Hyponatremia: Physiology
Authors: Mannat Dhillon Reviewers: Andrea Kuczynski Kevin McLaughlin* * MD at time of publication
Abnormal Renal H2O Handling (hypo-osmolar serum)
AKI/CKD Heart failure
↓ renal blood flow
↓ glomerular filtration
GFR < 25 mL/min, ↓ urine dilution ↑ H2O retention
Note:
• Plasma [Na+] is regulated by water intake/excretion, not by changes in [Na+].
• Artifactual hyponatremia can be differentiated by a normal or hyperosmolar serum.
Appropriate ADH secretion
↓ EABV
Hypovolemia: losses via GI, renal, skin, 3rd spacing, bleeding
Hypervolemia: heart failure, cirrhosis
↑ Na+/H2O absorption at PCT
↓ EABV, ↑ H2O retention
Urine [Na+] < 20 mmol/L
Hereditary: tubular disorders
(Bartter, Gitlemann syndromes).
Thiazide diuretics
Inappropriate: SIADH, hypothyroidism, AI
Normal EABV
Anti-diuresis
Primary polydipsia, eating disorder
↑ H2O or ↓ solute intake
↓ Osmoles
Impaired desalination
Block NCC
↑ H2O retention ↑ Na+/K+ excretion
Hyponatremia
Serum [Na+] < 135 mmol/L
Urine osmolality > 100 mmol/L
Urine osmolality < 100 mmol/L
Cerebral edema, ↑ intracranial pressure, vasoconstriction
If hypovolemic: ↓ JVP, ↓ blood pressure
Lethargy, altered mental status
Abbreviations:
AKI: Acute Kidney Injury
CKD: Chronic Kidney Disease
GFR: Glomerular Filtration Rate
H2O: Water
PCT: Proximal Convoluted Tubule
EABV: Effective Arterial Blood Volume
NCC: Na+/Cl- Co-Transporter
SIADH: Syndrome of Inappropriate ADH Secretion AI: Adrenal Insufficiency
Legend:
Pathophysiology
Mechanism
Sign/Symptom/Lab Finding
Complications
Published January 11, 2019 on www.thecalgaryguide.com