Eculizumab administration for myasthenia gravis also stabilizes thrombogenicity of catastrophic antiphospholipid syndrome

Background The co‐occurrence of myasthenia gravis and primary antiphospholipid syndrome (APS) is rare. Notably, both the diseases share common complement‐mediated mechanisms. Case Presentation A 36‐year‐old woman, who was previously diagnosed with myasthenia gravis and APS, developed multiple embolisms, involving the brain, kidney and spleen, with severe anemia and platelet reduction. She was diagnosed as catastrophic APS, and intensive immunotherapies, including plasma exchange, high‐dose corticosteroid and rituximab, were introduced. After these therapies, symptoms of both APS and myasthenia gravis worsened, consistent with elevation of immunoglobulin G anti‐beta‐2‐glycoprotein‐I antibody and anti‐acetylcholine receptor antibody. We started eculizumab, which resulted in stabilizing the disease activity of both diseases without notable adverse events. Conclusions Eculizumab can be effective for controlling multiple complement‐mediated pathophysiology.