Infectious Sacroiliitis in a Patient With a History of IV Drug Use

Discussion

Infectious sacroiliitis (ISI) is a rare form of infectious arthritis affecting the SIJ, with an incidence of 1 to 2 reported cases per year.¹ The literature on ISI currently consists only of case reports and case series. This infection is often diagnosed after the disease has progressed, with a mean time to diagnosis of 43.3 days.²

Infectious arthritis of any joint has a prevalence of 2 to 10 per 100,000 people. In 50% of cases, the knee is the joint most commonly affected, followed by the hip, shoulder, and elbow.³ Regardless of location, infectious arthritis is associated with significant morbidity and mortality due to sepsis and irreversible loss of joint function.⁴

Risk factors for ISI include IV drug use, pregnancy, trauma, endocarditis, and immunosuppression.¹ The decision to initiate the workup for ISI can be difficult to make because the condition may present without signs of an infectious etiology, such as toxic appearance, inflammatory changes surrounding the joint, or even fever—only 41% of affected patients in one case series were febrile.² The workup is often time-consuming, invasive, and expensive.

Although delayed diagnosis and treatment of septic arthritis is associated with significant adverse effects, there is unfortunately no consensus to guide the workup for ISI. As opposed to Kocher’s criteria for the differentiation of septic hip arthritis from transient synovitis in pediatric patients or well-known red-flags for further evaluation of low back pain, physicians are left without much guidance when considering laboratory workup or imaging decisions to evaluate for ISI.

Sacroiliac Joint

As previously noted, the SIJ is not commonly affected by infection. It is a diarthrodial, L-shaped joint comprised of the posterior ilium and sacrum, and is a near-rigid structure with very limited movement that provides stability to the axial skeleton.⁵ The SIJ is often overlooked as a secondary cause of low back pain in younger patients with rheumatologic conditions (eg, ankylosing spondylitis, Reiter syndrome), pregnancy-associated ligamentous laxity, and osteoarthritis in elderly patients. In one study, 88.2% of sacroiliitis cases were inflammatory, 8.8% infectious, and 2.9% degenerative.⁶

Signs and Symptoms

As our case illustrates, ISI often presents with nonspecific symptoms and physical findings.⁷ Patients typically present with fever, painful manipulation of the SIJ, and unilateral lumbo-gluteal pain.² The components of the history and physical examination suspicious for an infectious etiology include the subacute presentation; unresolved pain despite treatment; tenderness to palpation; decreased range of motion; and recent IV drug use, which increases the risk of infectious disease due to unsterile practices and direct inoculation of pathogens into the bloodstream⁸ and a further predilection into the axial skeleton. ⁹ It is important to obtain an accurate social history; however, patients may not be forthright about disclosing sensitive information such as sexual history and illicit drug use.

Physical Assessment

The SIJ is best appreciated in the seated patient by palpating one fingerbreadth medial to the posterior superior iliac spine as he or she slowly bends forward.¹⁰ Tenderness elicited while in this position is suggestive of SIJ inflammation. The area of tenderness may be lower than anticipated and lateral to the gluteal cleft, as synovial fluid is typically relegated to the lower half of the joint.

Several adjunctive physical examination maneuvers, such as the Gaenslen test and Flexion Abduction External Rotation test (FABER test or Patrick’s test) can isolate SIJ pathology or dysfunction. The Gaenslen test is performed by asking the patient to lie supine and flex the affected hip and knee, with the lumbar spine flat against the examination table. Hyperextending the contralateral thigh downward will reproduce pain in the affected SIJ.

The FABER test is a simple but less specific examination technique to assess joint pain in the hip, lumbar, and sacroiliac joints.¹¹ In this assessment, the clinician flexes the patient’s affected knee to 90°, externally rotates the hip, and applies downward pressure on the knee. Pain reproduced in the affected SI region is sensitive for joint inflammation.

Laboratory and Imaging Studies

Laboratory studies typically show inconsistent and nonspecific findings, such as the elevated ESR and CRP levels seen in our patient.^2,12 Imaging studies to assess the SIJ for signs of infection are therefore essential for confirming infection.

Magnetic resonance imaging is the preferred imaging modality to assess for ISI, since it has the highest sensitivity in visualizing joint effusion and bone marrow edema compared to other modalities. Computed tomography, however, can be helpful in visualizing associated abscesses and guiding arthrocentesis.¹² Plain X-ray may not demonstrate early changes in bone.¹³ The confirmatory study for ISI is synovial fluid analysis and culture.⁷

Treatment