The Afferent Threshold Hypothesis: A Unified Neural Framework for Functional Bladder Capacity and Nocturia

The Weiss nocturia indices have structured clinical thinking about nocturia for over 2 decades. By deriving simple ratios from the frequency-volume chart, they classify nocturia into nocturnal polyuria, reduced nocturnal bladder capacity, and mixed etiology. The arithmetic is sound. But the framework rests on an unexamined assumption: that the maximum voided volume, almost always the first morning void, represents "bladder capacity," and that the gap between this value and daytime voided volumes reflects normal physiology. We propose that what the frequency-volume chart measures during wakefulness is not capacity but a personal adaptive afferent threshold-a neurally determined set-point at which the individual initiates voiding, always below the structural capacity the detrusor permits. The first morning void approximates true structural capacity, revealed when sleep suppresses the cortical circuits that enforce the waking threshold. This reframing unifies the entire overactive bladder spectrum-normal voiding, frequency, urgency, urge incontinence-as positions on a single threshold continuum. It reveals nocturia from "reduced nocturnal bladder capacity" as the intrusion of the waking threshold into sleep when central afferent gating fails. The Weiss parameters captured the output of this system with numerical precision but encoded the threshold artifact as the reference norm, directing a generation of research toward the bladder in patients whose problem may be the brain.