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Next Lesson - Urinary Tract Infections
Contents
Abstract
- There are many causes of urinary tract obstruction - the most common of supra-vesical obstruction is kidney stones.
- Most kidney stones are made of Calcium Oxalate and are radiopaque so can be seen on an x-ray.
- A CT-KUB is the gold-standard imaging tool for assessing kidney stones.
- Urinary retention can be acute or chronic, and can be defined as low pressure or high pressure.
Core
Causes of Urinary Tract Obstruction
A urinary tract obstruction is any obstruction that occurs to the outflow of urine. This can happen at any level in the urinary tract, from the kidneys to the urethral opening.
The causes of urinary tract obstruction can be categorised into supra-vesical (above the bladder, i.e. ureteric) or infra-vesical (below the bladder i.e. urinary retention) pathologies. The causes are outlined below and will be discussed in more detail later.
Supra-vesical obstruction can be caused by either an extraluminal (external compression of the ureter) or intraluminal (pathology within the ureter or ureteric walls) abnormality.
- Retroperitoneal malignancy - malignancy originating in the retroperitoneal region or lymph node metastases commonly from breast, gynaecological, and prostate cancer.
- Direct obstruction by a tumour - bladder cancer can obstruct the ureters at the vesicoureteric junction (VUJ). Locally advanced prostate cancer can also obstruct the urethra.
- Retroperitoneal fibrosis
The most common intraluminal cause of ureteric obstruction is renal colic. This is usually caused by a stone within the urinary tract, but can also be caused by blood clots (clot colic) or sloughed papilla (necrosis of the renal papillae in which dead cells are discarded into the lumen of the ureter).
This is a rare condition in which excess connective tissue forms in the retroperitoneal space of the abdomen.
There are many causes:
- Idiopathic
- Malignancy - breast, lung, stomach, lymphoma, and others
- Autoimmune
- Drugs
- Abdominal Aortic Aneurysm (AAA)
This fibrosis can cause compression of the ureters as they (along with the kidneys) are retroperitoneal structures. This is therefore a cause of extra-luminal, supra-vesical obstruction.
Treatment for this includes surgical decompression of the ureters, exclusion of a possible underlying malignancy (which may require biopsies) and steroids/immunosuppression.
Renal colic is a very common cause of acute ureteric obstruction. Classically, it will present with colicky pain (pain which comes on in waves) with a loin to groin radiation, and haematuria, which may be non-visible (apparent only on urinalysis).
By far the most common cause of renal colic is kidney stones - though it may be caused by blood clots, and sloughed papillae. This usually occurs unilaterally, although it can be bilateral. Pyonephrosis can develop if there is an infection of the collecting system proximal to the obstruction.
Risk Factors for Kidney Stones
The following are risk factors for the development of kidney stones:
- Male Gender - men have a greater incidence of kidney stones compared to women
- Age - there are two peak ages for developing kidney stones: in the 20’s and 50’s
- Metabolic - hypercalcaemia can predispose a patient to develop kidney stones
- Family History
- Dehydration
- Hypertension
- Increased BMI
- Anatomical Abnormalities - for example: polycystic kidney disease, horseshoe kidney, and pelvoureteric junction (PUJ) stenosis
Kidney stones can be differentiated based on their chemical make-up:
- Calcium Oxalate - the most common type of stone
- Calcium Phosphate - includes apatite and brushite minerals
- Struvite Stones - magnesium ammonium phosphate stones associated with urease-producing bacteria and alkaline urine
- Uric Acid Stones - favoured by persistently acidic urine; ammonium urate stones form under different biochemical conditions
- Hydroxyapatite - a calcium phosphate mineral, so this overlaps with the calcium phosphate category above
- Cystine Stones - occur in cystinuria, an inherited transport disorder causing excessive urinary cystine; cystine is less soluble in acidic urine
One imaging technique is KUB Radiography (x-ray of the kidneys, ureters, and bladder). This will allow for the identification of calcium containing stones as these are radiopaque (so will appear white on x-ray). On plain KUB radiography, uric acid stones are radiolucent, whereas cystine stones have poor radiopacity and may be difficult to see. A negative KUB does not exclude a stone. Non-contrast CT KUB is more sensitive for stone detection; the choice of imaging depends on the clinical situation.
Kidney stones are likely to obstruct the ureter in three places, where the renal tract is the narrowest:
- Pelviureteric Junction (PUJ) - the junction between the renal pelvis and the ureter. The ureter is significantly smaller in diameter than the renal pelvis so stones can become stuck here.
- Pelvic Brim - this is where the ureters cross over the top of the common iliac arteries. This is an area of narrowing so again stones can become lodged here.
- Vesicoureteric Junction (VUJ) - the junction between the ureters and the bladder. The acute angle of entry of the ureters into the bladder causes a ‘u-bend’ which is a common site for stones to become lodged.
Some kidney stones will pass through the urinary tract without significant problems due to their small size, and the only management needed may be adequate analgesia and good hydration.
Surgical management may be needed in some cases:
- Stone is larger than 5mm
- Patient is in uncontrolled pain
- Bilateral kidney stones causing obstruction
- The patient only has one kidney which is obstructed by a stone
Stones can be treated surgically via laser lithotripsy (using a laser inside the ureters to break up the stone) or extracorporeal shockwave lithotripsy (ESWL [a non-invasive procedure aiming shockwaves at the location of the kidney stone]). ESWL has a number of risks, and is contraindicated in acute urinary tract infection or urosepsis, uncorrected bleeding disorders or coagulopathies, pregnancy, and abdominal aortic aneurysm.
Pyonephrosis is the accumulation of pus within an obstructed renal collecting system, and this is a urological emergency. Failure to decompress the obstruction may lead to death from sepsis or a permanent loss of renal function. Pyonephrosis should be suspected in patients with an obstructed kidney which develop signs of sepsis.
Pyonephrosis requires urgent drainage of the obstructed collecting system by a ureteric stent or percutaneous nephrostomy, alongside immediate antibiotics and sepsis management. Obtain urgent senior urological input to arrange source control; antibiotics alone do not relieve the infected obstruction.
Quiz
- 5419
