Hyponatremia (Low Blood Sodium)

Sodium is the body's primary extracellular electrolyte. It governs how much water is retained in and around cells, maintains nerve conduction, and keeps blood pressure stable. When it falls below 135 mEq/L, the result is hyponatremia: the most common electrolyte disorder seen in hospital practice. This article explains what hyponatremia is, its causes, symptoms, steps you can take to prevent it, and effective hyponatremia self-care tips.

What is Hyponatremia?

Hyponatremia is a serum sodium concentration below 135 mEq/L. The kidney regulates both sodium and water, and hyponatremia typically reflects a failure of one or both of those mechanisms, often compounded by an underlying medical condition or medication effect.

Severity is graded by the sodium level: 

• Mild: 130–134 mEq/L
• Moderate: 125–129 mEq/L
• Severe: Below 125 mEq/L. 

The rate of decline matters as much as the absolute value; a sodium level that drops rapidly to 128 mEq/L can be more dangerous than a chronic level of 120 mEq/L that the brain has adapted to over weeks.

Types of Hyponatremia

Hyponatremia is classified by volume status, which determines both the cause and the treatment approach:

• Hypovolaemic hyponatremia: Occurs when both sodium and water are lost but water is replaced more than sodium like from vomiting, diarrhoea, diuretics, or adrenal insufficiency. The body retains water to compensate for volume depletion, diluting remaining sodium.
• Euvolaemic hyponatremia: This is the most common type. Total body sodium is normal but water is retained in excess, typically due to SIADH (syndrome of inappropriate antidiuretic hormone secretion), hypothyroidism, or primary polydipsia.
• Hypervolaemic hyponatremia: Occurs in states of sodium and water excess with proportionally more water due to heart failure, cirrhosis, and nephrotic syndrome. Total body sodium is actually increased, but excess water drives the serum level down.

Symptoms of Hyponatremia

Hyponatremia symptoms are largely neurological, because brain cells are exquisitely sensitive to osmotic shifts. Their severity tracks with how fast sodium fell rather than just how low it is:

• Nausea, headache, and generalised malaise in mild cases
• Confusion, disorientation, and difficulty concentrating as levels fall further
• Lethargy, excessive sleepiness, and slurred speech
• Muscle cramps and weakness
• Seizures and loss of consciousness in severe or rapidly developing hyponatremia
• Respiratory arrest in the most extreme cases.

Causes of Hyponatremia

SIADH is the most common cause. In this ADH (antidiuretic hormone) is secreted inappropriately, causing the kidneys to retain free water against normal osmotic pressure. SIADH is triggered by CNS disorders (meningitis, stroke, subarachnoid haemorrhage), pulmonary disease (pneumonia, tuberculosis), malignancy (particularly small cell lung cancer), pain, nausea, and a long list of medications.

Thiazide diuretics are a major pharmacological cause. They block sodium reabsorption in the distal tubule while leaving ADH activity intact, causing dilutional hyponatremia, particularly in older women. 

Other causes include hypothyroidism, glucocorticoid deficiency, heart failure, cirrhosis, and excessive hypotonic fluid intake, including water overload in endurance athletes.

Risk Factors for Hyponatremia

People 65 or older are more prone to developing hyponatremia due to reduced renal concentrating ability, more comorbidities, and more medications. Other important risk factors are:

• SSRI antidepressants as they stimulate ADH secretion
• Medical conditions like heart failure, liver cirrhosis, and nephrotic syndrome
• Malignancy, particularly small cell lung cancer
• CNS disorders like head injury, stroke, meningitis
• Hypothyroidism and adrenal insufficiency
• Excessive fluid intake like in endurance sports without electrolyte replacement.

Complications of Hyponatremia

Acute severe hyponatremia causes cerebral oedema. As serum osmolality falls, water shifts into brain cells along the osmotic gradient, causing them to swell. The skull cannot expand, so intracranial pressure rises - the mechanism behind the headache, confusion, seizures, and brainstem herniation that define hyponatraemic encephalopathy.

Overcorrection carries its own serious complication: osmotic demyelination syndrome (ODS). Correcting sodium too rapidly in chronic hyponatremia allows brain cells that have adapted to a low-osmolality environment to shrink suddenly, causing demyelination of pontine and extrapontine neurons. ODS produces quadriplegia, pseudobulbar palsy, and locked-in syndrome that is often irreversible.

Diagnosis of Hyponatremia

• Blood work: Serum sodium below 135 mEq/L on a blood panel confirms hyponatremia. The workup then focuses on identifying the type and cause: serum osmolality, urine osmolality, urine sodium concentration, and clinical volume assessment are the four essential steps.
• Urine test: A urine osmolality above 100 mOsm/kg in the context of hyponatremia indicates that ADH is active and the kidney is not appropriately diluting urine, pointing toward SIADH or another ADH-driven mechanism. Urine sodium above 40 mEq/L with euvolaemia is highly characteristic of SIADH. 
• Hormone tests: Thyroid function and morning cortisol are checked to exclude hypothyroidism and adrenal insufficiency.

Treatment for Hyponatremia

Hyponatremia treatment depends on the type, severity, and rate of onset. The correction rate is as important as the treatment itself as too fast risks ODS.

Mild chronic hyponatremia is managed by addressing the underlying cause: 

• Stopping the offending drug
• Treating hypothyroidism
• Optimising heart failure or cirrhosis management
• Fluid restriction in SIADH.

SIADH is managed with:

• Fluid restriction (800–1,000 mL/day)
• Oral salt supplementation
• In persistent cases V2 receptor antagonists to block ADH at the renal collecting duct and produce a selective water diuresis (aquaresis) without sodium loss.

Hypovolaemic hyponatremia is treated with:

• Isotonic (0.9%) saline to restore volume
• Once volume is corrected, the stimulus for ADH secretion resolves and sodium self-corrects.

Severe symptomatic hyponatremia with seizures or reduced consciousness requires urgent 3% hypertonic saline. The target is a controlled rise of 1–2 mEq/L per hour for the first 2–3 hours, sufficient to stop cerebral oedema, then slower correction to avoid ODS.

When to See a Doctor for Hyponatremia

Any confusion, excessive drowsiness, or seizure in a patient known to have low sodium warrants immediate emergency care. Contact a doctor if:

• Nausea, headache, and fatigue that develop acutely, particularly in a patient on thiazides or SSRIs
• Sodium below 130 mEq/L found on any blood test, regardless of symptoms
• Worsening oedema or fluid retention in a patient with heart failure or cirrhosis.

Prevention of Hyponatremia

Fluid intake should match losses to prevent hyponatremia development. Additional preventive tips are:

• Endurance athletes must include electrolyte replacement, not plain water alone
• Patients on thiazide diuretics need periodic electrolyte monitoring, especially in hot weather
• SSRI users above 65 should have sodium checked within weeks of starting treatment
• Heart failure and cirrhosis management should include regular electrolyte panels as standard follow-up
• Any new confusion or neurological change in an older adult on multiple medications warrants an urgent sodium check.

Conclusion

Hyponatremia is easy to miss and dangerous to mismanage. It can present as vague fatigue in a well-looking outpatient or as seizures in an ICU patient. The treatment approach differs completely between those scenarios. Getting the type right, correcting at the right speed, and addressing the underlying cause are what determine the outcome. If you or your loved ones develop sudden confusion, excessive drowsiness, nausea, headache, muscle cramps, or seizure, don't brush it off and contact a doctor or go to the emergency department immediately.

FAQs

1. Is hyponatremia dangerous?

At mild levels it is manageable but not trivial. Severe or rapidly developing hyponatremia causes cerebral oedema, seizures, and brainstem herniation, all are life-threatening. Even moderate hyponatremia is associated with increased fall risk, cognitive impairment, and longer hospital stays. The danger scales with how low and how fast.

2. What is the normal sodium level in the blood?

Normal serum sodium is 135 to 145 mEq/L. Hyponatremia is defined as below 135 mEq/L. 

• Mild hyponatremia: 130–134 mEq/L
• Moderate hyponatremia: 125–129 mEq/L
• Severe hyponatremia: Below 125 mEq/L. 

3. Can drinking too much water cause hyponatremia?

Yes. Drinking water far faster than the kidneys can excrete it, as happens in psychogenic polydipsia or endurance athletes who over-hydrate with plain water, dilutes serum sodium. This is called dilutional or hypotonic hyponatremia and can develop rapidly during prolonged exercise without electrolyte replacement.

4. Can hyponatremia affect the brain?

Directly and significantly. Brain cells swell as water shifts in along the osmotic gradient when serum sodium falls. This causes the neurological symptoms like headache, confusion, seizures that characterise hyponatraemic encephalopathy. Paradoxically, correcting it too fast causes the opposite problem: osmotic demyelination syndrome, which can cause permanent neurological damage.

5. Is hyponatremia common in elderly people?

Very. Older adults have reduced renal concentrating ability, take more medications that affect sodium and water handling, and are more likely to have comorbidities like heart failure or hypothyroidism. Thiazide diuretics and SSRIs (both commonly prescribed in older adults) are among the most frequent pharmacological causes.

6. Can hyponatremia cause seizures?

Yes, particularly when sodium falls rapidly or drops below 120 mEq/L. Hyponatraemic seizures are a medical emergency requiring urgent medical intervention. Controlled rapid correction of 1–2 mEq/L per hour for the first few hours is the target to stop cerebral oedema without risking osmotic demyelination.

7. Is hyponatremia related to kidney disease?

Kidney disease impairs both sodium and water regulation, making hyponatremia more likely. Nephrotic syndrome causes hypervolaemic hyponatremia through reduced oncotic pressure and secondary ADH activation. CKD reduces the kidney's ability to dilute urine, limiting the response to water load. Renal function should always be assessed in a hyponatremia workup.

8. How quickly should hyponatremia be treated?

Urgency depends on symptom severity and onset speed. Acute symptomatic hyponatremia needs immediate correction with hypertonic saline. Chronic asymptomatic hyponatremia should be corrected slowly no more than 8–10 mEq/L per 24 hours, to avoid osmotic demyelination syndrome. Speed of correction is one of the most consequential decisions in management.

9. Can hyponatremia be prevented?

In most cases, yes. Matching fluid intake to losses with electrolyte replacement during endurance events, monitoring sodium in patients on thiazides or SSRIs, and managing heart failure and liver disease proactively all reduce risk. High-risk patients should have sodium checked routinely, not only when symptoms appear.

Dr. Utkarsh Deshmukh

MBBS, DNB (General Medicine), DNB (Nephrology)
Ganga CARE Hospital Limited, Nagpur

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