Correlation between limb muscle impairment and respiratory function in myotonic dystrophy type 1

<bold>Background:</bold> Myotonic dystrophy type 1 (MD1) is the most common neuromuscular disease in adults. Patients may present with limb muscle impairment, respiratory pump failure (RPF) and sleep disordered breathing (SDB). Whether limb muscle impairment correlates with the respiratory function is not known. <bold>Aim:</bold> To describe the correlation between 4-limb hypotonia, as assessed by the muscular impairment rating scale (MIRS), pulmonary function impairment and the need for non-invasive home mechanical ventilation (HMV) in MD1. <bold>Methods:</bold> We retrospectively analysed the data of all 121 patients followed from 2004 to 2023 in our neuromuscular disease outpatient clinic with MD1. <bold>Results:</bold> In our sample (54% male, mean age 39±17 ys.), MIRS score at first visit was available in 95 patients. MIRS was 1 in 8%, 2 in 36%, 3 in 41% and 4 in 15% of patients. HMV was prescribed in 49.6% of patients, 23.2% for SDB and 26.4% for RPF (pCO2 <45 mmHg and/or vital capacity (VC) <70% predicted). There was no statistically significant difference between the MIRS of patients with or without indication for HMV (p=0.7, χ2) and between patients ventilated for SBD or RPF (p=0.2, χ2). In contrast, in the whole sample, patients with reduced VC had significantly higher MIRS (p=0.002, χ2). In the RPF group, MIRS was significantly higher in reduced VC group with or without hypercapnia compared to patients with only hypercapnia (p=0.027, χ2). <bold>Conclusion:</bold> A high MIRS at the first visit seems to predict the development of functional impairment, as assessed by VC reduction. However, given the prevalence of other indications for HMV besides VC reduction, all MD1 patients should be followed in the pulmonary setting regardless of MIRS.