Urolithiasis Diagnosis
Classification
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
History
- 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
Physical Examination
- Should include the weight, blood pressure, costovertebral angle tenderness & lower extremity edema, as well as signs of primary hyperparathyroidism (HPT) & gout in the assessment
Laboratory Tests
- 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
Imaging
- 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