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Magnesium Ammonium Phosphate (MAP) Stones, Uric Acid Stones, & Cystine Stones

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Pathophysiology

Summary

Magnesium-ammonium-phosphate (MAP) stones are the second most common renal stones in adults, often in context of UTIs caused by urease-positive organisms such as Proteus, S. saprophyticus, and Klebsiella. Urease hydrolyzes urea to ammonia and CO2, creating an alkaline urinary environment that promotes the formation of MAP stones. These stones may present as struvite (‘staghorn’) calculi, and their crystals show a ‘coffin lid’ shape under microscopy.

Uric acid stones form mainly in conditions of hyperuricemia. This can occur in conditions like gout, disorders that cause rapid cell turnover such as leukemia and lymphoma, or in tumor lysis syndrome. Acidic urine predisposes to the formation of uric acid stones; therefore, treatment is aimed at urine alkalinization with agents such as potassium citrate and potassium bicarbonate.

Cystine stones are associated with cystinuria, an autosomal recessive defect impacting amino acid reabsorption in the PCT. Cystinuria leads to recurrent, non-calcium renal stones and typically presents in childhood. Diagnosis is confirmed via a sodium cyanide-nitroprusside urine test, which turns red-purple when positive. In severe instances, chelating agents like penicillamine are used to lower cystine levels.

Radiographically, uric acid & cystine stones are radiolucent, contrasting with the radiopaque nature of calcium & MAP stones.

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FAQs

What are MAP stones and what triggers their formation?

Magnesium-ammonium-phosphate (MAP) stones are the second most prevalent type of kidney stones in adults and can present as struvite or ‘staghorn’ calculi. They typically form during urinary tract infections (UTIs) caused by urease-positive organisms such as Proteus, S. saprophyticus, and Klebsiella. These organisms convert urea into ammonia and CO2, which results in the formation of ammonium hydroxide and a basic urine environment. In an alkaline environment, phosphate is more likely to exist in the ionized forms that have higher negative charges, like HPO4^2- and PO4^3-. These charged forms are less soluble and more prone to precipitate with cations like calcium and magnesium, forming crystals or stones. Under microscopy, MAP crystals exhibit a characteristic ‘coffin lid’ or rectangular prism shape.

What conditions predispose individuals to uric acid stone formation?

Uric acid stones are commonly associated with hyperuricemia, which can be due to conditions like gout, or in conditions that cause rapid cell turnover such as leukemia and lymphoma. Approximately 50% of patients with uric acid stones have idiopathic acidic urine. Metabolic acidosis, often caused by chronic diarrhea, can also contribute to uric acid stone formation by increasing hydrogen ion excretion. Alkalinization of the urine with agents like potassium citrate or bicarbonate can be an effective treatment and preventive measure.

What are the clinical features of cystinuria and what treatment options are available for severe cases?

Cystinuria is characterized by a defect in the proximal convoluted tubule (PCT) that impairs the reabsorption of amino acids like cystine, ornithine, arginine, and lysine. This condition often presents in childhood with recurrent, non-calcium renal stones, which may also manifest as ‘staghorn’ calculi. Under microscopy, cystine crystals exhibit a characteristic hexagonal shape. For severe cases, penicillamine is used as a chelating agent to lower cystine levels in the urine, thereby aiding in the prevention of cystine stone formation.

What diagnostic tests are used for cystinuria?

The primary diagnostic test for cystinuria is the sodium cyanide-nitroprusside urine test. In this assay, a positive result manifests as the urine turning a red-purple color, confirming the presence of cystine in the urine.

How do the appearances of magnesium-ammonium-phosphate (MAP), uric acid, and cystine stones differ under microscopy?

Under microscopic examination, each type of stone has a distinct appearance. Magnesium-ammonium-phosphate (MAP) crystals typically display a "coffin lid" or rectangular prism shape. Uric acid stones, on the other hand, present as diamond-shaped, yellow-brown crystals. Cystine crystals are characterized by their hexagonal shape. These unique morphologies can aid in the diagnosis and differentiation of the stone types.