VENTILATORY MUSCLE STRENGTH AND ENDURANCE IN MYASTHENIA-GRAVIS

Patients with generalized myasthenia gravis (MG) often have associated ventilatory muscle involvement. It is not known whether patients with isolated ocular muscle involvement have identifiable involvement of t heir ventilatory muscles. Most studies have assessed muscle involvemen t by measuring muscle strength; however, we hypothesized that measures of ventilatory muscle endurance may be more sensitive tests of ventil atory muscle involvement in myasthenia gravis. We studied 17 patients with myasthenia gravis (four with ocular involvement alone and 13 with varying degrees of generalized myasthenia gravis). Spirometry, ventil atory muscle strength (maximum inspiratory and expiratory pressures (M IP and MEP)) and endurance (2 min incremental threshold loading test) were measured before and 20 min after i.m. neostigmine. We compared th e results with those of 10 normal controls. We found no difference bet ween patients with isolated ocular involvement and controls, Ocular my asthenia gravis patients did not improve after neostigmine. The patien ts with generalized myasthenia gravis had reduced baseline ventilatory muscle strength (MIP 67 cmH(2)O (70% of predicted), MEP 86 cmH(2)O (5 0% of pred) and endurance (mean maximal load achieved = 246 g, mean pr essure at highest load ((P) over bar) =; 19.4 cmH(2)O) compared with c ontrols. After neostigmine, there was a significant increase in MIP in patients with generalized myasthenia gravis and a trend towards an in creased MEP. As a group, the patients with generalized myasthenia grav is did not demonstrate a change in their ventilatory muscle endurance after neostigmine; however, there was considerable interpatient variab ility in response. We conclude that patients with isolated ocular MG h ave 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 gr avis.