United Knowledge, Expert Care

Non-Calcium Stones

Struvite calculi
These are composed of magnesium, phosphate and ammonium (MAP). They occur in women and grow and recur rapidly. They often from staghorn stones and are infection stones related to urea splitting organisms (proteus, pseudomonas etc). The high ammonium concentration resulting from the urea metabolism produces an alkaline urine (pH 6.8-8.3), which results in precipitation of MAP. Anatomical abnormalities such as neuropathic bladders that predispose to infection increase the patient’s susceptibility to struvite calculi. Treatment is removal of the stones and any predisposing infective focus (such as catheters) along with antibiotic therapy and supportive measures.

Uric acid calculi
Less than 5% of stones are pure urate calculi. They are usually found in men and are radiolucent. Patients with gout, myeloproliferative disorders or those undergoing treatment for malignant conditions have a high incidence of uric acid calculi. A persistently low urinary pH increases the risks of crystallisation of uric acid and therefore therapy is aimed at increasing the urinary pH above the dissociation constant level of 5.75. Chemodissolution can be effective with alkalinisation techniques. Alternative treatments include allopurinol and reduction of dietary purines.

Cystine calculi
Cystine lithiasis is uncommon (1% of all stones) and is secondary to an autosomal recessive disorder of transmembrnae cystine transport manifested in the kidney and intestine. The stones are less radiopaque than calcium bearing stones and are hard and resistant to extracorporeal shockwave lithotripsy (ESWL). Treatment is aimed at clearing stone burden and then concentrating on prevention by reducing urinary cystine levels using high urine output, urinary alkalinisation and D-penicillamine and alpha-mercaptoproprionylglycine.

Xanthine calculi
Xanthinuria is a very rare autosomal recessive condition characterised by a deficiency of xanthine oxidase. Treatment involves a high fluid intake and urinary alkalinisation. Purine restriction may also prove valuable.

Drug related calculi
It is rare for drugs to be associated with urinary stone disease. Triamterine stones are rare (0.13% of all stones), and stones related to the use of protease inhibitors (predominantly indinavir) have been seen after treatment of human immunodeficiency virus type 1. Silicate calculi can occur after the ingestion of large quantities of antacids containing silicates (magnesium trisilicate).

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