Metabolic evaluation and disease characteristics of urolithiasis in children from 2020 to 2024: a retrospective single-center study at the national children's medical center

Abstract

Objective

To analyze the composition and disease characteristics of pediatric urolithiasis in a single center, providing a scientific basis for clinical diagnosis, treatment, and prevention.

Methods

This study included 266 pediatric patients with urolithiasis admitted from September 2020 to September 2024. Composition analysis was conducted on 174 stone samples using infrared spectroscopy, and 24-h urinary metabolic evaluations were performed on 124 patients. Additionally, genetic testing was conducted on 18 patients using the GenCap panel to explore the role of genetic factors in stone formation.

Results

Calcium oxalate stones were the most common type, accounting for 63.2% of all stones, followed by calcium phosphate and carbonate apatite stones. The high-incidence age range for stones was 6–12 years, with a male-to-female ratio of 1.86:1. The 24-h urinary metabolic analysis revealed that urinary components such as calcium, oxalate, and uric acid were closely related to stone formation. Genetic testing identified multiple different genes associated with stone formation, further confirming the importance of genetic factors.

Conclusion

The formation of pediatric urolithiasis is influenced by various factors, including metabolic abnormalities, urinary components, and genetic factors. Understanding these influencing factors contributes to the development of personalized prevention and treatment plans, reducing the recurrence rate and complications of stones.

Introduction

Over the past two decades, the incidence of pediatric urolithiasis has increased at an annual rate of 6% to 10%. While the prevalence of pediatric urolithiasis is relatively low in developed countries, it has significantly increased in some East Asian countries, reaching up to 30%. Although the overall prevalence of pediatric urolithiasis is only 0.36%-1.45% [1–3], symptoms are often more pronounced in children compared to adults, causing greater anxiety among parents. In severe cases, children may experience organ failure or even life-threatening complications. The etiology of pediatric urolithiasis is complex, with unique metabolic and renal physiological characteristics in children compared to adults. Clinical studies have shown that 16%-67% of pediatric patients with urolithiasis have metabolic abnormalities [4], and the condition is often caused by two or more risk factors. Climate, environment, diet, water quality, medications, metabolic diseases, and anatomical abnormalities can all contribute to the development of urolithiasis.

Metabolic evaluation is the gold standard for diagnosing the etiology of urolithiasis, including serological tests, urinalysis, and stone composition analysis. Patients with metabolic abnormalities have a significantly higher incidence and recurrence rate of stones compared to those without metabolic abnormalities. Children have unique phosphate and urate metabolism as well as renal physiological characteristics [5], making metabolic evaluation crucial for pediatric patients. The European Association of Urology (EAU) recommends that all patients with urolithiasis undergo 24-h urinalysis for metabolic evaluation, and repeat testing should be conducted if necessary to obtain accurate metabolic assessments [6].

Over the past four years, our center has collected stone samples from 174 pediatric patients, 24-h urinalysis results from 124 patients, and peripheral blood genetic testing results from 13 children. We now conduct a retrospective analysis of these metabolic evaluation results to provide guidance for the diagnosis and treatment of pediatric patients with urolithiasis.

Materials and methods

General information and sample collection

A total of 266 pediatric patients with urolithiasis admitted from September 2020 to September 2024 were selected. This retrospective study was approved by the Medical Ethics Committee of the Affiliated Children’s Hospital of Zhejiang University School of Medicine (Approval Number: 2024-IRB-0108-P-01). The committee waived the requirement for individual informed consent because the study utilized anonymized data collected during routine clinical care, and no additional interventions or risks were imposed on participants.

The inclusion criteria were:

1. Age < 18 years;

2. Confirmation of stone presence by ultrasonography and/or CT of the urinary system;

3. Obtainment of stone samples through surgical procedures or natural excretion;

4. Informed consent from the parents of the patients.

Stone composition analysis method

Take 5 mg of naturally air-dried stone sample and add 500 mg of dry potassium bromide. Grind the mixture thoroughly in an agate mortar. Take the grinded powder, press it for 20–30 s at a pressure of 16 MPa using a tablet press until it becomes transparent, and immediately place it into a Great GRD390 infrared spectrum analyzer. The computer will then generate a spectrum, automatically analyze it, and report the stone composition.

Stone classification criteria

Stones are classified based on the predominant mineral component (accounting for more than 50% of the total mineral matrix) [2]:

1. Oxalate stones: Calcium oxalate monohydrate, calcium oxalate dihydrate;

2. Calcium phosphate stones: Calcium phosphate, carbonate apatite, hydroxylapatite, dicalcium phosphate dihydrate, tricalcium phosphate;

3. Magnesium ammonium phosphate stones: Magnesium ammonium phosphate hexahydrate;

4. Uric acid stones: Anhydrous uric acid, ammonium urate;

5. Calcium carbonate stones: Calcium carbonate, calcite;

6. Cystine stones: L-cystine.

24-h urine metabolic analysis

Urine collection

Start timing at a fixed time (e.g., 8:00 AM), and first empty the bladder, but discard this urine as it is not included in the collection. Subsequently, collect every urine void during the next 24 h into the container, adding an appropriate amount of preservative to prevent urine decomposition. After the 24-h period, record the total urine volume in the container. Finally, take an appropriate amount of urine (e.g., 10 ml) as a sample for testing.

Observation indicators and evaluation criteria

The standard for 24-h urine metabolic indicators are: (1) Urinary calcium: < 0.1 mmol/kg/d; (2) Oxalate: < 45 mg/1.73 m²/d; (3) Uric acid: < 55 µmol/kg/d; (4) Citrate: > 365 mg/1.73 m²/d; (5) Cystine: < 13/1.73 m²/d; (6) Urinary creatinine/body weight: 5–40 mg/kg/d.

Additionally, 24-h urine metabolic analysis can also detect indicators such as urinary magnesium, urinary sodium, and calcium oxalate supersaturation to comprehensively understand the patient's renal function and metabolic status.

DNA library preparation, targeted gene enrichment, and sequencing

Extract DNA from peripheral blood using the QIAamp Kit, measure concentration with Nanodrop 2000, fragment to 150 bp, and prepare the library with MyGenostics reagents. Sequence using DNBSEQ-T7. Enrich target genes with GenCap, capture with probes, amplify, and purify. Resequence the enriched library. Process data with cutadaptor, map to UCSC hg19, remove duplicates, and correct errors. Detect variants (SNP/InDel) with Sentieon, convert to VCF, and annotate with ANNOVAR.

Statistical methods

Data analysis used SPSS 26.0 and R software. The Shapiro–Wilk test checked normal distribution. Normally distributed variables are reported as mean ± SD and compared with the t-test. Non-normal variables are reported as median (IQR) and analyzed with the Mann–Whitney U test. Categorical variables are described by case number and percentage, and tested with the Chi-Square Test or multifactorial regression. P < 0.05 is significant.

Results

Characteristics of pediatric urolithiasis

General baseline information

Among the 266 pediatric patients with urolithiasis, there were 173 males (65.04%) and 93 females (34.96%), with a male-to-female ratio of 1.86:1. The mean age was 6.10 ± 3.80 years, with males having a mean age of 5.74 ± 3.79 years and females having a mean age of 6.02 ± 3.74 years. The mean BMI was 16.64 ± 2.94 kg/m². A total of 37 patients (13.91%) had a clear family history of the condition. In this study, early recurrence of stones was defined as recurrence within 1 year after treatment, and 11 patients (4.14%) experienced early recurrence (Tables 1 and 2).

Table 1.

Patient demographics

Character	Value(%)
Age(yesrs)	6.10 ± 3.80
Gender
Male	173(65.04)
Female	93(34.96)
Follow-up time (years)	1.72 ± 1.12
Height(cm)	112.5 ± 27.16
Weight(kg)	22.77 ± 13.36
BMI(kg/m2)	16.64 ± 2.94
History family	37(13.91)
Early recurrence	11(4.14)

Table 2.

Information on past medical history

Past history	N (%)
Kidney-Related Conditions	30(11.28)
Urinary Tract Conditions & Surgeries	14(5.26)
Neurological & Musculoskeletal Conditions	5(1.88)
Cancer & Related Treatments	5(1.88)
Endocrine & Metabolic Disorders	2(0.75)
Post-Operative & Other Conditions	13(4.89)
Total	62(23.31)

Distribution of pediatric urolithiasis across different age groups

Based on dietary characteristics across different age groups, the patients were divided into four age brackets: 0–2 years, 2–6 years, 6–12 years, 12–16 years, and 16–18 years. The 6–12 years age group had the highest number of patients, accounting for 40.60% (108/266), followed by the 2–6 years age group with 33.46% (89/266) (Fig. 1). Across the entire age range, the number of male patients was consistently higher than that of female patients (the number of patients in the 16–18 years age group was too small for comparison) (Figs. 2).

Fig. 1.

Proportion of stone patients across different age groups

Fig.2.

Area chart showing the proportion of male and female stone patients across different age groups

Stone composition analysis

Qualitative analysis of stone composition

Using infrared spectroscopy, we analyzed the composition of 174 samples of pediatric urolithiasis. There were 95 cases of single-component stones(contains only one component without any other components), predominantly calcium oxalate (CaOx); and 79 cases of mixed stones, also predominantly CaOx-based.

Calcium oxalate (CaOx) stones were the most common type, accounting for 63.2% (110/174) of all stones, with pure calcium oxalate (Pure-CaOx) accounting for 54.5% (60/110) of CaOx stones. Calcium phosphate (CaP) stones accounted for 15.5% (27/174), carbonate apatite (CA) stones accounted for 13.7% (24/174), and stones with special components such as ammonium acid urate (AAU), L-cysteine (L-Cys), and calcite accounted for 7.47% (13/174).

Distribution of stones with different compositions in the urinary system

Lower urinary tract (LUT) stones were relatively rare, accounting for 19.0% (35/174) of all stones. CaOx stones were mainly distributed in the upper urinary tract (102/110), while CaP and CA stones were found in both the upper and lower urinary tracts, but were more prevalent in the upper urinary tract.

Stone type and location: The proportion of pure CaOx stones in the upper urinary tract was much higher than that in the lower urinary tract (93.3% vs 6.67%). Mixed stones and stones with special components were relatively evenly distributed in both the upper and lower urinary tracts, but the upper urinary tract still accounted for the majority.

Distribution of special stone types: AAU and L-Cys stones were mainly distributed in the upper urinary tract, accounting for 80.0% and 91.7% of their respective types.

See Table 3 for details.

Table 3.

Distribution of different types of stones by location

Component	n(%)
	UUT + LUT	UUT	LUT
Calcium Oxalate (CaOx)	110	102(92.73)	8(7.27)
Pure-CaOx	60	56(93.33)	4(6.67)
Mixed CaOx	50	46(92.00)	4(8.00)
Calcium Phosphate (CaP)	27	12(44.44)	16(59.26)
Dicalcium Phosphate Dihydrate (DCPD)	8	4(50.00)	4(50.00)
Struvite and Carbonate Apatite Mix (Struvite/CA)	20	8(40.00)	12(60.00)
Carbonate Apatite (CA)	24	15(62.50)	10(41.67)
Pure-CA	14	9(64.29)	6(42.86)
Mixed-CA	10	6(60.00)	4(40.00)
Special Composition	12	11(91.67)	1(8.33)
Ammonium Acid Urate (AAU)	5	4(80.00)	1(20.00)
L-Cystine (L-Cys) Calcite	7 1	7 0	0 0
Total	174	141(81.03)	35(20.11)

In the categories of calcium phosphate and carbonate apatite stones, there was one patient each who had stones in both the upper urinary tract (UUT) and the lower urinary tract (LUT). When summarizing, we counted them separately in both the UUT and LUT categories, but only once in the UUT + LUT category. Therefore, there may be a discrepancy in the number of patients listed in Table 3

Analysis results of children's past medical history and stones

Sixty-two(23.31%) patients had a history of previous illnesses, including hydronephrosis, past surgical history, hematological disorders, and so on (Table 4).

Table 4.

Information on stone types in patients with different past medical histories

Category	Stone Type	n
Hydronephrosis	Calcium oxalate monohydrate, Calcium oxalate dihydrate, Carbonate apatite, Struvite, Dicalcium phosphate dihydrate	15
Kidney Cysts	Calcium oxalate dihydrate + Carbonate apatite	1
Primary Hyperoxaluria	Calcium oxalate monohydrate	1
Post-Hypospadias Surgery	Calcium oxalate monohydrate + Calcium oxalate dihydrate + Carbonate apatite	5
Ureteral Reflux	Calcium oxalate monohydrate, Carbonate apatite	4
Bladder Conditions	Carbonate apatite + Tricalcium phosphate	1
Cerebral Palsy	Ammonium urate	1
Paralysis and Spinal Issues	Carbonate apatite, Carbonate apatite + Tricalcium phosphate	2
Leukemia	Carbonate apatite, Calcium oxalate monohydrate + Calcium oxalate dihydrate	4
Ovarian Teratoma	Calcium oxalate monohydrate	1
Hypothyroidism	Calcium oxalate dihydrate	1
Enzyme Deficiency	Calcium oxalate monohydrate + Calcium oxalate dihydrate	1
Post-Operative States	Dicalcium phosphate dihydrate, Struvite + Carbonate apatite + Oxalate stones	4
Hormone Treatment	Carbonate apatite, Calcium oxalate dihydrate + Carbonate apatite	2
Urinary Retention	Carbonate apatite	2

Results of 24-h urine metabolic analysis

Urine metabolic analysis results across different age groups

Figure 3 presents the correlation between different age groups (0–2 years, 2–6 years, 6–12 years, 12–16 years) and various urine components. The strength of the correlation between each age group and specific components is indicated by different colors and the width of the flow lines.

Fig. 3.

Sankey diagram showing the correlation between urine components and stone formation across different age groups

The analysis on the left side of the Sankey diagram focuses on the distribution of age groups, revealing that the 6–12 years age group has the highest incidence of stones and shows significant correlations with multiple urine components, including urinary calcium, oxalate, uric acid, citrate, magnesium, sodium, etc. The 2–6 years age group demonstrates strong correlations with urinary calcium and uric acid. Although the number of stone cases in the 12–16 years age group is relatively lower, it still correlates with components such as urinary magnesium and potassium.

The analysis on the right side of the Sankey diagram centers on the types of urine components. It is found that urinary magnesium and nitrogen are almost universally associated with stone formation across all age groups, particularly in the 6–12 years and 12–16 years age groups. Oxalate, uric acid, calcium, and citrate show particularly strong correlations with stone formation in the 6–12 years age group. Additionally, the urine creatinine-to-body weight ratio (Cr/BW) has a weak correlation with stone formation across different age groups.

Urine metabolic analysis results for patients with different stone types

1. Analysis on the left side of the Sankey diagram

   1. CaOx Pure: Represented by the widest bar in the diagram, it is associated with multiple urine components, including oxalate (Ox), urinary calcium (CaU), urinary magnesium (MgU), urinary nitrogen (NU), uric acid (UA), etc.
   2. CA Pure: Relatively less common, it correlates with urine components such as urinary magnesium and urinary calcium.
   3. CaOx Mixed: Related to various components including oxalate (Ox), urinary calcium (CaU), and urinary magnesium (MgU).
   4. Struvite with CA: Correlated with components such as urinary magnesium and urinary phosphorus (P).
   5. L-Cys (Cystine): Shows a strong correlation with cystine (Cys) and urinary sodium (Na+).
2. Analysis on the right side of the Sankey diagram

   1. Oxalate (Ox): Associated with both pure and mixed calcium oxalate stones.
   2. Urinary Calcium (CaU): Present in both calcium oxalate and calcium phosphate stone types.
   3. Uric Acid (UA): Strongly correlated with the formation of calcium oxalate-type stones.
   4. Citrate (Cit): Almost universally correlated with the formation of all stone types.
   5. Urinary Phosphorus (P): Related to stones containing magnesium ammonium phosphate.
   6. Urinary Nitrogen (NU) and Urinary Magnesium (MgU): Correlated with multiple stone types.

See Fig. 4 for details.

Fig. 4.

Correlation analysis between stone types and urine components

Analysis of factors related to stone formation

In this study, a comprehensive forest plot was used to demonstrate the impact of various variables on different stone types, with the Odds Ratio (OR) used to quantify these effects. The variables included urine components (such as UA, SSCaOx, P, etc.), gender, family history, and the creatinine-to-body weight ratio. The results showed that:

1. For CA (Mixed) stones, variables such as uric acid, family history (fam), calcium oxalate saturation, and urinary sodium have significant impacts on their formation. For CA (Pure) stones, they are mainly associated with factors such as urinary calcium, uric acid (UA), and family history.

2. For CaOx (Mixed) stones, uric acid, oxalate, urinary calcium, urinary magnesium, and citrate have significant influences. For CaOx (Pure) stones, calcium oxalate saturation and urinary calcium are important factors, and pure calcium oxalate stones are more likely to appear in the upper urinary tract.

3. L-Cys are related to factors such as cystine, urinary magnesium, and family history.

4. For Struvite with CA (magnesium ammonium phosphate stones, urinary phosphorus, urinary chloride, and urinary magnesium are important factors in their formation.

See Fig. 5 for details.

Fig. 5.

Forest plot analysis of the correlation between different factors and stone formation

Mutations detected in patients with urolithiasis

To identify disease-causing gene variants, a customized GenCap panel containing 93 genes associated with urolithiasis was utilized, and a capture strategy was implemented. A total of 18 patients underwent panel sequencing, and 13 of them were found to have a total of 20 mutated sites across 13 genes (Table 5). Among these, there were:

1. 6 cases of mutations related to calcium stones:

   1. SLC34A3 (not reported in ClinVar);
   2. SLC26A1 (not included in ClinVar);
   3. AGXT (not included in ClinVar);
   4. ALPL (not included in ClinVar).
2. 5 related to cystine stones:

   1. SLC3A1 (ClinVar: 1076782);
   2. SLC7A9 (not reported).
3. 2 related to xanthinuria:

   1. XDH (ClinVar: 1165590).
4. 2 related to 2,8-dihydroxyadenine stones:

   1. APRT (ClinVar: 884722).
5. 2 related to methylmalonic aciduria:

   1. LMBRD1 (not reported),
   2. MMACHC (ClinVar: 288244).
6. 1 related to uric acid stones:

   1. SARS2 (not included in ClinVar).
7. 1 related to Bartter syndrome:

   1. SLC12A1 (ClinVar: 3162818).
8. 1 related to Fanconi syndrome:

   1. NDUFAF6 (not included in ClinVar).
      Interestingly, both patients with XDH mutations shared the same mutation site (c.2794G>A), highlighting the significant role of alanine at position 932.

Table 5.

Mutations in patients with urolithiasis

Patient	Gene	Chromosome	Nucleotide change	REVEL	ACMG	Related disease	ClinVar Link
1	APRT	Chr16:88,876,532	NM_000485.3 c.346G > A	Damaging	Uncertain	Adenosine Phosphoribosyltransferase Deficiency	https://www.ncbi.nlm.nih.gov/clinvar/variation/884722/
2	APRT	Chr16:88,876,892	NM_000485.3 c.260G > A	Damaging	Uncertain	Adenosine Phosphoribosyltransferase Deficiency	Not reported
2	LMBRD1	Chr6:70,506,716	NM_018368.4 c.58C > T	Likely Benign	Uncertain	Methylmalonic Aciduria with Homocysteinuria	Not reported
3	XDH	Chr2:31,572,927	NM_000379.4 c.2794G > A	Uncertain	Uncertain	Type I Xanthinuria	https://www.ncbi.nlm.nih.gov/clinvar/variation/1165590/
4	SLC34A3	Chr9:140,128,880	NM_080877.2 c.1106C > T	Uncertain	Uncertain	Hypercalciuria	Not reported
4	ALPL	Chr1:21,889,610	NM_000478.6 c.305A > G	Uncertain	Uncertain	Hypophosphatase syndrome	Not included in ClinVar
4	SARS2	chr19:39,412,080	NM_017827.4 c.541G > A	Damaging	Uncertain	Hyperuricemia	Not included in ClinVar
4	MMACHC	Chr1:45,974,721	NM_015506.3 C.683C > T	Uncertain	Uncertain	Methylmalonic Aciduria with Homocysteinuria	https://www.ncbi.nlm.nih.gov/clinvar/variation/288244/
5	SLC7A9	Chr19:33,353,113	NM_014270.5 c.615G > T	Likely Benign	Uncertain	Cystine Stone	Not reported
6	SLC26A1	Chr4:985,353	NM_022042.4 c.139C > T	Likely Benign	Uncertain	Calcium Oxalate Kidney Stones	Not included in ClinVar
7	XDH	Chr2:31,572,927	NM_000379.4 c.2794G > A	Uncertain	Uncertain	Type I Xanthinuria	https://www.ncbi.nlm.nih.gov/clinvar/variation/1165590/
8	AGXT	Chr2:2,418,155,399–241,815,400	NM_000030.3 c.823_824dup		Pathogenic	Type I Hyperoxaluria	Not included in ClinVar
9	AGXT	Chr2:241,818,138	NM_000030.3 c.1079G > A	Damaging	Pathogenic	Type I Hyperoxaluria	Not included in ClinVar
9	AGXT	Chr2:241,812,466	NM_000030.3 c.595G > A	Likely Damaging	Likely Pathogenic	Type I Hyperoxaluria	Not included in ClinVar
10	NDUFAF6	Chr8:96,053,843	NM_152416.4 c.466G > T	Uncertain	Uncertain	Fanconi renal tubular syndrome	Not included in ClinVar
11	SLC12A1	Chr15:48,518,753	NM_000338.3 c.709G > A	Damaging	Uncertain	Type I Batter Syndrome	https://www.ncbi.nlm.nih.gov/clinvar/variation/3162818/
11	SLC7A9	Chr19:33,321,587	NM_014270.5 c.1403C > T	Uncertain	Uncertain	Cystine Stone	https://www.ncbi.nlm.nih.gov/clinvar/variation/328743/
12	SLC3A1	Chr2:44,539,756	NM_000341.4 c.1364C > T	Damaging	Likely Pathogenic	Cystine Stone	Not reported
13	SLC3A1	Chr2:44,539,765	NM_000341.4 c.1093C > T	Damaging	Pathogenic	Cystine Stone	https://www.ncbi.nlm.nih.gov/clinvar/variation/1076782/
13	SLC3A1	Chr2:44,528,223	NM_000341.4 c.1373G > T	Damaging	Pathogenic	Cystine Stone	Not reported

"Not reported" indicates no ClinVar entry exists for the variant

"Not included in ClinVar" indicates the variant is absent from ClinVar but may have supporting literature

Discussion

Pediatric urolithiasis is primarily caused by metabolic abnormalities (61.5%), urinary tract infections (18.3%), and anatomical abnormalities (17%) [7]. The most common metabolic factors contributing to stone formation include low urine volume, hypercalciuria, hyperoxaluria, hypocitraturia, cystinuria, and hyperuricosuria [8–10]. Preventing recurrence is critical, especially in children, due to the long-term nature of the disease. For instance, the 10-year recurrence rate for calcium oxalate stones can reach 50% [11]. Thus, reducing recurrences is a priority.

Stone composition analysis is a cornerstone of metabolic evaluation, guiding preventive measures, stone dissolution strategies, and further metabolic assessments [12]. A 24-h urine metabolic analysis, combined with stone composition data, helps identify the metabolic mechanisms behind stone formation and allows for tailored prevention and treatment plans. This enables patients to modify their diets and reduce the risk of recurrence.

Calcium-containing stones predominate in pediatric urolithiasis

Calcium-containing stones, primarily calcium oxalate and calcium phosphate, account for 79.3% of pediatric urolithiasis cases. Multivariate analysis reveals that calcium oxalate formation is strongly linked to elevated urinary calcium, oxalate, and uric acid levels. High urinary calcium and oxalate increase calcium oxalate saturation, promoting stone formation. Calcium phosphate stones are associated with high urinary calcium, uric acid, and a family history of stone formation.

Dietary factors also play a significant role in pediatric urolithiasis. Unlike adults, excessive calcium intake in children can contribute to stone formation due to differences in calcium absorption. Long-term consumption of formula milk with higher calcium content increases the risk of upper urinary tract stones in infants [13]. Therefore, unnecessary calcium supplements should be avoided unless a deficiency is confirmed.

Elevated uric acid levels, often linked to abnormal purine metabolism and high-purine diets, increase the risk of both calcium oxalate and calcium phosphate stones. A high-sodium diet exacerbates urinary calcium excretion, further promoting stone formation. Conversely, citrate-rich foods like lemon water can reduce the risk of calcium oxalate stones by binding with calcium to form soluble complexes. Thus, maintaining a balanced diet with adequate calcium and increased citrate intake is recommended for high-risk children.

Urinary tract stones and urinary tract infections

In our study, 62.0% of pediatric patients with urolithiasis developed urinary tract infections (UTIs), a proportion higher than previously reported [14]. Among patients with concurrent stones and UTIs, calcium oxalate stones were the most common, suggesting that UTIs may play a significant role in their formation. On the other hand, stone composition analysis revealed that 10.92% of patients had infectious stones (primarily composed of magnesium ammonium phosphate and carbonate apatite), which are associated with UTIs and high urinary phosphorus and magnesium concentrations. The link between urolithiasis and UTIs is multifactorial. The immature urinary systems of children are more susceptible to the irritation and infection caused by stones, and weakened immune systems and improper diets further exacerbate this situation. For these patients, controlling UTIs and regulating urine pH are essential preventive measures.

Urinary tract stones with congenital anomalies of the urinary tract

Urinary stones coexisting with urinary tract malformations are common in pediatric patients, and it is reported that 4.9% to 43.2% of children with urinary stones have concurrent underlying anatomical abnormalities of the urinary tract [15]. Ureteropelvic junction obstruction (UPJO) combined with kidney stones is common, and laparoscopic pyeloplasty combined with stone removal is a feasible treatment option with a high stone clearance rate. However, in special circumstances like renal malposition, long and narrow calyceal necks, or multiple stones, the combined use of flexible ureteroscopy or percutaneous nephrolithotomy is required to enhance stone removal efficiency. The combined dual-endoscope technique has played a crucial role in the treatment of these complex pediatric cases.

In our study, 9 pediatric patients with urolithiasis had urinary system malformations that required surgical intervention, including 7 cases of congenital UPJO and 2 cases of ureterovesical junction obstruction (UVJO). Five patients with UPJO underwent robot-assisted laparoscopic pyeloplasty (RALP) combined with holmium laser lithotripsy and stone extraction using a flexible ureteroscope. Two patients underwent laparoscopic pyeloureterostomy combined with minimally invasive percutaneous nephrolithotomy/flexible ureteroscopic lithotripsy. Two patients with UVJO and kidney stones underwent holmium laser lithotripsy and stone extraction using a ureteroscope during ureteral reimplantation. Postoperative stone analysis revealed that 5 patients with UPJO and 2 with UVJO had calcium oxalate stones, while 2 patients with UPJO had magnesium ammonium phosphate stones.

The recurrence rate within five years was 38% for patients with normal anatomical structures and 65% for those with anatomical abnormalities [16]. Therefore, it is essential to closely monitor changes in urine pH and composition after the surgery, and regularly perform urine routine tests and ultrasound examinations of the urinary system to promptly detect and manage any recurrence of stones.

Special types of stones

Cystine stones account for approximately 6% to 8% of pediatric urolithiasis cases [17]. These stones form due to a defect in the renal reabsorption of cystine, ornithine, lysine, and arginine (COLA) in the proximal tubules, leading to cystine supersaturation in urine. Cystinuria, a genetic disorder caused by mutations in the SLC3A1 and SLC7A9 genes [18], results in defective cystine transporters and low cystine solubility at normal pH levels, leading to stone formation. Cystinuria is the second most common autosomal recessive genetic disorder in Europe, typically presenting between ages 2 and 40, with a median onset of 12 years in males and 14 years in females [19].

In our study, 7 pediatric patients with cystine stones were identified, with a mean age of 6.17 ± 3.55 years for males and 10.5 ± 2.52 years for females. All had upper urinary tract stones. Cystine stones are highly recurrent and can cause renal damage, making early intervention and regular check-ups essential. Treatment aims to increase cystine solubility, which is pH-dependent, with higher solubility in alkaline urine. Maintaining a urine pH of 7.0–7.5 is recommended to prevent stone formation [20].

Additionally, several pediatric patients with ceftriaxone (CTX) stones were also identified, all with a history of ceftriaxone use prior to diagnosis. Ceftriaxone, which is excreted in urine (55%) and bile (45%) [21], can bind to calcium in urine to form CTX/Ca crystals [22]. Risk factors include low urine volume, high-dose CTX, and high urinary calcium. Given the friable nature of these stones, composition analysis was not possible. Clinicians should be cautious when prescribing ceftriaxone to pediatric patients with a history of urinary stones, carefully managing dosage and duration and monitoring urine volume and calcium levels. Increased fluid intake can help reduce the risk of stone formation. Prompt treatment and stone composition analysis are essential for patients with confirmed ceftriaxone stones.

Genetic testing for pediatric patients with special types of stones

Recent studies suggest that single-gene mutations contribute to a small number of pediatric urolithiasis cases. High-throughput sequencing of 30 genes associated with stone formation revealed pathogenic mutations in 21% of pediatric cases [23]. In our study, GenCap panel sequencing identified 20 mutations across 13 genes in 13 patients, highlighting the genetic complexity of urolithiasis. Notably, the AGXT gene variation in Patient 8 was labeled "Pathogenic" (not included in ClinVar), linked to Type I primary hyperoxaluria. Several patients with multiple gene variations, such as Patients 2, 4, 9, 11, and 13, exhibited greater disease complexity. Variations in genes like XDH (ClinVar:1165590) and SLC3A1 (ClinVar:1076782) were found in multiple patients, underscoring their importance in disease onset.

Genetic variations linked to rare disorders like Bartter syndrome and Fanconi syndrome were also identified. In Bartter syndrome, mutations in the SLC12A1 gene (ClinVar:3162818) affect renal ion reabsorption, while in Fanconi syndrome, mutations in NDUFAF6 (not included in ClinVar) disrupt energy metabolism in renal cells, increasing stone risk. These conditions require close monitoring for stone development and preventive measures [24, 25].

Despite the valuable insights genetic testing offers, it is not routinely performed in urolithiasis patients. Further research with larger cohorts is needed to expand the genotype–phenotype spectrum and develop targeted therapies.

Follow-up and prevention of pediatric urinary stones

Follow-up is also a crucial yet often overlooked aspect of metabolic assessment for pediatric urinary stones [26]. Studies have shown that within a median follow-up period of 3.29 years, 14.8% of children experience stone recurrence, with a median time to recurrence of 2.58 years [27]. Experts recommend reassessing 24-h urine metabolism 3 to 4 months after treatment initiation, followed by follow-ups every 6 months. If preventive measures fail, treatment adjustments and reanalysis of stone composition are necessary. Imaging exams should be repeated annually if no stones remain. Recurrent stones may have different compositions, requiring reanalysis.

The formation of urinary calculi in children is closely related to the patients' past medical history. Histories of urinary system diseases (such as hydronephrosis, ureteral stenosis, and reflux) and other diseases (such as hematological disorders) may affect the metabolic status or urine composition of children, thereby increasing the risk of calculus formation and recurrence. In this study, the most common past medical histories among the patients were hydronephrosis, post-urological surgery, and hematological disorders. The main types of calculi were calcium oxalate calculi and infectious calculi. In this study, the early recurrence rate among the pediatric patients was 4.14%. With the increase in the number of cases and the extension of follow-up time, the recurrence rate will further increase. Therefore, in clinical practice, it is essential to tailor treatment and prevention plans based on the specific causes, symptoms, and stone compositions to reduce the impact of stones on the pediatric urinary system.

Research limitations and future research directions

Although this study revealed the multifactorial characteristics of pediatric urolithiasis through metabolic evaluation and genetic testing, limitations remain. Due to the constraints of single-center sample size and data collection scope, we were unable to further subgroup patients by etiology (e.g., urinary tract abnormalities, metabolic disorders, genetic factors). Such subgrouping could more precisely clarify the association between different etiologies and stone types/metabolic indicators (e.g., urinary obstruction patients may be more prone to infectious stones, while metabolically abnormal patients may primarily form calcium oxalate stones). Future research will further explore the heterogeneous etiology of pediatric urolithiasis. We recommend multicenter collaborations to subgroup patients by urinary tract abnormalities, metabolic disorders, and genetic defects, combining long-term follow-up data to clarify the associations between etiologies, stone types, metabolic features, and recurrence risks. Additionally, expanding genetic testing cohorts and conducting functional validation experiments will help uncover the pathogenicity of rare genetic variants, providing a basis for precision prevention and treatment.

Conclusion

Children and adolescents aged 6–12 are at high risk for urolithiasis, with a male-to-female ratio of 1.86:1. Calcium oxalate stones are most common (63.2%). Urinary components like calcium, oxalate, and uric acid are linked to stone formation. Genetic factors, such as mutations in SLC12A1 and NDUFAF6, play a significant role. Various factors, including metabolic abnormalities and urinary tract infections, influence stone formation. Understanding these factors is key to developing personalized prevention and treatment plans to reduce recurrence and complications. Further research is needed to explore the genetic basis and improve treatment strategies.

Authors’ contributions

L.Z.H and K.J.H : Minimally invasive surgery and extracorporeal shock wave lithotripsy procedures; Writing and polishing papers; Data analysis. L.Z.H and K.J.H contributed equally to this work and should be considered co-first authors. L.L.J : Assist in extracorporeal shock wave lithotripsy and complete stone analysis. X.Y: Propose ideas, perform surgical procedures, and provide article guidance; revised the manuscript.

Funding

This study was supported by the Zhejiang Provincial "Leading Force and Leading Bird + X" R&D program (grant number: 2024C03196).

Data availability

Novel genetic variant data generated in this study have been deposited in ClinVar (National Center for Biotechnology Information, NCBI; https://www.ncbi.nlm.nih.gov/clinvar/) and are pending review. Submission IDs for individual variants are provided in the manuscript and accessible via the links above. Formal accession numbers will be assigned and updated upon completion of the curation process.

Declarations

Ethics approval and consent to participate

This study was approved by the Medical Ethics Committee of the Affiliated Children's Hospital of Zhejiang University School of Medicine (Approval Number: 2024-IRB-0108-P-01).

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.