In the hospital setting, a combination of factors affect the development of stress hyperglycaemia (). The mechanisms for this disorder probably vary with the patients’ underlying glucose tolerance, type and severity of disease, and stage of illness. The cause of hyperglycaemia in type 2 diabetes is a combination of insulin resistance and β-cell secretory defects. However, the development of stress hyperglycaemia is caused by a highly complex interplay of counter-regulatory hormones such as catecholamines, growth hormone, cortisol, and cytokines ().
34,59,60 The underlying illness might affect the scale of cytokine production and hormonal derangements. Complex feedforward and feedback mechanisms between hormones and cytokines exist,
61 and this neurohormonal environment ultimately leads to excessive hepatic glucose production and insulin resistance.
59,60 High hepatic output of glucose, especially through gluconeogenesis, seems to be the most important contributor to stress hyperglycaemia.
62,63 Excessive glucagon is the primary mediator of gluconeogenesis,
64 although epinephrine
65 and cortisol
66 also contribute. Tumour necrosis factor-α (TNFα) might promote gluconeogenesis by stimulating glucagon production.
67Insulin resistance during illness is characterised by an inability to suppress central hepatic glucose production. In the periphery, insulin resistance is mediated through two major pathways. Reduced insulin-mediated glucose uptake results from defects in postreceptor insulin signalling
68 and downregulation of glucose transporter (GLUT)-4.
69 Additionally, impaired non-oxidative glucose disposal probably results from reduced skeletal muscle glycogen synthesis.
70 Both excess cortisol
71 and epinephrine
72 reduce insulin-mediated glucose uptake. Cytokines such as TNFα,
73 and interleukin 1
74 inhibit postreceptor insulin signalling. Severity of illness is associated with a proportional rise in serum cytokines
25 and insulin resistance.
75 Furthermore, hyperglycaemia exacerbates the cytokine, inflammatory, and oxidative stress response, potentially setting up a vicious cycle whereby hyperglycaemia leads to further hyperglycaemia.
76–78 Resolution of hyperglycaemia is associated with normalisation of the inflammatory response.
78Insulin resistance ultimately promotes a catabolic state in which lipolysis takes place. Excessive circulating free fatty acids in turn exacerbate insulin resistance by disrupting end-organ insulin signalling
79 and glycogen synthase.
80 This lipotoxicity aggravates the inflammatory state, paralleling the effects of glucotoxicity.
81 Glucotoxicity, lipotoxicity, and inflammation are key components of what might be viewed as an exaggerated global insulin-resistance syndrome associated with acute illness. These components also promote endothelial dysfunction, which has a complex reciprocal cause–effect relation with insulin resistance.
82 Hyperinsulinaemia might impart additive consequences to that of hyperglycaemia, including exaggerated inflammatory and counter-regulatory hormone responses and impaired fibrinolysis.
83,84Despite reduced insulin-mediated glucose uptake, an early increase in whole-body glucose uptake takes place—mainly as a result of cytokine-mediated up regulation of GLUT-1.
85–88 GLUT-1 is a ubiquitous glucose transporter that is involved in non-insulin-mediated glucose uptake. Although non-oxidative metabolism (eg, glycogen synthesis) is impaired, oxidative glucose metabolism is upregulated early.
89 In addition to patient-specific factors, certain therapeutic interventions such as catecholamine infusions, corticosteroids, and enteral and parenteral nutrition can worsen or precipitate hyperglycaemia.
25 No studies of mechanisms comparing critically ill patients with diabetes or stress hyperglycaemia are available. Therefore, whether differences in pathophysiology explain differences in outcomes is unclear.