3. Clinical Presentation and Etiology of Sacroiliitis

The most common clinical manifestation of sacroiliitis is low back pain, which, generally, affects approximately 70% of individuals at some point in their lives ^[13][20]. Moreover, SIJ dysfunction is a major cause of chronic low back pain and may account for up to 20% of low back pain complaints in the general population ^[14][21]. This pain can be unilateral or bilateral, with unilateral pain occurring four times more frequently than bilateral pain ^[14][21]. The pain in the sacroiliac joint and surrounding structures can manifest itself in various patterns, such as lower back, pelvic, buttock, or sacral pain, as well as discomfort in the groin, posterolateral thigh, and abdomen Additionally, patients may experience acute abdominal pain localized in the lower quadrants and tenderness during rectal examination ^[15][16][22,23]. Despite the fact that sacroiliitis can manifest in various ways, its presentation is typically classified into acute and chronic ^[14][21]. Acute sacroiliitis often mimics other conditions and is frequently overlooked during its initial presentation. New-onset severe pain is a significant clinical symptom of acute sacroiliitis. This pain is typically intense and can be disabling for some patients. It intensifies during activities such as running, climbing stairs, or standing up from a seated position, which can hinder their ability to walk, sit, or even shift in bed ^{[14][17][18][19]}[21,24,25,26]. In the context of acute sacroiliitis, it is widely recognized that pyogenic sacroiliitis is the most frequently reported cause in the medical literature ^[14][21]. It typically presents unilaterally and often includes symptoms like a high or low-grade fever and intense, persistent pain originating from the affected sacroiliac joint ^[15][16][22,23]. Proposed factors that may increase the risk of pyogenic sacroiliitis encompass intravenous drug use, infections, especially those originating from skin tissues and infective endocarditis, as well as pregnancy ^[15][20][22,28].  Another worth-noting entity is acute brucellar sacroiliitis, which is clinically similar to pyogenic sacroiliitis. Bilateral involvement of the sacroiliac joints may be observed in up to 25% of patients. Additionally, healthcare providers should not overlook concomitant clinical signs indicative of brucellosis, such as fever, sweating, malaise, and hepatosplenomegaly ^[14][21]. SIJ aspiration, guided by ultrasound or CT, is a diagnostic procedure that plays a crucial role in evaluating acute sacroiliitis, particularly when septic or crystal-induced causes are under consideration ^[11][13]. The procedure is indicated when there is suspicion of septic sacroiliitis, as it allows for the collection of synovial fluid from the joint for microbiological analysis ^[21][30]. This analysis is instrumental in identifying the causative infectious agent, guiding the selection of appropriate antibiotic therapy ^[21][30]. Both ultrasound and CT can be employed to guide the needle placement during SIJ aspiration ^[11][13]. Ultrasound provides real-time imaging, particularly advantageous for visualizing soft tissues ^[22][32], while CT offers detailed cross-sectional images, valuable in certain clinical contexts ^[23][33]. The patient is positioned lying down, and local anesthesia is administered to minimize discomfort. Under the guidance of ultrasound or CT, a thin needle is carefully advanced into the sacroiliac joint space and synovial fluid is aspirated ^[24][34]. Dual-energy-computed tomography (DECT) emerges as a pivotal diagnostic tool in the comprehensive evaluation of sacroiliitis ^[25][35]. Its dual-energy approach enhances sensitivity, enabling the detection of subtle inflammatory changes such as bone marrow edema and synovial enhancement ^[26][36]. The aspirated synovial fluid is then subjected to laboratory analysis ^[27][39]. In cases of septic sacroiliitis, microbiological cultures are performed to identify the infectious organism ^[27][39]. For crystal-induced sacroiliitis, the fluid is examined for the presence of crystals, such as monosodium urate or calcium pyrophosphate crystals ^[28][40]. Positive findings in the synovial fluid analysis confirm the diagnosis of septic or crystal-induced sacroiliitis, providing critical information for targeted and effective treatment ^[29][41].

4. Radiological Findings and Imaging Modalities

4.1. Plain Radiographs

As shown in Figure 12, early sacroiliitis may show subtle erosions at the joint margins ^[30][44]. These erosions are typically irregular and may appear as small, well-defined areas of bone loss ^[30][44]. As the inflammation progresses, the joint may become more sclerotic, leading to increased bone density in the SI joint ^[31][45]. Progressive inflammation can lead to narrowing of the SIJ space, indicating joint damage and dysfunction As shown in Figure 23, early sacroiliitis may show subtle erosions at the joint margins ^[30][44]. These erosions are typically irregular and may appear as small, well-defined areas of bone loss ^[30][44]. As the inflammation progresses, the joint may become more sclerotic, leading to increased bone density in the SI joint ^[31][45]. Progressive inflammation can lead to narrowing of the SIJ space, indicating joint damage and dysfunction [1]. In addition, changes in the subchondral bone, including irregularities, sclerosis, and erosions, are common features of sacroiliitis ^[32][46].