Respiratory function vs sleep-disordered breathing as predictors of QOL inALS

Background: Most patients with ALS have evidence of respiratory muscle weak ness at diagnosis, and death is usually due to respiratory failure. Sleep d isruption, possibly due to apneas, hypopneas, orthopnea, or REM-related des aturation, is common. The relative impact of these factors on quality of li fe has not been established. Method: The authors recruited 23 subjects with probable or definite ALS. Quality of life was assessed using generic and s pecific instruments, and respiratory muscle strength by measurement of vita l capacity, maximum static pressures, and sniff nasal inspiratory pressure. Twenty-two subjects underwent polysomnography. Overall limb and axial musc le strength was estimated using a summated muscle score based on the Medica l Research Council clinical scale. Results: On univariate analysis, there w ere moderate to strong correlations between quality of life and all measure ments of respiratory muscle function (R = 0.42-0.82). The correlations with selected polysomnographic indices were weaker and less consistent (R = 0.4 4-0.59). Multivariate analysis showed that maximum static inspiratory press ure was the strongest independent predictor of quality of life. Conclusion: Quality of life was strongly and independently related to respiratory musc le function. Relations with polysomnographic indices were weaker and were a ttributable to respiratory muscle weakness. Respiratory muscle weakness is much more important than the frequency of apneas and hypopneas in determini ng quality of life in ALS.