Published: 18 Jun 2025

ICD9: 276.1      ICD10: E87.1      ICD11: 5C72

Hyponatremia, specifically with hypo-osmolality, is a condition characterized by low sodium levels in the blood (serum sodium concentration less than 135 mEq/L) accompanied by a low osmolality of the blood (less than 280 mOsm/kg H2O).
In simpler terms, the blood is more diluted than normal.

Let's break down the components:

Hyponatremia: This means "low sodium in the blood." Sodium is a vital electrolyte that helps regulate the amount of water in and around your cells, as well as playing a key role in nerve and muscle function.

Hypo-osmolality: This means "low concentration of dissolved particles in the blood." Osmolality reflects the balance of water and solutes (like sodium, potassium, chloride, glucose, and urea) in the blood. When osmolality is low, it indicates that there's relatively too much water compared to the amount of solutes.

Why is it important to distinguish 'hypo-osmolality' with hyponatremia?

Because not all cases of low sodium are truly caused by an excess of water. Sometimes the low sodium is an artifact of something else:

Hyperglycemia: Very high blood sugar can draw water into the bloodstream, diluting the sodium concentration. The osmolality might be normal or even high in this case.
Hyperlipidemia or Hyperproteinemia: High levels of fats or proteins in the blood can interfere with sodium measurement, giving a falsely low sodium reading, but the osmolality would be normal.
Pseudohyponatremia: (This is becoming less common with modern lab techniques) This occurs when the *fraction* of plasma that is water is reduced (e.g., due to very high lipid or protein levels) leading to a falsely *low* sodium concentration because the sodium is only measured in the water fraction of the plasma. Actual plasma osmolality is typically normal.

Causes of Hyponatremia with Hypo-osmolality:

The underlying causes of hyponatremia with hypo-osmolality are complex and involve problems with water regulation in the body. The main categories include:

Increased Water Intake:
Primary Polydipsia: Excessive thirst and drinking, often seen in psychiatric conditions. The kidneys can usually handle the excess water, but when intake overwhelms the kidneys' ability to excrete it, hyponatremia can develop.

Impaired Water Excretion: This is the most common cause and involves the body not being able to get rid of excess water properly. This can be due to several factors:
Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH): The body inappropriately releases too much antidiuretic hormone (ADH), also called vasopressin. ADH tells the kidneys to hold onto water, leading to dilution of the blood. SIADH can be caused by many things including medications, lung diseases, brain problems, and certain cancers.
Hypothyroidism: Underactive thyroid gland can affect water excretion.
Adrenal Insufficiency: Problems with the adrenal glands (e.g., Addison's disease) can lead to sodium loss and water retention.
Kidney Disease: Severe kidney problems can impair the ability to regulate water balance.
Medications: Many drugs can cause hyponatremia, including diuretics (especially thiazide diuretics), certain antidepressants (SSRIs), and some pain medications.
Heart Failure and Cirrhosis: These conditions can lead to activation of the renin-angiotensin-aldosterone system and ADH release, contributing to water retention.
Pain, Nausea, and Stress: These can also lead to ADH release.

Sodium Loss: While not directly *causing* hypo-osmolality (as it involves sodium *loss*), significant sodium loss (e.g., from vomiting, diarrhea, excessive sweating without sodium replacement) can eventually lead to increased ADH release as the body tries to maintain blood volume, which in turn can cause water retention and hypo-osmolality. However, these scenarios are often more complex because they also involve volume depletion.

Symptoms:

The symptoms of hyponatremia vary depending on how quickly the sodium level drops and how low it gets. Mild hyponatremia may cause few or no symptoms. More severe or rapidly developing hyponatremia can cause:

Nausea and vomiting
Headache
Confusion
Muscle weakness, spasms, or cramps
Fatigue
Seizures
Coma

Diagnosis:

Diagnosis involves:

Blood tests: To measure serum sodium, osmolality, and other electrolytes.
Urine tests: To measure urine sodium and osmolality.
Medical history and physical examination: To identify potential underlying causes.
Further testing: May be needed to determine the cause of the hyponatremia, such as thyroid function tests, adrenal function tests, or imaging studies.

Treatment:

Treatment depends on the severity of hyponatremia, the speed at which it developed, and the underlying cause. Treatment may include:

Fluid restriction: Limiting fluid intake to help raise sodium levels.
Sodium replacement: In severe cases, intravenous sodium chloride may be given. However, sodium levels must be corrected slowly to avoid a potentially dangerous complication called osmotic demyelination syndrome.
Treating the underlying cause: Addressing the condition causing the hyponatremia, such as stopping a medication or treating SIADH.
Medications: In some cases, medications that block the action of ADH (vasopressin receptor antagonists) may be used.

Important Considerations:

Hyponatremia can be a serious condition, and it's important to seek medical attention if you suspect you have it.
The treatment of hyponatremia is complex and should be managed by a healthcare professional. Rapid correction of hyponatremia can lead to serious complications.
The information provided here is for general knowledge and informational purposes only, and does not constitute medical advice. Always consult with a qualified healthcare professional for any health concerns or before making any decisions related to your health or treatment.