RELATIONSHIP BETWEEN CHRONIC HYPERCAPNIA AND INSPIRATORY-MUSCLE WEAKNESS IN MYOTONIC-DYSTROPHY

We studied 134 patients with Steinert's myotonic dystrophy (MD) in ord er to determine the prevalence of chronic hypercapnia, the level of mu scle weakness and forced expiratory volume at which hypercapnic respir atory failure is likely to occur, and how clinical assessment might he lp predict hypercapnic respiratory failure. Subjects were divided into five classes with a muscular disability rating scale (MDRS): 0 = no c linical impairment (n = 9), I = minimal signs of impairment (n = 11), II = distal weakness (n = 41), III = moderate proximal weakness (n = 6 2), and IV = nonambulatory (n = 11). The prevalence of hypercapnia (Pa -CO2 greater than or equal to 43 mm Hg) was found to be 0%, 27%, 29%, 45% and 55% for MDRS 0 to 4, respectively (p = 0.03). A multiple regre ssion analysis limited to clinical data showed that daytime hypersomno lence was a significant cofactor with the MDRS (p = 0.01) in predictin g Pa-CO2 (r = 0.40). Among respiratory parameters, T-VC, respiratory m uscle strength (RMS), and maximal inspiratory pressure against occlude d airways (Pl(max)) were found to be predictors of nearly equal streng th, explaining 16%, 15%, and 14% of the Pa-CO2 variance, respectively. In multiple regression analysis, sex, daytime sleepiness, and the exp ected/observed FVC ratio for a given RMS were found to be significant cofactors with Pl(max) in predicting Pa-CO2 (r = 0.51). It is conclude d that respiratory insufficiency should be suspected in MD patients wi th proximal weakness or daytime sleepiness. The likelihood of hypercap nia also increases with volume restriction and respiratory muscle weak ness. Our study suggests that the combination of inspiratory muscle we akness and loading plays a predominant role in the pathogenesis of chr onic alveolar hypoventilation in MD patients. The occurrence of daytim e hypersomnolence suggests that other factors, such as low central ven tilatory drive or sleep apnea, might play an additional role.