Effect of plasmapheresis on serum and CSF autoantibody levels in CNS paraneoplastic syndromes. We compared the effect of plasmapheresis on antineuronal autoantibody titers in the serum and CSF of 3 patients with CNS paraneoplastic syndromes. Plasmapheresis reduced the serum autoantibody titer to 20% of the initial levels in the 3 patients, but the CSF autoantibody titer decreased only in the patient with severe damage of the blood-brain barrier. 