Causes of Struvite Kidney Stones
Struvite kidney stones are primarily caused by urinary tract infections with urease-producing bacteria, which create an alkaline urinary environment and high ammonia concentration, leading to crystallization of magnesium ammonium phosphate (struvite). 1
Bacterial Infection as Primary Cause
- Struvite stones are also referred to as "infection stones" due to their strong association with urinary tract infections caused by specific organisms that produce the enzyme urease 1
- Urease-producing bacteria split urea into ammonia and hydroxide, creating an alkaline urinary environment that promotes stone formation 1
- Proteus species are the most common urease-producing organisms responsible for struvite stone formation, though other bacteria can also produce urease 2
- Unlike other stone types, bacteria reside within struvite stones (not just on the surface), making the stones themselves infected 1
Biochemical Mechanism of Formation
- The urease enzyme promotes generation of ammonia and hydroxide from urea, resulting in:
- Alkaline urinary pH
- High ammonia concentration
- Abundant phosphate and magnesium in urine 1
- These conditions promote crystallization of magnesium ammonium phosphate (struvite) 1
- The chemical composition of struvite stones is MgNH₄PO₄·6H₂O 3
- Struvite stones often contain calcium carbonate apatite as well 1
Contributing Factors
- Formation of an exopolysaccharide biofilm by bacteria facilitates stone development 1
- Incorporation of mucoproteins and other organic compounds into the stone matrix 1
- Repeated urinary tract infections with urease-producing organisms increase risk of stone formation 1
- Factors that predispose to urinary tract infections increase the likelihood of struvite stone formation 2
- Structural abnormalities of the urinary tract may increase risk 4
Clinical Significance
- Struvite stones often grow into a "staghorn" configuration, filling the renal pelvis and branching into several or all of the calices 1
- Untreated staghorn calculi can destroy kidney function and/or cause life-threatening sepsis 1
- Complete stone removal is crucial to:
- Eradicate causative organisms
- Relieve obstruction
- Prevent further stone growth
- Preserve kidney function 1
- Residual fragments may grow and be a source for recurrent urinary tract infection 1
Important Distinctions
- E. coli, the most common cause of UTIs, typically does not produce urease and is not associated with struvite stone formation 4
- Struvite stones differ from uric acid stones, which form in acidic urine (pH <5.5) 4
- Calcium oxalate or phosphate stones rarely grow in a staghorn configuration, unlike struvite stones 1
Prevention Strategies
- Aggressive management of urease-producing bacterial infections 5
- Antibiotic therapy and urease inhibitors offer the best form of preventative treatment 2
- Complete removal of existing stones is essential to prevent recurrence 1
- Chemical agents like pyrophosphate, acetohydroxamic acid, and trisodium citrate have shown inhibitory effects on struvite formation 3