The bladder is a muscular sac and expands (distends) as it fills with urine. As the bladder fills, the sensory stretch receptors elicit impulses to trigger the micturition reflex and empty the bladder. However, this process is also under voluntary control and the external urethral sphincter will not relax and allow urine to pass out unless a person wishes to do so, usually when the circumstances are appropriate for urinating.
The bladder’s ability to stretch extends beyond the point where the micturition reflex is triggered. A distended bladder may not be due to any pathology and can arise in case where a person consciously prevents voiding. However there are certain causes of a distended bladder which is a result of pathology where a person cannot pass out urine, despite the urge (tenesmus) and attempting to void. With an outlet obstruction, the strong micturition reflexes and detrusor contractions cannot involuntarily force out the urine (incontinence).
Distention of the bladder due to the retention of urine may be caused by one of two mechanisms :
- Obstruction of the urethra or bladder neck (bladder outlet obstruction)
- Damage or defects of the muscular bladder wall or nerve supply controlling micturition
Both these factors prevent emptying of the bladder. The urine produced by the kidneys pass down the ureters and accumulate in the bladder. With time, once the bladder distension reaches a point where the overstretched bladder can no longer contend with any additional urine, there will be a back flow of the urine. This will prevent further emptying from the kidney and can lead to a host of complications, including kidney failure. Urinary retention may be due to a obstruction higher up the urinary tract, like in the ureters, but this will not cause a distension of the bladder.
Urinary retention can be either acute or chronic. In acute cases, the ability to pass urine (void) is often totally impaired (anuria) and is considered as a medical emergency. In chronic cases, voiding is diminished (oliguria) but some urine is passed out occasionally albeit with great difficulty and often pain (dysuria). The urine passed out in partial obstruction is usually insufficient to prevent excessive distension of the bladder. Intervention, often by catherization, is necessary.
Causes of Bladder Outlet Obstruction
The following causes may affect both men and women :
- Bladder stones which are urinary stones that can start higher up the urinary tract (more often kidney stones) and lodge in the bladder neck. These stones are usually less than 5mm in diameter or they would not be able to enter or pass through the ureter. At times, the stone can start to form within the bladder most commonly due to an infection (cystitis). Due to its size, as well as additional growth in the bladder, it can obstruct the point of urine outflow from the bladder (i.e. bladder neck).
- Urethral strictures is a narrowing of the urethra which can occur in both men and women, but is rare in females. These strictures may be due to pathology within the urethra or its wall (intrinsic) or due to compression by surrounding structures (extrinsic ). Intrinsic strictures are usually due to inflammation or scar tissue of the urethral wall as a result of :
- urinary tract infections, especially STD’s (sexually transmitted diseases)
- injury to the urethra often due to the insertion of a catheter
- recurrent episodes of urethral inflammation (urethritis) due to various reasons
- congenital strictures, although a possibility, are rare
- urethral spasms
- urethral diverticula
- Tumors. Malignant tumors (cancer) are more common and include cancer of the colon, rectum, bladder, lymphomas or sarcomas. Other types of cancer are gender specific and discussed under distended bladder in men or women respectively. Benign tumors causing obstruction are less common. Other growths like fibroepithelial polyps of the urethra are rarely seen in adults.
- Blood clots may cause an obstruction of the urinary tract although it is more likely to affect higher up the tract, particularly the ureter and the junction with the bladder. This is a result of the one or more of the same causes of hematuria (blood in the urine).
- Fecal impaction in cases of severe acute or chronic constipation may cause significant distension of the rectum with compression on the bladder. It is acute and urinary retention is relieved following removal of the impacted stool from the rectum.
- Neurogenic bladder causes urinary retention by affecting the contraction of the detrusor muscle in the bladder wall as well as impairing relaxation of the urethral sphincter to allow for urine to flow out of the bladder. This may be due to a number of conditions affecting the central and peripheral nervous system, including diabetes, Parkinson’s disease, multiple sclerosis (MS), stroke and spinal cord injury.
- Iatrogenic bladder obstruction due to failure of the bladder neck and/or urethral sphincter to relax may be seen following general anesthesia in surgery or local anesthesia for pelvic procedures. The use of certain drugs, even OTC (over-the-counter) medication, may also cause bladder outlet obstruction. This is more frequently seen with anticholinergic drugs, calcium channel blockers, opioid analgesics and sedatives. Some of the more widely used drugs that may result in outlet obstruction includes :
Distended Bladder in Men
Most cases of urinary retention due to bladder outlet obstruction is seen in men.
- Prostatic enlargement is a leading cause and may be due to :
- benign prostatic hyperplasia (BPH) in older men
- prostatitis in younger men
- prostate cancer
- Iatrogenic causes as mentioned above can affect both men and women but a large number of cases are seen in men with pre-existing prostatic enlargement following surgery under general anesthesia.
- Bladder neck dyssynergia is more frequently seen in men in the 30 to 40 year age group and presents with symptoms similar to BPH. The spastic bladder neck muscles do not relax during voiding to allow urine to pass out to the urethra.
- Other causes in men may include :
- Phimosis or paraphimosis
- Use of penis enlargement devices or topical applications that cause severe localized inflammation
Distended Bladder in Women
- Tumors of the ovaries or uterus are more likely to affect the ureters but can also affect the bladder or urethra in women. Cervical cancer (malignant tumor of the cervix) is more likely to cause outlet obstruction in women.
The enlarged uterus in pregnancy is more likely to cause compression of the ureters and not bladder outlet obstruction.