Solute and Water Excretion Patterns in Patients with Nocturnal Polyuria

Background and Aims: Nocturia is a highly prevalent and troublesome lower urinary tract symptom, with 28-36% of adults usually voiding at least twice nightly. For many patients, nocturia arises from the presence of nocturnal polyuria (NP), usually defined as being present when more than 33% of the total 24-hour urine output occurs at night while the total 24-hour urine output remains normal. We report results of an innovative evaluation in a clinical sample of 29 nocturia patients found to have NP during evaluation of their nocturia symptoms.

Methods: Patients with nocturia collected 24-hour urine specimens split into two containers, one of which was collected during the daytime, the second collected during the nighttime. We analyzed patterns of solute and water excretion, comparing daytime to nighttime collections.

Results: Overall the group demonstrated remarkably abnormal patterns of solute and water excretion, with the rate of urine production dramatically greater at night (mean 118 mL/hr) than during the day (mean 56 mL/hr; p<0.001); solute excretion markedly increased at night (mean 41 mmol/hr) compared to during the day (mean 27 mmol/hr; p=0.001), and free water absorption marginally decreased at night (mean 18 mL/hr) compared to during the day (mean 33mL/hr; p=0.053). Within this broad pattern of abnormality, we found that patients fell into one of three categories: those with (1) Increased nocturnal solute diuresis, (2) Decreased nocturnal free water reabsorption, and (3) Nocturnal mixed diuresis. Nocturnal water diuresis and solute diuresis have both been noted in other studies of nocturics. Our study enhances evidence that nocturic patients may benefit from a more sophisticated evaluation than is afforded by analysis of a simple frequency-volume chart.Conclusions: A clinical categorization scheme based on these excretion patterns may help direct the clinician in choosing an appropriate therapy.