Disorders of Water Balance: Hyponatremia Flashcards
What is the definition of hypernatremia?
Hypernatremia is defined as serum or plasma [Na^+] > 145 mEq/L and hyperosmolality (serum osmolality > 295 mOsm/kg H2O).
What factors determine serum [Na^+]?
Serum [Na^+] is determined by total body Na^+, K^+, and water content.
What causes hypernatremia?
Hypernatremia can develop by a deficit in total body water and/or a gain of Na^+ or a combination of these events.
What are the two mechanisms that defend against hypernatremia in a healthy individual?
- Thirst * Excretion of a concentrated urine
What are the main reasons hypernatremia develops in patients?
- Cannot experience or respond to thirst * Have no access to water * Have salt loading * Excrete dilute urine with no or resistance to ADH
Who are the patients at risk for hypernatremia?
- Elderly * Children * Diabetics with uncontrolled glucose * Patients with polyuria * Hospitalized patients
What are the classifications of hypernatremia based on volume status?
- Hypovolemic hypernatremia * Hypervolemic hypernatremia * Normovolemic (euvolemic) hypernatremia
What are the primary steps in the approach to a patient with hypernatremia?
- Estimate volume status * History and physical examination * Diagnosis of hypernatremia * Obtain pertinent laboratory tests
What tests are important for diagnosing hypernatremia?
- Plasma and urine osmolalities * Urine Na^+ and K^+
What are the clinical manifestations of acute hypernatremia?
- Nausea * Vomiting * Lethargy * Irritability * Weakness * Seizures * Coma
What are the clinical manifestations of chronic hypernatremia?
- Weakness * Nystagmus * Depressed sensorium
What is diabetes insipidus (DI)?
DI is a condition characterized by hypotonic polyuria with urinary concentrating defect.
What are the types of diabetes insipidus?
- Central DI (ADH deficiency) * Nephrogenic DI (tubular resistance to ADH) * Gestational DI (placental vasopressinase degradation of ADH)
What is the treatment for central diabetes insipidus?
Desmopressin (DDAVP).
What causes nephrogenic diabetes insipidus?
- Tubular resistance to ADH despite adequate circulating levels * Hereditary forms (e.g., vasopressin V2 receptor mutation) * Acquired causes such as lithium, CKD, hypokalemia, hypercalcemia
What is primary polydipsia?
Primary polydipsia is excessive drinking due to psychiatric disorders (e.g., schizophrenia) or abnormal thirst due to osmoregulation dysfunction.
What is the water deprivation test used for?
The water deprivation test is used to diagnose polyuria by restricting fluids and measuring urine osmolality.
What do copeptin levels indicate in the diagnosis of diabetes insipidus?
- Random copeptin ≥21.4 pmol/L: Nephrogenic DI * Arginine-stimulated copeptin <3.8 pmol/L: Central DI * Arginine-stimulated copeptin >3.8 pmol/L: Primary polydipsia
What are the diagnostic characteristics of hypovolemic hypernatremia?
- Orthostatic changes: Yes (kidney) * Urine Na^+: >20 (kidney) * Urine osmolality: >100 (both kidneys & nonkidney) * Urine volume/day: Low (<1 L) * Edema: No
What role does brain adaptation play in hypernatremia?
When serum [Na^+] increases, the brain volume decreases due to exit of water, resulting in a decrease in brain size and an increase in osmolality.
What is transient hypernatremia?
Transient hypernatremia can occur from regular exercise and severe seizure activity due to water movement from extracellular to intracellular compartments.
