In this prospective study, PTD was evaluated in children with COVID-19. Approximately one-third of the patients had higher serum creatinine levels or proteinuria. Additionally, abnormal renal tubular phosphorus loss measured by TMP/eGFR was present in 10 patients (8.7%) and hyperuricosuria was present in 28.6%. Consequently, overall PTD was present in one of the five patients, and PTD associated with COVID-19 was significantly detected in younger children.
Some studies reported the kidney as a specific target for SARS-CoV-2 infection(23–26). The postmortem examination of the kidney revealed the accumulation of SARS-CoV-2 antigens in the renal epithelial cells(23). Despite the published articles, limited studies are emphasizing the effect of COVID-19 on proximal tubule functions (7, 22, 27). Detection of viral particles of SARS-CoV-2 in the proximal tubular epithelium and podocytes associated with foot process effacement, vacuolation, and detachment of podocytes from the glomerular basement membrane showed the involvement of the proximal tubules by SARS-CoV-2(26). Another renal impairment was SARS-CoV-2-related acute kidney injury and generally reported from critically ill patients (28, 29). In our patient's group, we didn’t observe any acute kidney injury related to SARS-CoV-2. This is mainly due to the properties of the study group. In our study. we mainly focused on children who didn’t require intensive care unit admission, which were less likely to effect from COVID-19 infections.
Werion et al. reported that 70-80% of the patients with COVID-19 had low-molecular-weight proteinuria, and 46% and 19% of the patients had an inappropriate urinary loss of uric acid and had phosphate, respectively (7). In another study, it was reported that 75% of the patients with COVID-19 had at least two of the four criteria for PTD (22). The recent studies reported that the patients requiring pediatric intensive care units were more likely to have signs of renal involvement such as proximal tubule injury (22). However, although the current study applied similar diagnostic criteria, the rate of PTD was 20.8% and lower compared to that study mentioned above. The higher rates of PTD in that previous study might have been depending on the limited number of patients and more of the patients followed up in the intensive care unit which was different from our study (22). As a result of having comorbid diseases and also much more nephrotoxic drugs administration to the patients in the intensive care unit may be associated with a higher rate of PTD. For this reason, to provide homogeneity of the study group and to prevent any possible confounding in our study, the patients in the intensive care units, patients receiving any nephrotoxic drugs, and patients with comorbid diseases were excluded. Thus, it may be speculated that PTD may be detected in a considerable rate of children with COVID-19, and including patients in intensive care units would probably increase the rate of PTD.
Our study revealed that the proximal tubule dysfunction is not only present in adults but also present in children with COVID-19. Children were reported to have fewer symptoms compared to adults and the attributable mortality in the children was far away from low compared to adults (30). Although the pathogenesis of age-related differences of severity was not clearly understood, the variety of ACE2 and TMPRSS2 was one of the important candidates for pathogenesis (31). One of the most popular hypotheses, depended on increased expression and affinity of ACE in addition to MTMPRSS2 with age, while against hypothesis supported the anti-inflammatory properties of ACE2(32–35). Proximal tubular cells were reported to express ACE2 and TMPRSS2 intensely thus resulting in them as a good target for SARS-CoV-2 at the early stage. (36). Besides ACE2 is present in the renal cells, such as the parietal epithelium of Bowman’s capsule, collecting ducts, a thick ascending limb of Helen, podocytes, proximal cell brush border, and mesangial cells, suggesting multiple sites are the target for SARS-CoV-2(37). Also as postulated above, the increased affinity of the spike protein of SARS-CoV-2 to ACE was associated with disease severity (38). A relatively low rate of PTD in children compared to adult studies supported this hypothesis. On the other hand, the significantly younger age in the. patients with PTD supported the ACE-related hypothesis and suggested unexplored complex mechanisms and interactions besides SARS-Cov-2 and ACE receptors on the proximal tubule cells.
One of the definition criteria for PTD was normoglycemic glycosuria. Werion et al, reported that although other elements of PTD were present in the patient cohort with different rates, no normoglycemic glucosuria was recorded (7). While, Korman et al, reported that normoglycemic glucosuria was present in 11 of the patients (28%) and most of the patients in this group were from intensive care units (22). In our study only 2 patients (1.73%) had normoglycemic glucosuria, suggesting the children had also a low rate compared to most of the adult studies. According to us, the difference between the adults and children regarding PTD and COVID-19 infections in the literature had several reasons. Besides the better prognosis of the COVID-19 infections, the relatively low burden of chronic diseases, and related medications, low rate of intensive care, and mechanical ventilation requirement at the children are the main reason for the difference.
Our study had several limitations, including a limited number of measurements for some markers, lack of renal biopsy, and failure to demonstrate SARS-Cov-2 at the proximal tubule cells, as well as the single-center design of the study. However, the results of the current study may be more reliable because of providing more unvarying groups by the exclusion of the patients in intensive care units who may have more possible confounding factors. A better marker such as low molecular weight proteins including beta-2 microglobulin and retinol-binding protein might be more helpful, however during the study period, we were not able to reach these sophisticated tests. However, up to our knowledge, this is the first large study focusing on the effect of SARS-CoV-2 infections on the proximal tubule functions in children with COVID-19 infections, and our result may play a role in developing a screening test. Also, the longer follow-up of the patients for recovery of PTD was missing, which would be planned in the future.
In conclusion, our findings suggested that 20.8% of the children with COVID-19 infections had proximal tubular dysfunction. Proximal tubular dysfunctions were observed not only in the adults but also in the children. Children with COVID-19 infections should be followed up for recovery of proximal tubule dysfunction. More detailed studies for the evaluation of renal effects of SARS-CoV-2 on pediatric patients are essential.