Differential Diagnosis for Female MS Patient on Rituximab

Single Most Likely Diagnosis

• Rituximab-induced cytokine release syndrome or infusion reaction: Although the last dose was 10 days ago, some reactions can be delayed. The high fever, severe back and bilateral shoulder pain, elevated CRP, and WBC count are consistent with a systemic inflammatory response that could be related to rituximab.

Other Likely Diagnoses

• Infection (e.g., pneumonia, urinary tract infection): The high fever and elevated inflammatory markers suggest an infectious process. Given the patient's immunocompromised state due to rituximab, she is at increased risk for infections.
• Multiple sclerosis relapse: Although there are no clear focal neurological deficits, MS relapses can sometimes present with more generalized symptoms, including pain. The absence of new sphincteric symptoms or sensory level does not rule out a relapse entirely.
• Osteomyelitis or septic discitis: The severe back pain could indicate an infectious process involving the bones or discs of the spine, especially in an immunocompromised patient.

Do Not Miss Diagnoses

• Spinal epidural abscess: This is a medical emergency that requires prompt diagnosis and treatment. The presence of severe back pain, fever, and elevated inflammatory markers, even without clear neurological deficits, mandates consideration of this diagnosis.
• Neurosarcoidosis or other granulomatous disease: Although less common, these conditions can present with systemic symptoms, including fever and pain, and can be exacerbated or triggered by immunomodulatory therapies.
• Sepsis: Given the patient's high fever, elevated WBC, and CRP, sepsis from any source (infectious or non-infectious) must be considered and promptly addressed.

Rare Diagnoses

• Rituximab-induced progressive multifocal leukoencephalopathy (PML): While PML is a rare but serious side effect of immunosuppressive therapies, the presentation typically includes focal neurological deficits, which are not described in this case.
• Hematologic malignancy: Rituximab is used in the treatment of certain lymphomas, but it can also increase the risk of other malignancies. The symptoms could potentially be related to a new or underlying malignancy, although this would be less likely given the acute presentation.