Sodium Flashcards
What is the normal range for serum sodium?
135–145 mEq/L.
What is the definition of hyponatremia?
Serum sodium <135 mEq/L.
What are the classifications of hyponatremia by severity?
- Mild: 130–135 mEq/L
- Moderate: 120–130 mEq/L
- Severe: <120 mEq/L
What are the three main types of hyponatremia based on tonicity?
- Hypotonic (true) hyponatremia (serum osmolality <275 mOsm/kg)
- Isotonic hyponatremia (serum osmolality 250–295 mOsm/kg)
- Hypertonic hyponatremia (serum osmolality >295 mOsm/kg)
What is hypovolemic hyponatremia?
Hyponatremia associated with decreased total body water and sodium loss.
What are renal causes of hypovolemic hyponatremia?
Diuretics, post-acute tubular necrosis diuresis, low aldosterone (primary adrenal insufficiency).
What are extrarenal causes of hypovolemic hyponatremia?
GI losses (vomiting, diarrhea), third-spacing (burns, pancreatitis), and poor intake.
How do urine sodium levels differentiate renal from extrarenal hypovolemic hyponatremia?
- Urine Na >40 mEq/L suggests renal salt loss
- Urine Na <40 mEq/L suggests extrarenal loss
What conditions cause euvolemic hyponatremia?
SIADH, primary polydipsia, beer potomania, tea and toast diet, hypothyroidism.
What lab findings differentiate SIADH from other causes of euvolemic hyponatremia?
SIADH: Urine osmolality >100 mOsm/kg, Urine Na >40 mEq/L
Primary polydipsia/beer potomania: Urine osmolality <100 mOsm/kg
What is hypervolemic hyponatremia?
Hyponatremia associated with excess total body water and sodium retention.
What are the causes of hypervolemic hyponatremia?
Heart failure, cirrhosis, nephrotic syndrome, advanced kidney failure.
What lab findings are characteristic of hypervolemic hyponatremia?
- Urine osmolality >100 mOsm/kg
- Urine Na <40 mEq/L (except in kidney failure where Urine Na is >40)
What is isotonic hyponatremia (pseudohyponatremia)?
A lab artifact caused by elevated serum lipids or proteins (e.g., hyperlipidemia, multiple myeloma).
What are causes of hypertonic hyponatremia?
Hyperglycemia, mannitol, sorbitol, radiocontrast agents.
How does hyperglycemia cause hyponatremia?
Hyperglycemia increases serum osmolality, causing a shift of water into the extracellular space, diluting serum sodium.
What are the neurological symptoms of hyponatremia?
Nausea, vomiting, headaches, confusion, lethargy, hyporeflexia, seizures, coma, respiratory arrest.
How is mild/asymptomatic hyponatremia managed?
Treat the underlying cause, fluid restriction, and salt tablets if appropriate.
When is hypertonic saline indicated in hyponatremia?
Severe hyponatremia (<120 mEq/L) with symptoms (e.g., seizures, altered mental status).
What is the maximum sodium correction rate to avoid osmotic demyelination syndrome?
6–8 mEq/L per 24 hours.
What is osmotic demyelination syndrome (ODS)?
A condition caused by rapid correction of chronic hyponatremia leading to irreversible demyelination.
What are the clinical features of osmotic demyelination syndrome?
Delayed onset (2–5 days) of dysphagia, dysarthria, paralysis, mental status changes, locked-in syndrome.
How can osmotic demyelination syndrome and cerebral edema be prevented?
Correct sodium levels slowly to prevent rapid shifts in osmolality.
What is the definition of hypernatremia?
Serum sodium >145 mEq/L.
What is the primary cause of hypernatremia?
Water depletion due to impaired thirst, GI losses, renal losses, or insensible losses.
What renal conditions cause hypernatremia?
Osmotic diuresis (e.g., hyperglycemia), diabetes insipidus (central or nephrogenic).
How is central DI differentiated from nephrogenic DI?
- Central DI: Low ADH (responds to desmopressin).
- Nephrogenic DI: Resistance to ADH (no response to desmopressin).
How is isovolemic hypernatremia managed?
Free water replacement (D5W or PO water).
How is hypovolemic hypernatremia managed?
IV isotonic saline initially, followed by free water replacement.
What is the maximum sodium correction rate to prevent cerebral edema?
12 mEq/L per day or 0.5 mEq/L per hour.
