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Findings:AP (A) and oblique (B) plain-film (KUB) images demonstrate a calcific-density to be projected over the region of the true pelvis, in this patient status-post multiple bowel resections. AP film (C) obtained during IVP, demonstrate persistence of ureteral contrast column on the left side, compared to that on the right. Image (D) demonstrates the "passed" specimen, shown here in actual size.

Diagnosis:Nephrolithiasis

Discussion: Nephrolithiasis, or renal calculus formation, is by far the most frequent cause of acute obstructive uropathy. Stone disease is most prevalent between the second and fourth decades of life, with males afflicted with frequency of four-times that of females.

Abdominal plain-film (KUB) analysis and intravenous pyelography (IVP) have been the traditional imaging mainstays for the detection of renal calculi, and for their resultant level and degree of obstructive uropathy, if present. Approximately 80-90% of renal calculi demonstrate plain-film radiopacity - most of these (75%) are composed primarily of calcium oxalate (monohydrate and dihydrate forms). The remainder of the sum-total of radiopaque stones are comprised of apatite, calcium phosphate, or are of mixed components, containing at least some proportion of calcium compound. Such is the case with "staghorn" calculi, composed of struvite - a magnesium ammonium phosphate compound, mixed, invariably with a small percentage of calcium phosphate; the latter component renders the calculi radiopaque. Cystine stones are quite uncommon - approximately 1% of the sum-total - and are at least faintly radiopaque, owing to their sulfur moiety.

Radiolucent calculi are much less common than their radiopaque counterparts. Of these approximately 80% are composed of uric acid, with small percentages being comprised of xanthine and protein-matrix. These varieties of calculi may be visualized, indirectly, on IVP as filling defects within a contrast column, or as the abrupt point of distal termination of a contrasted system that is (at least partially) obstructed.

In the majority of cases (approximately 75%), the exact etiology of nephrolithiasis is unknown; that is to say that the patient presents with no underlying disorder or risk factors. However, several disease entities have been associated with renal calculi of different compositions, for example:

• 1. calcium oxalate:
  • a. entities resulting in hyperoxaluria - oxalosis, inflammatory bowel disease (especially Crohn's Disease), pyridoxine (B6) deficiency, etc.
  • b. entities resulting in hypercalcuria - hyperparathyroidism, neoplastic disease, Cushing's Syndrome, hyperthyroidism, demineralization secondary to immobilization, hypervitaminosis D, sarcoidosis, etc.
• 2. apatite and calcium phosphate: entities resulting in hypercalcuria, as above.
• 3. struvite: urinary tract infections with urea-splitting organisms such as Proteus mirabilis, as well as urinary stasis.
• 4. cystine: related to the disease entity of hypercystinuria, an autosomal-recessive inherited defect of renal tubular reabsorption of cystine.
• 5. xanthine: Hyperxanthinuria results from a deficiency of xanthine oxidase, necessary for the conversion of xanthine to uric acid, in the final step of purine catabolism.
• 6. uric acid: hyperuricosuria is most often the result of treatment for gout (uricosuric agents), or for myeloproliferative disorders (chemotherapeutic agents).

Symptons related to nephrolithiasis are directly proportional to the size of the stone, which may range from sandy grit to a huge staghorn calculus, and to the grade of obstruction. Conventional teaching states that the most frequent levels for ureteral stone impaction are: 1. At the ureteropelvic junction (UPJ); 2. At the linea terminalis, or point where the ureter crosses the pelvic brim; and, 3. In proximity to the ureterovesical junction.

Smaller calculi may be entirely asymptomatic, or may lead the patient to be referred for additional workup, after incidental detection of microscopic hematuria. Larger stones, on the other hand, are more likely to become impacted, causing the patient to experience severe, colicky flank pain, gross hematuria, and nausea/vomiting. The patient may indeed present with an "acute abdominal picture", secondary to peritoneal irritation. The size of the calculus is also a determinant of the case with which it may be passed. Stones less than six millimeters in size are likely to pass spontaneously within a six-week period, while larger stones may necessitate surgical intervention for disimpaction and relief from obstructive symptoms.

References:
1. Bino M and Krestin GP. "Back and Flank Pain", pages 115-136. In: Acute Abdomen:
Diagnostic Imaging in the Clinical Context. Krestin GP and Choyke PL, eds.
Thieme Medical Publishers, Inc., 1996.

2. Brant WE. "Pelvicalyceal System, Ureter, Bladder, and Urethra", pages 783-800.
In: Fundamentals of Diagnostic Radiology. Brant WE and Helms CA, eds. Williams & Wilkins, 1994.

3. Ell SR. "The Kidney", pages 161-224, In: Handbook of Gastrointestinal and
Genitourinary Radiology. Mosby-Year Book, Inc., 1992.

4. Lange S. Teaching Atlast of Urologic Radiology. Thieme Meical Publishers, Inc., 1995.

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