Why does BPH cause urinary frequency?

[studylol]

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Antimuscarinics are contraindicated for men with BPH because it causes excess urinary retention. I can understand that: big prostate compresses the urethra.

Why does BPH cause urinary frequency, then? I would expect urinary retention in BPH people.

Thanks!

 

[SBR249]

BPH causes the patient to pee less each time (a slow short dribble instead of a nice high pressure arc). Given that urine production doesn't decrease, that means there's urine retention. Increased retention then stimulates the urge to pee more frequently. So instead of voiding a large volume a few times a day it becomes voiding a small volume many times a day.

 

[suarad32]

If I remember correctly, bladder outlet obstruction leads to detrusor hypertrophy and consequently overactivity/irritablity. That accounts for the urgency, and coupled with the incomplete bladder emptying in BPH, results in frequency.

 

[studylol]

Thanks a lot!

 

[iki4life]

the urgency is because the bladder can't completely empty out the urine because the hyperplastic prostate is squishing the bladder, so the patient wants to pee, and he does, but still he can't completely empty the bladder. urinary frequency is just that he goes to the bathroom over and over because he feels like he needs to urinate

 

[worldbeater]

Also a side note, in terms of treatment of BPH, the drug will end in a "-sin" and it's MOA will be an antagonist on the alpha 1 receptor.

The normal function of the alpha 1 receptor is associated with three specific things:
1) Vasoconstriction
2) Cycloplegia without mydriasis
3) sphincters "no pee"

So as an antagonist, it will act on #3, causing the sphincters to dilate and allow the patient to urinate.

In general, alpha 1 and beta 1 receptors are associated with constriction, alpha 2 and beta 2 are associated with "relaxation"

 

[SBR249]

alpha-1 antagonists are not the only drugs used for BPH. 5-alpha reductase inhibitors such as finasteride are also used because prostate enlarge is driven by dihydrotestosterone which is converted from testosterone by 5-alpha reductase.