To the Editor:
Sacroiliitis is associated with the spondyloarthropathies (SpA) including ankylosing spondylitis, psoriatic arthritis, and reactive arthritis (ReA), and may be visualized using magnetic resonance imaging (MRI). Here, we describe 4 cases of acute unilateral sacroiliitis with florid MRI appearances that mimicked infection but demonstrated a prompt and complete response to nonsteroidal antiinflammatory drugs (NSAID).
All patients in this report have given written consent for publication. Institutional review board approval for ethics was not required because patients were managed according to generally accepted standards of care prior to this case series report.
All subjects were HLA-B27–negative males and presented with a rapid symptom onset of unilateral sacroiliitis lasting from 2 days to 4 weeks (Table 1). One subject had a prior history of ulcerative colitis (Case 2) in remission and 1 had scalp psoriasis (Case 3). There were prodromal symptoms in 2 subjects (Cases 1 and 4) with short-lived fever at presentation. Case 4 had a sore throat preceding the presentation with neutrophilia (12.4 × 109/l), which prompted an infection screen. All 4 patients demonstrated a significant elevation in acute-phase markers, with a mean serum C-reactive protein (CRP) of 115 mg/l. There were no overt clinical features of systemic inflammatory response.
Clinical characteristics of 4 HLA-B27–negative subjects presenting with acute unilateral sacroiliitis.
Baseline MRI demonstrated florid bone marrow edema (BME) in 3 cases (Cases 1, 2, and 4) affecting > 75% of the sacroiliac joints (SIJ) and moderate (affecting 25–75%) in Case 3 (Figure 1). High signal was noted in surrounding muscle and soft tissue in all cases by the reporting radiologists, who observed the need to exclude infection. SIJ aspiration/biopsy was considered in all cases but not conducted because of the prompt symptom response following NSAID, with improvement in clinical variables and negative septic screen.
Coronal STIR MRI examination of the sacroiliac joints per case. Cases labeled by corresponding number. STIR: short-tau inversion recovery; MRI: magnetic resonance imaging.
Case 2 was advised to continue empirical combined oral antibiotics for 4 weeks. In addition, he continued NSAID therapy for 8 weeks until complete symptom resolution. Case 4 cultured group A Streptococcus from a throat swab and borderline antistreptolysin titer of 466 IU/ml and 406 IU/ml, respectively, suggesting plausible post-streptococcal ReA. Repeat MRI was performed in 3 patients at a mean followup of 5 weeks, which demonstrated improved but persistent inflammatory changes. Further imaging thereafter revealed significant improvement in BME changes in Cases 1 and 3, at 2 and 5 months, respectively.
Sacroiliitis typifies SpA but can also occur in sepsis where diffuse soft tissue edema, in addition to BME, is characteristic1. The symptom onset in SpA can be acute and may include fever and raised CRP, therefore mimicking infection. Bilateral sacroiliitis is invariably inflammatory; however, an acute unilateral presentation is frequently reported in the literature as pyogenic or suspicious for atypical organisms1. The current case series demonstrates that acute unilateral sacroiliitis with “extreme” MRI appearances, particularly with extensive sacroiliac BME and adjacent peri-articular muscle/soft tissue edema can, despite resembling infection, represent a reactive process suggestive of an inflammatory SpA. These cases illustrate the diagnostic challenge of differentiating infection versus inflammation. This is particularly important given that patients typically present through urgent appointments (Cases 3 and 4 presented to the emergency department requiring hospitalization). All patients demonstrated a good response to NSAID. Although the dose and duration of NSAID required to alter BME is unclear, our data support previous reports in the literature2. We acknowledge that the effect of NSAID cannot be quantitatively measured from these series, particularly as postinflammatory changes were still visible in 2 cases after 5 weeks. Remarkably, however, all patients were symptom-free within 8 weeks.
Acute unilateral sacroiliitis can be a manifestation of ReA3,4. During a Campylobacter jejuni outbreak, 1 in 15 cases of ReA presented with sacroiliitis4. Similarly, sacroiliitis is a rare manifestation of post-streptococcal ReA5. Pseudosepsis has been observed in psoriasis, palmoplantar pustulosis, acne, and the synovitis, acne, pustulosis, hyperostosis, and osteitis syndrome but is an unusual cause for de novo acute unilateral sacroiliitis6. The severity of sacroiliitis at baseline regardless of HLA-B27 status has been shown to be a predictor of poor prognosis for radiographic progression, but little is known specifically for acute ReA7. When managing such cases, it is essential not to overlook infectious sacroiliitis typified on MRI by periarticular muscle edema, although the cases presented here also demonstrate that inflammatory disease can mimic such appearances8. Interestingly and although within the spectrum of SpA, our cases could not be classified according to the Assessment of Spondyloarthritis international Society classification criteria given the acute onset of symptoms of < 3 months’ duration9,10.
These case reports highlight that significant reactive inflammatory sacroiliitis can yield MRI appearances mimicking infection. Thorough investigation should always be prioritized, but NSAID alone can be effective in resolving symptoms over several weeks, with eventual patient recovery.
Acknowledgment
The authors thank the patients for allowing their information to be shared for this report. This research is supported by the National Institute for Health Research (NIHR) Leeds Biomedical Research Centre. The views expressed are those of the authors and not necessarily those of the National Health Service (NHS), the NIHR, or the Department of Health, UK.
