## 3 Results
### Transmission Electron Microscopy  
Electron microscopy was performed in the first half of the patients (\(n=11\)), to further refine the morphological analyses. Ultrastruct examination by TEM showed skeletal muscle damage, with marked focal pathological alterations affecting the myofib, the surrounding endomysium, and the capillaries. No \"virus-like particles\" could be observed by TEM in the muscle samples investigated. Muscle fibers showed typical pathological ultrastructural characteristics attributed to myofibellar degeneration (Figure 3A-C), hydropic degeneration (Figure 3D-F), and necrosis (Figure 3G-I). These muscle fiber alterations were observed in all examined biopsy specimens, among healthy myofib. These abnormalities were observed in muscle samples collected, both upon admission, as well as after 7 days of ICU stay. The alterations were present to variable degrees and severities and were visible in the entire myofiber or in parts of it. Ultrastruct characteristics of these abnormalities were often observed in the same muscle fiber. Figures 4 and 5 show representative TEM images of the different degrees of these three abnormalities in neighboring muscle fibers. Ultrastruct signs of myofibrillar degeneration were most obvious at T0. Hydropic degeneration and severe necrosis were more pronounced at T1, and especially in \"swollen\" muscle fibers. Figure 6 shows representative images of two patients, one with ultrastructural characteristics of myofibrillar degeneration and decreased muscle fiber size on immunofluorescence (Figure 6A-C), and one patient showing hydropic degeneration at the ultrastructural level and an increase in muscle fiber size (Figure 6D-F). The ultrastructural alterations of degenerating myofib (Figure 3A-C) varied, from narrowing and splitting of the myofib with preserved sarcomere structure and normal thermoyofibrillar architecture, to more marked changes, including focal loss of myofibrils over one or more sarcomere lengths. The space vacated by the focal myofibrillar loss, contained large glycogen accumulations and numerous dark, elongated mitochondria that were mostly transversely oriented, in register with the \(Z\) line. In addition to myofilament loss, variable degrees of focal myofibrillar disorganization and disruption of sarcomeric myofilaments and \(Z\)-lines were observed. Myofibers altered by hydropic degeneration (Figure 3D-F) were characterized by vacuoles and membrane blebs. The mitochondria were swollen and vacuolated. Triads were displaced and showed swollen cisternae of the sarcoplasmic reticulum. The intermyofibrillar space, as well as the perinuclear space, contained large accumulations of glycogen. The nucleus was often located in a large membrane bleb and oriented towards a neighboring capillary. Necrotic fibers (Figure 3G-I) were characterized by segmental extensive loss of myofibrils, focal  
Figure 2: Representative images of muscle biopsy samples analyzed by light microscopy. Skeletal muscle biopsies showing (**A**) fiber atrophy and interstitial fibrosis. (**B**) Focal necrotic fibers being phagocyted by macrophages (black arrow). (**C**) perivascular fibrosis without thrombi or inflammatory infiltrates. (**D**) Early stage of intramuscular fibrosis. defects of plasma membrane, and irreversible nuclear changes, including condensation of chromatin (pyknosis) and fragmentation (karyorrhexis). Most necrotic cells were electron-dense. Their myofilament structure was lost completely and replaced by an amorphous granular material. The cytoplasm of these fibers usually showed numerous vacuolized mitochondria, autophagic vacuoles, and lysosomes. Some of these necrotic cells were characterized by hypercontraction of myofibrils. Other, less common, necrotic fibers, were electron-lucent and showed extensive myosin loss with preserved and misoriented Z-line material and related actin filaments. Sometimes, extrusion of the nucleus was observed. Figure 3: Representative electron micrographs of longitudinally sectioned myofibers.