What Is Hyponatremia (Low Sodium)?
Hyponatremia, or low blood sodium, can cause severe neurological symptoms.
Sodium and water levels in the body are tightly regulated to keep it functioning normally. Sodium concentration is higher in the bloodstream than inside cells. Regulatory mechanisms help control and maintain sodium levels. The hormones aldosterone (made in the adrenal gland) and anti-diuretic hormone (ADH) or vasopressin (made in the pituitary) adjust the way the kidneys deal with water and sodium to maintain the appropriate total amount of sodium and water in the body.
Water in the body is closely linked to the location of sodium in the body. If the concentration of sodium is too high in the bloodstream, water will leak from cells into the bloodstream to try to dilute and lower the sodium concentration. Conversely, if sodium levels in the bloodstream are too low, water will leave the blood and enter cells, causing them to swell.
Hyponatremia is the term used to describe low sodium levels in the bloodstream (hypo=low + natr=sodium + emia=blood). Acute hyponatremia describes the situation in which sodium levels drop quickly, while chronic hyponatremia describes situations with a gradual fall in sodium concentrations over days or weeks. Chronic hyponatremia is often well tolerated since the body has a chance to adapt.
Neurologic changes are the most concerning consequence of hyponatremia. Cerebral edema (excess fluid in the brain, leading to swelling) may occur with severe or acute hyponatremia. Water enters the brain cells causing them to swell. Because the brain is enclosed in a bony skull that cannot expand, the brain is compressed since there is no room for swelling to occur. As a result, brain function may be compromised significantly.
What Causes Hyponatremia?
Hyponatremia occurs because of an imbalance of water and sodium. Most frequently it occurs when excessive water dilutes the amount of sodium in the body or when not enough total sodium is present in the body. A common classification of hyponatremia is based on the amount of total body water that is present.
Normal volume (euvolemic) hyponatremia
The amount of water in the body is normal, but an anti-diuretic hormone is being inappropriately secreted (SIADH =syndrome of inappropriate ADH secretion) from the pituitary gland. This may be seen in patients with pneumonia, small cell lung cancer, bleeding in the brain, or brain tumors.
Excess volume (hypervolemic) hyponatremia
Too much total body water dilutes the amount of sodium contained in the body. This can be seen in heart failure, kidney failure, and liver diseases like cirrhosis. This situation is somewhat misnamed because, while there is increased total body water, there may be a relative decrease of fluid within the bloodstream. Because of the underlying disease, fluid leaks into the space between tissues (called the third space) causing swelling of the extremities or ascites, fluid within the abdominal cavity.
Inadequate volume (hypovolemic) hyponatremia
The amount of water in the body is too low as can occur in dehydration. The anti-diuretic hormone is stimulated, causing the kidneys to make very concentrated urine and hold onto water. This may be seen with excessive sweating and exercising in a hot environment. It can also occur in patients with excessive fluid loss due to vomiting and diarrhea, pancreatitis, and burns.
- Hyponatremia may be a side effect of medications, especially diuretics or water pills used to help control blood pressure. This class of drugs can cause excessive loss of sodium in the urine.
- Hormonal diseases such as Addison's disease or adrenal insufficiency and hypothyroidism may be associated with low sodium levels.
- Polydipsia, or excessive water intake, may cause "water intoxication," diluting sodium levels. This is occasionally associated with psychiatric illness.
- In some people who exercise, their concern about the potential for dehydration causes them to drink more water than they lose by perspiration. This may cause significant hyponatremia and has been known to be fatal in marathon participants who drink too much fluid without replacing lost sodium in excess of what their thirst mechanism dictates.
- In infants, hyponatremia may occur if the baby is fed tap water instead of formula or a balanced electrolyte solution like Pedialyte.
- Drug overdose with MDMA (ecstasy) is associated with hyponatremia.
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What Are the Symptoms and Signs of Hyponatremia?
The symptoms of hyponatremia tend to be neurologic. Patients may complain of headache, nausea and vomiting, lethargy, and confusion. If the sodium concentration drops quickly to critical levels, seizures, coma, and death may occur.
If dehydration is associated with hyponatremia, weakness and muscle aches and cramps may coexist.
When Should I Call a Doctor for Hyponatremia?
Often, hyponatremia is found incidentally when a patient is being evaluated for another condition. Blood tests for electrolyte status are done routinely to screen for many illnesses, and hyponatremia can be noted.
It is important to remember that the major problem with hyponatremia is cerebral edema. Therefore, medical care should be sought when individuals develop altered mental status, confusion, or lethargy; have had a seizure; or cannot be wakened. In these situations, activate emergency medical service (call 911).
What Are the Exams and Tests for Hyponatremia Diagnosis?
The diagnosis of hyponatremia is made by a blood test that measures the concentration of sodium in the bloodstream. The normal sodium level is between 135-145 mEq/l, and levels below 110 mEq/l constitute a true emergency.
Other tests may help decide what type of hyponatremia situation exists. The amount of sodium that is being excreted in the urine may be measured, as well as the concentration of urine. These results may guide the health care professional to decide whether a hypo-, hyper- or euvolemic hyponatremic situation exists and help establish the diagnosis.
The diagnosis of the cause of hyponatremia also depends upon a thorough history of the circumstances that lead to the patient's illness, and a thorough physical examination. Determining the severity of hyponatremia depends on accessing whether the sodium levels have decreased acutely, usually in less than 48 hours, or whether they have gradually decreased over a longer period of time. Since acute hyponatremia can cause potentially life-threatening brain swelling and death, it is imperative to find the reason for the hyponatremia and take appropriate action to correct it.
Past medical history, history of medication use, and recent activity, along with the blood and urine test results, will help guide the doctor to the diagnosis and direct the treatment.
Are There Home Remedies for Hyponatremia?
Prevention is important with respect to hyponatremia. It is important to respect the thirst mechanism to help decide about adequate hydration. As well, monitoring urine color may be helpful. Concentrated, or dark, urine is associated with dehydration, while clear, dilute urine usually indicates that the body has enough fluid.
Patients on diuretic medications and those with heart, kidney, and liver failure often have the electrolyte levels in their blood monitored routinely.
It is important to remember that the major issue with hyponatremia is cerebral edema. Therefore, emergency care is necessary for patients with altered mental status, confusion, or lethargy or those who have had a seizure or cannot be wakened.
What Is the Treatment for Hyponatremia?
If the patient appears to be in crisis with seizures or a coma, the first steps of therapy will be to make certain that the airway is protected, the patient is breathing, and has adequate blood pressure and pulse.
Once the patient is stable, the treatment will depend upon whether the hyponatremia is chronic or acute in nature.
Acute hyponatremia is less common, and the goal is to return the sodium levels to normal to prevent cerebral edema and brain death. In most patients, if the source of excess water intake is eliminated, the body's kidneys can correct the sodium abnormalities on their own. If however, coma or seizure exists, highly concentrated intravenous sodium (3% hypertonic saline) may need to be infused. The goal is to reverse the low sodium levels at a rate of 10-12 mEq/l in the first 24 hours.
Chronic hyponatremia is more common, and treatment should be given cautiously. If the sodium level is corrected too quickly, it may cause central pontine myelinolysis, a condition in which parts of the brain stem are damaged and cause stroke-like symptoms that do not resolve. For that reason, unless the patient is having a seizure or in a coma, the recommendation is to correct the sodium levels at a rate of no more than 8-10 mEq/l over the first day and no more than 18 mEq/l in the first 2 days.
What Is the Follow-up for Hyponatremia?
If the underlying illness or cause is identified and treated, no further treatment may be required. However, if the patient's situation is such that there is future potential for hyponatremia, routine monitoring with blood tests may be suggested.
How Do I Prevent Hyponatremia?
Studies have suggested that long-distance runners develop hyponatremia not infrequently after prolonged exercise and should drink adequate amounts of fluid to match their thirst requirements as well as replace sodium along with water during prolonged or extreme exercise. Ideally, a person should not lose more than 2% of their body weight during exercise in order to prevent dehydration and electrolyte abnormalities.
People who exercise and consume too much water, on the other hand, are at risk for water intoxication and low blood sodium levels. One should take care not to gain weight due to water consumption during exercise.
Infants should not be fed plain water because their kidneys are not able to adequately concentrate urine, leading to hyponatremia and other electrolyte disturbances.
What Is the Prognosis for Hyponatremia?
Hyponatremia may occur in a variety of situations and is due to an abnormality of water and sodium regulation. It is important for the health care professional to decide whether the low sodium occurred acutely (within 48 hours) or whether the hyponatremia is more chronic in nature.
Treatment is directed at correcting the underlying cause and then helping the body correct the sodium and water levels in the body. If the underlying cause is resolved, then observation may be all that is needed.
The urgency of treatment depends upon the neurologic status of the patient, with those in coma or having seizures needing emergent intervention. Intravenous saline in a variety of concentrations may be used to correct the sodium deficit in the body.
For those patients with chronic hyponatremia, correcting the sodium levels too quickly may be associated with central pontine myelinolysis, in which cells in the midbrain are damaged, leading to stroke-like symptoms.
Reviewed on 9/23/2022
Medically reviewed by John A. Daller, MD; American Board of Surgery with subspecialty certification in surgical critical care
Sterns, Richard H. "Overview of the treatment of hyponatremia in adults."
UpToDate.com. June 11, 2021. <https://www.uptodate.com/contents/overview-of-the-treatment-of-hyponatremia-in-adults#!>.