A 32-year-old male with a long history of low-intensity intermittent sciatica, conservatively treated, was referred to the radiology department, presenting with a 14-day history of pain in the lower back radiating posteriorly to the left limb and groin. He had been skiing the week before developing the pain but denied any trauma.
Although the neurologist recommended lumbar spine MRI based on the history and physical examination, during the initial stages of acquisition, large pelvic collections adjacent to the left sacroiliac joint drew attention. Therefore, the examination was shifted to a pelvic MRI, using the sacroiliac joint protocol.
Pelvic MRI revealed massive inflammatory changes in the left sacroiliac joint: enlarged articular space with fluid effusion; extensive periarticular bone marrow oedema; soft-tissue inflammation seen as inhomogeneous low T1-WI, high T2-WI/PD signal of the adjacent left hip muscles (gluteus muscles, iliac, iliopsoas, piriformis, internal obturator - at origin, superior gemellus - more subtle) and of left erector spinae muscles at insertion; numerous synovial fluid outpouchings arising from anterior, posterior and inferior aspects of sacroiliac joint, dissecting away into the depth of gluteus maximus and medius, piriformis, erector spinae muscles, and iliac muscle. The left sciatic nerve appeared edematous, in contact with the abscess wall within the piriformis muscle.
After contrast administration, enhancement of bone and muscle reactive oedema and thick rim enhancement of fluid outpouchings and collections were noted.
Interestingly, bone marrow oedema of the right aspect of sacrum involved left S2-S4 neural foramina, with inflammatory changes of the nerve roots.
Septic arthritis is an inflammatory condition of joints caused by infection with different microorganisms, most frequently bacterias (95%). The sacroiliac joints are a rare location of septic arthritis, accounting for 1-2% of all cases. [1]
Septic sacroiliitis (SSI) can occur from hematogenous spread, direct inoculation secondary to invasive medical procedures and trauma, or by contiguous spread from an adjacent infection process. The commonest etiological agent of SSI is Staphylococcus aureus; others include gram-negative microorganisms or Mycobacterium tuberculosis. [2,3]
As in our case, the heterogeneous and non-specific clinical manifestation of SSI can lead to a delayed diagnosis. Symptoms such as lombogluteal pain (most common), coxofemoral pain, or pelvic pain, to which is added the absence of fever (64,7% of cases), give rise to a diagnostic challenge for the clinician. The biological inflammatory syndrome, although suggestive of SSI, is inconsistent and variable in terms of amplitude. Therefore, the clinical and paraclinical ensemble may be interpreted as lumbar disc herniation, spondylodiscitis, hip pathology, or gynaecological/digestive pathology, leading to a protracted-time to diagnosis, with serious consequences on the patient's outcome. [4,5]
In order to achieve the diagnosis, imaging plays a crucial role.
X-ray is not usually of help, being unremarkable in the early stages. Ultrasound's role may be found in arthrocentesis guidance, although CT-guided procedures are more frequently used, as the sacroiliac joint is hard to reach, and so, its puncture has a high technical difficulty. [2,3]
CT and MRI are the imaging techniques with the highest reliability in diagnosing SSI. CT is able to show signs of inflammation such as widening of the sacroiliac joint, erosions, soft tissue swelling, and abscess formation. However, early abscesses may be easily missed; also CT's incapability to detect subtle bone changes or display bone fluid is well known. [3,6]
MRI has the highest sensitivity in detecting and characterizing in terms of extent and distribution the early inflammatory changes of the sacroiliac joint, such as joint effusion, periarticular bone oedema, soft-tissue swelling; also has a better performance in describing a possible spread of infection to muscle and abscess formation. MRI proves a 50% higher detection rate compared to CT and bone scintigraphy. Consequently, MRI is considered the reference imaging technique in diagnosing SSI. [3,5,7]
SSI is extremely rare a bilateral condition. Unilaterality is not pathognomonic for infectious aetiology, as it may also be seen in spondyloarthropathies, but the first diagnosis to exclude when facing unilateral sacroiliac joint changes on imaging is SSI. [9]
Microbiological analysis of the synovial fluid (usually obtained by CT-guided joint puncture) or positive blood cultures followed by antibiotic-sensitivity testing are important tools contributing to the diagnosis and mandatory requirements for treatment implementation, which is rapid and aggressive intravenous antibiotic therapy according to the implied microorganism. Surgical treatment is indicated in case of lack of response. [1,5]
Written informed patient consent for publication has been obtained.
Septic sacroiliitis
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The patient is a 32-year-old male who presents with lower back pain radiating to the left lower extremity and groin area for approximately two weeks. He has a history of chronic, intermittent sciatica. The current MRI primarily shows the following features:
Based on the above imaging and clinical presentation, the possible diagnoses include:
Taking into account the clinical presentation (lumbosacral to groin radiating pain, acute exacerbation, no obvious trauma) and the imaging findings of unilateral sacroiliac joint inflammatory changes, local soft tissue swelling, and possible purulent fluid, the most likely diagnosis is:
Infectious sacroiliitis (pyogenic sacroiliitis).
To confirm, a sacroiliac joint aspiration and culture could be performed, along with hematology tests (white blood cell count, CRP, ESR, etc.) to identify the pathogen and conduct drug susceptibility testing.
After the acute infection or fever subsides, a gradual exercise and functional training program can be introduced, following the FITT-VP principle:
Pay close attention to stability and posture control around the affected joint in the early stages of rehabilitation. If osteoporosis or other systemic issues are present, exercise caution and consider professional guidance from a rehabilitation therapist or physical therapist.
This report is based on imaging and existing clinical information and is for reference only. It cannot replace in-person consultation or the diagnosis and treatment advice of a professional physician. If you have any questions or if your condition changes, please seek medical attention promptly and follow your clinician’s guidance.
Septic sacroiliitis
