[Diaphragmatic weakness and its impact on respiratory function in primary myopathies]

Diaphragmatic weakness (DW) is a potential manifestation of primary myopathies. Prevalence and impact on respiratory function have not been defined.

Respiratory function (inspiratory vital capacity, IVC; maximal inspiratory muscle pressure, PImax; respiratory muscle effort, P (0.1)/PImax) and polysomnography/nocturnal capnometry (PtcCO(2)) was analysed in 49 patients with primary myopathies. DW was defined as > 25 % drop of IVC upon shift from upright to supine position.

19/54 (35 %) of patients, mostly AMD (68 %) and DMD (42 %) had DW. Restrictive ventilatory defect was moderate (IVC 37.2 +/- 26.2 %) in patients with and without DW (IVC 46 +/- 26 %, p = n. s.). DW caused a -33 %, respective - 25 %, supine drop of IVC and PImax, resulting in severe restriction in supine position (IVC (supine) 24.9 +/- 19.1 %, PImax (supine) 2.0 +/- 1.0 kPa, P (0.1)/PImax (supine) 19.9 +/- 12,8 %). All patients with DW and 75 % without had sleep-disordered breathing. This was significantly more severe in DW: 90 % (vs 17 % without DW) had continuous nocturnal hypoventilation (PtcCO(2) 62 +/- 2 mm Hg), 70 % (vs 18 % without DW) had combined nocturnal and diurnal hypercapnic respiratory failure (PaCO(2) 54,7 +/- 11.8 mm Hg). DW was an independent risk factor for sleep disordered breathing, for nocturnal and diurnal respiratory failure (r = 0.95, p < 0,05). Predictive thresholds thereof were accurate and identical to previously determined only for supine function data.

DW is common in primary myopathies and predictive of nocturnal and diurnal respiratory failure. Supine respiratory function tests are necessary for clinical diagnosis and respiratory risk stratification.