SLEEP HYPOXIA IN MYOTONIC-DYSTROPHY AND ITS CORRELATION WITH AWAKE RESPIRATORY-FUNCTION

Background - Tiredness and daytime respiratory failure occur frequentl y in myotonic dystrophy. Sleep hypoxaemia was studied in 12 patients w ith myotonic dystrophy and correlations were sought with their daytime lung and respiratory muscle function. Methods - All patients underwen t overnight sleep studies, clinical assessment, measurement of flow-vo lume loops and carbon monoxide transfer factor, arterial blood gas ana lysis, and physiological assessment of both thoracic muscle function a nd upper airways obstruction. Results - The mean nadir of oxygen satur ation during sleep was 75% (95% confidence interval 69% to 81%). A mea n of 3.4% of total sleep duration was spent at an oxygen saturation le vel below 85%. Five of the 12 patients had an apnoea index of >5, the group mean apnoea/hypopnoea index being 15.8 events/sleep hour. The me an awake arterial oxygen tension (PaO2) was 10.7 kPa. There was a tren d to hypercapnoea with a mean awake arterial carbon dioxide tension of 6.1 kPa; carbon dioxide retention worsened during sleep. Respiratory muscle dysfunction was mainly evident as a low maximum expiratory mout h pressure. Upper airway obstruction assessed by physiological criteri a was found in four of the 12 patients. The proportion of total sleep duration with oxygen saturation levels below 85% was directly related to body mass index (weight/height(2)) and inversely related to the awa ke PaO2. Body mass index was inversely related to the overnight nadir of oxygen saturation. Conclusions - Patients with myotonic dystrophy a re often hypoxic during sleep and the subgroup that are obese, or have symptoms of sleep apnoea, or both, are particularly at risk. Sleep st udies should be considered in this subgroup of patients with myotonic dystrophy.