Differential Diagnosis for Sacroilitis

The patient presents with sacroilitis that worsens at night, without other symptoms. Here's a categorized differential diagnosis:

• Single Most Likely Diagnosis

  • Ankylosing Spondylitis (AS): This chronic inflammatory disease primarily affects the spine and sacroiliac joints, often causing nocturnal pain that improves with activity. The absence of other symptoms does not rule out AS, as it can present with isolated sacroilitis early in the disease course.

• Other Likely Diagnoses

  • Psoriatic Arthritis: Although typically associated with skin and nail changes, some patients may present with isolated sacroilitis and nocturnal pain, especially if the psoriatic skin lesions are minimal or overlooked.
  • Reactive Arthritis: Following a gastrointestinal or genitourinary infection, reactive arthritis can cause sacroilitis. Nocturnal pain is common, and other symptoms like conjunctivitis or urethritis might be absent or mild.
  • Inflammatory Bowel Disease (IBD)-associated Arthropathy: Patients with Crohn's disease or ulcerative colitis can develop sacroilitis as an extraintestinal manifestation. The gastrointestinal symptoms might be mild or absent, making the diagnosis challenging.

• Do Not Miss Diagnoses

  • Osteomyelitis or Septic Sacroilitis: Infectious causes of sacroilitis can present with nocturnal pain and might not have systemic symptoms like fever early in the course. Missing this diagnosis could lead to severe consequences, including bone destruction and sepsis.
  • Tumor or Metastasis: Although rare, a tumor in the sacrum can cause nocturnal pain due to increased pressure at night. Missing this diagnosis could delay necessary oncologic treatment.

• Rare Diagnoses

  • Brucellosis: This zoonotic infection can cause sacroilitis among other symptoms. It's rare in many parts of the world but should be considered in individuals with exposure to infected animals or their products.
  • Sarcoidosis: This systemic granulomatous disease can rarely cause sacroilitis. The diagnosis might be challenging without other systemic symptoms or findings suggestive of sarcoidosis.