What can cause increased urinary frequency and incontinence?

Hi doctor,

A few months ago after having a couple of falls, my mother was diagnosed with progressive supranuclear palsy by a neurologist. Last week, she suddenly started to experience urinary incontinence. Over the past few days she has complained of the urge to pass urine quite often and sometimes as much as several times within an hour. When she does visit the toilet, she sometimes passes little or no urine. Last night we got her tested for UTI and the test came back negative. We are concerned and exhausted. We look forward to getting your input. Thank you.