Hyperosmolar Hyperglycaemic State (HHS)
Hyperosmolar hyperglycaemic state is a severe acute metabolic complication of diabetes characterised by extreme hyperglycaemia, marked hyperosmolality, and profound dehydration, without significant ketosis or acidosis. It occurs most commonly in individuals with Type 2 diabetes and often develops insidiously over days to weeks. HHS reflects a state of relative insulin deficiency rather than absolute absence. Understanding the pathophysiology explains why blood glucose levels become far higher than those seen in diabetic ketoacidosis and why neurological impairment is often the dominant clinical feature.
What You Need to Know
Hyperosmolar hyperglycaemic state develops when circulating insulin levels are sufficient to prevent lipolysis and ketone formation but insufficient to promote effective glucose uptake into cells or suppress hepatic glucose production. Blood glucose therefore rises progressively, often to extreme levels, while ketogenesis remains minimal. Because insulin is still partially active, metabolic acidosis is absent or mild, distinguishing HHS from diabetic ketoacidosis at a physiological level.
As hyperglycaemia worsens, plasma osmolality increases and water shifts out of cells into the intravascular space. The kidneys attempt to excrete excess glucose, but this results in profound osmotic diuresis with loss of water and electrolytes over days to weeks. Unlike DKA, symptom onset is gradual, allowing dehydration and hyperosmolality to become severe before clinical recognition, particularly in older adults or those with limited access to fluids.
Several interrelated processes drive clinical deterioration in HHS:
Extreme hyperglycaemia leading to marked hyperosmolality
Severe osmotic diuresis causing progressive dehydration and electrolyte loss
Reduced circulating volume impairing renal perfusion and glucose clearance
As dehydration progresses, renal function declines and glucose clearance falls, allowing blood glucose and osmolality to rise further. Increasing hyperosmolality disrupts neuronal function, contributing to altered mental status, seizures, or coma. Because ketosis and acidosis are minimal, early warning signs may be subtle, making delayed presentation common and increasing the risk of significant morbidity and mortality once HHS is established.
Beyond the Basics
Relative insulin deficiency and absence of ketosis
In hyperosmolar hyperglycaemic state, residual insulin activity remains sufficient to suppress lipolysis within adipose tissue. Insulin inhibits hormone-sensitive lipase, limiting release of free fatty acids and preventing significant hepatic ketone production. For this reason, metabolic acidosis is absent or mild despite extreme hyperglycaemia.
At the same time, this residual insulin is inadequate to support effective glucose uptake by muscle and adipose tissue or to suppress hepatic gluconeogenesis. Glucose production continues while peripheral utilisation remains impaired, allowing blood glucose levels to rise progressively, often to levels far exceeding those seen in diabetic ketoacidosis.
Progressive hyperosmolality and cellular dehydration
As blood glucose rises, plasma osmolality increases, creating a strong osmotic gradient between the intravascular space and intracellular compartments. Water moves out of cells to restore osmotic balance, resulting in widespread cellular dehydration.
Neuronal cells are particularly vulnerable to osmotic shifts. Loss of intracellular water within the central nervous system disrupts neuronal function and synaptic signalling, contributing to confusion, seizures, and coma. Although gradual development allows some cerebral adaptation, once compensatory capacity is exceeded neurological deterioration may accelerate rapidly.
Renal dysfunction and worsening hyperglycaemia
Severe dehydration leads to reduced intravascular volume and declining renal perfusion. As glomerular filtration falls, the kidneys become less able to excrete glucose. Reduced clearance allows blood glucose concentrations to rise further, increasing plasma osmolality and intensifying osmotic diuresis.
This creates a self-perpetuating cycle in which hyperglycaemia worsens dehydration and dehydration further worsens hyperglycaemia. Electrolyte disturbances develop due to urinary losses and fluid shifts between compartments. Sodium levels may appear elevated due to free water loss, representing hypertonicity rather than true sodium excess, and contributing to neurological and cardiovascular instability.
Delayed recognition and disease severity
Because ketone production and acidosis are minimal, early warning features such as abdominal pain, vomiting, and deep compensatory respiration are usually absent. This delays recognition and allows metabolic derangement to progress silently over days to weeks.
By the time hyperosmolar hyperglycaemic state is diagnosed, total body water loss is often profound and neurological impairment advanced. This delayed presentation contributes to the higher morbidity and mortality associated with HHS compared with diabetic ketoacidosis, despite the absence of significant acidosis.
Clinical Connections
Hyperosmolar hyperglycaemic state commonly presents with severe dehydration, hypotension, tachycardia, and marked neurological impairment ranging from confusion to coma. Neurological features may include focal deficits that resemble stroke, arising from extreme cellular dehydration and hyperosmolality rather than primary structural brain injury. As osmolality rises, neuronal function becomes increasingly unstable, accounting for fluctuating consciousness and seizure risk.
Clinical priorities centre on stabilising circulation and correcting hyperosmolality in a controlled manner.
Features that indicate significant physiological compromise include:
Profound hypotension or tachycardia due to intravascular volume depletion
Altered mental state or focal neurological signs associated with severe hyperosmolality
Reduced urine output indicating impaired renal perfusion
Management focuses first on cautious fluid replacement to restore circulating volume and improve renal perfusion. Gradual reduction in plasma osmolality is essential, as rapid shifts in water balance can precipitate cerebral oedema. Insulin therapy is introduced after initial fluid resuscitation to lower blood glucose, with lower doses often required than in diabetic ketoacidosis because ketosis is minimal and insulin sensitivity improves as volume status is corrected.
In HHS fluid management is the cornerstone of treatment and careful pacing of correction is critical. Stabilising perfusion and osmolality reduces neurological risk and supports safer normalisation of glucose levels in a condition where abrupt metabolic change can cause serious harm.
Concept Check
Why does HHS produce extreme hyperglycaemia without significant ketosis?
How does rising plasma osmolality lead to neurological dysfunction?
Why does dehydration worsen hyperglycaemia in HHS?
How does HHS differ physiologically from diabetic ketoacidosis?
Why must hyperosmolality be corrected gradually during treatment?