Differential Diagnosis for the Complication

The patient's symptoms of high fever, dyspnea, and hypotension following intravesical BCG instillation for high-grade T1 urothelial carcinoma suggest a systemic reaction. Here is a categorized differential diagnosis:

• Single most likely diagnosis

  • BCG sepsis or BCGosis: This is a known complication of intravesical BCG therapy, where the live attenuated BCG bacteria can cause a systemic infection, leading to symptoms such as high fever, dyspnea, and hypotension. The timing of the symptoms, 12 hours after the instillation, supports this diagnosis.

• Other Likely diagnoses

  • Sepsis from another source: Although less likely than BCG sepsis given the context, sepsis from a urinary tract infection or another source could present similarly and should be considered, especially if the patient has other risk factors or symptoms.
  • Systemic inflammatory response syndrome (SIRS) due to the BCG: This could be considered a variant of the systemic reaction to BCG, where the body's response to the BCG does not involve a clear infection but still leads to significant systemic symptoms.

• Do Not Miss diagnoses

  • Sepsis from a urinary tract infection: While the context points towards BCG as the cause, missing a urinary tract infection that has progressed to sepsis could be catastrophic. It's crucial to rule out other sources of infection.
  • Anaphylactic reaction: Although rare, an anaphylactic reaction to BCG or another component of the instillation could present with similar symptoms and requires immediate recognition and treatment.

• Rare diagnoses

  • Tuberculosis (TB) reactivation: In patients with a history of TB, intravesical BCG could theoretically reactivate the disease, although this would be extremely rare and typically not present so acutely.
  • Other drug reactions: If the patient is on other medications that could cause systemic reactions, these should be considered, though they are less likely given the temporal relationship to the BCG instillation.