Multicystic dysplastic kidney (MCDK) is a non-inherited cystic kidney disease resulting in a non-functioning kidney due to abnormal kidney development [1]. In 1955, MCDK was distinguished as a separate disease entity from polycystic kidney disease, a classification that remains current [2]. Although the developmental etiology of MCDK is not fully understood, two main hypotheses have been proposed. First, the obstruction theory, suggests that MCDK results from severe fetal ureteral obstruction. The second attributes MCDK to abnormal interactions between the ureteric bud and metanephric mesenchyme during kidney development [3,4]. Bilateral MCDK results in oligohydramnios and fatal pulmonary hypoplasia due to absent kidney function in the fetus and newborn, leading to nonviability in most cases. In contrast, unilateral MCDK is associated with a functioning contralateral kidney and typically presents without significant clinical issues, except for rare cases with a large abdominal mass [1].

Unilateral MCDK is among the most common kidney anomalies detected by prenatal ultrasound, with an incidence of 1 in 2,200 to 1 in 4,300 births [5]. While previous studies reported hypertension and malignancy as potential complications, recent long-term follow-up studies and systematic reviews have lessened these concerns [6-8]. However, vesicoureteral reflux (VUR) in the contralateral functioning kidney has been reported in approximately 30% of cases, and there is increasing interest in the growth and long-term function of the solitary kidney [5]. Compensatory hypertrophy of the contralateral kidney is present in about one-quarter of patients at birth and in 77% at follow-up, and is directly associated with involution of the MCDK [9,10]. In childhood and adolescence, several studies have examined the impact of compensatory hypertrophy of the contralateral kidney on residual kidney function [5,11].

Using long-term follow-up data from Korean pediatric patients with unilateral MCDK, this study analyzed the clinical course of unilateral MCDK and the contralateral kidney, including changes in kidney length and comorbid complications, and explored their associations. In particular, we evaluated the incidence and clinical significance of compensatory hypertrophy by comparing age-specific contralateral kidney length with normative data from Korean children.

MCDK, one of the most common congenital anomalies of the urinary tract, is characterized by multiple, non-communicating cysts of varying sizes separated by dysplastic parenchyma and absence of a normal pelvicalyceal system [3].

Unilateral MCDK has been described extensively in the literature, with clinical characteristics varying among studies and geographic regions [3,9,13-15]. Aslam and Watson [13] in the UK reported that 33% of MCDK cases had fully involuted by age 2 years in a 10-year follow-up, 47% by age 5 years, and 59% by age 10 years. VUR was observed in 19% of contralateral kidneys, with 96% of these cases being mild to moderate reflux. No patients developed hypertension, proteinuria, or malignancy during follow-up. A German study of 75 unilateral MCDK patients found total involution in 25% and size reduction in 60%. No malignancy was reported; VUR was present in only 4.5%, and compensatory hypertrophy of the contralateral kidney occurred in 43% [9]. In a U.S. study by Mansoor et al. [15], analysis of 101 children with unilateral MCDK showed VUR in 16.8%, ureteropelvic junction obstruction in 4.1%, and megaureter in 2.4% of contralateral kidneys. By age 5 years, 60% of MCDK had completely involuted, and compensatory hypertrophy was confirmed in 74.1%. Notably, the risk of CKD and hypertension was significantly increased when contralateral anomalies were present [15]. In a Japanese study of 128 children with MCDK, 50% were diagnosed antenatally, with a mean age at diagnosis of 2.8 years. VUR was found in 9.5% of contralateral kidneys, and compensatory hypertrophy was observed in 59.4% during a 1-year follow-up. One patient developed hypertension, and seven patients (5.5%) underwent nephrectomy [3]. A Korean study from 2006 reported that among 46 unilateral MCDK patients, 24% experienced complete involution and 41% partial involution over a 30-month follow-up. The average time to complete involution was 37 months. One patient developed hypertension and decreased kidney function; no cases of malignancy were observed [14]. In the present study of 71 patients, the mean age at diagnosis was 1.1 months, and 63 patients (88.7%) were diagnosed by prenatal ultrasonography. Aside from VUR diagnosed in three patients, additional kidney and urinary tract anomalies were found in 16 patients (22.5%): 10 in the contralateral kidney and six in the ipsilateral MCDK. Complete or partial involution occurred in 66 patients (93.0%) within 2 years of age. All patients (n=5, 7.0%) who did not experience regression by age 2 years underwent nephrectomy due to continued MCDK growth. Despite differences in age at diagnosis and follow-up duration among published studies, most report complete or partial involution in over 70% to 80% of cases during follow-up. Notably, this study found that 93% of patients had complete or partial involution by age 2 years. The particularly young mean age at diagnosis in this study likely reflects the high proportion of prenatal diagnoses via ultrasonography.

There is ongoing debate regarding the necessity of kidney scintigraphy and VCUG in the evaluation of unilateral MCDK [16,17]. Kidney scintigraphy demonstrates the absence of kidney function in MCDK, offering a different clinical perspective from ultrasonography. Hannallah et al. [16] retrospectively reviewed 172 unilateral MCDK cases to assess the necessity of kidney scintigraphy, comparing the prevalence of VUR, recurrent UTIs, and CKD between patients diagnosed solely by ultrasonography and those assessed with both modalities. They concluded that kidney scintigraphy is not essential for diagnosis, as no significant differences were found between groups, though it may be useful in select follow-up scenarios. In the present study, DMSA scans confirmed non-functioning kidneys in 66 of 71 patients (93.0%). VCUG is sometimes performed in all unilateral MCDK patients, depending on institutional protocols, as previous reports indicate VUR may be present in 4.5% to 47% of cases [17-19]. A recent study by Blachman-Braun et al. [17] found that the likelihood of severe VUR was low when the contralateral kidney appeared normal on ultrasound, suggesting that VCUG should be reserved for patients with suspected contralateral structural abnormalities. In this study, VCUG was performed in 73.2% of patients, specifically those with anomalies of the contralateral kidney on ultrasound or a history of UTI. Among these patients, only three cases of VUR were identified, all of which were grade 3 or lower and did not require surgical intervention. These findings suggest that the necessity of routine VCUG in MCDK patients warrants reconsideration.

Most children with a congenital solitary functioning kidney exhibit compensatory hypertrophy of the contralateral kidney [6,11]. Gaither et al. [6] reported that among patients diagnosed with unilateral MCDK before age 2 years, the mean time to involution was 5.5 years, and the mean time to contralateral hypertrophy was 2.7 years. After adjusting for MCDK side, sex, age, and cohort status, each additional year of involution after age 2 was associated with a 0.35 cm increase in contralateral kidney length compared to patients without involution. Wang et al. [11] reported that in pediatric patients with a solitary kidney, each 1 cm increase in kidney length due to contralateral hypertrophy was associated with a 7.8 mL/min/1.73 m² increase in GFR. Matsell et al. [20] noted that prognosis depends on the etiology of congenital solitary functioning kidney. Compared to unilateral MCDK, unilateral kidney agenesis is more frequently associated with genetic syndromes and genitourinary malformations, resulting in a poorer long-term prognosis due to higher rates of hypertension and proteinuria. In this study, compensatory hypertrophy was most prevalent in the first month of life, declined to its lowest at 3 to 4 years, and subsequently increased through adolescence, reaching 81.8% at 9 to 10 years. Unlike previous studies that reported a continuous increase in compensatory hypertrophy with age, our findings revealed a biphasic age-related pattern. We propose that this pattern may reflect two distinct mechanisms: an early adaptive response in neonates and progressive compensatory growth during later childhood and adolescence. Furthermore, patients with complete involution of the dysplastic kidney by 7 to 8 years showed a 2.2-fold higher likelihood of developing compensatory hypertrophy in the contralateral kidney, although this association did not reach statistical significance. Therefore, complete involution of MCDK may serve as a useful predictor of compensatory hypertrophy in the contralateral kidney. Notably, patients with left-sided MCDK had significantly higher rates of contralateral compensatory hypertrophy compared to those with right-sided MCDK, indicating more frequent compensatory hypertrophy in the right kidney. We speculate that this may be due to the typically smaller baseline size of the right kidney, allowing greater potential for compensatory growth when left-sided MCDK is present.

Conversely, according to the hyperfiltration hypothesis, sustained increases in filtration load on the contralateral kidney may result in glomerular injury, ultimately leading to glomerulosclerosis, systemic hypertension, proteinuria, and impaired kidney function [13-15,21,22]. In a systematic review by Hutchinson et al. [21], among patients with congenital solitary functioning kidney due to unilateral kidney agenesis or MCDK, the incidence rates during follow-up were 10.1% for proteinuria, 7.4% for hypertension, and 8.4% for reduced kidney function. In the present study, among patients followed up to 10 years of age, the mean contralateral kidney length was 10.6±0.8 cm, and the mean eGFR was 102±27.2 mL/min/1.73 m². Hypertension was observed in only one patient (1.41%), and three patients (4.23%) developed kidney insufficiency during follow-up. Recently, Poggiali et al. [23] proposed a clinical predictive model for kidney injury in children with congenital solitary functioning kidney, identifying serum creatinine, recurrent UTI, and contralateral kidney length as three independent predictors. In 2022, the Italian Society of Pediatric Nephrology issued consensus recommendations for the management of congenital solitary kidney, categorizing patients into three risk groups based on long-term outcomes and outlining management strategies for each group. Cases with compensatory hypertrophy were classified as low risk, those without compensatory hypertrophy or with additional genitourinary malformations as medium risk, and those with reduced kidney function, hypertension, or proteinuria as high risk. For patients in the medium- and high-risk groups, regular monitoring of kidney function was recommended [24].

This study had several limitations. First, as a retrospective single-center study, the sample size was small, and observation periods varied among patients. Consequently, it was not possible to assess the relationship between final eGFR values and compensatory hypertrophy of the contralateral kidney. Additionally, the absence of long-term follow-up data limited the evaluation of complications such as proteinuria and decreased kidney function in patients with a solitary functioning kidney. Second, potential interobserver variability in serial kidney size measurements should be considered. Despite these limitations, this study presents long-term follow-up data on unilateral MCDK in Korean children and is notable for providing the first analysis of compensatory hypertrophy of the contralateral kidney using age-specific normative data for Korean children.

In conclusion, if complete or partial involution of unilateral MCDK is confirmed by age 2 years in Korean children, spontaneous resolution is likely, and universal screening for associated VUR may not be required. Furthermore, compensatory hypertrophy of the contralateral kidney increases progressively from birth and is observed in approximately 80% of patients by age 10 years.