Quest for a detrusor overactivity index

Urge incontinence, which is the predominant type of geriatric incontinence, is generally attributed to detrusor overactivity. However, detrusor overactivity is present in up to half of continent elderly individuals. We postulated that detrusor overactivity associated with urge incontinence would be more severe but there are no established criteria for judging severity. Using urge incontinence frequency as a yardstick and controlling for nonurodynamic contributing factors we sought intrinsic lower urinary tract parameters that reflect detrusor overactivity severity. We postulated that parameters in 1 or more of 5 domains would be important, namely characteristics of uninhibited contraction, bladder capacity, bladder proprioception, detrusor contractility and sphincter adequacy.

We analyzed data on 79 community dwelling incontinent individuals older than 60 years old. All subjects had urge incontinence on a 4-day voiding record and underwent multichannel videourodynamics. We examined the associations of urge incontinence frequency with the postulated key factors.

Multivariable analysis revealed that 24-hour urine output and functional bladder capacity consistently predicted urge incontinence frequency. Bladder proprioception was significant in some models. Uninhibited contraction pressure was another predictor. Surprisingly higher uninhibited contraction pressure was associated with lower urge incontinence frequency. This negative correlation was more pronounced in a subgroup with a less adequate sphincter but absent in those with good sphincter function, implying that low uninhibited contraction pressure does not necessarily indicate less severe detrusor overactivity but rather reflects sphincter inadequacy in many patients. Age was not independently associated with urge incontinence frequency.

We identified functional bladder capacity as a measure of detrusor overactivity severity. The measure commonly used, namely uninhibited contraction pressure, is inappropriate because it is severely confounded by sphincter function, especially in older individuals. Furthermore, we confirmed that urine output, and possibly bladder sensation and sphincter strength modify the clinical manifestation of detrusor overactivity.