Stone composition and morphology: A window on etiology

Abstract

Physical methods, namely, X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) reliably identify speci fi c forms of nephrolithiasis involving a single component such as cystine, 2,8-dihydroxyadenine, xanthine, uric acid, struvite, and drugs as well as common-type stones made of calcium oxalate (CaOx) and/or calcium phosphate. However, for the latter, these methods do not provide etiologic information in clinical practice because a same-stone composition may be the result of very different lithogenic processes A comprehensive stone analysis method combining morphological examination followed by XRD or FTIR analysis of the core, middle layers, and surface of calculi provides a more complete contribution to etiologic diagnosis than compositional analysis alone. Using this method, stones may be classi fi ed into 7 types subdivided in 2 subtypes. Among CaOx stones, type Ic COM calculi are pathognomonic of primary hyperoxaluria. Among calcium phosphate stones, a peculiar morphology of carbapatite stones (type IVa2) is closely associated with distal tubular acidosis, whereas in primary hyperparathyroidism calculi are predominantly made of carbapatite mixed with eddellite or of brushite (type IVd). Ammonium urate calculi of type IIId are found in patients with low phosphate intake and chronic diarrhea due to laxative abuse or in children with endemic urolithiasis. Uric acid calculi are mainly suggestive of low urine pH related to insulin resistance as observed in metabolic syndrome or type 2 diabetes or in case of colon resection. Among common, idiopathic CaOx stones, predominance of whew lite (type I morphology) is mainly associated with high urinary oxalate concentration, whereas predominance of weddellite (type II morphology) is associated with hypercalciuric states. This method is of decisive interest for early diagnosis�and therefore proper treatment�of severe diseases such as primary hyperoxaluria and 2,8-dihydroxyadeninuria.