Sodium – The Master Electrolyte of the Body
Sodium is the most abundant extracellular electrolyte in the human body, primarily located in the extracellular fluid (ECF). It plays a vital role in:
- Maintaining fluid balance and osmotic pressure
- Regulating acid–base balance
- Ensuring proper nerve impulse conduction
- Facilitating muscle contraction
- Driving active and passive transport mechanisms
The normal serum sodium range is 135–145 mEq/L. Deviations from this range result in:
- Hypernatremia: >145 mEq/L (too much sodium).
- Hyponatremia: <135 mEq/L (too little sodium).
Both are common electrolyte disturbances seen in hospitalized patients and are associated with high morbidity and mortality if not corrected promptly.
The Physiology of Sodium Balance
Sodium balance is regulated through the interplay of:
- Kidneys: Filter and reabsorb sodium according to body needs.
- Aldosterone: Increases sodium reabsorption in the distal nephron.
- ADH (antidiuretic hormone): Indirectly regulates sodium by controlling water reabsorption.
- Thirst mechanism: Helps restore fluid and sodium balance when disturbed.
The balance between sodium and water is more important than sodium alone. Sodium imbalance is usually classified based on volume status:
- Hypovolemic: Loss of both sodium and water.
- Hypervolemic: Excess water relative to sodium.
- Euvolemic: Sodium low or high without significant fluid shifts.
Hypernatremia: Too Much Sodium (>145 mEq/L)
Definition
Hypernatremia is a state of serum sodium concentration >145 mEq/L, usually due to either excess sodium intake or loss of free water.
Mnemonic for Symptoms: “BIG & BLOATED”
- F – Flushed skin
- R – Restless, anxious, confused, irritable
- I – Increased BP & fluid retention
- E – Edema (pitting)
- D – Decreased urine output, dry mouth
- S – Skin flushed & dry
- A – Agitation
- L – Low-grade fever
- T – Thirst, dry mucous membranes
Hypernatremia makes cells dehydrated as water shifts out of them, leading to neurological dysfunction.
Causes & Risk Factors of Hypernatremia
1. Increased Sodium Intake
- Excess oral intake (e.g., salt poisoning, tube feeds).
- Iatrogenic: Hypertonic saline or sodium bicarbonate infusion.
2. Loss of Free Water (Hemoconcentration → increased sodium)
- Fever, watery diarrhea.
- Diabetes insipidus (lack of ADH or resistance to ADH).
- Excessive sweating.
- Burns or infections with high insensible losses.
- Kidney dysfunction or failure.
Clinical Features of Hypernatremia
- Neurological: Irritability, restlessness, confusion, lethargy, seizures, coma.
- Cardiovascular: Hypertension (fluid retention) or orthostatic hypotension if fluid depleted.
- Renal: Low urine output, concentrated urine.
- General: Dry mucous membranes, thirst, fever, edema.
Diagnosis of Hypernatremia
- Serum sodium >145 mEq/L.
- Serum osmolality >295 mOsm/kg.
- Urine osmolality: Helps differentiate renal vs extrarenal water loss.
- Imaging if central diabetes insipidus suspected.
Management of Hypernatremia
- If due to fluid loss: Administer hypotonic fluids (D5W, 0.45% saline) slowly to avoid cerebral edema.
- If due to sodium gain: Give diuretics that promote sodium excretion.
- Restrict sodium intake (dietary counseling).
- Correct underlying cause (e.g., treat diabetes insipidus with desmopressin).
Hyponatremia: Too Little Sodium (<135 mEq/L)
Definition
Hyponatremia is defined as serum sodium <135 mEq/L, and it can be hypovolemic, euvolemic, or hypervolemic. It is the most common electrolyte disorder in hospitalized patients.
Mnemonic for Symptoms: “SALT LOSS” (Hypovolemic Hyponatremia)
- S – Stupor/coma
- A – Anorexia, nausea, vomiting
- L – Lethargy
- T – Tachycardia, thready pulse
- L – Limp muscles (weakness)
- O – Orthostatic hypotension
- S – Seizures/headache
- S – Stomach cramps, hyperactive bowels
Types of Hyponatremia
1. Hypovolemic Hyponatremia (↓ sodium & ↓ water, sodium loss > water loss)
- Causes: GI loss (diarrhea, vomiting, drains), diuretics, burns.
2. Hypervolemic Hyponatremia (↑ body water > sodium)
- Causes: Heart failure, kidney disease, liver cirrhosis, SIADH.
- Causes: SIADH (syndrome of inappropriate ADH), adrenal insufficiency, hypothyroidism, excessive water intake.
Causes & Risk Factors of Hyponatremia
- Increased sodium excretion: Diuretics, diarrhea, vomiting, drains.
- Inadequate sodium intake: Low-salt diet, fasting, NPO.
- Dilutional causes: SIADH, heart failure, kidney disease, adrenal crisis.
Clinical Features of Hyponatremia
- Neurological: Headache, confusion, irritability, seizures, coma.
- GI: Nausea, vomiting, abdominal cramps.
- Musculoskeletal: Weakness, spasms.
- Cardiovascular: Hypotension (hypovolemic), edema (hypervolemic).
Diagnosis of Hyponatremia
- Serum sodium <135 mEq/L.
- Serum osmolality <275 mOsm/kg.
- Urine sodium/osmolality: Helps determine type and cause.
- Endocrine workup (adrenal, thyroid) if indicated.
Management of Hyponatremia
Mnemonic: “ADD SALT”
- A – Administer sodium chloride IV (if hypovolemic).
- D – Diuretics (if hypervolemic).
- D – Daily weights: Monitor fluid status.
- S – Safety: Fall precautions (risk of confusion).
- A – Airway protection: Confused patients are aspiration risks.
- L – Limit water intake: Especially in SIADH or hypervolemic hyponatremia.
- T – Teach: Educate about low-salt diets, hidden sodium in processed foods, and fluid restriction.
Sodium correction must be slow and controlled. Rapid correction risks osmotic demyelination syndrome (ODS).
Complications of Sodium Imbalance
- Hypernatremia: Intracranial hemorrhage, seizures, coma, death.
- Hyponatremia: Cerebral edema, seizures, permanent neurological damage if corrected too rapidly.
- Both are medical emergencies when severe.
Quick Clinical Comparison Table
| Feature | Hypernatremia (>145 mEq/L) | Hyponatremia (<135 mEq/L) |
|---|---|---|
| Mnemonic | BIG & BLOATED | SALT LOSS |
| Neuro | Restlessness, confusion, coma | Confusion, seizures, stupor |
| CV | Hypertension, edema, low urine | Hypotension (hypo), edema (hyper) |
| Renal | Oliguria, concentrated urine | Polyuria, dilute urine |
| GI | Dry mucosa, thirst | Nausea, vomiting, cramps |
| Causes | Sodium gain, water loss | GI loss, SIADH, heart failure |
| Treatment | Hypotonic fluids, diuretics | Fluid restriction, IV saline, diuretics |
Frequently Asked Questions (FAQ)
Q1. What is the normal sodium range in blood?
135–145 mEq/L.
Q2. Which is more dangerous: hypernatremia or hyponatremia?
Both are serious, but hyponatremia is more common and often more dangerous because of its effects on brain swelling.
Q3. How fast should sodium be corrected?
No more than 8–10 mEq/L in 24 hours to prevent central pontine myelinolysis.
Q4. Can dehydration cause sodium imbalance?
Yes. Dehydration often leads to hypernatremia due to water loss exceeding sodium loss.
Q5. What foods are high in sodium?
Processed foods, canned soups, chips, pickles, sauces, and packaged snacks. Patients with sodium imbalance should regulate intake based on medical advice.
