Hyponatremia (Low Blood Sodium)

What hyponatremia is

Sodium is the main salt dissolved in the fluid outside your cells. It controls where water goes in the body. The normal blood level is about 135 to 145 mmol/L (millimoles per liter, equivalent to mEq/L).

How fast sodium falls matters as much as how low it goes. Acute hyponatremia develops within 48 hours, and the brain has no time to adapt, so symptoms can be severe even at moderate levels. Chronic hyponatremia develops over more than 48 hours. The brain adjusts, so people may feel almost well at a level of 125 mmol/L, although they are still at higher risk of falls and fractures.

Most hyponatremia is dilutional: the body has kept too much water, usually because the hormone ADH (antidiuretic hormone, vasopressin) is acting when it should not. Less often, the body has lost both sodium and water, and the lost water has been replaced with plain fluids.

Symptoms of low sodium

Mild to moderate symptoms

• nausea and loss of appetite
• headache
• tiredness, low energy and lethargy
• muscle cramps and weakness
• unsteady walking and falls, especially in older adults
• poor concentration, forgetfulness and irritability

Severe symptoms

• vomiting
• confusion, disorientation or abnormal behavior
• extreme drowsiness
• seizures
• breathing problems and coma, from brain swelling

Mild chronic hyponatremia often causes no symptoms and is found on a routine blood test. In older people, subtle gait problems and attention changes are common and are a real cause of falls.

Neurological assessment for low sodium

Because low sodium mainly affects the brain, doctors look for neurological signs:

• Level of consciousness: alert, drowsy, or responding only to voice or pain (Glasgow Coma Scale)
• Orientation and attention: knowing the date, place and person, and simple tasks like counting backward
• Gait and balance: walking heel to toe, and swaying when standing with eyes closed
• Reflexes and muscle strength
• Signs of raised brain pressure: worsening headache, vomiting, slowing heart rate and, late on, changes in the pupils

Any decline in these signs, or a seizure, means sodium must be raised promptly in hospital.

Causes of hyponatremia

Doctors group causes by the person's fluid status.

Normal fluid volume (euvolemic)

• SIADH (syndrome of inappropriate antidiuretic hormone): the body releases ADH without a normal trigger. Causes include lung infections and lung cancer (especially small cell), brain injury, stroke, meningitis, surgery, pain and nausea.
• Medicines: SSRIs and other antidepressants, carbamazepine and oxcarbazepine, some antipsychotics, some chemotherapy drugs, desmopressin and MDMA.
• Underactive thyroid (hypothyroidism) and low cortisol from pituitary disease.
• Drinking too much: psychogenic polydipsia, or endurance athletes drinking large volumes of plain water.
• "Tea and toast" diet or heavy beer drinking: very little salt and protein means the kidneys cannot excrete enough water.

Too much fluid (hypervolemic)

• Heart failure
• Cirrhosis of the liver
• Kidney failure and nephrotic syndrome

In these conditions the body senses low effective blood flow, holds on to salt and water, and releases ADH. Swelling of the legs or belly is usually obvious.

Too little fluid (hypovolemic)

• Thiazide diuretics such as hydrochlorothiazide and chlorthalidone. These are one of the commonest causes, particularly in older women.
• Vomiting and diarrhea replaced with water rather than salty fluids.
• Addison's disease (adrenal insufficiency), which causes salt loss.
• Heavy sweating in long exercise.
• Cerebral salt wasting after brain injury or bleeding.

Who is at higher risk

Older adults, people in hospital (especially after surgery), people taking thiazides or antidepressants, women during endurance events, and people with heart, liver or kidney disease.

How hyponatremia is diagnosed

A basic metabolic panel measures sodium. To find the cause, doctors add:

• Serum osmolality: confirms the blood is truly dilute and rules out "false" low sodium from very high glucose or fats.
• Urine osmolality: shows whether ADH is acting. A very dilute urine suggests overdrinking.
• Urine sodium: helps tell salt loss from SIADH and from heart, liver or kidney causes.
• Thyroid function and morning cortisol
• Glucose, kidney and liver tests
• Chest X-ray or CT if a lung or brain cause is suspected

The doctor will also review every medicine and supplement you take and how much you drink.

Treatment

Treatment depends on symptoms, speed of onset and cause.

Severe symptoms

Seizures, confusion or coma are treated in hospital with hypertonic (concentrated) saline given in small, measured amounts, with sodium checked every few hours.

Mild or chronic hyponatremia

• Stop or change the culprit medicine, such as a thiazide or SSRI, where possible.
• Fluid restriction, often to around 800 to 1,000 ml (about 1 quart) a day for SIADH, as advised by your doctor.
• Normal saline IV for people who are dehydrated from vomiting, diarrhea or diuretics.
• Treat the cause: thyroid hormone, steroid replacement in adrenal insufficiency, or better control of heart or liver failure.
• Other options for SIADH: oral urea, salt tablets with a diuretic, or vasopressin-blocking medicines (vaptans) in selected people under specialist care.

Why sodium must rise slowly

If chronic low sodium is corrected too quickly, brain cells can be damaged, causing osmotic demyelination syndrome: trouble speaking and swallowing, weakness and sometimes permanent disability. Guidelines limit the rise to about 8 to 10 mmol/L in the first 24 hours, and lower in high-risk people such as heavy drinkers and the malnourished. This is why moderate and severe cases are managed in hospital with frequent blood tests.

Complications

• seizures, brain swelling and death in severe acute cases
• falls, fractures and attention problems in chronic mild cases
• osmotic demyelination if correction is too fast
• longer hospital stays and higher mortality in heart failure and cirrhosis

Prevention

• If you take a thiazide diuretic or antidepressant, ask whether a sodium check is due, especially in the first weeks.
• During long endurance events, drink to thirst rather than on a fixed schedule, and use drinks containing electrolytes.
• Do not force large volumes of water to "flush" your system.
• Follow fluid limits set for heart failure, cirrhosis or SIADH.

Frequently asked questions

What are the first signs of low sodium?

Early symptoms are usually nausea, headache, tiredness, muscle cramps and feeling "foggy". Older adults may become unsteady or confused. Many people with mild hyponatremia have no symptoms at all, and the low level is found on a routine blood test.

What sodium level is dangerous?

Levels below 125 mmol/L are classed as severe and carry the highest risk of confusion and seizures. However, a rapid drop to 128 to 130 mmol/L within a day or two can also be dangerous, because the brain has no time to adapt. Symptoms matter as much as the number.

Can drinking too much water cause hyponatremia?

Yes. Drinking water faster than the kidneys can excrete it, about 0.8 to 1 liter an hour, can dilute blood sodium. It is seen in endurance athletes, after MDMA use and in people with compulsive water drinking. Drinking to thirst prevents it in healthy people.

Should I eat more salt if my sodium is low?

Not usually, unless your doctor advises it. Most low sodium is caused by excess water, not a lack of salt, and extra salt can worsen swelling in heart, liver or kidney disease. Treatment depends on the cause, so follow your doctor's plan.

How long does it take to recover from hyponatremia?

Mild cases often improve over a few days once the cause is removed, such as stopping a diuretic. Severe cases are corrected over 2 to 3 days in hospital. Symptoms like unsteadiness and poor concentration usually clear as sodium normalizes.

Which medicines cause low sodium?

The commonest are thiazide diuretics, SSRI antidepressants, carbamazepine and oxcarbazepine, some antipsychotics, desmopressin and certain chemotherapy drugs. Do not stop a prescribed medicine on your own. Ask your doctor whether a blood test or change is needed.