In this study, we conducted an epidemiological investigation of omphalitis and urachal remnants in Japan using data from the JMDC Claims Database from 2005 to August 2023. We specifically examined the prevalence of urachal remnants among patients  ≥ 15 years of age who presented with symptoms of omphalitis.

Patients with urachal remnants have symptoms other than omphalitis. In cases of infection or bleeding on the bladder side of the ureteral duct, blood in the urine or cystitis, and infection in the ureteral duct that is not in contact with the umbilical cord or bladder, lower abdominal pain may occur. However, owing to the nonspecific nature of tract infections and lower abdominal pain, which can be associated with various conditions and departments (e.g., urology), we selected only patients with omphalitis symptoms using insurance claims data to improve the specificity of our study.

While urachal remnants occur in 1.03–1.63% of children  < 15 years of age [4, 5], they are found in only 0.063% of adults [18,19,20]. However, among patients with symptoms of omphalitis, the prevalence of urachal remnants was notably high (16.0%). In male patients, the prevalence was 22.0%, highlighting a marked sex difference. These findings underscore the significance of age at onset and sex-based differences.

The median umbilical ligament, which connects the bladder to the umbilicus, is a remnant of the embryonic urachus that closes during early fetal development. Urachal remnants occur when closure is incomplete [21]. When these remnants are accompanied by symptoms, such as omphalitis, the condition is termed a urachal remnant disease [21, 22]. Given this developmental mechanism, remnant urachal disease is most common in newborns and children [1]. However, adult cases have also been reported, particularly in young adults [7, 23, 24]. This can be attributed to the existence of childhood- and adult-onset urachal remnant disease [23]. Childhood-onset type results from the persistence of a patent urachal cavity. As such abnormalities are discovered and treated at birth, childhood-onset urachal disease is rare in adults [7]. These cases usually involve anomalies of the urinary system and therefore require careful examination and testing [7].

Conversely, the adult-onset type occurs when a previously closed urachus reopens later in life, typically between 20 and 40 years of age. The urachal remnant cases identified in patients  ≥ 15 years of age in our study were primarily considered the adult-onset type. Urachal remnants without infections are usually asymptomatic and are discovered incidentally during imaging [4, 24]. However, symptomatic patients presenting with omphalitis or abdominal pain require medication or procedures, and surgery may be necessary if outpatient treatment is insufficient. Awareness of urachal remnants as a potential cause of omphalitis facilitates prompt diagnostic imaging and timely surgical intervention. This is particularly relevant for male patients, in whom more than one in five cases involves urachal remnants, making diagnostic testing crucial. Moreover, in the peak age range of adult-onset urachal remnant disease (20–34 years), the prevalence of omphalitis in males was 30%. Although urachal remnants were also present in females in this age group, the sex disparity was striking (Fig. 3). Various hypotheses regarding this difference have been proposed, including anatomical differences, hormonal influences, and genetic predispositions. However, the exact cause remains unclear and warrants further investigation.

Our study showed that 39.7% of patients  ≥ 15 years of age with urachal remnant disease (728 of 1836 patients) underwent surgery, indicating that 2 of 5 patients required surgical intervention. Several studies have explored the treatment strategies for childhood-onset urachal remnants. Nogueras-Ocaña et al. reported spontaneous regression in 8 of 13 patients (61.5%) [25], while Galati et al. reported spontaneous regression in 80% of patients  < 6 months of age [26]. Childhood-onset urachal remnant disease caused by incomplete closure during fetal development may resolve spontaneously [20] and usually improves without surgery [25]. However, the adult-onset type, which results from the reopening of a previously closed urachus, is less likely to resolve spontaneously [25]. Moreover, in most cases, urachal remnant-associated omphalitis cannot be definitively treated with outpatient care alone, owing to persistent structural abnormalities, with a reported recurrence rate of 30% [27]. Consequently, surgical treatment is recommended due to the risk of recurrence [25, 28].

The potential for malignant transformation is another factor that influences the decision to perform surgical intervention in urachal remnant cases [27]. Urachal cancer occurs in approximately 1 in 550,000–5.5 million individuals [29, 30]. However, > 80% of cases are diagnosed at Sheldon Stage ≥ III [31]. Furthermore, owing to the challenges of early detection, the prognosis remains poor, with a 5-year survival rate of 6.5–10% [29]. In our study, only one case of urachal cancer was identified among 1836 patients with urachal remnants, indicating that the frequency of urachal cancer in this population was significantly higher than that in the general population.

Moreover, we found that the median time to surgery was relatively short (two months). Of the two onset types, adult-onset urachal remnant disease may make surgical intervention more acceptable to both patients and clinicians. Given its pathogenesis, spontaneous resolution is less likely and prolonged and recurrent infections are common [22], leading to short observation periods before surgery [2, 20].

Patients who did not undergo surgery underwent outpatient treatment. Treatment methods included cleaning and disinfection of the wound, intravenous infusion and oral administration of antibiotics, and incision and drainage. Some patients were treated in the hospital, rather than on an outpatient basis. A few patients who did not undergo surgery experienced recurrence; however, most of the recurrent cases emerged after 1 year. Surgery is usually performed for recurrence within 1 year. Nevertheless, if the time to recurrence is long, patients may prefer to undergo follow-up.

Notably, surgical techniques have evolved from open to laparoscopic procedures. Although the surgeries in our dataset began in 2006, laparoscopic procedures have been performed since 1992. Following the approval of insurance coverage for laparoscopic urachal surgery in Japan in 2014 [29], the number of laparoscopic procedures has steadily increased [32], surpassing that of open surgeries after 2018. Furthermore, single-incision laparoendoscopic single-site surgery has garnered considerable attention [33, 34]. In terms of hospital size, there was a lag of approximately 5 years between the adoption of laparoscopic surgery in large hospitals (≥ 200 beds) and small- and medium-sized hospitals (< 199 beds). This suggests that adopting new surgical techniques across different hospital settings requires approximately 5 years.

This epidemiological study revealed a high prevalence of urachal remnants in patients with omphalitis, with a substantial proportion requiring surgical intervention. Our findings suggest that when treating recurrent or prolonged omphalitis in an outpatient setting, clinicians should consider the possibility of urachal remnants and proactively pursue imaging studies. Young individuals, particularly males 10–40 years of age, are more likely to have residual ureters. Therefore, further testing was required.

Additionally, patient characteristics may predict urachal remnants. Our study showed that patients with urachal remnants had better outcomes than those without urachal remnants, exhibiting most items in the metabolic syndrome evaluation criteria. This suggests that the presence of omphalitis symptoms in patients without abnormalities in lifestyle or health status (such as susceptibility to infections due to obesity or diabetes) is caused by ureteral remnants.

For less invasive screening, abdominal ultrasound is recommended [32], whereas more definitive assessments, such as abdominal computed tomography or magnetic resonance imaging, may be necessary to confirm the presence of urachal remnants [32].

We included only patients  ≥ 15 years of age because there is no standardized distinction between children and adults in Japan. Individuals  > 15 years of age are treated as adults and are likely to make their first visit to an adult outpatient department. Therefore, we defined adults in our study as those  > 15 years of age. Data on urachal excision surgery were first recorded in Japan in 2006. However, owing to the nature of insurance data, confirming the number of laparoscopic surgeries before 2014 is challenging, as laparoscopic surgeries were not covered by insurance. Therefore, we did not compare open and laparoscopic surgeries performed before insurance coverage. Our case identification relied on diagnostic codes for omphalitis and urachal remnant diseases. While these codes are entered by physicians during patient evaluation, they do not guarantee clinical accuracy, which is a common limitation of large-scale claims database research. Additionally, because “omphalitis” is not a standardized diagnosis code, some adult cases may have been recorded as “neonatal omphalitis,” potentially capturing recurrent rather than initial adult-onset cases. This coding ambiguity may introduce minor inaccuracies. However, as diagnostic codes are required to justify medical procedures and our study included all diagnoses related to the umbilicus, we believe that the risk of underestimating omphalitis cases was minimal. The database primarily included information from individuals enrolled in employment-based insurance programs, resulting in limited data for patients  > 70 years of age. Nonetheless, as omphalitis predominantly affects younger individuals, this age-related gap likely has a minimal impact on our conclusions. Regarding the treatment procedures, some surgical records may have been missed if they were not covered by insurance, as urachal remnant treatment is a relatively recent development. Despite this, the consistent trend toward minimally invasive techniques suggests that our data capture a broader trajectory of surgical evolution. Finally, while only one case of urachal cancer was identified in our dataset, it is important to note that our study focused on patients with omphalitis and did not provide a comprehensive estimate of the urachal cancer prevalence among all patients with urachal remnants. Future studies should aim to determine the true incidence of urachal cancer in this population.

Our epidemiological study of urachal remnants among patients  ≥ 15 years of age presenting with omphalitis symptoms highlights the potential significance of early imaging in guiding treatment strategies for adults with refractory or recurrent omphalitis given the high prevalence of urachal remnants in this population.