Jeroen van Doorn

Congenital myopathies, the respiratory muscles and ultrasound are outlined in Chapter 1. Ultrasound is increasingly used to assess respiratory muscle function in neuromuscular disorders, the intensive care unit and pulmonary medicine, both in clinical practice and as outcome measure in research. The first part of this thesis – Current practice – discusses technical application of respiratory muscle ultrasound in congenital myopathies and its interpretation. The second part of this thesis – Critical appraisal – scrutinizes respiratory muscle ultrasound methodology. This thesis thus aimed to determine the current status and future perspectives of respiratory muscle ultrasound in clinical practice and research.

Part 1 – Current practice
Chapter 2 is a review of the literature on methodology, technological developments, clinical applications and limitations of ultrasound and magnetic resonance imaging of the respiratory muscles in neuromuscular disorders. Accessory respiratory muscle weakness has been identified as clinically significant in patients with neuromuscular disorders, however, the majority of studies only aimed at measuring the diaphragm. Furthermore, a lack of standardization in methodology lead to large variability in reported data and longitudinal studies with respiratory muscle imaging are lacking. Ultrasound has been used to characterize respiratory muscle function in several neuromuscular disorders, but is rarely used in clinical practice. Echogenicity assessment, strain imaging through speckle tracking, and ultrafast shear wave elastography are identified as promising methods to assess the respiratory muscles in neuromuscular disorders.

Chapter 3 evaluated respiratory function in the Dutch cohort of nemaline myopathies (NM). Median diaphragm thickening in typical NM was below the centre-specific lower limit of normal (Chapter 5) together with an increased median echogenicity z-score, while not in mild NM or Childhood-onset NM with slowness. No statistical differences were found between NM types, in contrast to pulmonary function testing and respiratory muscle strength testing, indicating that these volitional tests may be more sensitive to differences in diaphragm function than diaphragm ultrasound. When stratifying on pulmonary function and ventilation status, patients with impaired pulmonary function and used mechanical ventilation had decreased diaphragm thickening and increased diaphragm echogenicity, in line with differences in pulmonary function and respiratory muscle strength. Both these findings lead to the hypothesis that diaphragm ultrasound reflect pulmonary function as well as diaphragm strength. No differences in diaphragm and intercostal resting end-expiration thickness were found in both analyses, indicating that an increase of respiratory muscle involvement does not lead to atrophy in NM.

Chapter 4 evaluated respiratory function in the Dutch cohort of LAMA2 related muscular dystrophy (LAMA2-MD) and SEPN1 related congenital myopathy (SELEONON-RM), and the change in respiratory function during a 1.5 year follow-up period. There were no changes in respiratory function in LAMA2-MD during the follow-up period, both as assessed with pulmonary function tests, respiratory muscle strength tests and respiratory muscle ultrasound. The diaphragm as measured with ultrasound was normotrophic and thickening was normal, while median echogenicity z-score ranged between 1.7 and 2.1, where 2.0 marks an increase. In contrast, the diaphragm was atrophic and diaphragm thickening was decreased in SELENON-RM. As a result, diaphragm echogenicity could not be analysed. However, these parameters did not change during the follow-up period in line with respiratory muscle strength tests but in contrast to pulmonary function tests. The analyses in both patient groups reinforces the hypothesis that diaphragm ultrasound reflects diaphragm strength testing and pulmonary function testing, but may be less sensitive to change as volitional tests.

Part 2 – Critical appraisal
In Chapter 5, diaphragm thickness, thickening and echogenicity data are collected with ultrasound in a representative sample of young to old healthy participants. Diaphragm echogenicity is strongly influenced by age. Echogenicity that would be normal in older individuals would be substantially increased in younger individuals, leading to the need for controlling age as a statistical variable in interpretation of diaphragm echogenicity analysis. Diaphragm thickness is influenced by sex and body mass index (BMI), although less pronounced as with echogenicity. Diaphragm thickening is not influenced by age, sex, or BMI, and a lower limit of normal is provided. Normative data or matched controls have to be available for valid interpretation of diaphragm ultrasound in clinical practice or research. We provided a detailed protocol for acquisition and analysis of diaphragm ultrasound, as well as methods to calculate reference data, as the proposed standard.

In Chapter 6, the complex relationship between diaphragm thickening as measured with ultrasound, force generated by the diaphragm and changes in lung volume is elucidated. The relation has been subject of considerable scientific debate and with the experiments performed in this chapter, an in-depth physiological analysis is performed. Diaphragm thickening has been considered as a surrogate of force generated by the diaphragm. Participants that performed maximal isometric contractions had no increase in diaphragm thickness, while diaphragm thickness increased substantially during unloaded concentric contractions. This suggested that diaphragm thickening did not reflect diaphragm force generation, but rather an increase in lung volume. however, intended leakage in the system prevented the isometric manoeuvre from being strictly isometric, and physiological resistance in the respiratory system prevented the unloaded concentric manoeuvre from being strictly unloaded. Therefore, tapered flow resistance breathing was used to control force generation as well as inspiratory volume simultaneously at different loading levels. Diaphragm thickening did not change between different loading levels, but increased along increasing lung volume. Diaphragm thickening should not be interpreted as surrogate measure of force effort generated by the diaphragm.

The use of shear wave elastography (SWE) was assessed as an alternative to measure respiratory effort in Chapter 7. Previous studies that used SWE in the diaphragm indicated that it is a surrogate of inspiratory effort generated by the diaphragm. This study was repeated with healthy participants, and was extended with congenital myopathy patients, reliability analyses and the accessory respiratory muscles. SWE had poor intra-observer, inter-observer and test-retest reliability, did not show differences between healthy participants and patients with congenital myopathies and did not correlate with the gold standard of inspiratory effort generated by the diaphragm, transdiaphragmatic pressure. Therefore, SWE was not a suitable method to assess respiratory muscle function. Importantly, the SWE methodology used by the manufacturer may have had considerable impact on these results and may be different to devices from different manufacturers.

General discussion
Chapter 8 discussed the methodological value to assess respiratory muscle tissue remodelling with static ultrasound imaging and to asses respiratory muscle function with dynamic ultrasound imaging. Furthermore, the value of respiratory muscle ultrasound in congenital myopathies are discussed. Echogenicity analysis and atrophy assessment are reliable and valid methods to assess tissue remodelling. Assessing respiratory muscle function, in particular diaphragm function, is difficult with analysis of diaphragm movement, thickening and shear wave elastography. Spirometry and pressure-based measurements remain more sensitive for functional assessment in congenital myopathies, while reduced diaphragm thickening as measured with ultrasound is indicative of diaphragm weakness. Current research efforts focus on assessing the accessory respiratory muscle and performing long-term follow-up studies.