Causes of Acute Hyponatremia
Acute hyponatremia is most commonly caused by syndrome of inappropriate antidiuretic hormone (SIADH), excessive fluid intake, medications, and acute volume depletion states. 1 Understanding the underlying mechanisms and classification is essential for proper management.
Classification by Volume Status
Hyponatremia can be classified based on the patient's volume status:
1. Hypovolemic Hyponatremia
- Gastrointestinal losses: Vomiting, diarrhea
- Renal losses:
- Diuretic use (especially thiazides) 2
- Cerebral salt wasting
- Adrenal insufficiency
- Third-space losses: Burns, pancreatitis
- Excessive sweating
2. Euvolemic Hyponatremia
- SIADH (most common cause of euvolemic hyponatremia) 3, 4
- Malignancies (especially small cell lung cancer)
- Pulmonary disorders (pneumonia, tuberculosis)
- CNS disorders (meningitis, encephalitis, stroke)
- Pain, nausea, stress
- Postoperative state
- Medications 5:
- Psychotropics (SSRIs, antipsychotics)
- Anticonvulsants (carbamazepine, oxcarbazepine)
- Antineoplastic agents
- Opioids
- Ecstasy (MDMA)
- Endocrine disorders:
- Hypothyroidism
- Glucocorticoid deficiency
- Primary polydipsia (excessive water intake)
- Reset osmostat syndrome
3. Hypervolemic Hyponatremia
- Heart failure
- Liver cirrhosis 3
- Nephrotic syndrome
- Renal failure
Pathophysiological Mechanisms
The development of acute hyponatremia involves several mechanisms:
Excessive ADH secretion: Non-osmotic stimuli like pain, nausea, medications, and certain diseases can trigger inappropriate ADH release, leading to water retention 3
Impaired water excretion: In conditions like cirrhosis, the renin-angiotensin-aldosterone system is activated due to systemic vasodilation and decreased effective plasma volume, causing excessive sodium and water reabsorption 3
Iatrogenic causes:
- Administration of hypotonic fluids, especially in patients with elevated ADH levels 3
- Post-surgical states
- Medication administration
Rapid free water intake: Exceeding the kidney's excretory capacity (typically >1L/hour)
Special Considerations
Small Cell Lung Cancer
Small cell lung cancer deserves special mention as 10-45% of cases produce ADH, with 1-5% developing symptomatic SIADH 3. This paraneoplastic syndrome manifests as:
- Hyponatremia (serum sodium <134 mEq/L)
- Hypoosmolality (plasma osmolality <275 mosm/kg)
- Inappropriately high urine osmolality (>500 mosm/kg)
- Inappropriately high urinary sodium concentration (>20 mEq/L)
Pediatric Considerations
In children, acute hyponatremia is often iatrogenic, resulting from:
- Incorrect replacement of transepidermal water loss
- Inadequate water intake
- Excessive sodium intake during transition phase 3
- Administration of hypotonic maintenance intravenous fluids 3
Clinical Manifestations by Severity
The symptoms of hyponatremia depend on the severity and acuity of onset:
- Mild (130-135 mEq/L): Often asymptomatic
- Moderate (125-129 mEq/L): Nausea, headache, confusion, general weakness
- Severe (<125 mEq/L): Delirium, confusion, impaired consciousness, seizures, coma, and potentially death 6
Diagnostic Approach
When evaluating a patient with suspected acute hyponatremia:
Assess volume status: Look for signs of hypovolemia (orthostatic hypotension, dry mucous membranes), euvolemia, or hypervolemia (edema, ascites)
Measure serum and urine osmolality:
- Low serum osmolality (<275 mosm/kg) confirms true hyponatremia
- High urine osmolality (>100 mosm/kg) suggests impaired water excretion
Measure urine sodium:
- <20 mEq/L suggests hypovolemic or hypervolemic hyponatremia
20 mEq/L suggests SIADH or other euvolemic causes
Screen for underlying conditions:
- Medications review
- Evaluation for malignancy, particularly small cell lung cancer
- Thyroid and adrenal function tests
Understanding these causes and mechanisms is essential for proper diagnosis and management of acute hyponatremia, which can be life-threatening if not addressed promptly.