The mechanisms underlying ICUAW are complex and involve functional and structural alterations in both the muscles and the nerves.
In CIP, the pathological finding is axonal degeneration [
15]. The pathogenesis of such axonal degeneration remains incompletely understood, in part explained by the invasiveness of nerve biopsies. Factors that play a role are microvascular changes in the endoneurium evoked by sepsis [
16], which promotes vascular permeability and allows penetration of toxic factors into the nerve ends [
17]. The endoneural edema resulting from increased permeability may impair energy delivery to the axon followed by axonal death. Direct toxic effects and mitochondrial dysfunction evoked by hyperglycemia may contribute to this process [
18‐
20]. Animal experiments also identified channelopathy as a cause of rapidly occurring but reversible neuropathy [
21]. Whether such a channelopathy also occurs in ICU patients, as a functional precursor of axonal degeneration that is found later in the course of critical illness, remains unclear [
22].
In CIM, several factors are thought to play a role by negatively affecting muscle structure and function, all interacting in a complex manner [
22]. A detailed overview of mediators and molecular mechanisms involved was published recently [
23]. First, muscle atrophy can occur quite early during critical illness [
24]. Muscle atrophy is brought about by increased breakdown and decreased synthesis of muscle protein [
24,
25]. Several processes during critical illness may promote such wasting of muscle protein, which preferentially involves myosin [
25,
26]. These include inflammation, immobilization, the endocrine stress responses, the rapidly developing nutritional deficit, impaired microcirculation, and denervation [
22,
27,
28]. Key proinflammatory mediators involved include tumor necrosis factor alpha, interleukin-1, and interleukin-6 [
23]. Recently, another stress-induced cytokine, growth and differentiation factor-15 (GDF-15), a member of the transforming growth factor beta super family, was identified as a mediator of muscle atrophy during critical illness [
29]. In addition to muscle atrophy, other factors may contribute to decreased muscle function. Muscle membrane inexcitability induced by sodium channel dysfunction is an early finding in CIM [
30] and predicts the development of clinical muscle weakness [
31]. An altered intracellular calcium homeostasis, affecting the excitation–contraction coupling, has also been shown to contribute to reduced contractility in animal sepsis models [
32,
33]. The muscles of septic patients show signs of bioenergetic failure, comprising oxidative stress, mitochondrial dysfunction, and ATP depletion [
34]. Finally, muscle atrophy and weakness are not synonymous [
22]. Indeed, muscle quality might be even more important than muscle mass in determining muscle function and the development of ICUAW [
35]. Autophagy, which is a cellular housekeeping system that mediates removal of damaged large organelles and protein aggregates, is an important factor in maintaining muscle fiber integrity [
36], and deficient autophagy may have a crucial role in the development of ICUAW [
35].