Ravilya _ Pediatrics_ Nephro_ Cystinuria

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pathphys

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genetic

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Homozygous cystinuria is characterized by lifelong, recurrent urolithiasis that is difficult to manage, either surgically or medically.

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Urinalysis

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Urinalysis

Cystine is one of the sulfur-containing amino acids; therefore, the urine may have the characteristic odor of rotten eggs.
Urinalysis may show typical hexagonal or benzene crystals, which are essentially pathognomonic of cystinuria. Microscopic crystalluria is present in 26%-83% of patients.
Disappearance of cystine crystals in the first morning urine is a good index of treatment efficacy.

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This is a rapid, simple, and qualitative determination of cystine concentrations

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Sodium cyanide–nitroprusside test

This is a rapid, simple, and qualitative determination of cystine concentrations.
Cyanide converts cystine to cysteine. Nitroprusside then binds, causing a purple hue in 2-10 minutes.
The test detects cystine levels of higher than 75 mg/g of creatinine.
False-positive test results occur in some individuals with homocystinuria or acetonuria and in people taking sulfa drugs, ampicillin, or N -acetylcysteine. In persons with Fanconi syndrome, a false-positive test result can result from generalized aminoaciduria

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For individuals with positive cyanide-nitroprusside test findings, perform

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For individuals with positive cyanide-nitroprusside test findings, perform ion-exchange chromatographic quantitative analysis of a 24-hour collected urine sample.

The normal excretion rate is 40-80 mg/d (0.166-0.333 mmol/d).
Heterozygotes excrete 200-400 mg/d (0.8-1.7 mmol/d).
Homozygotes always excrete 600-1400 mg/d (2.5-5.8 mmol/d).

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Management algorithm

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Management algorithm

Overall, for a patient with cystinuria who does not have a stone, first-line therapy in most cases is a conservative approach, including large-volume fluid intake (urine output >2.5 L/d), regular urine pH monitoring (urine pH level of 7.5 and < 8), dietary restrictions, and urinary alkalization with potassium citrate.
If this standard therapy fails to achieve the urinary cystine concentration of 300 mg/L, then medical therapy with D-penicillamine, alpha-MPG, or captopril must be added.
Treat patients with stone disease according to the location of the stone. The expertise of a urologist and a radiologist is important for decision-making processes, and stone site and size also influence further management. See treatment algorithm in image below.

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first-line alkalinizing drug

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Potassium citrate is the first-line alkalinizing drug. The typical adult dose is 60-80 mEq/d divided into 3-4 doses (15-20 mL/d), titrating the dose as needed to maintain a urine pH level within the target range of 7-7.5.