Evaluation of the etiology of nocturia in men: the nocturia and nocturnal bladder capacity indices.

To determine and quantify the cause of nocturia in men, we describe and evaluate the relative contribution of two complementary indices of nocturia: the nocturia index (Ni), a measure of nocturnal urine overproduction, and the nocturnal bladder capacity index (NBCi), reflective of nocturnal bladder capacity. The records of 100 consecutive men with lower urinary tract symptoms (LUTS), having undergone video-urodynamic studies (VUDS), were prospectively studied. Evaluation included American Urological Association symptom score (AUASS), micturition diary (day, night, and 24-hr voided volume), and VUDS. Voiding diary analysis was carried out as previously described by us, determining the Ni, NBCi, and nocturnal polyuria index (NPi) (nocturnal urine volume/24-hr urine volume). In the case of AUASS question #7 (degree of nocturia), the odds of having a severe AUA question #7 response was found to be 4.09 times higher for patients with NBCi > 2.0 compared with patients whose NBCi was </= 2.0 using logistic regression analysis. In comparing patients with severe nocturia and low NBCi with those having mild nocturia and low NBCi, Ni performed in a fashion superior to NPi in identifying relative nocturnal urine overproduction as the suspected explanation for their nocturia (Ni = 3.42 vs. 1.42, P = 0.0002 cf. Npi = 0.44 vs. 0.27, P = 0.018, Mann-Whitney test, respectively). We suggest a discriminating threshold of NBCi > 2 as highly significant in defining diminished NBC as a factor in the etiology of nocturia. In addition, we propose Ni of 1.5 as a threshold greater than which nocturia may be attributed to nocturnal urine overproduction in excess of maximum bladder capacity. Together, these indices describe in quantitative fashion the relative contributions of nocturnal urine overproduction and diminished NBC in identifying the etiology of nocturia in male patients. Neurourol. Urodynam. 18:559-565, 1999. Copyright 1999 Wiley-Liss, Inc.