Abstract
PurposeCystine stones are widely considered hard and difficult to treat. Hounsfield Units (HU) are used in other stone types to estimate ‘hardness’ and treatments based on that finding. Our objective was to report mean HU of cystine stones in vivo in a large case series of cystinuria patients and assess for differences in genotype.MethodsA prospective case series of cystinuria patients referred to a specialist centre was analysed. CT imaging was assessed by two independent radiologists to determine in vivo attenuation of cystine calculi. Mean HU was compared for both cystinuria genes (SLC3A1 and SLC7A9) using an independent t-test.Results164 adult cystinuric patients were identified (55% male), median age 43 years (range 18–80). Median follow up was 31 months (IQR 10–62). Genetic data available for 153/164 (93%) demonstrated 97 SLC3A1 (63%) and 55 (36%) SLC7A9 mutations (39 homozygous, 16 heterozygous) and one heterozygous for both SLC3A1/SLC7A9. 107 patients had CT images available demonstrating calculi. Median HU across the cohort was 633 (5th to 95th centile 328–780). There was no difference in mean HU between SLC3A1 and SLC7A9 genotypes (p = 0.68) or homo and heterozygous SLC7A9 (p = 0.70). Mean HU correlated with stone size (Pearson correlation coefficient = 0.51, p < 0.001).ConclusionIn this large single centre cystinuria cohort, mean HU was low for stones that are difficult to treat. Calculi of < 800 HU should prompt consideration of a cystinuria diagnosis. Attenuation was not associated with genotype, and distinct ‘smooth’ and ‘rough’ stones were not observed. Calculi with HU > 1000 are unlikely pure cystine, and in a known cystinuric would suggest conversion to another stone type.
Highlights
IntroductionIt is not known how different gene mutations affect the degree of protein dysfunction and how this translates to clinical phenotype
Cystinuria is a phenotypically heterogeneous condition in which defective renal tubular transport leads to supersaturation and crystallisation of the amino acid cystine in the Electronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.urinary collecting system
We have previously reported 57 mutations in one of two genes (SLC3A1 and SLC7A9) in a large UK cohort of cystinuria patients [2]
Summary
It is not known how different gene mutations affect the degree of protein dysfunction and how this translates to clinical phenotype. We have previously reported 57 mutations in one of two genes (SLC3A1 and SLC7A9) in a large UK cohort of cystinuria patients [2]. We have modelled the degree of protein dysfunction and its association with disease severity [3], and in sibling studies suggested that patient genotype alone does not determine phenotype [4]. In vitro and in vivo studies have assessed the utility of computed tomography attenuation in predicting stone composition [5, 6], the standard of care remains analysis with X-ray diffraction and infra-red spectroscopy following extraction or spontaneous passage.
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