AIMS OF STUDY:
Factors in the etiology of nocturia remain obscure. While we have recently proposed methods to quantify parameters of nocturia, the relationship of such parameters to the presence or absence of a urodynamic diagnosis of detrusor instability and to changes in nocturnal bladder capacity has not been previously explored. Specifically, we have previously identified a parameter NBCi (defined in Methods section) which compares actual with predicted numbers of nightly voids (1-3). Patients having NBCi # 0-2 arise to void with volumes close to functional bladder capacity. In contrast, patients with NBCi > 2 arise more frequently than predicted, owing to diminished nocturnal bladder capacity. We herein compare voiding diary-derived parameters of nocturia with presence or absence of detrusor instability to further identify the causes of nocturia in men and women.

METHODS:
Voiding diary analysis was prospectively carried out in 137 consecutive men and women with or without a urodynamic diagnosis of detrusor instability (DI). The nocturia index (Ni) was defined as nocturnal urine volume (NUV) divided by functional bladder capacity (FBC, the largest volume voided in a 24 hour time period). Nocturnal bladder capacity (NBC) was defined as the largest volume voided during the hours of sleep. Nocturia due to nocturnal urine overproduction relative to FBC occurs when Ni>1. The nocturnal bladder capacity index (NBCi) is a measure of bladder capacity during sleep. It was calculated by subtracting the predicted # nightly voids (PNV) from the actual # nightly void (ANV). PNV was Ni minus 1 and rounded up to the next highest integer if Ni is a mixed number. The nocturnal polyuria index (NPi) was calculated as NUV divided by 24 hour voided volume. Comparisons of the following nocturia parameters were made between groups of patients with and without a urodynamic diagnosis of detrusor instability: ANV, NUV, Ni, NBC, FBC, NBCi and NP. The subset of patients with NBCi # 0-2 was compared with those having NBCi >2 in order to correlate the NBCi with severity of nocturia (ANV) and the NBC. Statistical comparisons utilized two-tailed Student's t-test.

RESULTS:
There were no statistically significant differences in parameters of NBC, FBC, Ni, NPi, NUV or ANV whether or not patients harbored a urodynamic diagnosis of detrusor instability (see Table 1). While patients with DI had significantly lower NBCi than those without DI, in both cases the NBCi was # 0-2.1, the latter range having been previously found to lack relation to nocturia driven by pathology of the bladder or bladder outlet. However, patients with NBCi # 0-2 had significantly higher NBC (average 284.2 ml) than those with NBCi > 2 (193.9, p = 0.01) and significantly lower nocturnal voiding episodes (ANV = 3.0) than those with NBCi > 2 (ANV = 5.4, p = 0.0002) (see Table 2). Table 1.

Table 2.

CONCLUSIONS:
The etiology of nocturia cannot be discerned through a diagnosis of detrusor instability derived from awake urodynamic studies. However, nocturia owing to diminished nocturnal bladder capacity may be diagnosed and quantitated by NBCi and NBC, parameters easily gleaned from a simple arithmetic analysis of the 24 hour voiding diary.

References:
1. Weiss, JP, Blaivas, JG, Stember, DS and Brooks, MM: Nocturia in adults: Etiology and Classification. Neurourology and Urodynamics, 17:467-472, 1998.
2. Weiss, JP, Stember, DS, Chaikin, DC and Blaivas, JG: Evaluation of the Etiology of Nocturia in Men: The Nocturia and Nocturnal Bladder Capacity Indices. Neurourol. Urodyn., 18: 559, 1999.
3. Weiss, JP and Blaivas, JG: Nocturia. J Urol 163: 5-12, 2000.

Financial support provided by the Pharmacia-Upjohn Corporation.