Among all established calcium stone risk factors, we could identify only hypercalciuria as discriminating between normal and stone forming children. In particular, otherwise critical differences in urine chemistry found between adult normals and stone formers, such as hyperoxaluria, hypocitraturia, abnormal urine pH, or low urine volume, were not found. Because our groups were selected by a range of physicians in many practices, for having stones or relatives with stones or for being a normal child with no family history of stones, the isolated differences of urine calcium cannot reasonably be ascribed to anything but stones themselves. Moreover, since stone forming cannot change urine calcium, we can presume the hypercalciuria was selected for because it caused the stones. From this reasoning, we suggest that among children the principal risk factor for calcium stone formation is hypercalciuria.
Because hypercalciuria is inherited [3
], we are not surprised to find it far more prevalent among siblings of stone formers than among unselected normal children. However, we would not have a priori predicted that the magnitude of hypercalciuria might be greater in those with stones than among their siblings. The fact that hypercalciuria of siblings is significantly less marked than that of stone formers strongly supports the hypothesis that hypercalciuria itself is a principal cause of stones.
The accepted link between urine calcium and stones is degree of supersaturation, because SS is a gauge of the thermodynamic driving force for crystal nucleation and growth [6
]. In fact, both CaOx SS and CaP SS of stone formers exceeded those of siblings and normals in a pattern that paralleled differences in urine calcium. This parallel is expected since the other main determinants of SS, urine volume, oxalate and citrate excretion and pH, did not differ between the three groups. Taken with differences in urine calcium, the SS measurements support the hypothesis that hypercalciuria in children is a principal stone risk factor acting via increase of both CaOx SS and CaP SS. The vast majority of pediatric stones are calcium [18
], and recruiting practices for this study excluded those with detectable systemic disease or stones known not to be calcium. From this we would presume a majority had CaOx stones, which would be promoted by a high CaOx SS. We have shown that an initial CaP nucleation may be a crucial step in the initiation of CaOx stones [19
], so the increased CaP SS in stone forming children may play as crucial a role as the increased CaOx SS.
The upper limit of metastability (ULM) is an empirical estimate of the SS needed to initiate crystallization [20
]. The smaller the difference between the ULM and SS, the closer is the ambient SS to that needed for crystallization [21
] and therefore the greater the risk that crystallization and stones will occur. We have observed among adult men and women that the CaOx ULM rises proportionally to CaOx SS so that the ULM – SS distance remains reasonably constant comparing stone formers to normals, but the CaP ULM – SS distance is smaller in both male and female stone formers than corresponding normals because CaP ULM does not rise in proportion to CaP SS [24
]. This, too, is consistent with the idea that CaP nucleation plays a role in calcium stone genesis even when stones are predominantly CaOx.
Our children with stones manifest the same pattern as we observed in adults: among stone formers the CaP ULM – SS distance is less than that of siblings or normals. Remarkably, among the siblings CaP SS is higher than normal because of hypercalciuria, but CaP ULM is also higher so that the ULM – SS distance is normal whereas among stone formers, the CaP ULM remains identical to that of siblings despite a marked rise in CaP SS. In other words, siblings are hypercalciuric and consequently have a higher CaP SS than normal, but can increase CaP ULM so that the critical ULM – SS distance remains normal. Adult stone formers and children with stones do not seem able to maintain this distance, for reasons that are not within the scope of this research. Until now, ULM – SS distances have not been documented for hypercalciuric non stone forming cohorts such as our siblings, so this is a unique and novel observation that clearly points toward a need for additional research.
The loss of CaP ULM – SS distance was most striking in children in >2 stones. This is most compatible with the supposition that the CaP ULM – SS distance is a crucial determinant of whether stones will form. In fact, the CaP ULM of children with >2 stones was below that of the other groups, accentuating the loss of distance as CaP SS rose. We have no reason to presume that forming more stones will reduce CaP ULM; at present, no mechanisms exist to create such a link. For this reason, we propose that the CaP ULM – SS difference is indeed a graded risk factor, at least among children, and marks not only whether stones will form or not but possibly whether stone recurrence is more likely. This supposition deserves new research that goes beyond the scope of the present work.
The existence of a ULM – SS distance in a 24-hour urine demonstrates that factors in the urine are retarding crystallization; simple solutions supersaturated with CaOx and CaP, like our urines, would not remain stable for a period of days. Many crystallization inhibitors are known to be in urine, including small molecules, proteins, proteoglycans and glycosaminoglycans [5
]. Their aggregate activity has been assayed by ourselves and others using seeded and unseeded crystal assays [29
Our one assay of nucleated CaOx crystal growth reveals a gradual loss of urine inhibition with age. One would assume that such inhibition would slow and even stop CaOx initial crystallization, and that loss of inhibition with age might permit stone formation to increase. It is certainly true that stone rates are much higher among adults than unselected children [36
], and because IH is inherited the trait is present from birth; therefore our findings suggest a possible reason why a fixed risk factor – hypercalciuria – might manifest its consequence – stone formation – increasingly with age. However, we were unable to detect any differences in growth retardation between our three groups, so at least this one assay fails to provide any insight as to why stones formed in these children compared to their siblings and normal children. Likewise, our one assay offers no clue as to the low ULM – SS distance in the stone formers.
Adults display a wider range of urine risk factors compared to children, which includes reduced urine citrate, increased urine oxalate, and increased uric acid excretions [5
]. Possibly these all reflect diet variations that conspire to increase stone risk [2
]. That children with stones should differ from normal children only in urine calcium and adults with stones should differ in many more traits suggests that some real differences exist between the adult and childhood populations. One may be urine volume, which averages about 1.1 liters daily in children () and about 1.4 – 1.5 liters daily in adults [39
]. Isolated hypercalciuria may be more effective in stone genesis given low volumes, whereas in adults more abnormalities are needed. This is a speculation that can be considered in other clinical researches.
In the course of our work, we noted lower urine potassium excretion in stone formers and their siblings than in normals; moreover, stone forming boys, but not girls, had significantly lower urine ammonia and urea nitrogen excretions than their normal counterparts. This pattern is compatible with a higher intake of fruits and vegetables in normal children and lower protein intake in the boys of stone forming families compared to normal boys. Lower protein would imply a lower acid load and therefore a lesser demand for ammonia excretion. This was noted incidentally and is not part of our primary research intent. However, because acid load from protein increases urine calcium [2
], the lower protein intake of the hypercalciuric stone formers and their siblings would have fostered a lesser increase of urine calcium. Urine sodium was not different in SF compared to the other groups, which suggests hypercalciuria in children is not as diet dependent as in adults since neither sodium nor protein intake was higher in SF. Though diet calcium intake cannot be estimated from the urine calcium excretion, much of diet calcium would come from dairy products which also provide protein so it seems unlikely that calcium intake was higher in SF. We might speculate that the diets of stone formers contained more carbohydrate and fat in place of protein, but because we did not make specific measurements we do not propose that our data can substantiate this speculation. However, we did find that BMI, which has been shown to correlate with measures of body fat in children and adolescents, was higher in SF boys than in their normal or sibling counterparts [41
]. This supports the idea of dietary differences contributing to stone risk in boys, as high BMI has been shown to be associated with increased CaP SS in children with nephrolithiasis [42
Our results are in agreement with those of previous studies that have shown hypercalciuria to be the primary risk factor for calcium stone formation in children [10
]. Like DeFoor et al., we have also shown calcium excretion to be significantly higher in subjects with more stones [12
]. Other investigators have identified hypocitraturia as a common risk factor along with hypercalciuria [11
], and still others have reported hypocitraturia alone to be the most important risk factor [43
]. We have found no evidence for hypocitraturia among the stone formers we studied here. This disparity may perhaps be attributed to geographic differences in study populations, as the studies reporting the primacy of hypercalciuria were conducted in the United States while those reporting hypocitraturia were carried out elsewhere (Turkey, Argentina, and Scotland).
DeFoor et al. found supersaturation levels of CaOx, but not CaP, to be significantly higher in stone forming children compared to normal controls [13
]. Our results are similar for CaOx, but, in addition, we found higher CaP SS in SF compared to the normal or sibling groups. It is possible that the larger, age-matched population in the present study allowed the difference in CaP SS to be detected. In fact, our results point to CaP SS as a key determinant of stone risk as evidenced by the dramatic loss of resistance to CaP crystallization (reduced ULM – SS distance) in children with >2 stones.
A crucial limitation of our study is rigorous ascertainment of controls. Some siblings of stone formers without manifest stones may indeed have formed stones which were silent. Visualization studies were impossible in our study design. Likewise, we do not have stone analyses for all of the stone forming patients although we can be very certain from their urological history and treatment that they did indeed form kidney stones. Uric acid and cystine stones have evident clinical characteristics, and the latter is often familial, so it is likely that most if not all of our patients formed calcium stones: i.e. clinicians excluded the two uncommon stone types effectively.
Overall, children with calcium stones differ from normal children matched by age and sex in having a higher level of urine calcium excretion with resulting high SS for CaOx and CaP, and in having a reduced ULM – SS distance for CaP, which is unexplained at the present time. The remaining established risk factors found in adults are not apparently playing a role in children. Because IH is a common trait, by definition representing 5% of humans, and stones are much less prevalent, especially in children, other abnormalities are presumably present [45
]. Management of these children should follow that used for adults. For the siblings, maintaining a high urine volume and low sodium intake, both measures known to reduce stones [46
], seems practical and prudent.