Simple classification of renal calculi closely related to their micromorphology and etiology

Abstract

Background: Classification of renal calculi with clear correlation with the main urinary etiological conditions has been previously established. However, such information is complex and difficult to adapt to clinical routine practice. Methods: A simple classification of renal calculi based on their structure and composition is proposed and applied to 2500 renal calculi to achieve the percentage of each category. The urines of 700 individuals chosen randomly have been analyzed and the results compared with those obtained with 51 healthy subjects. Results: 12.9% calculi corresponded to calcium oxalate monohydrate papillary calculi, 16.4% to calcium oxalate monohydrate unattached calculi, 33.8% to calcium oxalate dihydrate calculi, 11.2% calcium oxalate dihydrate/hydroxyapatite mixed calculi, 7.1% hydroxyapatite calculi, 4.1% struvite calculi, 0.6% brushite calculi, 8.2% uric acid calculi, 2.6% calcium oxalate/uric acid mixed calculi, 1.1% cystine calculi and 1.9% various infrequent calculi. Based on the corresponding urinary analytical studies, each kind of calculus is related with the more frequently associated urinary alterations. Conclusions: An important aspect of this classification is the possibility to establish, by means of the correct study of the calculus, some of the main possible etiologic factors closely related to its formation.

Introduction

Initial studies on urinary calculi were developed before 1800 by Scheele (1742–1786) and Bergman (1734–1794), who firstly identified uric acid calculi. The chemical characterization of urinary calculi was initiated by J.F. Heller in the nineteenth century, who in 1860 proposed a scheme for chemical investigation of urinary calculi based on the colour, hardness and chemical reactions performed directly on the dry material. In fact, these studies on renal calculi can be considered the beginning of modern Clinical Chemistry [1]. In spite of this early commencement that suggested that this pathology would be one of the first to be solved, it was not and still now there are diverse aspects that need to be clarified.

The analysis of renal calculi implying wet chemistry qualitative reactions in order to identify the different anions and cations present in the calculus was the unique methodology used during the first four decades of the twentieth century and in fact, this process is still often applied in routine clinical laboratories using specifically designed kits [2]. Unfortunately, the inadequacies of elemental chemical methods of calculi analysis were not recognized until the beginning of 1950, when it was demonstrated that the structure and internal arrangement of calculi, crucial in determining the mechanism of formation of the different kinds of stones, were impossible to identify using chemical methods and it was necessary to use compositional physical techniques, like X-ray diffraction. Thus, in 1947 appeared Prien's papers [3], [4] in which it was clearly pointed out that the lack of basic and fundamental knowledge concerning the exact composition of renal calculi was largely due to inadequate methods of analysis. These papers constituted the first attempt to set up a classification of renal calculi useful for clinical purposes and also the first effort to find the relationships between pathogenesis, structure and composition of calculi, yet no connections with urinary parameters were established. The papers published in the following two decades [5], [6], [7], [8] must be considered in the same line. Thereafter, new detailed studies on the composition and micromorphology of renal stones that incorporate scanning electron microscopy as a fundamental tool appeared [9], [10]. Finally, in 1993, Daudon et al. [11] established the first classification of renal calculi with a clear correlation with the main urinary etiologic conditions. However, this information is complex and probably is difficult to adapt to clinical routine practice, in spite of its interest for scientific purposes. Consequently, it is necessary to establish a classification of renal calculi, in accordance to its composition and fine structure, clearly correlated with specific pathophysiologycal conditions as the main urinary alterations, adapted to the common clinical practice, thus providing important information about calculus treatment. The objective of this paper is to contribute to establish such a classification.