Applied Radiology

Diagnosis

Multiple bladder stones composed of half uric acid and half ammonium acid urate secondary to urinary stasis from benign prostatic hypertrophy. The abdominal radiograph (figure 1) showed multiple sub-centimeter spherical opacities in the pelvis in the region of the bladder, without focal discrete calcifications. The IVU (figure 2) revealed multiple spherical and nodular filling defects in the bladder ranging from 5 to 10 mm in size. These defects moved slightly with positioning and with bladder filling. Whether these nodules floated or sank in the contrast could not be determined from supine or upright plain radiographs. Mild bladder wall thickening was present and later verified on CT scan. The mobile, intraluminal, and dependent nature of the filling defects was determined by supine (figure 3) and prone (figure 4) CT scans of the pelvis. CT further verified absence of dense calcification and the lack of bladder wall mass. Attempted stone evacuation via rigid cystoscopy was unsuccessful due to the tremendous stone burden and the enlarged prostate. The patient subsequently underwent open prostatectomy and the bladder stones were removed (figure 5).

Discussion

Uric acid stones represent 5 to 10% of urinary stones in the US. If pure, they are classically radiolucent and small. More than half are mixed with calcium salts, which make them appear both larger and more radiopaque. Risk factors for uric acid stone formation include hyperuricosuria, myeloproliferative disorders, gout, and causes of concentrated or acidic urine.¹ Up to one-half of patients with uric acid stones will have gout. Uric acid stones are more common in males and are usually familial, whether or not gout is present. Uric acid stones are formed when concentrated acidic urine becomes supersaturated with undissociated uric acid, which forms crystals and stones that are colored red-orange in urine and are strongly birefringent. Rarely, uric acid crystals can plug the collecting tubules, causing acute renal failure or chronic renal insufficiency.² Seventy percent of bladder stones are associated with obstruction, stasis, or poor emptying. Thirty percent have associated gram-neg-ative urinary tract infections.³ Less common causes include bladder diverticula, foreign body, urinary stent, chronic catheterization, or a migrant stone from the upper tract.^1-3 No definite correlation between specific stone type and the cause of bladder obstruction has been identified.⁴ Bladder stone diagnosis may be difficult to make on plain radiography due to overlying rectosigmoid feces, poor bladder opacification with contrast, or similarity to phleboliths.¹ Bladder stones have become increasingly less common in the western hemisphere in the last century, most likely due to dietary and nutritional improvements. The incidence in other parts of the world remains higher, however. In India, 13 people in 100,000 may be affected. A recent review of 300 bladder stones found 50% were made up of mostly magnesium ammonium phosphate, 31% of calcium salts, and 5% of uric acid origin.⁶ Although bladder stones usually are solitary, multiple stones have been found in up to 25% of patients.¹ Patients with bladder stones may present with hematuria, recurrent UTIs, pelvic pain, irritative or obstructive voiding symptoms, or with no symptoms at all. Treatment options include surgical extraction, lithotripsy, or alkalinization of urine, although the latter has demonstrated poor results.⁸ Diagnosis of bladder calculi is made with a CT scan or cystogram from an intravenous urogram or a retrograde study. The differential diagnosis for the nodular filling defects in this case includes blood clots, fungus balls, papillary tumor, eosinophilic cystitis, cystitis cystica, and cystitis glandularis. Nonopaque or partly opaque stones can be differentiated from tumor by showing mobility on a plain radiograph, ultrasound, or CT scan. Uric acid stones will show up as a bright, well defined area of medium-high attenuation, with CT numbers of greater than 150 HU, although usually of less than stones with a more calcific component.⁹ CT scans may also address the questions of prostate size and bladder wall thickening versus mass. CT has recently proven to be a useful tool in the evaluation of ureteral stones in the setting of acute flank pain.¹⁰ It also may assist in the diagnosis and characterization of bladder stones.

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