[Soft lithiasis ("mous")].
OBJECTIVES: To describe the most relevant features of matrix lithiasis, an infrequent type of lithiasis seldom reported since the generalization of extracorporeal shock wave lithotripsy. METHODS: We report a clinical case and analyze the most important bibliography found by MEDLINE search (1950-1999). We will focus on the physiopathology and etiology of these stones to understand its development and the clinical presentation showed by these patients. RESULTS: Matrix lithiasis is a very rare entity, described almost one century ago. It is mainly composed of lithiasis matrix, and the percentage of mineral content is really low in comparison with usual stones. Persistent urinary tract infection is another constant in these patients, which to some extent conditions the modification of the usual urinary components, creating a series of the disturbances within the urinary tract that favour the aggregation of the components of the lithiasis matrix up to the formation of a stone, without need of aggregation of mineralized crystals. CONCLUSIONS: A high degree of suspicion is necessary for the diagnosis of matrix lithiasis, because it does not show the usual clinical picture of a renal stone. Exhaustive lavage of caliceal cavities and elimination of bacteriuria are essential elements for treatment, in order to avoid recurrence of the disease.