Struvite and Triple Phosphate Renal Calculi

Excerpt

The relationship between infections and renal calculi has been known since the time of Hippocrates, but it was not until 1817 that Marcet identified the connection between urinary alkalinity, infection, phosphate calculi, and increased urinary ammonia. In 1901, Brown first suggested that urea splitting bacteria activity increased urinary ammonia production, which was the immediate cause of higher urinary alkalinity levels and phosphate (struvite) stone formation. The 1926 description of urease, the first enzyme ever isolated and purified, earned Sumner the Nobel Prize for Chemistry in 1946. Struvite was first discovered in bat droppings by Swedish geologist Georg Ulex who named it after his friend, Russian diplomat, and naturalist, Baron von Struve.

Struvite is a crystalline compound made up of magnesium ammonium phosphate (MgNH4PO4.6H20). Struvite stones are actually a mixture composed of three cations (calcium, magnesium, ammonium) and one anion (phosphate). They are therefore also known as triple phosphate stones even though pure struvite actually contains no calcium. The mixture is composed of struvite (MgNH4PO4.6H20) and calcium phosphate (Ca10.[PO4]6.CO3); hence carbonate ions are also usually found. They can form in the kidney or bladder in patients with catheters or urinary stasis. For example, about 8% of patients with spinal cord lesions will form stones, and 98% of these will be struvite. If left untreated, renal struvite stones have a tendency to grow more rapidly compared to calcium-based stones and may eventually fill up the entire collecting system. This can lead to a staghorn stone or branched calculus formation. Among all stone formers, the percentage of struvite stones was found to be 5% to 15%.

Struvite stones only form in alkaline environments (pH >7) and are always associated with urinary tract infections from urease-producing bacteria such as Proteus. Virtually all infectious urinary organisms make urease except E. coli, Citrobacter freundii, Streptococci, and Enterococci. It should be pointed out that staghorn stones, while predominantly made of struvite or triple phosphate, may also be composed of mixtures of calcium oxalate, calcium phosphate, uric acid, and cystine. However, for the purpose of this review, we shall focus exclusively on struvite staghorn calculi.

Staghorn stones are also defined as "partial" or "complete." A "partial" staghorn stone would include at least two calyces, while "complete" would indicate that at least 80% of the renal collecting system was involved.