Urinary tract infections: Pathology review

Definitions & Key takeaways

Urinary tract infections (UTIs) are common bacterial infections that can occur in any part of the urinary tract, including the kidneys, bladder, ureters, and urethra. They are most commonly caused by bacteria, such as E.coli, Klebsiella pneumoniae, Proteus mirabilis, and Staphylococcus saprophyticus.

Symptoms include pain or burning sensation when urinating, frequent urge to urinate, cloudy or strong-smelling urine, pain or pressure in the lower abdomen or back, and in more severe cases, fever or chills.

Risk factors for UTIs include female gender, sex, diabetes mellitus, indwelling catheters, pregnancy, and kidney stones. Complications of untreated or recurrent UTIs can include cystitis, pyelonephritis, and sepsis. Treatment typically involves a course of antibiotics and plenty of fluids to help flush out the bacteria.

Chapters:

Watch video only

Case study0:00–0:50

I Two people arrive at the nephrology ward. The first one is 25-year-old Carmen, who recently returned from her honeymoon and now complains of suprapubic pain, dysuria, and urinary frequency.
The other one is 35-year-old Pamela, who has had dysuria, fever, nausea, and flank pain for the past two days. On physical exam, she has costovertebral angle tenderness.
CBC and urinalysis were done for both people. They showed that Carmen had a normal white blood cell count, but on the dipstick test, leukocyte esterase and nitrites were both positive.
For Pamela, the white blood cell count was high. The dipstick test showed positive leukocyte esterase and nitrites, and on microscopy, there's white blood cell casts in the urine.

Pathology0:50–2:47

Both Pamela and Carmen have urinary tract infections or UTIs. This includes the lower portion of the tract like the bladder and urethra, and the upper portion of the tract like the kidneys and ureters.
UTIs are almost always caused by an ascending infection where bacteria moves from the rectal area to the urethra and then migrate up the urethra and into the bladder.
Normally bacteria would be washed away with urination, but in some cases, like with E. coli, that doesn't happen.
Instead, E. coli uses little thread-like extensions called fibriae to bind the uroepithelial cells and colonize the bladder mucosa, causing cystitis.
From the bladder, the infection can go up the ureter and into the kidney where they attract neutrophils into the renal interstitium, causing pyelonephritis.
As neutrophils die off, they make their way through the urinary tract and appear in the urine. The neutrophils and the surrounding inflammatory protein debris is even casted into the shape of the tubule, creating white blood cell casts and hyaline casts.
For your exams, remember that other factors like sexual intercourse or an indwelling catheter can also let bacteria into the urinary tract.
So remember, in ascending infections, bacteria moves from the rectal area to the urethra and then migrate up the urethra and into the bladder.
In most cases, ascending infections are caused by Essarechia coli, but other bacteria like Staphylococcus saprophyticus, Klepsiella pneumonia, or Proteus mirabilis can also cause ascending infections.

Cystitis2:47–4:44

Now descending infections are also possible in rare cases. This is when the kidneys get infected via hematogenous infection or spread through the bloodstream.
In this case, acute pyelonephritis can be a consequence of septicaemia or bacteremia. In these situations, the most common organisms are Staphylococcus species and again E.
coli. OK, let's talk about cystitis.
There are some risk factors associated with cystitis. It's more common in people with female anatomy because the urethra is shorter, making it easier for the bacteria to ascend, and because the urethra is closer to the rectum where the primary culprits live.
Another risk factor is frequent sexual intercourse, which can lead to honeymoon cystitis, especially in younger individuals with female anatomy.
Remember, this type of infection is often caused by Staphylococcus saprophyticus. Other risk factors include diabetes mellitus, indwelling catheter, and impaired bladder emptying, which can happen when there's a bladder tumor, for example.
Symptoms include dysuria, which is painful urination, urinary frequency and urgency, and suprapubic pain. CBC, urinalysis, and urine cultures are done to confirm the diagnosis and find the culprit.
On the CBC, the white blood count is normal. The appearance of the urine is often cloudy, and the dipstick test shows positive leukocyte esterase, which signifies pyuria or the presence of white blood cell in the urine.
Dipstick also shows positive nitrites. And this is caused by enterobacteria CA like E.
coli, Proteus, and Klebsiella converting normal urine nitrates into nitrates. On microscopy, there are more than 10 white blood cells per high power field.

Acute pyelonephritis4:44–6:29

The best way to identify the organism for treatment is to do a urine culture. Now, sometimes cystitis can be mistaken for urethritis, which is the inflammation of the urethra.
Urethritis can present with the same symptoms as cystitis, but the difference is that urethritis is most commonly caused by neisseria gonorrhea and chlamydia trachomatis.
Both of these are sexually transmitted infections. With both, urinalysis shows sterop pyuria, meaning that leukocyte esterase is positive, but when the cultures are done, no bacteria are found.
Moving on to acute pyelonephritis, risk factors include indwelling catheters, diabetes mellitus, and pregnancy. Another risk factor is any obstruction in the urinary tract like kidney stones.
This causes urinary stasis, making it easier for bacteria to adhere to and colonize the tissue. Now another major factor is vesicoureteral reflux or VUR.
This is where the one-way valve at the vesicoureteral junction is loose, which allow urine to move from the bladder back into the ureters and kidneys.
VUR can be a result of a primary congenital defect, or it can be caused by bladder outlet obstruction like a bladder tumor, which increases pressure in the bladder and distorts the valve.
For symptoms, people with acute pyelonephritis can have a similar presentation as cystitis, where they have dysuria, urinary frequency, and urinary urgency.

Chronic pyelonephritis6:29–7:30

The person might have flank pain, implying there's something going on with the kidneys. The distinguishing feature for pyelonephritis is that there are systemic symptoms like fever, chills, nausea, or vomiting.
On clinical examination, there will be costovertebral angle tenderness. A CBC, urinalysis, and urine cultures are done.
The urinalysis is positive for leukocyte esterase and nitrites like cystitis, but white blood cells and hyaline casts are sometimes present on the CBC.
The white blood cell count is high, and this will help you differentiate between cystitis and pyelonephritis. Now, remember, imaging methods are not necessary for diagnosis, but if done, CT should show striated parenchymal enhancement, also known as striated nephrogram.
Now, recurrent episodes of acute pyelonephritis can result in chronic pyelonephritis, where the renal interstitium undergo fibrosis and scarring, leading to tubule atrophy.

Proteus infections7:30–8:04

These changes are generally found on the upper and lower poles of the kidney where a CT scan will show blunt or flattened renal calys.
Other complications of acute pyelonephritis include renal papillary necrosis, perinephric abscess, and urosepsis, which is most commonly caused by E.
coli. Histology will show dilated tubules filled with a glassy proteinaceous material that forms colloid-like cast as a result of chronic inflammation.

Review8:04–9:20

Since the tubules filled with colloids look similar to thyroid tissue, this process is sometimes referred to as thyroidization of the kidney.
Now important risk factors for chronic pyelonephritis include anything that causes chronic obstruction and urine stasis like VUR or kidney stones.
A special type of kidney stone called staghorn stone. Due to their large size and branching nature, are associated with proteus infections.
One high yield fact is that proteus infections alkalinize the urine, giving it a distinct ammonia smell. Chronic proteus infections can lead to a very serious type of pyelonephritis called xanthogranulomatous pyelonephritis, or XGP, which is characterized by necrotic hemorrhagic masses containing foamy macrophages.
These masses are often mistaken for tumors on imaging studies. OK, let's review.
UTIs are almost always caused by an ascending infection, and the most common bacteria involved are E. coli, Klebsiella pneumonia, Proteus mirabilis, and Staphylococcus saprophyticus.

Summary9:20–11:27

Risk factors for UTIs include female urinary anatomy, frequent sexual intercourse, diabetes mellitus, indwelling catheters, pregnancy, VUR, and obstruction like with kidney stones.
Lower UTIs include cystitis, where symptoms are dysuria, urinary frequency, urinary urgency, and suprapubic pain. Lab work shows normal WBC, positive leukocyte esterase, positive nitrites, and more than 10 WBCs per high power field on microscopy.
Upper UTIs include pyelonephritis, where symptoms include fever, chills, nausea, vomiting, and flank pain. Lab work here shows high WBC along with WBC and hyaline casts in the urine.
Recurrent episodes of acute pyelonephritis can lead to chronic pyelonephritis, where the calys appear blunted on imaging and on histology, there's thyroidization of the kidney.
Coming back to our cases, Carmen has honeymoon cystitis that presented with typical symptoms like suprapubic pain, dysuria, and frequency.
The labs showed positive leukocyte esterase and positive nitrites, but there's no increase in WBC. Her infection will clear out with a short course of antibiotics.
On the other hand, Pamela has acute pyelonephritis that presented with some of the symptoms of cystitis like dysuria. But she also has systemic symptoms like fever and nausea.
Her white blood count was high, and there were white blood cell casts in the urine, which confirms the diagnosis. In her case, she will need a longer course of antibiotic treatment.