Classification of urinary stones

Size

• Usually given in one or two dimensions & is stratified to those measuring up to 5, 5-10, 10-20 & 20 mm in largest diameter

Location

• Classified according to anatomical position: upper, middle or lower calyx, renal pelvis, upper, middle or distal ureters & urinary bladder

X-ray characteristics

• Classified according to plain X-ray appearance (eg kidney-ureters-bladder (KUB) radiography), according to mineral composition
• Non-contrast computed tomography (NCCT) scan is used to classify stones according to density, structure & composition

Etiology of formation

• Non-Infectious (eg calcium oxalate, calcium phosphate, uric acid)
• Infectious (eg magnesium ammonium phosphate, carbonate apatite, ammonium urate)
• Genetic causes (eg cystine, xanthine, 2,8 dihydroxyadenine)
• Further diagnostic tests & management depends on the composition of the stone
• Risk status of the stone formers should be assessed because it will define the probability of recurrence or regrowth & is imperative for the choice of pharmacological treatment

• Preferred analytical procedures are infrared spectroscopy (IRS) & X-ray diffraction analysis of urinary stones
• Repeat stone analysis is needed in cases of:
  • Recurrence after pharmacological intervention
  • Early recurrence after interventional therapy w/ complete stone clearance
  • Late recurrence after a prolonged stone-free period since stone composition may change overtime

Stone types

Calcium stones (oxalate & phosphate)

• Most common type of kidney stone
• Formed when there is high level of calcium in the urine
• Characterized as either large & smooth or rough & spiky
• Diseases & disorders related to calcium stones:
  • Hypercalciuria (inherited condition)
  • Renal tubular acidosis
  • Nephrocalcinosis
  • Primary hyperparathyroidism
  • Kidney disease
  • Sarcoidosis (granulomatous disease)
  • Primary hyperoxaluria
  • Enteric hyperoxaluria

Struvite or infection stones

• May originate de novo or grow on pre-existing stones infected w/urea-splitting bacteria
• Predisposing factors for stone formation:
  • Neurogenic bladder
  • Spinal cord injury or paralysis
  • Continent urinary diversion
  • Ileal conduit
  • Foreign body
  • Stone disease
  • Indwelling urinary catheter
  • Urethral stricture
  • Benign prostatic hyperplasia
  • Bladder diverticulum
  • Cystocele
  • Caliceal diverticulum
  • Uteropelvic junction (UPJ) obstruction

Uric acid & ammonium urate stones

• Associated w/ hyperuricosuria or low urinary pH
• Hyperuricosuria may be due to dietary excess, endogenous overproduction (enzyme defects), myeloproliferative disorders, tumor lysis syndrome, drugs, gout & catabolism
  • Ammonium urate stones are rare & are associated w/ inflammatory bowel disease (IBD), ileostomy diversion, laxative abuse, potassium deficiency, hypokalemia & malnutrition
  • Forms in the urine at pH <6.5 (ammonium urate crystals) & <5.5 (uric acid stones)

Cystine stones

• Poorly soluble in urine & crystallizes spontaneously w/in the physiological urinary pH at 6.0
• Clinical manifestations are the same for patients who are genotypic or phenotypic type of cystinuria

Other stone types

2,8-Dihydroxyandenine stones & xanthine stones

• Both stone types were rare & the diagnosis is similar to that of uric acid
• Genetically determined defect of adenine phosphoribosyl transferase that causes high urinary excretion
• Decreased levels of serum uric acid are seen in patients who forms xanthine stones

Drug stones

• These are induced by pharmacological treatment & exists as:
  • Stones formed due to unfavorable changes in urine composition under drug therapy & by the crystallized compounds of the drug
• Compounds that causes drug stones:
  • Allopurinol/oxypurinol
  • Amoxicillin/ampicillin
  • Ceftriaxone
  • Quinolones
  • Ephedrine
  • Indinavir
  • Magnesium trisilicate
  • Sulphonamides
  • Triamterene
  • Zonisamide
• Substances impairing urine composition:
  • Acetazolamide
  • Allopurinol
  • Aluminium magnesium hydroxide
  • Ascorbic acid
  • Calcium
  • Furosemide
  • Laxatives
  • Methoxyflurane
  • Vitamin D
  • Topiramate

Matrix stones

• Pure matrix stones are extremely rare
• More prevalent among females
• Main risk factors are:
  • Urinary tract infections (UTIs) due to Proteus mirabilis or Escherichia coli
  • Previous surgery for stone disease
  • Chronic renal failure
  • Hemodialysis

• A detailed history from the patient should be elicited
• Thorough review of medical records should include:
  • Number & frequency of episodes
  • Previous imaging studies, interventions, evaluations & treatments

• Family history that may reveal genetic predisposition:
  • Cystinuria (type A, B & AB)
  • 2,8 Dihydroxyandeninuria
  • Xanthinuria
  • Renal tubular acidosis (RTA type 1)
  • Primary hyperoxaluria
  • Lesch-Nyhan syndrome
  • Cystic fibrosis

• General factors:
  • Early onset of urolithiasis (especially in children & teenagers)
  • Familial stone formation
  • Brushite-containing stones (calcium hydrogen phosphate)
  • Uric acid & urate-containing stones
  • Infection stones
  • Solitary kidney

• Dietary history of the patient:
  • Average daily intake of fluids (amount & specific beverages)
  • Eating habits (meals & snacks)
  • Calcium, sodium, high oxalate-containing food
  • Fruits & vegetables

• Nutritional factors associated w/ stone diseases:
  • Calcium intake that is below or significantly above the recommended dietary allowance (RDA)
  • Low fluid intake
  • High sodium intake
  • Limited intake of fruits
  • Vegetables & high intake of animal-derived purines

• Complete list of current prescription & over-the-counter drugs, as well as vitamins & supplements should be obtained; stone-provoking medications include:
  • Probenecid
  • Some protease inhibitors
  • Lipase inhibitors
  • Triamterene
  • Chemotherapy
  • Vitamins C & D
  • Carbonic anhydrase inhibitors (eg Topiramate, Acetazolamide, Zonisamide)

• Conditions associated w/ stone disease:
  • Obesity
  • Gout
  • Hyperparathyroidism
  • Renal tubular acidosis type I
  • Diabetes mellitus type II
  • Bone disease
  • Primary hyperparathyroidism
  • Bariatric surgery
  • Bowel or pancreatic disease
  • Nephrocalcinosis
  • Sarcoidosis
  • Due to jejunoileal bypass & intestinal resection

• Anatomical abnormalities associated w/ stone formation:
  • Medullary sponge kidney (tubular ectasia)
  • Ureteropelvic junction (UPJ) obstruction
  • Calyceal diverticulum, calyceal cyst
  • Ureteral stricture
  • Vesico-uretero-renal-reflux
  • Horseshoe kidney
  • Ureterocele

• Should include the weight, blood pressure, costovertebral angle tenderness & lower extremity edema, as well as signs of primary hyperparathyroidism (HPT) & gout in the assessment

• Serum chemistries should include electrolytes (eg sodium, potassium, chloride, bicarbonate, calcium, creatinine, & uric acid) to uncover hypokalemia or renal tubular acidosis (RTA)
• Parathyroid hormone (PTH) level should be measured if there is high normal or elevated serum & urine calcium concentration
• Level of 25-hydroxy vitamin D should also be investigated to rule out the possibility of vitamin D deficiency in patients w/ elevated PTH
• Urinalysis should include dipstick, microscopic evaluation (urinary pH, indicators of infection & identification of crystals that are pathognomonic of stone type)
• 24-hour urine collection/metabolic testing
  • The cornerstone for which the therapeutic recommendations are based
  • At least two samples are collected, while consuming their usual diet & volume of fluid
  • Metabolic testing should analyze total volume, pH, calcium, oxalate, uric acid, citrate, sodium, potassium & creatinine
  • Urinary potassium measured at baseline can be compared to urinary potassium obtained during follow-up, to gauge compliance w/ medication regimens
  • Urinary cystine should additionally be measured in stone formers w/ known cystine stones or a family history of cystinuria or for those in whom cystinuria is suspected
  • Primary hyperoxaluria should be suspected when urinary oxalate excretion exceeds 75 mg/day in adults w/o bowel dysfunction
  • These patients should be considered for referral for genetic testing &/or specialized urine testing

• Urine (eg dipstick test of spot urine sample, urine microscopy &/or culture)
• Serum blood sample (eg creatinine, uric acid, ionized calcium, sodium & potassium)
• Blood cell count
• Coagulation test, if intervention is likely or planned

Basic laboratory analysis in non-emergency cases

• Biochemical work-up is similar for all patients but if there is no planned intervention, then sodium, potassium, C-reactive protein (CRP) & blood coagulation time can be omitted

• Choice of imaging modality will depend on the clinical situation of the patient
• Indicated for patients w/ fever or solitary kidney & when diagnosis is doubtful
• Used to differentiate ureteral stones from renal stones

Ultrasound (UTZ)

• Used as the primary diagnostic tool
• Identifies presence of stones in the calices, pelvis, pyeloureteric & vesicoureteric junctions, & in patients w/ upper urinary tract dilatation

Kidneys, Ureter & Bladder (KUB) radiography

• Helpful in differentiating radiolucent & radiopaque stones
• Used for comparison during follow-up
• Should not be performed if non-contrast computed tomography scan (NCCT) is being considered

Evaluation of patient w/ acute flank pain

Non-Contrast Computed Tomography (NCCT)

• First choice in confirming stone diagnosis in patients w/acute flank pain
• Significantly more accurate than intravenous urethrogram (IVU)/intravenous pyelogram (IVP) in evaluating patients w/ acute urolithiasis
• Used to determine the diameter, density, inner structure & skin-to-stone distance that affects the outcome of extracorporeal shockwave lithotripsy (SWL)
• Can detect uric acid & xanthine stone that are radiolucent on plain films

Low-Dose Computed Tomography (CT) Scan

• Can reduce radiation risk
• Used to detect ureteric stones in patients w/ a body mass index (BMI) of <30