The cardinal symptom of respiratory muscle weakness is dyspnoea. When muscle weakness becomes more obvious, symptoms may also occur at rest, dyspnoea, hypercapnia and/or speech problems disable the patient. In the case of severe expiratory muscle weakness, reduced cough efficiency may become an important handicap and patients may become ventilator dependent. Only in severe respiratory muscle dysfunction, vital capacity is generally reduced as a consequence of the respiratory muscle weakness and may become a better predictor of morbidity than measurements of respiratory muscle strength