Polyuria

Urine output of >3 litres per day.  Note that it is usually accompanied by urinary frequency, but that frequency itself may not indicate polyuria (i.e. frequently passing small vs large volumes).

Pathophysiology

  • In general, polyuria can be a result of anything that causes
    • Increased water intake (polydipsia)
    • Decreased ADH (antidiuretic hormone) secretion (central diabetes insipidus)
    • Decreased peripheral ADH sensitivity (nephrogenic diabetes insipidus)
      • ADH promotes water reabsorption in the renal collecting ducts
    • Solute diuresis (most commonly seen in uncontrolled diabetes mellitus- where high glucose concentrations cause a passive diuresis)

Assessment

  • History
    • Define the extent of polyuria and distinguish between urinary frequency i.e. how much urine
    • If polyuria is present, explore this
      • Onset, Duration, Progression, Triggers, Exacerbating/Alleviating factors
      • Associated symptoms- specifically thirst/drinking (polydipsia); weight changes, (also night sweats)
    • Specific things to ask about include
      • Any recent IV fluids/tube feeds; recent catheterisation/urinary obstruction; recent head trauma/surgery or stroke
        • Note patients can be polyuric following urinary obstruction
    • PMHx
      • Diabetes mellitus
      • Psychiatric disorders – on lithium treatment
      • Sickle cell disease
      • Sarcoid/amyloidosis
      • Hyperparathyroidism
      • Hypertension – on diuretics
      • Alcohol and caffeine intake
      • Smoking history
  • Examination
    • General examination of
      • Blood pressure/pulse
      • Weight
      • Mucous membranes (dry?)
      • Skin (dry, pigmented lesions, ulcers/nodules)
    • Neurological/psychiatric exam (doesn’t have to be extensive but may be further explored if there is any suspicion or positive findings
      • e.g. papilloedema; visual fields etc

Red Flags

  • Abrupt onset or onset in children
  • Night sweats, weight loss (particularly where there is a smoking history too)
  • Psychiatric disorder

Investigations

  • Serum or fingerprick (BM) glucose measurement and urinalysis to rule out diabetes mellitus
  • Where hyperglycaemia is absent
    • U&Es including serum Calcium
      • Hypernatraemia suggests excess water loss due to diabetes insipidus
      • Hyponatraemia suggests excess free water intake (polydipsia)
    • Urine osmolarity
      • usually low with water diuresis and high with solute diuresis
  • If a diagnosis is not yet clear, a water deprivation test can be done
    • (NB only to be done as an inpatient under supervision)
      • Typically a baseline set of weight, bloods and observations are performed in the morning.  The patient is then deprived of water.  Where possible, hourly samples of urine are tested for osmolarity and sodium concentrations.  Once the patient shows signs of deprivation (i.e. orthostatic hypotension; postural tachycardia; >=5% weight loss OR if the urinary concentration does not increase by >30mOsm/kg), baseline measurements are repeated and a bolus of exogenous ADH administered.  One hour later, measurements are repeated again.
    • Interpretation
      • Normal
        • Maximal urine osmolarity after dehydration (>700mOsm/kg), and osmolarity does NOT increase >5% following ADH injection
      • Central diabetes insipidus
        • Urine osmolarity unchanged during water deprivation but concentrates following ADH administration
      • Nephrogenic/peripheral diabetes insipidus
        • Urine osmolarity shows no significant change after either water deprivation or ADH administration
      • Psychogenic polydipsia
        • Initial urine osmolarity is low (<100mOsm/kg), but urine concentration will normalise with water deprivation (essentially normal response)
    • Other tests which may be appropriate include
      • Pituitary function tests
      • Serum lithium concentration
      • Autoantibody screen

Causes

polyuria

  • Other causes include
    • Cushing’s syndrome/disease
    • CKD
    • Hypercalcaemia
    • Fanconi’s syndrome
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