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Medullary sponge kidney (MSK), was first described in 1948. The prevalence of the disorder is unknown but is said to occur in 0.5% of all unselected excretory urograms and is often clinically disregarded. However, we and others have found MSK to be a frequent association in patients with recurrent calcium calculi (RCRC) particularly in females (F). Although 3 large series of MSK and RCRC have been published1, 2, 3, more data are required, particularly with respect to stone pathogenesis4. This study was designed to address these issues.

Clinical and biochemical parameters were investigated in a group of patients with medullary sponge kidney (MSK) and compared with tho se of 2 groups of patients with idiopathic calcium nephrolithiasis, single and recurrent stone-formers (SSF and RSF). No sex differences, a more common family history of nephrolithiasis, no tendency to over weight and a higher urinary magnesium excretion were found to be the distinctive features of the patients with MSK with respect to SSF and RSF. Although the high magnesium excretion has been postulated to inhibit calcium oxalate crystallization, its protective effect in MSK is probably overwhelmed by other risk factors such as distal aci dification defects and prolonged transit time in the ectatic ducts.

Structural abnormalities of the urinary tract are usually thought to predispose to renal stone formation. Medullary sponge kidney is probably a congenital abnormality. Because of such structural abnormalities, a higher incidence of tubular dysfunction may be anticipated. However, a broad investigation of renal tubular function has not been performed in larger series. The renal acidification capacity has only been investigated in a few patients and found to be defective in some of these1. In some cases the diagnosis of renal tubular acidosis (RTA) might have been overlooked2,3. It seems important to diagnose the acidification defect since it seems to be associated with a more severe stone disease4.

A defect in distal tubular acidification, consistent with the anatomic site of the lesions, has been reported to occur in a substantial proportion of patients having medullary sponge kidney (MSK) with bilateral tubular ectasia (1). However, a lower incidence of this disorder has been found by others (2) or in patients with unilateral and segmental MSK (3). An increased pH of the tubular fluid within the ectatic lesions could lower the solubility of apatite. This is a frequent constituent of stones produced by MSK patients although calcium oxalate (CaOx) appears to be the most frequent and abundant component of calculi in such patients (4). Defective acidification could be associated with hypocitraturia that may favor CaOx crystallization. On the other hand, hypercalciuria, possibly another manifestation of impaired function of ectatic ducts, has been reported as a frequent lithogenic factor in patients with MSK (5, 6). However, urinary citrate and calcium excretion have not been evaluated, as yet, in connection with urinary acidification in patients with MSK complicated by calcium nephrolithiasis.

In 1972, Coe et al postulated the existence of a renal tubular leak of calcium to account for the hypercalciuria of some idiopathic stone formers1. More recently, it has been suggested that such patients have an overall defect in the proximal tubular reabsorption of fluid and electrolytes2. To shed more light on this controversial topic, we have examined the question as to whether or not the apparent renal leak of calcium is due to dietary factors, or to some general renal tubular dysfunction.

The project described in this presentation has been in progress for the past five years and has two main objectives. The first is to define the incidence of medullary sponge kidney in patients with calcium stones and the second is to attempt to determine the relationship between the milder degrees of the condition frequently encountered in our study and stone formation. During the course of our study, we have obtained all available intravenous urograms performed on our patients and reviewed them jointly with our radiologist colleague (S.J.) for evidence of tubular ectasia, a term which we prefer to “medullary sponge kidney” which we consider to be inappropriate for the milder degrees of the disorder. When the quality of the available x-rays was considered to be inadequate, infusion nephrotomography has, when feasible, been performed in an attempt to obtain better quality films.

Idiopathic calcareous calculi remains a disorder of unknown cause, or causes. However, this affliction occurs predominantly in nonblack men, is affected by undefined geographic or climatic factors, and may be influenced by dietary habits1. In addition, the familial occurrence of idiopathic calculi has suggested that hereditary factors may exist that increase susceptability to this disorder2. Searching for a possible index of renal tubular dysfunction, McGeown3 investigated the amino acid excretion of 110 patients with calcareous calculi. She reported that, except for 30 patients with some increase in urinary cystine, the amino acid excretion was reduced in the calculous patients as compared with the amino acid excretion of normal subjects. Of these 110 patients, twenty-three had hyperparathyroidism and the mean excretion of amino acid nitrogen was less in these than in the group as a whole. Subsequently, Hum and associates4 reported the urinary amino acid values of 15 patients with calcareous calculi, but they found a reduced excretion only in those patients with impaired renal function.