AssessmentsUrinary tract infections: Pathology review
USMLE® Step 1 style questions USMLE
A 45-year-old woman presents to the clinic with flank pain, malaise, and weight loss over the past several weeks. She states, “I just have no appetite for anything these days.” She has a history of recurrent urinary tract infections and several episodes of nephrolithiasis. Her temperature is 38°C (100.4°F), pulse is 94/min and blood pressure is 154/62 mmHg. Physical examination shows a palpable left sided flank mass. She subsequently undergoes a biopsy of the mass with the following histologic finding demonstrated below:
Reproduced from: Wikipedia
Which of the following pathogens is most commonly implicated in this disease process?
Two people came to 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 2 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 count was high, the dipstick test showed positive leukocyte esterase and nitrites, and on microscopy, there’s white blood cell casts in the urine.
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 fimbriae 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 Escherichia coli, but other bacteria like Staphylococcus saprophyticus, Klebsiella pneumoniae or Proteus mirabilis can also cause ascending infections.
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 these situations, the most common organisms are Staphylococcus species and again E. coli.
Okay, let’s talk about cystitis.
There are some risk factors associated with cystitis.
It’s more common in female individuals, because they have a shorter urethra, 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 female individuals.
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.
On microscopy, there are more than 10 white blood cells per high power field.
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.
Both of these are sexually transmitted infections.
With both, urinalysis shows sterile pyuria, meaning that leukocyte esterase is positive, but when cultures are done, no bacteria are found.
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- "Diagnosis and Management of Acute Ureterolithiasis" American Journal of Roentgenology (2000)