How to Prevent Hyponatremia in Athletes and Elderly Patients

Understanding Sodium Homeostasis and Serum Osmolality

Sodium plays a critical role in maintaining extracellular fluid volume, blood pressure, and neuromuscular function. Serum sodium concentration is tightly regulated within the range of 135–145 mmol/L. The primary mechanism of regulation involves a dynamic interplay between antidiuretic hormone (ADH), the renin-angiotensin-aldosterone system (RAAS), and natriuretic peptides. The hypothalamus senses plasma osmolality via osmoreceptors and modulates the release of ADH (vasopressin) from the posterior pituitary in response to small changes (1–2%) in serum osmolality.

Water balance is closely linked with sodium regulation. Any deviation in water intake or excretion directly impacts sodium concentration, often with acute and severe neurological consequences.

What Is Hyponatremia?

Hyponatremia is defined as a serum sodium concentration less than 135 mmol/L. It is categorized as:

• Mild: 130–134 mmol/L
• Moderate: 125–129 mmol/L
• Severe: <125 mmol/L

The pathophysiology of hyponatremia revolves around water retention that exceeds sodium retention or intake, leading to dilution of serum sodium. It can result from conditions that either increase water intake or impair water excretion. The most dangerous forms of hyponatremia occur rapidly, such as in exercise-associated hyponatremia or in the syndrome of inappropriate antidiuretic hormone secretion (SIADH).

Classification Based on Volume Status

Hyponatremia can be further classified based on a patient’s volume status:

1. Hypovolemic hyponatremia – Sodium and water are both lost, but sodium loss exceeds water loss. Common in vomiting, diarrhea, and diuretic use.
2. Euvolemic hyponatremia – Total body water increases, but sodium content remains unchanged. SIADH and primary polydipsia are common causes.
3. Hypervolemic hyponatremia – Both sodium and water increase, but water retention is disproportionately higher. Often seen in congestive heart failure, nephrotic syndrome, and liver cirrhosis.

Understanding these categories is vital for appropriate therapeutic intervention.

Symptoms and Clinical Presentation

The clinical manifestations of hyponatremia depend on both the severity and rapidity of onset. Acute hyponatremia (developing in <48 hours) can cause:

• Nausea and vomiting
• Headache
• Confusion and lethargy
• Seizures
• Coma and death (especially if serum Na <120 mmol/L)

Chronic hyponatremia may be asymptomatic or present with non-specific symptoms such as fatigue, gait disturbances, and cognitive impairment—often dismissed in elderly patients. It is critical to maintain a high index of suspicion.

The Role of Excessive Water Intake in Hyponatremia

Excessive water consumption is one of the less commonly discussed yet significant causes of euvolemic hyponatremia. Water intoxication can dilute serum sodium levels when renal water excretion mechanisms are overwhelmed. Healthy kidneys can excrete up to 15–20 liters of water per day, but the rate of excretion is limited to approximately 0.8 to 1.0 L/hour.

Thus, drinking water at a rate faster than this limit—particularly in the setting of ADH elevation (due to nausea, stress, or pain)—can precipitate dangerous hyponatremia. This scenario is common in:

• Marathon runners
• Psychiatric patients with psychogenic polydipsia
• Military trainees during intense physical exercise
• Individuals participating in water-drinking contests

In one documented case, a 28-year-old female died after drinking nearly 6 liters of water within a 3-hour period during a radio contest. Autopsy revealed cerebral edema due to acute water intoxication-induced hyponatremia.

The Science of Water Balance: How Much Water Is Too Much?

Public health campaigns commonly recommend “8 glasses per day” without scientific grounding. Actual water requirements vary considerably based on:

• Age
• Sex
• Body weight
• Activity level
• Climate and environmental exposure
• Diet composition
• Renal function

The Institute of Medicine (IOM) provides an adequate intake (AI) recommendation:

• Men: 3.7 liters/day (from all beverages and food)
• Women: 2.7 liters/day

However, these values are not prescriptive. The kidneys regulate water balance by adjusting urine output and osmolality, making overhydration a more plausible risk than previously believed—especially when intake far exceeds physiologic demand.

ADH Dysregulation and SIADH

SIADH is the prototypical cause of euvolemic hyponatremia due to inappropriate ADH secretion despite normal or low plasma osmolality. Common causes include:

• Malignancies (e.g., small-cell lung carcinoma)
• CNS disorders (stroke, hemorrhage, trauma)
• Pulmonary diseases (pneumonia, TB)
• Medications (SSRIs, carbamazepine, cyclophosphamide)

In SIADH, despite a normal or low serum osmolality, ADH remains elevated, resulting in water retention and hyponatremia. Patients often have inappropriately concentrated urine (>100 mOsm/kg) and low serum uric acid.

Impact of Hyponatremia on the Brain

The brain is highly sensitive to osmotic shifts. Rapid-onset hyponatremia causes water to move into brain cells, leading to cerebral edema. If untreated, this results in increased intracranial pressure, brain herniation, and death.

In contrast, in chronic hyponatremia, brain cells adapt by extruding osmolytes, including potassium, glutamate, and taurine, to reduce intracellular osmotic load. Rapid correction in such cases can cause osmotic demyelination syndrome (ODS), especially when serum sodium is increased by more than 8–10 mmol/L in 24 hours. ODS is often irreversible and causes quadriparesis, pseudobulbar palsy, and locked-in syndrome.

How to Safely Prevent Hyponatremia

Individualized Water Intake

Instead of arbitrary intake targets, hydration should be guided by:

• Thirst sensation (a highly reliable indicator of water need)
• Urine color (light yellow suggests adequate hydration)
• Activity and environmental conditions

A more scientific estimation of minimum water needs can be calculated using formulas based on body weight. One common approach:

Daily water intake (mL) = 35 × body weight (kg)

This translates to approximately:

• 2.45 L/day for a 70 kg person
• Increased requirements in high heat, intense exercise, or illness

Monitoring Sodium in High-Risk Patients

Patients on thiazide diuretics, SSRIs, or those with renal impairment should have serum sodium levels monitored regularly. Additionally, post-operative patients and elderly individuals should be assessed frequently, especially when fluid intake is encouraged.

Caution in Endurance Sports

Athletes should be trained to drink to thirst, not beyond. Commercial sports drinks, although marketed as sodium replacements, may still be hypotonic and can worsen hyponatremia if consumed excessively.

Medical Management of Hyponatremia

The treatment of hyponatremia is nuanced and based on:

• Severity of symptoms
• Duration of hyponatremia (acute vs. chronic)
• Volume status

Hypertonic saline (3%) is indicated in severe or symptomatic hyponatremia but must be used with caution to avoid ODS. The rate of correction should not exceed:

• 8 mmol/L in 24 hours
• 18 mmol/L in 48 hours

Vaptans (vasopressin receptor antagonists) such as tolvaptan are FDA-approved for euvolemic and hypervolemic hyponatremia but are costly and require close monitoring.

A Note on Water Intoxication in Psychiatric Populations

Psychogenic polydipsia occurs in patients with schizophrenia and other psychiatric disorders. These patients may consume over 10 liters of water per day, leading to recurrent, and at times fatal, hyponatremia. Treatment includes fluid restriction, behavioral therapy, and addressing underlying psychiatric pathology.