Hyponatremia: Primary polydipsia

Hyponatremia is excess of water relative to sodium and almost always due to increased ADH.

  • SIADH: inappropriate increase (Conrad: any lung, brain, drugs and cancer)
  • SADH: appropriate increase such as in hypovolemia or hypervolemia with decreased effective arterial volume

Evaluation of hyponatremia begins with the following laboratory datas

  1. Plasma osmolality: to determine if the patient really has hyponatremia
    • Pseudohyponatremia: will be high in patients with osmotic substances that draw water to dilute the concentration of sodium
  2. Urine osmolality: useful in limited circumstances  -> Uosm will be < 100
    • malnutrition(decrease solute intake)
    • primary polydipsia(increase water intake)
  3. Urinary sodium: used to determine the kidney’s concentrating capabilities(basically how the body is responding to this state, or did it cause it)
    • Normally should be low < 20

Clinical features: generally any change in hyper or hypo osmolality causes water shifts and may result in change in brain cell volume -> AMS or seizures may precipitate

  1. Neurologic: osmotic water draws water increasing ICF volume causing brain cell swelling or cerebral edema
    • Altered mental status, headache, delirium, irritability
    • Muscle twitching, weakness, hyper-DTRs
    • IICP(Cushing triad of irregular respirations caused by impaired brainstem function, bradycardia, systolic hypertension with widening pulse pressure), seizures, coma
  2. Gastrointestinal: n/v, ileus, watery diarrhea

Here are some algorithms that may prove helpful.


Here is another algorithm that most books, including pocket medicine, implements -> assessment of volume status. It included the assessment of vital signs, orthostatics, JVPs, skin turgor, mucous membranes, peripheral edema, BUN/Cr and uric acid 


Primary polydipsia/Malnutrition(“tea and toast” or “beer potomania”):

  • More common in psychiatric patients(also called psychogenic polydipsia)
  • Possibly due to a central defect in thirst regulation(decreased serum osmolality should inhibit thirst reflex). -> This causes the kidney to increase water excretion and diluted the urine(Uosm < 100 mOsm/kg)
    • if water intake > kidney excretion -> hyponatremia develops

Q 2657


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