Respiratory muscle strength and ventilatory failure in neuromuscular diseases: Myotonic dystrophy and motor neurone disease

This thesis has explored the use of pulmonary function tests, sleep studies, volitional and nonvolitional respiratory muscle strength assessment techniques in the evaluation of patients with neuromuscular diseases. I have particularly concentrated on patients with myotonic dystrophy and motor neurone disease, which comprised the largest group of patients apart from Duchenne muscular dystrophy, referred for assessment to Glasgow Respiratory units with expertise in non-invasive ventilation. In a group of 20 myotonic dystrophy (MyD) and 5 subjects with MND, I went on to establish that spirometry remains a useful basic tool. Supplemented with mouth pressures, we can derive useful information regarding global respiratory muscle strength. Additional routine evaluation of static lung volumes and diffusion capacity did not add in useful information that would be useful in routine clinical care. It is well known that sleep worsens respiratory failure. The use of screening sleep studies in assessing patients with neuromuscular diseases remains debatable. 25 subjects with MyD and 12 with MND were studied. Routine sleep studies did not seem to add any useful information that could not be predicted from the daytime pulmonary function tests apart from 12% of MyD patients where it was useful in identifying nocturnal respiratory disturbance. However it is debatable whether treating these abnormalities affects prognosis in the long term. There has been significant contribution to our understanding of respiratory muscle strength in various circumstances such as in children and intensive care setting with the introduction of magnetic phrenic nerve stimulation technique. After establishing the methodology at the laboratory, in a group of 10 patients with myotonic dystrophy, we found this to be useful in providing nonvolitional diaphragm strength. It would be immensely useful in this group as mouth pressures are frequently reduced due to facial muscle weakness. We showed that underestimation of respiratory muscle strength using volitional methods is common in MyD [Mean (sd) Sniff Pdi of 67.1(30.7)cm H2O vs bilateral Tw Pdi of 17.1(9.4) cm H2O]. BAMPS was also a useful daytime marker of nocturnal respiratory disturbances and low Tw Pdi identified 75% (3 out of 4) subjects with sleep related breathing disorder during daytime. Repeat volitional and nonvolitional tests 4-6 months' apart confirmed stable respiratory muscle strength. Lastly, I studied patients with motor neurone disease. This group had severe respiratory muscle weakness even at the outset with marked symptoms. Non-invasive ventilation improved symptoms in its users. Interestingly, we found that the inspiratory and expiratory positive airway pressures required for these patients were significantly lower compared to patients with hypventilation due to obesity and post-polio syndromes