Hyperosmolar hyperglycemic state pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Husnain Shaukat, M.D [2]

Overview

The hyperosmolar hyperglycemic state (HHS) is the result of relative insulin deficiency and excess of counter-regulatory hormones like glucagon, growth hormone, catecholamine, and cortisol. The decrease in insulin-to-glucagon ratio puts the body in the catabolic state and leads to hyperglycemic and hyperosmolar state. The hyperglycemia is secondary to activation of gluconeogenesis, glycogenolysis and decreased peripheral utilization of glucose. The increase in plasma osmolality is secondary to osmotic diuresis and dehydration. Advanced age and other underlying comorbidities such as congestive heart failure or chronic kidney disease, decrease in fluid intake and osmotic diuresis leading to activation of renal angiotensin aldosterone system (RAAS) further aggravate the increase in plasma osmolality. There is enough endogenous insulin secretion in the hyperglycemic hyperosmolar state (HHS) to prevent unrestrained ketosis but not enough to prevent hyperglycemia.

Pathophysiology

Glucose homeostasis

Anabolic state during meals

• During fed-state, high glycemic levels cause increased insulin release from pancreatic beta cells.
• Increased insulin levels inhibit the secretion of glucagon from pancreatic alpha cells which leads to increased insulin-to-glucagon ratio.
• High insulin-to-glucagon ratio favors anabolic state, during which insulin mediated uptake of glucose occurs in the liver and muscles where it is stored as glycogen.
• Insulin dependent uptake of glucose also drives potassium into the cells.
• The high insulin-to-glucagon ratio also favors uptake of amino acids by muscle.

Catabolic state between meals

• Between meals, the decrease in insulin and rise in glucagon leads to low plasma insulin-to-glucagon ratio which favors the catabolic state.
• During catabolic state, there is a breakdown of glycogen in the liver and muscles and gluconeogenesis is initiated in the liver.
• Both these processes maintain plasma glucose concentration within normal range.
• The low insulin-to-glucagon ratio also favors lipolysis and ketone body formation.
• Several insulin-independent tissues such as the brain and kidneys utilize glucose as a major source of energy, regardless of the insulin-to-glucagon ratio.

Pathogenesis

The progression to hyperosmolar hyperglycemic state (HHS) can occur due to the reduction in the net effective concentration of insulin relative to glucagon and other counter-regulatory stress hormones (catecholamines, cortisol, and growth hormone), which may be seen in a multitude of settings:

• In type 1 diabetics, there is an immune-mediated destruction of insulin-producing pancreatic β cells, which leads to either an absolute or relative deficiency of insulin in the body.
• In type 2 diabetics, although the major mechanism of hyperglycemia is peripheral insulin resistance and there is some basal production of insulin; patients may develop a failure of pancreatic β cells at late stages of the disease.
• Increased levels of counter-regulatory stress hormones may also cause insulin resistance. The levels of counter-regulatory stress hormones may increase during an acute illness (e.g., infections such as UTI or pneumonia, myocardial infarction [MI], or pancreatitis), stress (e.g., surgery or trauma), when counter-regulatory hormones are given as therapy (e.g., dexamethasone), and as a result of their overproduction (eg, in Cushing's syndrome).
• Some pharmacologic agents may also cause insulin resistance. Notable pharmacologic agents which may lead to insulin resistance include:
  • Antipsychotics (such as clozapine, olanzapine, risperidone)
  • Immunosuppressive agents (such as cyclosporine, interferon, pentamidine)
  • Sympathomimetic agents (such as albuterol, dobutamine, terbutaline)
• All these situations may cause a decrease effective insulin-to-glucagon ratio which may lead to hyperosmolarity and hyperglycemia seen in the hyperosmolar hyperglycemic state (HHS).

Hyperglycemia in hyperosmolar hyperglycemic state (HHS)

Hyperglycemia in HHS develops as a result of three processes:

(a) Increased gluconeogenesis

• Gluconeogenesis takes place in the liver and it increases in HHS due to:
  • Increased gluconeogenic precursors, such as:
    • Amino acids (alanine and glutamine), which are increased due to proteolysis and decreased protein synthesis.
    • Lactate, which comes from muscle glycogenolysis.
    • Glycerol, which comes from lipolysis.
  • Increased activity of gluconeogenic enzymes, which are further stimulated by stress hormones including:
    • Phosphoenolpyruvate carboxykinase (PEPCK)
    • Fructose-1,6-Biphosphatase
    • Pyruvate carboxylase
    • Glucose-6-phosphatase
• The low insulin-to-glucagon ratio also inhibits production of an important metabolic regulator, fructose-2,6-biphosphate (FBPase-2) by triggering the production of cyclic adenosine monophosphate (cAMP), which activates a cAMP-dependent protein kinase. This kinase phosphorylates the phosphofructokinase 2 (PFK-2) and FBPase-2 enzymes which leads to activation of FBPase-2 activity and inhibition of PFK-2 activity, thereby decreasing the levels of fructose 2,6-bisphosphate in the cell. With decreasing amounts of fructose 2,6-bisphosphate, glycolysis is inhibited while gluconeogenesis is activated.
• Reduction of fructose-2,6-biphosphate stimulates the activity of fructose-1,6-bisphosphatase (an enzyme that converts fructose-1,6-biphosphate to fructose-6-phosphate) and inhibits phosphofructokinase (PFK), the rate-limiting enzyme in the glycolytic pathway.

(b) Increased glycogenolysis

• The low insulin-to-glucagon ratio promotes glycogenolysis by stimulating glycogen phosphorylase, a key enzyme of glycogen breakdown.

(c) Impaired glucose utilization by peripheral tissues

• The low insulin-to-glucagon ratio also decrease the insulin dependent uptake of glucose by peripheral tissues.

Lipid and ketone metabolism in hyperosmolar hyperglycemic state (HHS)

The main mechanisms involved in ketone metabolism in hyperosmolar hyperglycemic state include the following:

• The low insulin-to-glucagon ratio affects lipid metabolism and mobilization because insulin normally promotes triglycerides clearance from circulation and inhibits lipolysis.
• The low insulin-to-glucagon ratio stimulates lipolysis from adipose tissue by activating hormone - sensitive lipase (HSL). This lipolysis leads to the abundant supply of free fatty acids (FFA) to the liver.
• The free fatty acid levels may be as high in the hyperosmolar hyperglycemic state (HHS) as in diabetic ketoacidosis (DKA).
• The increase in free fatty acids in the liver diverts the hepatic fatty acid metabolism toward the ketogenesis.
• The ketogenesis occurs in the hepatic mitochondria and the transport of free fatty acids across the mitochondrial membrane is enhanced by the glucagon-mediated decrease in the cytosolic malonyl-CoA, which removes inhibition of carnitine palmitoyltransferase 1 (CPT1).
• The activation of carnitine palmitoyltransferase I allows free fatty acids to cross the mitochondrial membrane in the form of fatty acyl-CoA after their esterification with carnitine.
• The insulin-to-glucagon ratio in the hyperosmolar hyperglycemic state (HHS) does not decrease to a level where unrestrained ketoacidosis occurs.
• The hyperosmolar hyperglycemic state (HHS) was also named hyperosmolar hyperglycemic nonketotic state, but findings of moderated ketonemia in several patients has lead to the use of current term.

Hyperosmolarity in hyperosmolar hyperglycemic state (HHS)

• The hyperosmolar state in HHS is a combination of a decrease in total body water, loss of electrolytes, dehydration, and hyperglycemia.
• The osmotic diuresis in HHS results when the glucose concentration reaches greater than 180 to 200 mg/dl.
• A glucose concentration greater than 180 to 200 mg/dl saturates the reabsorbing capacity of the proximal tubular transport system in the kidneys.
• The saturation of glucose transport system prevents further reabsorption and glucose eventually starts losing in the urine along with water and electrolytes and causing a decrease in the total body water.
• The blood glucose concentration keeps on rising due to continued gluconeogenesis, glycogenolysis, and decrease in total body water which further increases the plasma osmolarity.
• The increase in plasma osmolarity and water loss stimulates antidiuretic hormone (ADH) secretion, which leads to increased water reabsorption through the collecting ducts in the kidney.
• The renal water loss in the hyperosmolar hyperglycemic state (HHS) leads to dehydration especially in the elderly and in the patients who are dependent on others for care as they have decreased oral water intake.
• The decrease in effective circulatory volume due to dehydration leads to activation of renal angiotensin aldosterone system (RAAS), which conserves water but further exacerbates hyperglycemia due to oliguria which decreases renal excretion of glucose.
• The decrease in effective circulatory volume or hypotension eventually leads to coma due to a decrease in tissue perfusion, and the massive activation of renal angiotensin aldosterone system eventually leading to a renal shutdown.

References

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