Send letter to journal:
Re: Re: Urinary tract calculi in children

Dear Editor,

We read with great interest the article by S-A Hulton on urinary tract calculi in children published in the April 2001 issue of your journal [1]. There are several issues we would like to address.

First, it needs to be clarified that although adenine phosphoribosyltransferase (APRT) deficiency is a disorder of purine metabolism, it is not a disorder of uric acid metabolism as incorrectly indicated in Table 1 and Figure 1. Affected patients excrete 2,8- dihydroxyadenine (2,8-DHA) in their urine which in untreated individuals frequently results in the formation of radiolucent kidney stones or even renal failure due to tubulointerstitial fibrosis and/or intratubular deposition of 2,8-DHA crystals [2]. Biochemical analysis of urinary tract stones does not distinguish 2,8-DHA from uric acid and frequently results in the erroneous diagnosis of uric acid nephrolithiasis. The differentiation of these two disorders is very important as allopurinol therapy is indicated in all patients with APRT deficiency, but is only recommended in those individuals with uric acid stones who are found to have increased uric acid excretion. In our experience skillful urine microscopy, revealing the typical round reddish-brown 2,8-DHA crystals, is the single most important procedure for the diagnosis of APRT deficiency [2,3].

Secondly, we do not agree with Dr. Hulton's recommendations regarding the choice of imaging studies in children suspected of urinary tract stones. We believe that in all cases an attempt should be made to classify stones as radio-opaque or radiolucent as this greatly influences the differential diagnosis [2]. We, therefore, recommend that a renal ultrasound examination should always be complemented with at least a plain abdominal film or an intravenous pyelography.

Finally, we believe that the classification of hypercalciuria into absorptive and renal subtypes is not indicated in clinical practice due to evidence suggesting that these two disorders constitute a variable expression of the same phenomenon and impose the same risk to affected indviduals [4]. Moreover, evidence from the literature strongly suggests that even a modest decrease in dietary calcium intake may increase the risk of nephrolithiasis [5]. A recent study in children with hypercalciuria demonstrated a significantly reduced bone mineral content in 30% of the patients and a population-based cohort study in adults with nephrolithiasis revealed an increased vertebral fracture risk amongst the affected individuals [6,7]. In view of the above evidence we believe that even a modest restriction of calcium intake in children with hypercalciuria and/or nephrolithiasis should be avoided. The proper therapeutic regimen for children with calcium nephrolithiasis and hypercalciuria should, therefore, always include ample fluid intake and a moderate sodium restricion. In patients with symptomatic hypercalciuria and/or recurrent stone formation, thiazide diuretics should be added to the therapeutic regimen as they have been shown to decrease urinary calcium excretion and improve bone mineral content.

Vidar Edvardsson, MD
Department of Pediatrics
Section of Pediatric Nephrology
Landspitali - University Hospital
Reykjavik, Iceland

Runolfur Palsson, MD
Renal Unit and Department of Medicine
Landspitali - University Hospital
Reykjavik, Iceland

Thröstur Laxdal, MD
Department of Pediatrics
Landspitali - University Hospital
Reykjavik, Iceland

References

(1) Hulton SA. Evaluation of urinary tract calculi in children. Arch Dis Child 2001 Apr; 84(4):320-3.
(2) Edvardsson V, Palsson R, Olafsson I, Hjaltadottir G, Laxdal T. Clinical Features and Genotype of Adenine Phosphoribosyltransferase Deficiency in Iceland. Am J Kidney Dis (in press)
(3) Laxdal T: 2,8-dihydroxyadenine crystalluria vs urolithiasis. Lancet 340:184, 1992
(4) Aladjem M, Barr J, Lahat E, Bistritzer T. Renal and absorptive hypercalciuria: a metabolic disturbance with varying and interchanging modes of expression. Pediatrics 1996 Feb; 97(2):216-19
(5) Curhan GC, Willett WC, Rimm EB, Stampfer MJ. A prospective study of dietary calcium and other nutrients and the risk of symptomatic kidney stones. N Engl J Med 1993 Mar 25;328(12):833-8
(6) Garcia-Nieto V, Ferrandez C, Monge M, de Sequera M, Rodrigo MD. Bone mineral density in pediatric patients with idiopathic hypercalciuria. Pediatr Nephrol 1997;11(5):578-83
(7) Melton LJ 3rd, Crowson CS, Khosla S, Wilson DM, O'Fallon WM. Fracture risk among patients with urolithiasis: a population-based cohort study. Kidney Int 1998 Feb;53(2):459-64

Send letter to journal:
Re: Re: Re: Urinary tract calculi in children

Dear Editor,

This article was written expressly with the general paediatrician in mind and attempts to provide a succinct approach to the problem. It in no way presumes to be a definitive treatise on the metabolic investigation of renal stones in children. For this reason the term “uric acid stones” was used to help simplify the figure and table. Edvardsson et al are absolutely correct in pointing out that APRT deficiency is a disorder of purine metabolism and that the scientific classification should be entitled as such, with separate subdivisions of “Uric Acid Stones”, “Dihydroxyadenine Stones” and “Xanthine Stones”.

Similarly, my comments on idiopathic hypercalciuria attempt to summarise extensive literature and controversy on this topic. Many investigators have abandoned characterising hypercalciuria as absorptive or renal and my choice of wording the article was careful to highlight this, i.e. “the distinction between these two forms MAY be considered relevant”.

Finally, I have no wish to adopt a dogmatic approach to the evaluation of children with possible renal stones and would encourage all physicians reading this article to use their clinical acumen when investigating their patients. For this reason I do not believe that plain abdominal radiography needs to be performed in every single patient and I would certainly disagree with Edvardsson et al that intravenous pyelography, with its attendant risks, should be done on a regular basis as part of the investigation.