Case 32: Diagnosis & Conclusions

Case 32 Index

Diagnosis: Urinary tract infection (complicated) with struvite stone

Case Summary: This is a young woman with recurrent episodes of urinary tract infection (UTI). The frequency of her infections should prompt further evaluation for predisposing factors.

Take a look at factors predisposing to recurrent UTI:
Local Factors
Impaired bladder emptying Bladder or kidney calculi Kidney cysts Anatomic abnormality (i.e. horseshoe kidney) Indwelling urinary catheter Bladder instrumentation Vaginal atrophy (often postmenopausal)
Systemic Factors
Diabetes mellitus Immunosuppression Pregnancy

In this case, our exploration reveals evidence of stone disease. Let’s pause for a second and review the wide differential for stone and crystal disease in the kidney. Though we won’t discuss these stones and crystals here (stay tuned for future cases!), the patient’s clinical history (particularly use of medications, toxin ingestions, and co-morbidities), urine pH, and stone appearance can help narrow down the differential diagnosis. Urine microscopy and stone analysis by a pathology lab are critical in correctly identifying the stone or crystal in question. 

Kidney Stones & Crystals

Back to our case – here we also have an elevated urine pH and infection with a urease-producing organism. This presentation is most consistent with struvite stones (see the “coffin shaped” stones.  These stones are a mixture of magnesium ammonium phosphate and carbonate apatite, and occur more commonly in women than men. They are also referred to as as “triple phosphate” stones. 

Urine sediment with struvite, coffin-shaped stones under light microscopy. Image courtesy of Dr. Jose A. T. Poloni.

Struvite stones grow quickly over weeks to months. Elevated urine pH is characteristic, which is caused by upper UTI with urease-producing organisms. Examples of these organisms include Proteus (as in our case) and Klebsiella.  So what is “urease”, anyway? In the presence of urease, urea is hydrolyzed (water is added) and ammonium is produced – which alkalinizes the urine and thereby decreases the solubility of phosphate. Under these conditions, magnesium ammonium phosphate precipitates. 

On x-ray, struvite stones have a characteristic “staghorn” appearance as seen in the left kidney below . The kidney ultrasound in this case revealed a dense, calcified object with posterior acoustic shadowing. 

Staghorn (struvite) calculus, in left kidney. Image courtesy of Dr. Natalie Yang (Radiopaedia.org , Case 9733)

The treatment? Once the stone has already formed, the gold standard is percutaneous nephrolithotomy (PCNL), or stone removal via a small puncture wound through the skin.  Other approaches include urease inhibitors, acidification therapy to prevent stone formation, dissolution therapy, extracorporeal shockwave lithotripsy, ureteroscopy, and anatrophic nephrolithotomy.

Case 32 Index
Case 32 Introduction
Case 32 Physical Exam
Case 32 Diagnostic Testing