Acute Sacroiliitis – Unilateral: A Comprehensive Clinical and Radiologic Review

Keywords: Acute sacroiliitis, unilateral sacroiliitis, sacroiliac joint inflammation, MRI sacroiliitis, pelvic pain, infectious sacroiliitis, sacroiliitis treatment.

Introduction

Acute sacroiliitis is an inflammatory disorder involving one or both sacroiliac (SI) joints, with the unilateral form most commonly associated with infectious, reactive, or traumatic etiologies. The sacroiliac joint is a critical articulation that transmits axial loads from the spine to the pelvis and lower extremities, and inflammation here can cause profound pain and disability.

In unilateral acute sacroiliitis, timely diagnosis is crucial because the underlying cause often requires urgent intervention — particularly in infectious cases. This post explores the cause, etiology, pathophysiology, epidemiology, clinical presentation, imaging characteristics, treatment, and prognosis of acute unilateral sacroiliitis, illustrated with MRI case findings.

Case Summary

Patient: 36-year-old male
Chief Complaint: Pelvic and lumbar pain on the left side.
Laterality: Left sacroiliac joint involvement.

MRI Findings

[Figure 1] Axial T1-weighted image: Low signal intensity involving both articular surfaces of the left SI joint.

[Figure 2] Axial T2-weighted image: High signal intensity consistent with edema and inflammation.

[Figure 3] Coronal T1-weighted image: Hypointense marrow changes localized to the left SI joint.

[Figure 4] Axial T2 fat-saturated image: Marked bone marrow edema and joint effusion with periarticular muscle involvement.

[Figure 5] Coronal T2 fat-saturated image: Widespread inflammatory changes across the joint space and adjacent musculature.

Interpretation: Extensive bone marrow edema on the iliac and sacral surfaces of the left SI joint with associated joint effusion and myositis of the ipsilateral iliacus, piriformis, gluteus medius, and minimus muscles. The right SI joint appears normal.

Causes and Etiology

Acute unilateral sacroiliitis arises from multiple possible etiologies:

Infectious Causes (Pyogenic Sacroiliitis)
- Bacterial: Staphylococcus aureus is the most common pathogen, followed by Streptococcus spp., and Gram-negative bacilli.
- Routes of infection: Hematogenous spread, direct extension from adjacent infection, or penetrating trauma.
Inflammatory Causes
- Seronegative spondyloarthropathies (ankylosing spondylitis, psoriatic arthritis, reactive arthritis) often begin as bilateral but may initially appear unilateral.
- Autoimmune-mediated synovitis and enthesitis trigger inflammation.
Post-traumatic or Mechanical Causes
- Direct injury to the pelvis or repetitive microtrauma may cause localized inflammation.
Postpartum Sacroiliitis
- Relaxin-mediated ligamentous laxity combined with childbirth trauma can lead to acute unilateral SI joint inflammation.
Other Causes
- Tuberculosis (Pott’s disease extension), brucellosis, and fungal infections in immunocompromised patients.

Pathophysiology

The sacroiliac joint is partly synovial (anterior) and partly fibrous (posterior). Its unique anatomy makes it prone to infection and inflammation due to:

Limited vascularity: Infections can persist once established.
High mechanical stress: The joint absorbs axial and torsional loads.
Immune-mediated injury: Activated T-cells and inflammatory cytokines lead to osteitis, cartilage destruction, and periarticular edema.

In infectious cases, hematogenous spread is the predominant pathway, especially in younger adults, with bacterial seeding of the synovium causing rapid inflammatory destruction. Myositis of adjacent muscles is often a secondary phenomenon due to local spread.

Epidemiology

Incidence: Sacroiliitis accounts for 1–2% of all cases of septic arthritis.
Age distribution: Most infectious unilateral cases occur between 20–40 years of age.
Gender: No strong sex predilection for infection; ankylosing spondylitis (usually bilateral) is more common in men.
Risk factors:
- Intravenous drug use
- Pregnancy/postpartum state
- Immunosuppression (HIV, chemotherapy)
- History of pelvic trauma
- Chronic inflammatory arthropathy

Clinical Presentation

Patients typically present with:

Pain location: Lower back, buttock, or posterior pelvis, often radiating to the thigh or groin.
Pain characteristics: Worsens with weight-bearing, hip extension, or prolonged standing.
Associated symptoms: Fever (in infection), morning stiffness (in autoimmune causes), reduced range of motion.
Gait changes: Antalgic gait due to pain avoidance.

On physical examination, provocative maneuvers such as the FABER (Flexion, Abduction, External Rotation) test may reproduce pain localized to the affected SI joint.

Imaging Features

MRI is the gold standard for early detection of acute sacroiliitis, particularly in the unilateral form, as radiographs may appear normal in the early stages.

MRI Findings in This Case

Bone marrow edema
- Low signal on T1 ([Figure 1], [Figure 3])
- High signal on T2 and fat-suppressed images ([Figure 2], [Figure 4], [Figure 5])
Joint effusion
- Hyperintense on fluid-sensitive sequences.
Soft tissue changes
- Edema and inflammation of surrounding muscles, especially the iliacus and gluteal group.
Absence of chronic features
- No sclerosis, erosions, or ankylosis, supporting acute onset.

Treatment

The therapeutic approach depends on etiology:

Infectious Acute Sacroiliitis
- Antibiotic therapy: IV antibiotics tailored to culture results; empiric therapy often includes anti-staphylococcal agents.
- Drainage: Ultrasound- or CT-guided aspiration if abscess is present.
- Analgesia and rest: NSAIDs for pain and inflammation.
Non-infectious Acute Sacroiliitis
- NSAIDs: First-line treatment for inflammation.
- Disease-modifying antirheumatic drugs (DMARDs): Methotrexate or sulfasalazine for autoimmune forms.
- Biologic therapy: TNF-α inhibitors for refractory cases.
Rehabilitation
- Gradual return to activity with physiotherapy focused on core stabilization and hip mobility.

Prognosis

Infectious cases: With prompt antibiotic therapy, prognosis is generally good, although delayed diagnosis may lead to chronic pain, joint destruction, or ankylosis.
Inflammatory cases: Prognosis depends on the underlying systemic disease; early control reduces long-term disability.
Postpartum or traumatic cases: Often resolve with conservative therapy within weeks to months.

Figures and Captions

Figure 1: Axial T1-weighted MRI showing hypointense marrow changes in the left sacroiliac joint.

Figure 2: Axial T2-weighted MRI demonstrating hyperintense marrow edema.

Figure 3: Coronal T1-weighted MRI showing low signal intensity across the left SI joint articular surfaces.

Figure 4: Axial T2 fat-saturated MRI highlighting bone marrow edema, joint effusion, and adjacent muscle inflammation.

Figure 5: Coronal T2 fat-saturated MRI showing widespread inflammatory changes across the joint and periarticular tissues.

Quiz

1. In the presented case of acute unilateral sacroiliitis, which MRI sequence best demonstrates the bone marrow edema as high signal intensity in the affected sacroiliac joint?

A. Axial T1-weighted image

B. Axial T2-weighted image

C. Coronal T1-weighted image

D. CT scan without contrast

2. Which associated muscle groups showed oedematous infiltration in this patient’s MRI findings?

A. Gluteus maximus and quadratus lumborum

B. Iliacus, piriformis, gluteus medius, and gluteus minimus

C. Rectus femoris and tensor fascia latae

D. Psoas major and adductor magnus

3. What is the most common bacterial pathogen responsible for acute infectious unilateral sacroiliitis in adults?

A. Escherichia coli

B. Staphylococcus aureus

C. Streptococcus pneumoniae

D. Mycobacterium tuberculosis

Answer & Explanation

1. Answer: B. Axial T2-weighted image. Explanation: Bone marrow edema is hypointense on T1-weighted images and hyperintense on fluid-sensitive sequences such as T2-weighted or fat-suppressed images. In this case, the Axial T2-weighted image ([Figure 2]) clearly shows hyperintense marrow signal in the left sacroiliac joint due to acute inflammation.

2. Answer: B. Iliacus, piriformis, gluteus medius, and gluteus minimus. Explanation:

The MRI findings revealed joint effusion with oedematous infiltration of periarticular muscles on the left side, specifically involving the iliacus, piriformis, gluteus medius, and minimus. This pattern is typical of infectious or severe inflammatory sacroiliitis due to local spread of inflammation ([Figure 4] and [Figure 5]).

3. Answer: B. Staphylococcus aureus. Explanation: Staphylococcus aureus is the leading cause of acute pyogenic sacroiliitis, followed by streptococcal species and Gram-negative bacilli. Infections typically occur via hematogenous spread and can progress rapidly if not treated promptly. Mycobacterium tuberculosis is more often associated with chronic granulomatous infection rather than acute presentation.

References

[1] P. A. Howard et al., “Septic sacroiliitis: A report of 20 cases,” Clin Infect Dis, vol. 36, no. 3, pp. 319–325, 2003.
[2] J. M. Vyskocil, R. J. McIlroy, and J. B. Brennan, “Septic sacroiliitis: Review of 166 cases,” Medicine (Baltimore), vol. 70, no. 3, pp. 188–197, 1991.
[3] H. A. Braun and J. Sieper, “Sacroiliitis and spondyloarthropathy: Pathophysiology and imaging,” Radiology, vol. 286, no. 1, pp. 29–42, 2018.
[4] L. Puhakka et al., “Acute and chronic changes in sacroiliitis: MRI findings,” Radiographics, vol. 24, no. 2, pp. 577–592, 2004.
[5] J. Y. Lee et al., “MRI diagnosis of septic sacroiliitis,” AJR Am J Roentgenol, vol. 203, no. 1, pp. 143–148, 2014.
[6] E. F. Resnick and D. Niwayama, “Diagnosis of inflammatory sacroiliitis by MRI,” Semin Musculoskelet Radiol, vol. 16, no. 3, pp. 282–291, 2012.
[7] G. A. Govender, “Infectious sacroiliitis in adults: Clinical and MRI features,” S Afr Med J, vol. 97, no. 1, pp. 44–46, 2007.

MediAI

Search This Blog