Chapter 17: Fluid Balance Disorders in the Elderly

Disorders of fluid balance are common in the elderly and often are caused by age-related alterations in urinary tract function, which can present clinically as urinary frequency, nocturia, and incontinence. Among the factors predisposing to the development of these clinical disturbances are aging changes in the renal and hormonal systems that control water and sodium excretion along with changes in the reservoir function of the bladder1 (Table 1). This chapter will examine the effect of aging on the systems involved in urine formation and consider how these changes interact with those of bladder function and lead to urinary frequency, nocturia, and incontinence. In young, healthy persons there is a circadian pattern to urine production in which nighttime urine flow rate is less than daytime flow rate. In association with the normal aging process, there is an increase in nocturnal production of urine, so that nighttime urine flow rate equals or exceeds daytime production rate.2 When of sufficient magnitude to result in nocturia, this change has been termed nocturnal polyuria syndrome. The establishment of a circadian rhythm of urine flow takes place during childhood, generally between the ages of 2 to 5 yr. In normal children with a mean age of 7 yr, daytime rate of urine production is two to three times that of the nighttime period. Delay or failure of the circadian rhythm to develop during childhood is associated with the presence of nocturnal enuresis. In adulthood, the ratio of daytime to nighttime urine production is usually greater than 2:1, so that about 25% or less of daily urine output occurs during sleep. Typical urine production rates are approximately 70 to 80 ml/h during the waking period and 30 to 40 ml/h during sleep. This circadian pattern seems to be linked to the day-night sleep pattern. The circadian pattern of urine flow is paralleled by similar rhythms of renal plasma flow and GFR. The circadian pattern is maintained in healthy persons until around age 60 yr when a shift to a greater proportion of urine production to the nighttime period becomes evident. With further increase in age, there is often reduction in the ratio of day to night urine flow to the point that nighttime flow rates become equal to or exceed daytime rates. Despite the change in circadian pattern of urine excretion, total urine production per 24 h is not affected.3