Background: Most patients with ALS have evidence of respiratory muscle weakness at diagnosis, and death is usually due to respiratory failure. Sleep disruption, possibly due to apneas, hypopneas, orthopnea, or REM-related desaturation, is common. The relative impact of these factors on quality of life has not been established.
Methods: The authors recruited 23 subjects with probable or definite ALS. Quality of life was assessed using generic and specific instruments, and respiratory muscle strength by measurement of vital capacity, maximum static pressures, and sniff nasal inspiratory pressure. Twenty-two subjects underwent polysomnography. Overall limb and axial muscle strength was estimated using a summated muscle score based on the Medical Research Council clinical scale.
Results: On univariate analysis, there were moderate to strong correlations between quality of life and all measurements of respiratory muscle function (R = 0.42-0.82). The correlations with selected polysomnographic indices were weaker and less consistent (R = 0.44-0.59). Multivariate analysis showed that maximum static inspiratory pressure was the strongest independent predictor of quality of life.
Conclusion: Quality of life was strongly and independently related to respiratory muscle function. Relations with polysomnographic indices were weaker and were attributable to respiratory muscle weakness. Respiratory muscle weakness is much more important than the frequency of apneas and hypopneas in determining quality of life in ALS.