Polyuria
The definition of polyuria is arbitrary. A daily urine volume exceeding 3l in adults and 2l/m2 in childs is generally considered as polyuria.
Demarcation
Frequent urination is not equivalent to polyuria, the increased daily volume of excreted urine, it is named pollakisuria if only few drops are expelled frequently. This situation occurs during infections or in neurogenic bladder.
As the volume of diluted urine is also determined by the intake of water, polydipsia might be the primary cause of an increased renal water excretion.
Clinical Findings
Polyuria is often not so obvious as polydipsia, the increased fluid intake. Urine is colourless. Because by polyuria the body loses a large amount of water and minerals, various disturbances of water metabolism and sodium balance ensue.
In children soon a failure to thrive, muscular weakness, and idleness becomes apparenct.
Diagnosis
Volume and osmolality of daily renal excretion has to be measured in order to calculate free water clearance and osmolar clearance. The osmolar clearance can be understood as the amount of water necessary to excrete solutes in urine at the same osmolality as in plasma. The free water clearance is the differens of water required to obtain the volume of water actually excreted. As urine might be diluted or concetrated, free water clearance might be positive or negative, respectively.
In hereditary disorders, polyuria generally manifests early in childhood: antenatal Bartter syndrome prenatal by polyhydramnios, nephrogenic diabetes insipidus after the first week, infantile Bartter syndrome in the first months, central diabetes insipidus after the first year, classical Bartter syndrome in the first years. Gitelman syndrom is the exception with an onset in young adults.
The plasma sodium concentration provides clues whether disturbed water retention or solute excretion is the leading cause of disease. In water diuresis, such as diabetes insipidus, sodium concentration is rather high-normal whereas salt-loosing nephropathies tend to lower sodium concentration.
Plasma ADH concentration should be measured only on conjunction with a water restriction test. The exception to this rule are newborns and very young infants, and patients with diluted urine (below plasma osmolality). In the latter group of patients only baseline ADH concentration is measured. In all other patients stimulated ADH concentration is measured after plasma osmolality reached 295 to 300 mosmol/kg.
By ADH stimmulation, we are able to demonstrate central diabetes insipidus. If plasma osmolality reaches its maximum and ADH administration can further increase urinary osmolality, this condition proves defective ADH secretion, central diabetes insipidus.
Classification
Polyuria may be divided into water diuresis and osmotic diuresis.
water diuresis
Water diuresis is characterized by reduced urine osmolality (<250mosmol/kg) wheras total solute excretion is normal (600 to 900 mosmol/day). Free water clearance is markedly increased (>0).
osmotic diuresis
In osmotic diuresis urine osmolality is normal or increased (>300mosmol/kg), the total excretion of osmotic substances is markedly increased (>600-800 mosmol/d), and free water clearance is diminished (<0).
Physiology
Total body water equals 60% in a young man and 50% in a young woman. Of that water, 65% is found in the cellular compartment and 35% extracellular. The normal daily water intake is 1.0-1.5L. Almost the same volume of water is eliminated by the kidney. Cellular metabolism produces additionally about 0.5L of water. This balances the excretion by stool, sweat and respiration.
