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Aminoaciduria – Causes, Symptoms and Treatment

Aminoaciduria refers to the excessive excretion of amino acids in the urine. It can indicate metabolic disorders or kidney disease.

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Things worth knowing about "Aminoaciduria"

Aminoaciduria refers to the excessive excretion of amino acids in the urine. It can indicate metabolic disorders or kidney disease.

What is Aminoaciduria?

Aminoaciduria is a condition in which abnormally high amounts of amino acids are excreted in the urine. Amino acids are the building blocks of proteins and play a vital role in human metabolism. Under normal circumstances, the kidneys reabsorb nearly all amino acids filtered from the blood, so only trace amounts appear in the urine. When aminoaciduria occurs, this reabsorption process is impaired.

Causes

There are two main forms of aminoaciduria, classified by their underlying cause:

Renal Aminoaciduria

In the renal form, the problem lies in a defect in amino acid reabsorption within the renal tubules. Blood amino acid levels are usually normal. Causes include:

Hereditary transporter defects (e.g., cystinuria, Hartnup disease)
Acquired kidney disorders (e.g., Fanconi syndrome)
Toxic damage to the renal tubules

Overflow Aminoaciduria

In this form, amino acid concentrations in the blood are so elevated that they exceed the reabsorptive capacity of the kidneys. This is commonly caused by inherited metabolic disorders that impair the breakdown of specific amino acids:

Phenylketonuria (PKU): impaired phenylalanine metabolism
Maple syrup urine disease: impaired catabolism of branched-chain amino acids
Homocystinuria: disrupted methionine metabolism
Liver failure with reduced amino acid degradation

Symptoms

Aminoaciduria itself often does not cause direct symptoms. Clinical signs depend on the underlying condition:

Developmental delays and neurological abnormalities in inherited metabolic disorders
Kidney stones, particularly in cystinuria
General weakness and growth disturbances in children
Characteristic odors in the urine (e.g., musty smell in PKU, maple syrup odor in maple syrup urine disease)

Diagnosis

Diagnosis is established through laboratory investigations:

Urine analysis: Detection of elevated amino acid concentrations using amino acid chromatography or tandem mass spectrometry
Blood tests: Measurement of plasma amino acid levels to distinguish between renal and overflow forms
Genetic testing: To identify hereditary metabolic defects
Newborn screening: Many countries conduct expanded newborn screening programs for amino acid metabolism disorders

Treatment

Treatment depends on the underlying cause:

Dietary management: In conditions such as PKU, a strict low-phenylalanine diet is essential to prevent neurological damage
Medication: Depending on the condition, treatments may include sodium benzoate for ammonia detoxification or tiopronin for cystinuria
Treatment of the primary condition: In Fanconi syndrome, addressing the root cause is the primary goal
Supplementation: Replacing essential amino acids and other nutrients lost through the kidneys

References

Scriver CR et al. - The Metabolic and Molecular Bases of Inherited Disease, 8th Edition, McGraw-Hill (2001)
Blau N et al. - Physician's Guide to the Diagnosis, Treatment, and Follow-Up of Inherited Metabolic Diseases, Springer (2014)
World Health Organization (WHO) - Newborn Screening for Inborn Errors of Metabolism, WHO Technical Report (2022), available at: https://www.who.int

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