[Nephrocalcinosis in children]

Nephrocalcinosis is defined by calcium phosphate or calcium oxalate deposits in the kidney parenchyma, particularly in tubular epithelial cells and interstitial tissue. It should be differentiated from urolithiasis where calcium salts deposits are located in the kidney and urinary tract. The epidemiology of nephrocalcinosis in children is unknown but the condition is not so rare, with an increased incidence in preterm infants. Often detected as an incidental finding, nephrocalcinosis may be classified according to the radiological type: medullary, cortical or diffuse. Nephrocalcinosis in children can be caused by a variety of etiology. The most common causes concern medullary nephrocalcinosis and include hereditary tubular disorders, in particular distal renal tubular acidosis and Dent disease, metabolic disorders such as idiopathic hypercalciuria and hyperoxaluria, and iatrogenic causes such as vitamin D intoxication. In the newborn, the main cause is hypercalciuria of the premature baby, whose multifactorial origin is largely iatrogenic. Primary hyperoxaluria which can lead to early onset nephrocalcinosis and usually to chronic kidney disease should always be considered and further investigated. In order to provide a specific diagnosis, it is essential to take into account the family history, the clinical context and complete laboratory data. Early initiation of an appropriate etiological treatment is recommended and may prevent or delay the progression to chronic kidney disease in some cases.

[Primary hyperoxaluria: A review]

Primary hyperoxalurias (PH) are inborn errors in the metabolism of glyoxalate and oxalate with recessive autosomal transmission. As a result, an increased endogenous production of oxalate leads to exessive urinary oxalate excretion. PH type 1, the most common form, is due to a deficiency of the peroxisomal enzyme alanine: Glyoxylate aminotransferase (AGT) in the liver. PH type 2 is due to the deficiency of the glyoxylate reductase/hydroxypyruvate réductase, present in the cytosol of hepatocytes and leucocytes. PH type 3 is linked to the gene HOGA1, encoding a mitochondrial enzyme, the 4-hydroxy-2-oxo-glutarate aldolase. Recurrent urolithiaisis and nephrocalcinosis are the markers of the disease. As a result, a progressive dysfunction of the kidneys is commonly observed. At the stage of severe chronic kidney disease, plasma oxalate increase leads to a systemic oxalosis. Diagnostic is often delayed and it based on stone analysis, cristalluria, oxaluria determination and DNA analysis. Early initiation of conservative treatment including high fluid intake and long-term co-administration of inhibitors of calcium oxalate crystallization and pyridoxine, could efficiently prevent end stage renal disease. In end stage renal failure, a combined liver-kidney transplantation corrects the enzyme defect.

[Tubular renal acidosis]

Renal tubular acidosis (RTAs) are a group of metabolic disorders characterized by metabolic acidosis with normal plasma anion gap. There are three main forms of RTA: a proximal RTA called type II and a distal RTA (type I and IV). The RTA type II is a consequence of the inability of the proximal tubule to reabsorb bicarbonate. The distal RTA is associated with the inability to excrete the daily acid load and may be associated with hyperkalaemia (type IV) or hypokalemia (type I). The most common etiology of RTA type IV is the hypoaldosteronism. The RTAs can be complicated by nephrocalcinosis and obstructive nephrolithiasis. Alkalinization is the cornerstone of treatment.