Hyponatremia (serum sodium less than 135 mEq/L) may be classified based upon serum ADH level, duration of hyponatremia, serum osmolality and volume status. The various classification systems enable accurate identification of the cause of hyponatremia and hence translate into optimal management based on the condition of the patient.

Hyponatremia is defined as serum sodium less than 135 mEq/L (mmol/L) ^[1] .There are different classifications for hyponatremia based on duration, severity, volume status, ADH level and serum osmolality.

Hyponatremia is classified based on serum sodium level into the following types ^[2] :

• Mild : Serum sodium 130– 135 mmol/L
• Moderate:  Serum sodium ≤125–129 mmol/L
• Severe: Serum sodium <124 mmol/L

Hyponatremia may be classified based on duration into the following types:^[3]

• Hyper acute ^[4]: Develops in a few hours, excess water intake, impaired water excretion, runners, users of the recreational drug (Ecstasy)

• Acute: Rapid onset <48 hours, surgeries, colonoscopy preparation, polydipsia, diuretics
• Chronic: Gradual onset >48 hours, caused by chronic disease ( including cardiac, renal, hepatic and other conditions)

( Etiologies cause hyperacute and acute hyponatremia are applicable to each category interchangeably depending on the onset of symptoms)

Hyponatremia may be classified into the following types based on ADH levels:

• ↑ ADH: Volume depletion (GI loss, Renal loss) , decreased perfusion ( CHF, Cirrhosis), increased ADH secretion, reset osmostat

• ↓ ADH: Primary polydipsia, ↓ dietary solute intake, advanced renal failure

Hyponatremia may be classified into the following types based on serum osmolality:^[5]

• Hypertonic hyponatremia: Serum osmolality >295 mOsm/kg
• Hypotonic hyponatremia: Serum osmolality < 275 mOsm/kg
• Normotonic hyponatremia: Serum osmolality 275–295 mOsm/kg

Hyponatremia may be classified into the following types according to volume status :