AQP4 and granulomatosis with polyangiitis: A-ON can be caused by auto-immune diseases such as Neuromyelitis Optica Spectrum Disorders (NMOSD) with Anti-Aquaporin-4 IgG antibodies (AQP-4 IgG), Myelin Oligodendrocyte Glycoprotein IgG antibody associated disorders (MOGAD); Systemic Lupus Erythematosus (SLE); Behcet’s disease or Granulomatosis with Polyangiitis (GPA) or systemic inflammatory diseases such as Sarcoidosis.