High-pressure versus low-pressure chronic urinary retention
In chronic urinary retention (a bladder that has long failed to empty), high pressure can damage the kidneys, but with low pressure the kidneys are not usually affected.
Remember
In chronic urinary retention, kidney involvement shows that the pressure is high: look for hydronephrosis (swelling of the kidney's collecting system as urine backs up) and abnormal urea and electrolytes (blood tests of kidney function). Painless bladder stretching and a weak stream occur in both forms.
Memory aid
High pressure hurts the kidneys, low pressure spares them.
Ask whether the pressure in the bladder is passed back to the kidneys.
In chronic urinary retention the bladder has failed to empty properly for a long time. Over months or years the detrusor (the muscular bladder wall) is stretched and becomes less sensitive, which is why the retention is painless. A patient may still pass small amounts of urine, but is likely to have a weak stream and hesitancy (a delay in starting to pass urine). Chronic retention affects women as well as men.
The deciding feature is whether the pressure is passed upstream. In high-pressure retention, a blockage at the bladder outlet, such as benign prostatic hyperplasia (BPH, a non-cancerous enlargement of the prostate), keeps the detrusor working against resistance, so the pressure inside the bladder stays high. That pressure is transmitted back up the ureters (the tubes carrying urine from the kidneys to the bladder) to the kidneys. Urine cannot drain from them, which causes hydronephrosis and abnormal urea and electrolytes (U&Es).
In low-pressure retention the bladder is large and floppy, and a weak detrusor can let it fill to a large volume. The kidneys are not usually affected, so U&Es and renal imaging are typically normal. Because the two forms can look alike at the bedside, the kidney findings help to tell them apart. A painless bladder can look reassuring, yet high-pressure retention can still damage the kidneys.
Management involves catheterisation (placing a catheter), often long term, especially in high-pressure retention. In a man with chronic retention, check serum creatinine and refer for specialist assessment.
Key facts
- Why painless
- the detrusor is stretched and desensitised (made less sensitive) over time, so even a very full bladder is not painful.
- Volume retained
- large volumes of urine can build up in the bladder, from 300 to 4000 ml.
- High-pressure form
- bladder pressure is passed back to the kidneys, causing hydronephrosis and abnormal U&Es.
- Low-pressure form
- the kidneys are not usually affected, so U&Es and renal imaging are typically normal.
- Shared features
- a weak stream, hesitancy and a painless stretched bladder occur in both forms.
- Overflow incontinence
- leaking from a bladder that is already too full, so leaking does not mean the bladder is emptying well.
- Bedside action
- check serum creatinine in a man with chronic retention and refer for specialist assessment.
Exam traps
- Painless passing of small volumes of urine does not separate the high-pressure form from the low-pressure form, because it is a hallmark of chronic retention and appears in both.
- Overflow incontinence does not separate the high-pressure form from the low-pressure one, because it can accompany chronic retention of either kind and is not specific to the high-pressure form.
- A large, painless bladder does not make the retention low-pressure, because both chronic forms leave one; bilateral hydronephrosis (both kidneys swollen with trapped urine) with a rising creatinine shows that pressure is being passed up to the kidneys.
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