Evidence for shrunken pore syndrome in children

Creatinine-based formulas are not ideal to estimate glomerular filtration rate in selected pediatric patients: data from a tertiary pediatric nephrology center

BACKGROUND

Evaluating glomerular filtration rate (GFR) remains challenging in pediatrics; new formulas were developed to increase performance of GFR estimation (eGFR). We aimed to evaluate the recently published formulas as applied to another pediatric population.

METHODS

A retrospective study was conducted in a cohort of 307 patients with a "kidney risk" (mean age 12.1 ± 4.5 years, sex ratio 1/1) assessed in a tertiary pediatric nephrology center and a mean measured GFR (mGFR) using plasma iohexol clearance of 85.5 ± 25.3 mL/min/1.73 m2; creatinine levels were measured by IDMS-standardized enzymatic method and cystatin C by immunonephelometry. The following eGFRs were calculated: Schwartz2009, Schwartz-Lyon, CKiDU25creat, and EKFC for eGFR using creatinine (eGFR-creat), CKiDU25cys and FAScys for eGFR using cystatin (eGFR-cys) as well as combined SchwartzCreat-Cys, average (CKiDU25creat-CKiDU25cys), and average (EKFC-FAScys) for eGFR using both biomarkers. The performance of the different formulas was evaluated compared to mGFR by absolute bias measurement and accuracy (p10%, p30%). Results are expressed as mean ± SD.

RESULTS

Creatinine-based formulas and especially the new CKiDU25 and EKFC overestimate GFR, even in children with normal kidney function. However, the bias is constant with these two formulas whatever the age group or gender, contrary to the previously published formulas. In contrast, cystatin C-based equations and combined formulas showed good performance in all age groups and all medical conditions with an acceptable bias and p30%.

CONCLUSIONS

In our pediatric population, the performance of all creatinine-based formulas is inadequate with significant GFR overestimation, mainly in subjects with mGFR > 75 mL/min/1.73 m2. Conversely, cystatin C-based or combined formulas have acceptable performance in patients followed in a tertiary pediatric nephrology unit.

Is shrunken pore syndrome also a reality in children?

BACKGROUND

Shrunken pore syndrome (SPS) is defined as cystatin C-based-eGFR (eGFRcys)/creatinine-based-eGFR (eGFRcreat) <0.6 or 0.7 and is associated with an increased cardiovascular risk. SPS has been described in children, but no link to increased morbi-mortality was demonstrated.

OBJECTIVES

Study the prevalence of SPS in a pediatric population using several glomerular filtration rate (GFR) estimating formulas and measured GFR and evaluate the potential link with cardiovascular risk.

METHODS

In 307 renal risk pediatric patients, we studied prevalence of SPS either with CKiDU25creat and cyst or with FAScreat and cyst and EKFCcreat. The characteristics of patients with SPS (defined with Full-age spectrum equation (FAS) and/or European Kidney Function Consortium equation (EKFC)) were compared.

RESULTS AND CONCLUSION

The prevalence of SPS varies widely depending on the threshold and the formulas used. Higher C-reactive protein (CRP) and phosphate levels and smaller size are observed in children with SPS defined with FAS and/or EKFC and might be associated with long-term increased cardiovascular risk. Further studies in wider general pediatric populations are warranted.

The complexity of kidney disease and diagnosing it - cystatin C, selective glomerular hypofiltration syndromes and proteome regulation

Estimation of kidney function is often part of daily clinical practice, mostly done by using the endogenous glomerular filtration rate (GFR)-markers creatinine or cystatin C. A recommendation to use both markers in parallel in 2010 has resulted in new knowledge concerning the pathophysiology of kidney disorders by the identification of a new set of kidney disorders, selective glomerular hypofiltration syndromes. These syndromes, connected to strong increases in mortality and morbidity, are characterized by a selective reduction in the glomerular filtration of 5-30 kDa molecules, such as cystatin C, compared to the filtration of small molecules <1 kDa dominating the glomerular filtrate, for example water, urea and creatinine. At least two types of such disorders, shrunken or elongated pore syndrome, are possible according to the pore model for glomerular filtration. Selective glomerular hypofiltration syndromes are prevalent in investigated populations, and patients with these syndromes often display normal measured GFR or creatinine-based GFR-estimates. The syndromes are characterized by proteomic changes promoting the development of atherosclerosis, indicating antibodies and specific receptor-blocking substances as possible new treatment modalities. Presently, the KDIGO guidelines for diagnosing kidney disorders do not recommend cystatin C as a general marker of kidney function and will therefore not allow the identification of a considerable number of patients with selective glomerular hypofiltration syndromes. Furthermore, as cystatin C is uninfluenced by muscle mass, diet or variations in tubular secretion and cystatin C-based GFR-estimation equations do not require controversial race or sex terms, it is obvious that cystatin C should be a part of future KDIGO guidelines.

Shrunken pore syndrome in childhood cancer survivors treated with potentially nephrotoxic therapy

Childhood cancer survivors (CCS) are at risk of kidney dysfunction. Recently, the shrunken pore syndrome (SPS) has been described, which is characterized by selectively impaired filtration of larger molecules like cystatin C, while filtration of smaller molecules like creatinine is unaltered. It has been associated with increased mortality, even in the presence of a normal estimated glomerular filtration rate (eGFR). The aim of this study was to evaluate the prevalence of SPS in CCS exposed to potentially nephrotoxic therapy. In the Dutch Childhood Cancer Survivor Study (DCCSS)-LATER 2 Renal study, a nationwide cross-sectional cohort study, 1024 CCS ≥5 years after diagnosis, aged ≥18 years at study, treated between 1963-2001 with nephrectomy, abdominal radiotherapy, total body irradiation, cisplatin, carboplatin, ifosfamide, high-dose cyclophosphamide or hematopoietic stem cell transplantation participated, and 500 age- and sex-matched controls form Lifelines. SPS was defined as an eGFR_cys/eGFR_cr ratio <0.6 in the absence of non-GFR determinants of cystatin C and creatinine metabolism (i.e. hyperthyroidism, corticosteroids, underweight). Three pairs of eGFR-equations were used; CKD-EPI_cys/CKD-EPI_cr, CAPA/LMR, and FAS_cys/FAS_age. Median age was 32 years. Although an eGFR_cys/eGFR_cr ratio <0.6 was more common in CCS (1.0%) than controls (0%) based on the CKD-EPI equations, most cases were explained by non-GFR determinants. The prevalence of SPS in CCS was 0.3% (CKD-EPI equations), 0.2% (CAPA/LMR) and 0.1% (FAS equations), and not increased compared to controls. CCS treated with nephrotoxic therapy are not at increased risk for SPS compared to controls. Yet, non-GFR determinants are more common and should be taken into account when estimating GFR.

Reduced renal elimination of larger molecules is a strong predictor for mortality

Renal dysfunction is a major risk factor for premature death and has been studied extensively. A new renal syndrome, shrunken pore syndrome (SPS), confers higher mortality in all studied populations. SPS is a condition in which cystatin C-based estimation of glomerular filtration rate (eGFR_{cystatin C}) is ≥ 60% than creatinine-based estimation of glomerular filtration rate (eGFR_creatinine). We aimed to study the impact of SPS on mortality in a cohort of patients with follow up of up to 10 years. This was a retrospective single centre cohort study. We enrolled 3993 consecutive patients undergoing elective cardiac surgery. Outcome was evaluated using Kaplan Meier analysis and multivariable Cox regression. 1-, 5- and 10-year survival for patients with SPS was 90%, 59% and 45%, and without SPS 98%, 88% and 80% (p < 0.001). SPS was found to be an independent predictor for mortality with an HR of 1.96 (95% CI 1.63-2.36). SPS negatively affected survival regardless of pre-operative renal function. SPS is an independent predictor for mortality after elective cardiac surgery, equal to or greater than risk factors such as diabetes, impaired left ventricular function or renal dysfunction. SPS affected mortality even in patients with normal eGFR.Clinical registration number: ClinicalTrials.gov, ID NCT04141072.

Impaired selective renal filtration captured by eGFRcysC/eGFRcrea ratio is associated with mortality in a population based cohort of older women

Deranged renal filtration of mid-sized (5-30 kDa) compared to smaller molecules (< 0.9 kDa) results in increased plasma levels of cystatin C (cysC) compared to creatinine resulting in a low eGFRcysC/eGFRcrea ratio. A ratio below 0.6 or 0.7, is termed shrunken pore syndrome (SPS), which in patient based studies is associated with mortality. Reference values for eGFRcysC/eGFRcrea ratio, the prevalence of SPS and the consequence of low eGFRcysC/eGFRcrea ratio in the general, elderly population are unknown. 75-yr old women (n = 849) from the population-based OPRA cohort, followed for 10-years had eGFR calculated with CKD-EPI study equation, and eGFRcysC/eGFRcrea ratio calculated. Mortality risk (HR [95% CI]) was estimated. Women with sarcopenia or on glucocorticoids were excluded. Almost 1 in 10 women (9%) had eGFRcysC/eGFRcrea ratio < 0.6 at age 75 and this did not increase appreciably with age. Women with ratio < 0.6 had higher 10-yr mortality risk compared with ratios > 0.9 (HRadj 1.6 [95% CI 1.1-2.5]). In elderly women eGFRcysC/eGFRcrea ratio < 0.6 is common and associated with increased mortality. Our results confirm patient-based findings, suggesting that identifying individuals with SPS may be clinically relevant to assessing mortality risk in the elderly.

Glomerular filtration and shrunken pore syndrome in children and adults

A major function of the kidney is to, by glomerular filtration, maintain the overall steady-state of 5-30 kDa proteins, many of which are signalling molecules. This function of the kidney has been overlooked, since predominantly low-molecular-mass substances <1 kDa have been used to measure or estimate glomerular filtration rate (GFR). The use of cystatin C (13 kDa) as a marker of GFR has allowed the discovery that the filtration of 5-30 kDa molecules can be selectively impaired defining the shrunken pore syndrome. The discovery, pathophysiology, morbidity (mainly cardiovascular manifestations) and mortality of this syndrome are described.

Combined use of creatinine and cystatin C improves the detection of renal dysfunction in children undergoing home parenteral nutrition

BACKGROUND

Renal dysfunction can complicate home parenteral nutrition (HPN). The aims were, in the context of pediatric HPN, to assess renal function using the measured glomerular filtration rate (mGFR), determine the most accurate formula(s) to estimate GFR, and identify possible underlying mechanisms of renal impairment.

METHODS

A retrospective study was performed in 2 centers. Patients receiving HPN and aged 2-16 years without medical history of nephropathy were included. GFR was measured using iohexol clearance. Estimated GFR (eGFR) was calculated using creatinine, cystatin C-based, and combined (eGFR_cr+cyst ) Schwartz formulas.

RESULTS

A total of 36 patients (18 females) were included; they received HPN for 8 (2-16) years. The primary digestive disease was short-bowel syndrome for 16 (44%) patients, gastrointestinal motility disorder for 10 (28%), or congenital diarrhea for 10 (28%). The median (range) mGFR was 99 (33-136) ml/min/1.73 m² ; 9 (25%) patients had mildly decreased mGFR (<90 and ≥60 ml/min/1.73 m² ), and 2 (6%) had mildly to severely decreased mGFR (<60 ml/min/1.73 m² ). The eGFR_cr+cyst formula was the most accurate and precise to estimate GFR. A significant negative correlation between mGFR and PN duration was found for patients receiving PN for 6-7/7 days (P = .008). Activation of the renin-angiotensin system was identified in 15 of 36 (42%) patients.

CONCLUSION

Renal dysfunction was frequent and correlated with the duration of PN only for patients with the most severe intestinal failure. The use of eGFR_cr+cyst improves its detection in these patients. Chronic dehydration may be an underlying mechanism.

Shrunken pore syndrome - a common kidney disorder with high mortality. Diagnosis, prevalence, pathophysiology and treatment options

Invasive studies show that the glomerular sieving coefficients for 5-30 kDa plasma proteins in the human kidney may be selectively reduced compared to those for small molecules < 0.9 kDa, commonly used to measure glomerular filtration rate (GFR). Identification of this pathophysiological state, called shrunken pore syndrome (SPS), can easily and non-invasively be done by comparing estimations of GFR using cystatin C (13.3 kDa) and creatinine (0.113 kDa). SPS is present if the estimate of GFR using cystatin C is lower than 60 or 70% of the estimate using creatinine in the absence of non-renal influences on cystatin C or creatinine. All studies of SPS show that the 3- or 5-year mortality is strongly increased and high hazard ratios for mortality associated with SPS have been observed for many different patient cohorts, including cohorts with normal measured GFR, no albuminuria and no diagnosis. The prevalence of SPS in the cohorts so far investigated is between 0.2 and 36%. Proteome studies of SPS demonstrate that the high mortality associated with the syndrome might be caused by the accumulation of 10-30 kDa signalling proteins promoting development of atherosclerosis and thus suggesting use of monoclonal antibodies to reduce the levels of the most detrimental signalling proteins as a treatment option. The KDIGO recommendations for classification of chronic kidney disease (CKD) comprise determination, or estimation, of GFR and analysis of albuminuria and therefore cannot identify a large fraction of the patients with SPS. The high prevalence and mortality of SPS and the possible treatment options strongly suggest that the KDIGO recommendations should be expanded to include determination of cystatin C to be able to identify all patients with SPS.