After establishing that a patient has respiratory failure due to myasthenia gravis the dose of anticholinesterase drugs should be optimised. The vital capacity should be monitored before and after small (2 mg) doses of intravenous edrophonium. Swallowing is usually impaired and a nasogastric tube is often needed. Pyridostigmine should be given orally or via the nasogastric tube. Doses more than 90 mg three hourly are rarely necessary. When enteral fluids cannot be absorbed, neostigmine should be given intramuscularly, substituting each 60 mg of oral pyridostigmine with 1 mg of parenteral neostigmine.
Patients with myasthenia gravis respond so dramatically in the short term to PE that a controlled trial has never been undertaken. We used to use 50 ml/kg exchanges on alternate days until an adequate response has been achieved, in most cases after two to five exchanges. Improvement was usually noticeable after the second exchange and lasts for about four to six weeks. However similar clinical benefit and fall in anti-acetylcholine receptor antibody titre have been reported followed IVIg, and we now use IVIg as our treatment of choice for myasthenic crises. This practice has now been endorsed by a randomised trial which showed similar benefit from PE and IVIg.80,81 Cosi et al.82 reported clinical improvement 12 days after a standard course of 0 4 g/kg in 70% of 37 patients treated and the improvement lasted for 60 days in 57%. Arsura et al.83 reported improvement in 11 of 12 patients commencing 36 days after IVIg treatment began, reaching a maximum after 86 days, and lasting an average of 52 days. Sustained improvement has been maintained with repeated courses in a small number of cases.84
As IVIg or PE provide only temporary relief, immunosuppressive treatment should be started, or increased, at the same time. We use oral prednisolone 15 mg/kg on alternate days initially, bring the disease under control, and then taper the dose. In patients with early respiratory failure steroids should be introduced slowly and cautiously because of the danger of a transient worsening during the first week or two of the course. In patients with established respiratory failure on artificial ventilation, this cautious approach is superfluous and a full dose of steroids can be started immediately. Very large doses of steroids, including boluses of intravenous methylprednisolone, should be avoided because of the danger of inducing acute myopathy.42,85 For patients who are inadequately controlled with steroids we add azathioprine. The addition of azathioprine to steroids has been shown to achieve better control of chronic myasthenia.86 If azathioprine is not tolerated, other immunosuppressive agents, such as cyclophosphamide or methotrexate, can be tried. When the respiratory failure due to a myasthenic crisis has been controlled, younger patients and those with thymoma should be assessed for thymectomy.
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