In a patient with prior biopsy-proven IgG4-related pachymeningitis who develops recurrent neurologic symptoms years after discontinuing immunosuppression, how do you determine whether this represents relapse versus alternative etiologies?  

Patient previously had biopsy-proven IgG4-related pachymeningitis with mental status changes, treated with rituximab and mycophenolate mofetil (both discontinued >1 year ago). He now has recurrent symptoms (tinnitus and memory difficulty) but normal brain MRI and CSF.

PET scan was negative except for mild focal uptake in the prostate. Serum IgG levels remain normal and there is no eosinophilia. He improved with a 1 month trial of steroids, but symptoms recurred off therapy.

Would you recommend repeat brain imaging or evaluation of the prostate lesion to assess for possible IgG4 disease activity, and when would you consider resuming immunosuppression?