polyuria / polydipsia

Polyuria

When we talk of polyuria, we must first make sure we know what it means.

·         Polyuria is defined by a urine output of more than 3literes per day (hence 24 hour urine collection is essential)

·         It must be distinguished from frequency of urination which is just more frequent urination and not necessarily the amount of urine output.

Potential mechanisms of polyuria includes;

·         Excretion of nonabsorbable solutes (e.g. glucose) SOLUTE DIURESIS

·         Excretion of water (usually from a defect in ADH production or renal responsiveness) WATER DIURESIS

How do we know whether the patient is having a solute / water dieresis?

·         Urine osmolality measurements

o    Usual person excretes between 600-800 mosmol of solutes per day

§  As urea and electrolytes

o    Urine output is >3litres per day, and urine is dilute (<250mosmol / Litre), then total amount of solutes excreted is normal, and patient is having WATER DIURESIS.

§  Arising from

·         Polydipsia

·         Inadequate secretion of ADH (central DI)

·         Failure of renal tubules to respond to vasopressin (nephrogenic DI)

o    Urine output is >3 litres per day, and urine is >300mosmol/litre, then a SOLUTE DIURESIS is present, and the solute responsible for it must be found out, like;

§  Glucose

§  Mannitol

§  Urea

§  Diuretics

o    Poorly controlled diabetes with glucosuria is the most common cause of solute dieresis, leading to volume depletion and serum hypertonicity.

polydipsia

The primary stimulus for water ingestion is thirst, which is mediated by …

·         Increase in effective osmolality

·         Decrease in ECF volume or blood pressure

The osmoreceptors

·         Located in the anterolateral hypothalamus

·         Stimulated by a rise in tonicity

Urea and glucose does NOT play a role in stimulating thirst.