Urinary Tract Obstructions (w/Kidney Stones)

Obstructive uropathy refers to obstructions in the urinary tract that impair urine flow. Obstructive nephropathy refers to dysfunction that arises from obstruction, which can lead to permanent damage and end-stage renal disease. Urinalysis, bladder catheterization, cystourethroscopy (in which a cystoscope is used to examine the inside of the bladder and urethra), and imaging with MRI, CT, and ultrasound. In children, congenital anomalies are the most common causes of obstruction; in our example, we show posterior urethral valves, in which a membrane blocks the urethral lumen. In adults, kidney stones are the most common cause of urinary tract obstruction; we'll learn about different types of kidney stones in the second half of the tutorial. In older males, benign prostatic hyperplasia and prostate cancer are common causes of urethral blockage. Anatomic anomalies include bladder neck contracture, ureter polyps or strictures, abnormal urethral valves (as we saw in the congenital example earlier), and injuries or diverticula. Functional examples include drug-induced uropathy (for example, in patients taking anticholinergic drugs), neurogenic bladder, and uteropelvic/uterovesical junction dysfunction. Compressive obstruction can be due to pregnancy, abscesses, and benign prostatic hyperplasia (as discussed earlier in older males). Mechanical obstruction of the lumen can be caused by kidney stones, blood clots, fungus balls (caused by various pathogens, including C. albicans), and carcinoma. Effects vary by the location and degree of blockage and the time since onset. Proximal to obstruction, we see increased luminal pressure, local ischemia, urinary tract infection, and stone formation (which exacerbates obstruction). Distal to the point of obstruction, urine flow is impaired, which can lead to oliguria (the production of small quantities of urine); complete blockage of the urethra or urinary bladder produces absolute anuria. Anuria and oliguria can lead to volume expansion and hypertension, which typically resolves after the blockage is cleared. Be aware that urine output and frequency varies depending on tubular damage, and that we need to distinguish "urinary urgency," which is the "urge" to urinate, with urinary frequency and polyuria (polyuria = large volume of urine passed). As an example, we show the effects on the urinary tract when blockage occurs in the urethra:

• First, urine retention; be aware that urine retention, aka, urinary stasis, is a risk factor for urinary tract infections.
• The bladder wall distends and thickens, and the ureters dilate.
• The kidney swells and distends as urine accumulates – this is called hydronephrosis.

These changes can produce "renal colic," which is characterized by radiating pain, difficulty voiding, and nausea and vomiting. Pain Patterns: when the obstruction is proximally located, pain tends to radiate upwards along the flank; distally located obstructions tend to cause pain that radiates towards the inguinal and groin areas. Be aware that severity of renal colic varies, and may even be absent, depending on the location, degree of obstruction, and how long the obstruction has been in place. Aka, renal calculi Estimated to occur in 1 in 10 adults in the U.S. Nephrolithiasis are stones that form within the kidney. Urolithiasis are stones that form anywhere along the urinary tract, including the kidney. Stones outside of the kidney can impair urine flow and cause renal colic and hematuria. Risk factors include dietary factors (for example, high sodium and high protein diets) and other health conditions, such as Crohn's disease, diabetes mellitus, gout, and gallstones. Indicate that low urine volume promotes supersaturation and crystal formation. Treatments: When symptomatic, kidney stones can be quite painful, but most pass on their own in the urine; NSAIDs can be provided to alleviate the pain. When needed, alpha blockers or other medications can be used to relax the smooth muscle of the urinary tract and facilitate stone passage. Larger stones may require lithotripsy, which breaks the stone into smaller pieces using ultrasound shock waves, or even surgical removal. Urinary tract obstruction can lead to permanent damage and end-stage renal disease, so clearance is important. Recurrence is common; it occurs in approximately 50% of patients. Calcium oxalate stones: Approximately 80% of stones comprise calcium oxalate, with and without elements of calcium phosphate. On X-ray and CT these stones are radiopaque. Crystals look like envelopes, and they look like spiky balls on gross examination. Causes include low urine pH, hypercalciuria, and hyperoxaluria. Hypercalciuria can be idiopathic or secondary to hyperparathyroidism, vitamin D excess, etc.; hyperoxaluria can be due to diet or malabsorption. Calcium phosphate stones: Radiopaque. Their crystals are needle or star-shaped, and they look like smoother versions of the calcium oxalate stones. They are associated with high urine pH. Struvite stones: Comprise ammonia, magnesium, and phosphate. They are radiopaque with "coffin-lid" crystals. They tend to have large projections (staghorns). Struvite stones are caused by urease-positive pathogens that cause urinary tract infections; the bacteria convert urea to ammonia, one of the key elements of the stone. Uric acid stones: Radiolucent on X-ray and visible on CT. Crystals are rhomboid-shaped, and they have a pitted appearance. Uric acid stones are associated with low urine pH, gout (which is associated with uricemia), cancer, and metabolic syndromes. Cystine stones: Faintly radiopaque. Hexagonal crystals and irregularly-shaped; they can become large. Cystine stones are caused by an autosomal hereditary disorder in which the renal tubules cannot reabsorb cystine (due to defects in the genes SLC3A1 and SLC7A). Because the defect is genetic, stone formation is recurrent, difficult to prevent, and causes renal insufficiency in affected patients.

• For references, please see full tutorial.