Hypernatremia (High Blood Sodium)

Why sodium rises

Sodium concentration reflects the balance between sodium and water in the body. Normally, a rise in sodium triggers strong thirst and the release of antidiuretic hormone (ADH), which makes the kidneys conserve water. Hypernatremia develops when this protective system fails or is overwhelmed, usually because a person cannot respond to thirst.

Because water moves out of cells into the blood, brain cells shrink. This causes the neurological symptoms. Over 1 to 2 days, brain cells adapt by making particles that hold water in. That adaptation is why sodium must be lowered slowly.

Causes

Too little water intake

• older adults with reduced thirst, dementia, stroke or limited mobility
• infants who cannot ask for water, especially with breastfeeding difficulties
• people who are sedated, intubated or unconscious
• lack of access to water

Water loss greater than sodium loss

• gut losses: diarrhea and vomiting, especially in children
• skin losses: fever, sweating, heat exposure, burns
• kidney losses: - diabetes insipidus: central (low ADH) or nephrogenic (kidneys do not respond, for example from lithium) - osmotic diuresis from very high blood sugar (hyperosmolar hyperglycemic state), mannitol or high-protein tube feeds - loop diuretics - recovery phase of acute kidney injury

Sodium gain (uncommon)

• IV hypertonic saline or sodium bicarbonate
• incorrectly prepared infant formula
• swallowing large amounts of salt or seawater
• rarely, excess aldosterone or cortisol (mild only)

Symptoms

Symptoms depend on how high sodium is and how fast it rose. Early symptoms are easy to miss, especially in older adults.

• intense thirst (often absent in older adults)
• tiredness, weakness and lethargy
• irritability and restlessness, a high-pitched cry in babies
• confusion and difficulty concentrating
• muscle twitching, cramps and stiffness
• seizures
• reduced consciousness and coma in severe cases

Signs of the underlying cause, such as dry mouth, low blood pressure and fast pulse in dehydration, or passing large amounts of urine in diabetes insipidus, are often present.

Diagnosis

• Blood sodium: above 145 mmol/L confirms hypernatremia. Above about 160 is severe.
• Blood glucose, urea, creatinine and potassium, and blood osmolality.
• Urine osmolality and sodium: concentrated urine suggests water loss elsewhere (gut, skin) or low intake. Dilute urine despite high sodium suggests diabetes insipidus.
• Urine volume measurement.
• Water deprivation test or copeptin levels in specialized settings to diagnose diabetes insipidus.

Treatment

Replace water carefully

• Mild cases: drinking water if the person can safely swallow.
• Moderate to severe cases: IV fluids, usually 5% dextrose or dilute saline, or water via a feeding tube, given in hospital with frequent sodium checks.
• Correction speed: for hypernatremia that developed over more than about 48 hours, sodium is usually lowered by no more than about 10 to 12 mmol/L per day. Correcting too quickly can cause brain swelling and seizures, especially in children. Very acute hypernatremia (within hours, for example from salt ingestion) can be corrected faster.
• If the person is also dehydrated with low blood pressure, normal saline is given first to restore circulation.

Treat the cause

• treat fever, diarrhea and vomiting
• desmopressin for central diabetes insipidus
• adjust lithium, diuretics or tube feed composition
• control blood sugar
• ensure regular access to water for people who cannot drink independently

Outlook

Mild hypernatremia corrected promptly usually leaves no lasting effects. Severe hypernatremia, particularly in frail older adults or very ill patients, is linked to high mortality, often reflecting the seriousness of the underlying illness. In babies, rapid shifts in sodium can cause brain injury, so careful hospital management is important.

Prevention

• offer drinks regularly to older adults, people with dementia and those with limited mobility, especially during illness or hot weather
• monitor wet diapers and feeding in newborns
• prepare infant formula exactly as instructed
• people with diabetes insipidus should have access to water at all times and wear medical ID

Frequently asked questions

What causes hypernatremia?

It is usually caused by losing more water than you take in: not drinking enough (especially in older adults or babies), fever, diarrhea, vomiting, sweating, diabetes insipidus or very high blood sugar. Less often, it results from taking in too much sodium.

What are the symptoms of high sodium?

Early symptoms include thirst, tiredness, weakness and irritability. As levels rise, confusion, muscle twitching, seizures and coma can develop. Older adults may not feel thirsty, so confusion may be the first sign.

Can eating too much salt cause hypernatremia?

Rarely. In healthy people, salty food triggers thirst and the kidneys excrete the extra sodium. Hypernatremia from salt usually happens only with very large amounts, such as swallowing seawater or incorrect infant formula, or when someone cannot drink.

Why is hypernatremia treated slowly?

Brain cells adapt to high sodium within a day or two. If sodium is lowered too quickly, water rushes into these adapted cells, causing brain swelling and seizures. Doctors usually lower chronic high sodium gradually over 1 to 3 days.

What is a dangerous sodium level?

Levels above 145 mmol/L are high. Levels above about 160 mmol/L are considered severe and can cause confusion, seizures and coma. Any high sodium level with neurological symptoms needs urgent hospital treatment.