To explore the mismatch between functional bladder capacity and nocturnal urine production, and to study the pathophysiology of an increased nocturnal urine production in older patients with urinary incontinence.
The present prospective observational study included adults aged ≥65 years with urinary incontinence. Participants completed questionnaires, frequency volume charts and renal function profiles. The nocturnal lower urinary tract symptom index was defined as nocturnal urine output/maximum voided volume; the nocturnal polyuria index as nocturnal/24 h urine output.
The median age (n = 95) was 74 years (69-79), 87% were women and 73% had nocturnal lower urinary tract symptoms (nocturnal urinary incontinence or nocturia ≥2). Participants with nocturnal lower urinary tract symptoms had a significantly higher nocturnal urine output (809 mL vs 650 mL; P = 0.001) and no significant difference in maximum voided volume (350 mL vs 437 mL; P = 0.079) compared with participants without nocturnal lower urinary tract symptoms. Participants (nocturnal polyuria index >33% [n = 56], nocturnal polyuria index >40% [n = 42], nocturnal lower urinary tract symptom index >1.87 [n = 51]) showed higher night-time diuresis rates, free water and sodium clearance compared with during the daytime. Controls (nocturnal polyuria index ≤33% [n = 26], nocturnal polyuria index ≤40% [n = 40], nocturnal lower urinary tract symptom index ≤1.87 [n = 44]) had no circadian rhythm in their diuresis rate or sodium clearance, but more nocturnal free water clearance compared with during the daytime.
The majority of older adults with urinary incontinence present nocturnal lower urinary tract symptoms. An increased nocturnal sodium diuresis seems to be the only mechanism differentiating patients with nocturnal lower urinary tract symptoms from controls.