Pyogenic sacroiliitis

Clinical History

A 40 year old male patient presented with left lumbar pain and tenderness radiating to the left gluteal region, with night sweats and chills in the last 48 hours. Occasional user of intravenous cocaine. Analysis: CRP 58.5 mg /l and ESR 92mm.

Imaging Findings

Pelvic radiograph (Fig. 1) was normal. CT with intravenous contrast showed subtle enlargement of the left sacroiliac joint, irregularities in the articular surface and bony erosions [Figure 2 a, b,c], an enlargement of the iliopsoas, obturator and gluteus minimus muscles, and multiple collections in these muscles suggestive of abscesses [Figure 3 a, b].

Discussion

Pyogenic sacroiliitis is an uncommon infection often diagnosed late because of poor localization of symptoms and inadequate physical examination. It accounts for 1-2% of all cases of septic arthritis.[1]
Initial symptoms are usually non-specific and difficult to differentiate from sciatica or septic arthritis of hip, and sometimes may mimic acute abdomen and sepsis, so clinical examination is important for correct orientation of the diagnosis, and therefore evaluation with appropriate imaging is usually required for definitive diagnosis.
Delay in diagnosis may lead to several severe complications such as abscess, prolonged period of sepsis, long-term joint deformity and disability and even death. The most common causative agent is Staphylococcus aureus.[1, 2]
Plain radiograph is rarely helpful. Bone scintigraphy is a sensitive test, but lacks specificity. CT clearly shows bone and soft tissue involvement, and may have a role in aspiration or biopsy. MRI is sensitive for bone marrow change.[3]
In the presence of joint space narrowing, it is important to differentiate between inflammatory, infectious and degenerative conditions. Joint inflammation is characterized by bony erosion, osteopenia, soft-tissue swelling. The hallmark of joint inflammation is erosion of the bone. If joint inflammation is limited to a single joint, infection must be the first concern.
Features of septic arthritis encompass those of any inflammatory arthritis, periarticular osteopenia, soft-tissue swelling, and bone erosions. Not all findings may be present simultaneously. Furthermore, the joint space may be initially widened owing to the effusion.[4]
The unilateral involvement, rapid progression of erosions, joint destruction and abscess formation are important clues for differential diagnosis between inflammatory arthritis (Rheumatoid Arthritis, seronegative spondylarthropathy, …) and septic arthritis.
For an appropriate therapy it is essential to make a specific diagnosis, particularly a differentiation between tuberculous arthritis and pyogenic arthritis. Insidious onset of disease, substantial osteopenia, minimal sclerosis, and relative preservation of joint space favour the diagnosis of tuberculous arthritis, as pyogenic arthritis usually has the more aggressive course. However, tuberculosis may have a virulent pattern of destruction. Ultimate diagnosis can be made by isolating the causative organism from the synovial fluid or by performing a synovial biopsy.[5]
In our patient, non-imaging clues such as the personal history (the drug abuse), the symptoms and analytic results were useful for the correct diagnosis. We performed a fine needle aspiration of the abscess in the muscle and S. aureus was isolated, then the patient was treated with iv cloxacillin, with good result and complete resolution of the abscesses after 28 days without the need of surgical drainage.

Differential Diagnosis List

Final Diagnosis

Infectious sacroiliitis complicated by pelvic muscle abscesses

URL:	https://www.eurorad.org/case/10303
DOI:	10.1594/EURORAD/CASE.10303
ISSN:	1563-4086