V Ve en nt ti il la at to or ry y m mu us sc cl le e s st tr re en ng gt th h a an nd d e en nd du ur ra an nc ce e i in n m my ya as st th he en ni ia a g gr ra av vi is s We studied 17 patients with myasthenia gravis (four with ocular involvement alone and 13 with varying degrees of generalized myasthenia gravis). Spirometry, ventilatory muscle strength (maximum inspiratory and expiratory pressures (MIP and MEP)) and endurance (2 min incremental threshold loading test) were measured before and 20 min after i.m. neostigmine. We compared the results with those of 10 normal controls. We found no difference between patients with isolated ocular involvement and controls. Ocular myasthenia gravis patients did not improve after neostigmine. The patients with generalized myasthenia gravis had reduced baseline ventilatory muscle strength (MIP 67 cmH 2 O (70% of predicted), MEP 86 cmH 2 O (50% of pred) and endurance (mean maximal load achieved = 246 g, mean pressure at highest load (P) = 19.4 cmH 2 O) compared with controls. After neostigmine, there was a significant increase in MIP in patients with generalized myasthenia gravis and a trend towards an increased MEP. As a group, the patients with generalized myasthenia gravis did not demonstrate a change in their ventilatory muscle endurance after neostigmine; however, there was considerable interpatient variability in response.We conclude that patients with isolated ocular MG have normal ventilatory muscle strength when tested conventionally. The measurement of ventilatory muscle endurance by incremental threshold loading does not provide additional information to the measurement of ventilatory muscle strength in patients with generalized myasthenia gravis.

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