Sodium Balance:
• The human body contains 1 g Na / Kg of BW
• Sodium is located: 95% extracellularly
5% intracellularly.
• Daily balance of sodium is 6 gr (150 meq)
• Daily losses = 150 meq = 100 meq in urine + 35 meq in sweat + 15 meq in feces
Sodium reabsorption
• Sodium is reabsorbed almost completely
(~ 99% ) esp. in proximal tubule.
• The percent amount of sodium that is excreted in the urine is called F•Na and is calculated by the formula:

FENa (%) = Urinesodium/Plasmasodium X 100
Urinecreatinine/Plasmacreatinine

Hyponatremia:
Plasma Na < 135meq /L
• Almost always due to •ADH Secretion
• Appropriate
• Inappropriat
• One Exception: Primary Polydipsia • supression of ADH Secretion BUT still overwhelms kidney’s diluting ability • Free water retention & Hyponatremia

Epidemiology of Hyponatremia:
Hyponatremia is among the most common electrolyte disorders encountered in clinical medicine, with an incidence of 0.97% and a prevalence of 2.48% in hospitalized adult patients when plasma [Na+ ] concentration below 130 mEq/L is the diagnostic criterion.

Clinical Manifestations:
• < 125 mEq/l •
• Malaise - Muscle cramps
• Nausea, Vomiting, Headache
• Hypotension – Tachycardia

• < 110 mEq/L •
• Confusion, convulsions, coma

Type of Hyponatremias:
1) Hypotonic hyponatremias:
• Hypervolumic
• Euvolumic
• Hypovolumic

2) Hypertonic hyponatremia
3) Isotonic hyponatremia

Hypovolemic Hypotonic Hyponatremia:
• Primary Na loss • Secondary Water gain

Renal Losses (FENA > 1%)
• Diuretics
• Hypoaldosteronism
• Salt-wasting Nephropathy

Extra-renal Losses (FENA < 1%)
• GI losses
• Third Spacing
• Insensible losses

Euvolemic Hypotonic Hyponatremia:

• Psychogenic Polydipsia:
• Requires intake of >10 L/day
• Uosm < 100 mosm/kg
• Low Uric Acid

• Reset Osmostat:
• ADH physiology reset to secrete at subnormal serum osmolality threshold (<280 mosm/kg)
• Seen in: Elderly, Pulmonary processes (e.g. TB), Malnutrition

Euvolemic Hypotonic Hyponatremia:
• SIADH

• Diagnostic Criteria:
• Euvolemic state
• Normal renal, thyroid and adrenal function
• Hypoosmolar serum (<270 mosm/Kg)
• Inappropriately concentrated urine (>100 mosm/Kg)
• High urinary Na (>40 meq/L) with normal salt and water intake

• Etiologies:
• Endocrinopathies: Hypothyroidism, Adrenal Insufficiency
• Pulmonary Pathology: Pneumonia, Asthma, COPD, PTX
• Intracranial Pathology: Trauma, Infection, Hemorrhage
• Malignancies: Small Cell Lung ca. Intracranial Tumors
• Drugs: Antipsychotic, Antidepressants, Thiazides

Hypervolemic Hypotonic Hyponatremia:
• Decreased Effective Arterial Volume
• Congestive Heart Failure
• Cirrhosis
• Nephrotic Syndrome
• Advanced Renal Failure

Workup:
• Determine Tonicity…
• Osmolality = 2 (Na meq/L) + Glucose(mg/dl) + BUN(mg/dl)
18 2.8
For Hypotonic Hyponatremia:
• Determine Volume Status…

Treatment:
• Hypovolemic Hyponatremia:
• Volume replacement with 0.9% NaCl
• Na Deficit =
0.6 X Body Wt. X (140 – Measured Na) (X 0.85 in women)

• Hypervolemic Hyponatremia:
• Sodium Restriction to 1-3 g/day
• Water Restriction: 1.0-1.5 L/day
• Diuretics
• Na <110 meq/l + CNS symptoms: judicious administration 3% saline with diuretics
• Emergency dialysis

• Euvolemic hyponatremia :
• Free Water Restriction
• Careful Na correction
• Asymptomatic but Na <120 meq/l : 0.9% saline + frusemide maybe used
• In case of Neurological Emergencies
• Loop Diuretics + Fluid Replacement with Hypertonic Saline ( 3% )
• If Chronic • Demeclocycline 300-600 mg twice daily
• Fludrocortisone
• Selective vasopressin V2 antagonist