Types of Urolithiasis Based on Stone Composition (Dorit E. Zilberman, Michael N. Ferrandino, and Glenn M. Preminger)
Calcium Stones• Stones with calcium components develop as a result of the greatest variety of metabolic disorders and often patients present with a combination of stone forming risk factors. Stone Composition & Possible Clinical Associations
Uric Acid Stones• Patients with pure uric acid urolithiasis generally have a low urinary pH (<5.5) and are therefore defined as having gouty diathesis. Infection Stones• High urinary pH (>7.5) is associated with infection. Urea splitting organisms leads to an increase in the ammonia concentration, which promotes infection stone formation, which compose the majority of stag-horn calculi.
Cystine-Based Stones• Cystine urolithiasis is the result of an autosomal recessive trait, which disrupts the transepithelial transport of cystine, ornithine, lysine, and arginine. The concentration of cystine rises to levels above the saturation point, leading to precepitation of cystine crystals.• Even though cystine stone formers all have definitive genetic disorders, they have been found to have a number of other metabolic risk factors, which can contribute to their stone formation
Metabolic Evaluation of Stone DiseaseWhy?• The main goal of metabolic evaluation is to prevent recurrent stone formation in high-risk stone producers, as well as to prevent further growth of any existing stones.
Metabolic Evaluation of Stone DiseaseWho?1. Patient with stone who wishes to undergo metabolic evaluation2. Single stone with risk factors: a. Family history b. Intestinal disease c. Chronic diarrhea d. UTI e. Gouty f. Osteoporosis g. Skeletal fractures3. Recurrent stone formers4. Children5. Controversial: uric acid/struvite/cystine stones
Metabolic Evaluation of Stone DiseaseWhen?• Many authors suggest that one should wait at least 1 month after stone passage or stone removal, allowing the patient to return to their normal routine, otherwise the metabolic evluation may yield unhelpful information.
Metabolic Evaluation of Stone DiseaseHow?• History & Examination• Stone Analysis• Serum Chemistry• Urine Culture• Urinalysis• 24 hour urine collection• Radiological Imaging
Metabolic Work-up of Stone Disease• The basic metabolic evaluation begins with a thorough history and physical exam, (similar previous conditions or family history)• Stone Analysis: For patients with less common stone compositions – cystine, pure struvite, and pure uric acid – treatment regimens could begin immediately.• Patients with calcium phosphate stones are known to be at increased risk from renal tubular acidosis and primary hyperparathyroidism, while patients with calcium oxalate stone formation have a mixture of metabolic diagnoses.
Metabolic Work-up of Stone Disease• Serum chemistry consists of basic metabolic panel (e.g., sodium, potassium, chloride, carbon dioxide, blood urea nitrogen, creatinine) as well as calcium and uric acid.• Hypercalcemia may be indicative of hyperparathyroidism and would warrant further evaluation with a parathyroid hormone assay. The presence of hypokalemia and hyper- chloremia is strongly suggestive of metabolic acidosis
Metabolic Work-up of Stone Disease• Urine Cultures positive for Klebsiella, Proteus, Pseudomonas, or other urea-splitting organisms may indicate the presence of struvite stone• Urine Analysis with PH between 6.8 and 7.2 suggestive of RTA, a pH greater than 7.5 correlated with urinary tract infection whereas a pH less than 5.5 defines the diagnosis of gouty diathesis.
Metabolic Work-up of Stone Disease• 24 hour collection of urine to measure: total volume, pH, calcium, phosphorus, oxalate, citrate, sodium, magnesium, potassium, uric acid, and sulfate.• It is our routine to have the patient collect two 24 h urine samples on two different days (either consecutive or separate) with the patient on their “normal routine” (normal diet, fluid intake, medications, physical exercise, etc) .
Metabolic Work-up of Stone Disease• Many Authors consider radiologic imaging as a part of the basic metabolic evaluation, as there are insights which can be gleaned from the different imaging techniques.• For example, multiple researchers have reported the use of CT to ascertain stone composition based on measurement of Hounsfield units