice. The patient’s clinical progression improved during hospitalization, which is also consistent with early empirical antibiotic therapy for infectious sacroiliitis.
Based on the consideration of “acute pyogenic sacroiliitis,” it is recommended to proceed with the following management, after ruling out other chronic infections or organic diseases:
Disclaimer: This report provides a professional reference analysis based on the existing imaging and clinical information and does not replace a face-to-face consultation or a professional physician’s opinion. If you have any questions or notice any changes in your symptoms, please seek medical advice from a specialist promptly.
Acute pyogenic sacroiliitis
