Hyponatremia, or low sodium levels in the blood, is one of the most common and clinically significant electrolyte disturbances encountered in medicine. While it might sound like a minor lab abnormality, the consequences can range from mild confusion to seizures and even coma.
This article delves into the definition, causes, signs and symptoms, diagnosis, treatment, and uses the mnemonic “SALT LOSS” to help students and professionals easily remember the key clinical manifestations.
What is Hyponatremia?
Hyponatremia is defined as a serum sodium concentration less than 135 mEq/L. It results from excess water relative to sodium in the body and may be due to water retention, sodium loss, or both.
Classification of Hyponatremia
| Serum Sodium Level | Severity |
|---|---|
| 130–134 mEq/L | Mild |
| 125–129 mEq/L | Moderate |
| <125 mEq/L | Severe |
| <115 mEq/L | Critical, life-threatening |
Based on Duration:
- Acute (<48 hours): higher risk of cerebral edema
- Chronic (>48 hours): brain adapts, but prone to osmotic demyelination if corrected rapidly
Causes of Hyponatremia
Hyponatremia can result from numerous pathologies. Causes are often grouped based on volume status:
1. Hypovolemic Hyponatremia
Loss of both sodium and water, but more sodium lost:
- Vomiting, diarrhea
- Diuretics (especially thiazides)
- Addison's disease
2. Euvolemic Hyponatremia
Normal body volume with excess free water:
- SIADH (Syndrome of Inappropriate ADH secretion)
- Hypothyroidism
- Psychogenic polydipsia
- Postoperative states
3. Hypervolemic Hyponatremia
Excess total body water:
- Congestive heart failure
- Liver cirrhosis
- Nephrotic syndrome
- Renal failure
Clinical Manifestations of Hyponatremia
The symptoms of hyponatremia are primarily neurological and depend on:
- Severity of sodium drop
- Rapidity of onset
To make it easier to recall, we use the mnemonic:
SALT LOSS
Each letter represents a key sign or symptom.
Mnemonic: SALT LOSS
| Mnemonic | Symptom/Sign | Explanation |
|---|---|---|
| S | Stupor/Coma | Due to cerebral edema and increased intracranial pressure |
| A | Anorexia (Nausea, Vomiting) | Gut irritation, often an early symptom |
| L | Lethargy | Brain swelling leads to decreased alertness |
| T | Tendon reflexes decreased | Neuromuscular irritability reduction |
| L | Limp muscles (Weakness) | Muscle fatigue, flaccidity |
| O | Orthostatic hypotension | Seen in hypovolemic states |
| S | Seizures/Headache | Sign of impending brain herniation |
| S | Stomach cramping | GI irritation due to altered cellular metabolism |
Let’s break down each one.
S – Stupor/Coma
When sodium falls below 115 mEq/L, it leads to severe cerebral edema, resulting in:
- Confusion
- Drowsiness
- Coma
- Risk of brain herniation, especially in acute hyponatremia
A – Anorexia (Nausea and Vomiting)
The GI tract becomes sensitive to hyponatremia. Early symptoms include:
- Loss of appetite
- Persistent nausea
- Emesis due to increased ICP
L – Lethargy
Fatigue and slowed responsiveness are common due to:
- Cerebral cell swelling
- Decreased neurotransmission
T – Tendon Reflexes Decreased
Hyporeflexia is an important neurological sign, indicative of:
- Depressed central nervous system activity
- Impaired neuromuscular conduction
L – Limp Muscles (Muscle Weakness)
Patients report:
- Generalized muscle weakness
- Difficulty standing or walking
- Risk of falls in elderly patients
O – Orthostatic Hypotension
Common in hypovolemic hyponatremia, signs include:
- Drop in BP on standing
- Dizziness or fainting
- Tachycardia
S – Seizure/Headache
As sodium drops quickly, brain cells swell, causing:
- Raised intracranial pressure
- Headache, visual changes
- Seizures
This is a medical emergency needing ICU management.
S – Stomach Cramping
Caused by:
- Sodium shifts affecting gut smooth muscle tone
- Electrolyte imbalance causing abdominal discomfort or diarrhea
Diagnostic Approach to Hyponatremia
Step 1: Confirm Hyponatremia
- Serum Na⁺ <135 mEq/L
Step 2: Check Serum Osmolality
- Hypotonic (<275 mOsm/kg): True hyponatremia
- Isotonic: Pseudohyponatremia (e.g., hyperlipidemia)
- Hypertonic: Seen with hyperglycemia
Step 3: Assess Volume Status
- History and physical exam to determine hypovolemia, euvolemia, or hypervolemia
Step 4: Check Urine Sodium and Osmolality
- Helps identify causes like SIADH or renal losses
Treatment of Hyponatremia
Treatment is based on:
- Severity
- Duration
- Symptoms
- Underlying cause
| Situation | Treatment |
|---|---|
| Asymptomatic, chronic, mild | Water restriction, treat underlying cause |
| Hypovolemic hyponatremia | IV normal saline |
| SIADH (euvolemic) | Fluid restriction, demeclocycline, urea |
| Hypervolemic hyponatremia | Diuretics + fluid and sodium restriction |
| Severe symptomatic hyponatremia | 3% hypertonic saline with ICU care |
Rapid correction (>8–10 mEq/L in 24 hours) can lead to Osmotic Demyelination Syndrome (Central Pontine Myelinolysis).
Complications of Hyponatremia
| Complication | Description |
|---|---|
| Cerebral edema | Especially in rapid onset |
| Seizures and coma | Due to brain swelling |
| Herniation and death | From severe untreated hyponatremia |
| Central pontine myelinolysis | From rapid overcorrection |
| Muscle cramps and falls | Especially in elderly |
Prevention of Hyponatremia
- Careful IV fluid management in post-op and ICU patients
- Avoid hypotonic fluids unless indicated
- Monitor serum sodium during diuretic therapy
- Educate endurance athletes about water intoxication
Summary Table: SALT LOSS Mnemonic for Symptoms
| Mnemonic | Symptom | Mechanism |
|---|---|---|
| S | Stupor/Coma | Cerebral edema |
| A | Anorexia, Nausea, Vomiting | CNS and gut irritability |
| L | Lethargy | Decreased CNS function |
| T | Tendon reflexes ↓ | Depressed neuromuscular excitability |
| L | Limp muscles | Muscle hypotonia |
| O | Orthostatic hypotension | Volume depletion |
| S | Seizures/Headache | Increased ICP |
| S | Stomach cramping | Electrolyte imbalance in GI smooth muscle |
Frequently Asked Questions (FAQs)
Q1. What sodium level is considered critical in hyponatremia?
A: Serum sodium <115 mEq/L is critical and can lead to coma and death.
Q2. What is the most common cause of hyponatremia in hospitalized patients?
A: SIADH and inappropriate IV fluid therapy.
Q3. Can hyponatremia be asymptomatic?
A: Yes, especially if chronic and mild.
Q4. What is the danger of correcting sodium too quickly?
A: Central Pontine Myelinolysis, a potentially irreversible brain injury.
Q5. How do I distinguish SIADH from hypovolemia?
A: SIADH presents with euvolemia, concentrated urine, and high urine sodium despite normal fluid status.
