A prospective clinical study on the mechanisms underlying critical illness myopathy-A time-course approach

Abstract

Background: Critical illness myopathy (CIM) is a consequence of modern critical care resulting in general muscle wasting and paralyses of all limb and trunk muscles, resulting in prolonged weaning from the ventilator, intensive care unit (ICU) treatment and rehabilitation. CIM is associated with severe morbidity/mortality and significant negative socioeconomic consequences, which has become increasingly evident during the current COVID-19 pandemic, but underlying mechanisms remain elusive.

Methods: Ten neuro-ICU patients exposed to long-term controlled mechanical ventilation were followed with repeated muscle biopsies, electrophysiology and plasma collection three times per week for up to 12 days. Single muscle fibre contractile recordings were conducted on the first and final biopsy, and a multiomics approach was taken to analyse gene and protein expression in muscle and plasma at all collection time points.

Results: (i) A progressive preferential myosin loss, the hallmark of CIM, was observed in all neuro-ICU patients during the observation period (myosin:actin ratio decreased from 2.0 in the first to 0.9 in the final biopsy, P < 0.001). The myosin loss was coupled to a general transcriptional downregulation of myofibrillar proteins (P < 0.05; absolute fold change >2) and activation of protein degradation pathways (false discovery rate [FDR] <0.1), resulting in significant muscle fibre atrophy and loss in force generation capacity, which declined >65% during the 12 day observation period (muscle fibre cross-sectional area [CSA] and maximum single muscle fibre force normalized to CSA [specific force] declined 30% [P < 0.007] and 50% [P < 0.0001], respectively). (ii) Membrane excitability was not affected as indicated by the maintained compound muscle action potential amplitude upon supramaximal stimulation of upper and lower extremity motor nerves. (iii) Analyses of plasma revealed early activation of inflammatory and proinflammatory pathways (FDR < 0.1), as well as a redistribution of zinc ions from plasma.

Conclusions: The mechanical ventilation-induced lung injury with release of cytokines/chemokines and the complete mechanical silencing uniquely observed in immobilized ICU patients affecting skeletal muscle gene/protein expression are forwarded as the dominant factors triggering CIM.

Keywords: critical illness myopathy; mechanical ventilation; membrane exitability; muscle paresis; myosin loss.

Figure 1

Myosin:actin ratios, compound muscle action potential (CMAP) amplitudes and single muscle fibre contractile measurements: (A) myosin:actin ratio and CMAP amplitude in response to supramaximal stimulation of (B) the median, (C) fibular and (D) tibial nerves at the six different measurements during the 12 day observation period. (E) Single muscle fibre cross‐sectional area, (F) specific force and representative single muscle fibres from (G) the first and (H) final muscle biopsy. Each patient is given the same colour in (A)–(F). The horizontal bar denotes 100 μm.

Figure 2

Clustered expression patterns of differentially expressed genes (DEGs) from the neuro‐intensive care unit (ICU) patients during 12 day observation and the functional enrichment analysis: (A) time‐course series in the seven neuro‐ICU patients from whom muscle samples were collected at six time points during the 12 day period. Membership varying from green to red colour indicates the degree of genes suitable to the cluster. (B) Comparison of top 5 enriched GObp, GOcc and Kyoto Encyclopedia of Genes and Genomes (KEGG) terms among four clusters. Heatmap of DEGs in (C) the GObp term ‘muscle system process’ from Cluster 1 (left half) and Cluster 3 (right half), (D) the GOcc term ‘neuromuscular junction’ from Cluster 1 (upper half) and Cluster 3 (lower half) and (E) the KEGG term ‘p53 signaling pathway’ from Cluster 2 (upper half) and Cluster 4 (lower half). Terms are ranked by −log (false discovery rate [FDR]). Dot size represents the number of DEGs involved in each term.

Figure 3

Gene set enrichment analysis (GSEA) of differentially expressed genes (DEGs) from first biopsy versus control: (A) the top GObp and GOcc terms and (B) Kyoto Encyclopedia of Genes and Genomes (KEGG) terms were selected for demonstration; GSEA plot of the gene set enrichment analysis for (C) ‘muscle contraction (GObp)’, (D) ‘inflammatory response (GO bp)’, (E) ‘cAMP signaling pathway (KEGG)’ and (F) ‘calcium signaling pathway (KEGG)’. NES, normalized enrichment score

Figure 4

Zinc ion homeostasis in neuro‐intensive care unit (ICU) patients: the alteration tendency of (A) plasma zinc concentration and (B) differentially expressed genes (DEGs) related to ‘cellular zinc ion homeostasis’ in tibialis anterior (TA) muscle tissue at six time points of biopsies from neuro‐ICU patients compared with healthy control

Figure 5

Clustered expression patterns of plasma differentially expressed proteins (DEPs) from the neuro‐intensive care unit (ICU) patients: (A) heatmap in time‐course series of 28 DEPs, (B) DEP alterations of Cluster 1 and Cluster 2 at the six time points of biopsies and (C) enriched KEGG terms of DEPs from Cluster 1 at the six time points of biopsies in seven neuro‐ICU patients