Differential Diagnosis for Sudden Weakness of Bilateral Leg after Lumbar Sacral Fusion

Single Most Likely Diagnosis

• Epidural Hematoma: This is the most likely diagnosis given the recent history of lumbar sacral fusion and the sudden onset of bilateral leg weakness. The CT findings of a fluid collection extending into the central spinal canal with moderate mass effect on the spinal sac are consistent with an epidural hematoma, which can compress the spinal cord or cauda equina, leading to neurological deficits.

Other Likely Diagnoses

• Epidural Abscess: Although less likely than an epidural hematoma in the immediate postoperative period, an epidural abscess could present with similar symptoms and imaging findings. The presence of fever, elevated white blood cell count, or other signs of infection would increase the likelihood of this diagnosis.
• Seroma or Pseudomeningocele: These are fluid collections that can occur after spinal surgery. While they might not typically cause significant mass effect or neurological symptoms, a large seroma or pseudomeningocele could potentially compress neural structures.

Do Not Miss Diagnoses

• Spinal Epidural Abscess with Sepsis: Although the primary concern might be a hematoma, missing an epidural abscess, especially one associated with sepsis, could be catastrophic due to the potential for rapid neurological deterioration and high mortality rate. Immediate antibiotic therapy and possible surgical intervention are crucial.
• Arachnoiditis: This is an inflammatory condition of the arachnoid membrane that surrounds the spinal cord and nerve roots. It can be a complication of spinal surgery and might present with symptoms similar to those described, although the imaging findings might be less specific.

Rare Diagnoses

• Spinal Cord Infarction: This could occur due to various reasons including embolic events, vasculitis, or atherosclerosis affecting the spinal cord blood supply. It's a rare but serious condition that could present with acute neurological deficits.
• Tumor (e.g., Metastasis or Primary): Although extremely rare in the immediate postoperative period after a lumbar sacral fusion, a spinal tumor could potentially cause similar symptoms and imaging findings, especially if there was an undiagnosed tumor at the time of surgery.

Should Antibiotics be Started?

Given the clinical presentation and imaging findings, starting antibiotics empirically might be considered, especially if there's a high suspicion of an epidural abscess or another infectious process. However, the decision to start antibiotics should be made in consultation with a neurosurgeon or an infectious disease specialist, as the primary concern is likely to be a non-infectious cause (e.g., epidural hematoma) that requires surgical intervention.