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Monday, 30 December 2019

DIAGNOSIS OF CYSTINURIA BY DIRECT VISION IN THE URINARY SEDIMENT


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Written by Alexia Rubio Peral | Ana Velasco Romero | M. Francisca García Codesal

Figure 1. Images taken in ordinary light (oc x10, obj x40) in which cystine crystals are observed at different degrees of maclation, more intense in the below image.

Cystinuria is a hyperaminoaciduria due to a defective transport of cystine and dibasic aminoacids (ornithine, arginine and lysine) in the apical membrane of the intestinal epithelium and the renal proximal tubule, which leads to an excess removal of cystine through urine. It is transmitted with an autosomal recessive inheritance pattern, with an average prevalence in neonates of 1/7000.
The only known clinical manifestation of Cystinuria is renal cystine lithiasis, which constitutes one of the most infrequent types of renal lithiasis, with a prevalence between 1 and 3% of the total lithiasis, reaching 5% of the stones in children.

Cystine calculus is formed due to its excessive concentration in urine and its high insolubility at acid pH. In addition to obstruction, cystine stones result in significant urinary infections with pyuria, hematuria, nephritic colic and flank pain. In rare cases it can progress to renal failure.

The simplest diagnostic procedure is the direct visualization in the urinary sediment of the flat and transparent hexagonal crystals typical of cystine, although only 19-26% of homozygous patients are detected in the urine. They are more easily observable in the urine of the first urine in the morning, since this is more acid and more concentrated.

The case presented is a cystinuria discovered in an ordinary urine test, in a 35-year-old patient without history of renal lithiasis, who has given birth 6 months ago, so her diagnosis is important for newborn follow-up. The lack of lithiasic symptomatology is surprising given the characteristics of the crystals found.

The pH 5 urine shows hexagonal prism crystals with six equal faces (Figure 1). The six axes and planes of symmetry provide cystine a high capacity of maclation, whose presence constitutes an excellent tool that can be used in the prediction of lithiasic risk.

There are 3 types of maclation which can be seen in figure 2:

  1. Stepping maclation: consists of the telescopic growth on the vertical axis of symmetry of new crystals. Usually one or more sides are erased, which gives it a distinctive and unmistakable appearance (figure 2A)
  2. Lateral maclation: consists of the formation of new crystals from any horizontal axis. The figure obtained is a surface growth, in the style of a “honeycomb” in which hexagons share one or more faces (figure 2B)
  3. Mixed maclation: consists in the growth of new crystals from the vertical and horizontal axes. They are difficult to find because they form large masses and are easily fractured during centrifugation. The geometric figure is complex and results from the combination of the previous types of maclation (Figure 2C). Its presence indicates a high lithogenic risk.
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Figure 2. Images taken in ordinary light (oc x10, obj x40) enlarged for detail. We observe the different types of maclation of cystine crystals. (A)Intense stepping maclation of innumerable faces. (B) Complex lateral maclation. (C) Large mixed maclation in which lateral and vertical expansion is observed (areas that are out of focus).

In daily clinical practice we can find phenotypic aspects in cystinuric patients who do not follow an expected pattern of behavior, as in the case presented, not all homozygotes develop clinical lithiasis , the debut may occur from infancy to senile age, or not present. It may even happen that among homozygous siblings some develop renal lithiasis and others do not, the environmental factors, diet and similar metabolism.

In this case the diagnosis is important, both for the clinical follow-up of the patient, whose crystals found have a very high lithogenic risk, and for the implementation of prevention measures for the newborn, among which are high fluid intake , urine alkalization and use chelating agents. It is also necessary a regular check of the cystine levels in urine to adjust the doses of the treatment and evaluate the response to it.
References
  1. C.M. Cabrera-Morales. Cystinuria: diagnosis and therapeutic approach. An Sist.Sanit.Navar. 2011:34(3):453-461.
  2. F. Dalet Escribá. Atlas del sedimento urinario. Roche Diagnostics S.L. 2010: 291-300.
  3. F.Rousaud, M.Palacín ,V. Nunes. Cistinuria. Nefrología. Vol XXIII Suplemento I: 52-59. 2003.

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