COMPARATIVE EFFECTS OF PLASMA-EXCHANGE AND PYRIDOSTIGMINE ON RESPIRATORY MUSCLE STRENGTH AND BREATHING PATTERN IN PATIENTS WITH MYASTHENIA-GRAVIS

Background - Pyridostigmine, an acetylcholinesterase antagonist, is us eful in improving respiratory function in patients with myasthenia gra vis. More recently, plasma exchange has been employed in myasthenia gr avis because it acts presumably by removal of circulating antibodies a gainst acetylcholine receptors. Surprisingly, comparative data on the effects of pyridostigmine and plasma exchange on lung volumes, respira tory muscle strength, and ventilatory control system in patients with myasthenia gravis are lacking. Methods - Nine consecutive patients wit h grade IIb myasthenia gravis were studied under control conditions an d after a therapeutic dose of pyridostigmine. In a second study the pa tients were re-evaluated a few days after a cycle of plasma exchange, before taking pyridostigmine. In each subject pulmonary volumes, inspi ratory (MIP) and expiratory (MEP) muscle force, and respiratory muscle strength, calculated as average MIP and MEP as percentages of their p redicted values, were measured. The ventilatory control system was eva luated in terms of volume (tidal volume, VT) and time (inspiratory tim e, TI, and total time, TTOT) components of the respiratory cycle. Mean inspiratory flow (VT/TI) - that is, the ''driving'' - and TI/TTOT - t hat is, the ''timing'' - components of ventilation were also measured. Results - In each patient treatment relieved weakness and tiredness, and dyspnoea grade was reduced with plasma exchange. Following treatme nt, vital capacity (VC) increased on average by 9.7% with pyridostigmi ne and by 14% with plasma exchange, and MIP increased by 18% and 26%, respectively. In addition, with plasma exchange but not with pyridosti gmine forced expiratory volume in one second (FEV(1)) increased by 16% and MEP increased by 24.5%, while functional residual capacity (FRC) decreased a little (6.8%). The change in respiratory muscle strength w as related to change in VC (r(2)=0.48). With plasma exchange, VT incre ased by 18.6% and VT/TI increased by 13.5%, while neither TI nor TI/TT OT changed. Conclusions - Plasma exchange can be used in patients with myasthenia gravis when symptoms are not adequately controlled by anti cholinesterase agents. Plasma exchange increases respiratory muscle fo rce and tidal volume due to changes in ''driving'' but not ''timing'' of the respiratory cycle.