Imaging bladder sensations

Normal bladder sensations progress in a continuum of gradually increasing intensity and unpleasantness, from first sensation of bladder filling, through first desire to void, to strong desire to void. The abnormal sensation of urgency is not part of this continuum and is one of the defining symptoms of the overactive bladder (OAB) syndrome. Functional magnetic resonance imaging of the brain can show which regions respond to filling during various sensations. We have imaged these cerebral responses in female volunteers, either with normal bladder control or with urge incontinence (poor control). Normal bladder filling sensations are mapped mainly in the insula, shifting anteriorly as sensation becomes stronger and more unpleasant. In subjects with poor bladder control, sensations and brain responses are different from normal. Responses are relatively small at low bladder volumes (with mild sensation), but become exaggerated (with strong sensation) above a certain volume threshold, even when there is no actual detrusor overactivity. This may perhaps account for Yamaguchi's observations, reported in this supplement, of a similar volume threshold for patient-reported urgency. Based on these results, it is unlikely that abnormal (OAB) responses to bladder filling can be accounted for simply by increased bladder afferent activity, that is, bladder hypersensitivity. Apparently, either the nature of the afferent signals or the handling of these signals in the brain is abnormal.