Introduction

Kernicterus, once prevalent due to neonatal hyperbilirubinemia, is now rare owing to improved care and treatment methods [1,2,3,4,5]. However, bilirubin neurotoxicity, encompassing a spectrum known as kernicterus spectrum disorder (KSD), remains a concern [1, 5, 6]. Phototherapy, alongside exchange transfusion in severe cases, is the standard treatment for hyperbilirubinemia in newborns [7, 8]. Despite its efficacy, concerns persist regarding potential adverse effects such as allergy, neurological disorders, and cancer [9,10,11,12]. Previous studies have suggested an increased risk of seizures or epilepsy in children undergoing neonatal phototherapy, yet determining causality between hyperbilirubinemia, neonatal phototherapy, and epilepsy remains challenging [10, 13]. In this study, we sought to contribute further evidence to this topic using Danish registry data from a university hospital.

Methods

Study design and study population

We conducted a cohort study utilizing data from the Danish Medical Birth Registry [14] to identify all live singleton births with a gestational age of ≥ 35 weeks at Aarhus University Hospital between January 1, 2002, and November 30, 2016. The study population comprised infants with available data on birth weight, who survived the neonatal period, and remained resident in Denmark on day 29 post-birth (N = 65,365; see Supplementary Fig. 1). In Denmark, each resident is assigned a unique identification number in the Danish Civil Registration System, facilitating accurate data linkage across national registers [15, 16]. Only children born at Aarhus University Hospital were included in this study due to incomplete information on neonatal phototherapy in other regions of Denmark [17]. We used data from the Danish registries and no consent forms are needed according to Danish data protection regulation. Our research adhered to the ethical principles outlined in the Declaration of Helsinki and the study was approved by the Danish Data Protection Agency (reference number: 2016–051-000001, serial number 1515, approval date: 11. June 2019). We utilized ChatGPT to assist in revising the language of our manuscript, but we carefully reviewed and incorporated the revisions.

Information on measurements of bilirubin

Data on bilirubin measurements, including total serum bilirubin and unconjugated bilirubin, were extracted from the clinical laboratory information system (LABKA). Implemented in 2000 within the Central Denmark Region, LABKA offers comprehensive coverage within this region, encompassing Aarhus University Hospital [18]. The system aggregates test results from samples collected in both public and private hospitals, as well as those collected by general practitioners and submitted to clinical biochemistry departments.

Key test items during the neonatal period included total serum bilirubin (TsB) of neonates (NPU04145), total serum bilirubin (NPU01370), unconjugated bilirubin (NPU01366), and conjugated bilirubin (NPU17194), with the NPU terminology employed for result identification and communication across clinical laboratories [18].

The dataset contains details such as sampling date and time, results, and units of tests conducted. For this study, bilirubin measurement primarily pertains to total serum bilirubin and/or unconjugated bilirubin, given the negligible presence of conjugated bilirubin during the neonatal period. Multiple bilirubin measurements could be recorded for a child during this period, with the highest value selected for analysis.

Information on neonatal phototherapy

Data on neonatal phototherapy treatment were extracted from the Danish National Patient Register (DNPR) [19]. Children receiving neonatal phototherapy were identified if they were coded with procedure codes BNGC (Phototherapy) or BNGC0 (Phototherapy to neonates) in the DNPR, with treatment dates falling within the neonatal period (up to 28 days of age). Clinical practices adhere to phototherapy guidelines for neonatal hyperbilirubinemia adapted by the Danish Pediatric Society in 1992 and 2012 from guidelines established by the American Academy of Pediatrics (see Supplementary Table 1a) [2, 20]. The information about the devices, light spectrum, intensity used for neonatal phototherapy at Aarhus University Hospital could be found in the Supplementary Table 1b. Since 2011, the Danish recommendation of intensity for neonatal phototherapy is a minimum 30 microwatts/cm2/nm, measured at initiation. Phototherapy usually last for 24 h before taking a bilirubin measurement to judge if the treatment should continue or not (personal communication with coauthor JPP).

Information on diagnosis of epilepsy

Diagnoses of epilepsy were retrieved from the DNPR [21]. Children were classified as having epilepsy if they were registered with ICD-10 codes G40-G41 as either a primary or secondary diagnosis after the neonatal period. The date of epilepsy onset was defined as the date of hospital contact that led to the diagnosis.

Information on covariates

Data on date and time of birth, sex of the child, gestational age at birth, birth weight, Apgar score at five minutes, maternal age at the time of birth, and parity were obtained from the Danish Medical Birth Registry. The age of the child in hours at the time of blood sample collection for bilirubin tests was determined by utilizing both the date and time of birth and blood sample collection.

Children were classified based on gestational age and birth weight percentile: those with a birth weight below the 10th percentile for their gestational age were categorized as small for gestational age (SGA), those with a birth weight above the 10th percentile were classified as large for gestational age (LGA), and those with a birth weight between the 10th and 90th percentile were deemed appropriate for gestational age (AGA). Information on family disposable income was obtained from Statistics Denmark [22].

Diagnoses of major congenital malformations in the neonatal period (within 28 days after birth) were extracted from the DNPR. Information on major congenital malformations (see Supplementary Table 2), other neonatal factors (see Supplementary Table 3) during the neonatal period, as well as maternal factors during pregnancy (see Supplementary Table 4), were also obtained from the DNPR.

Statistics

Characteristics of children were summarized for children with neonatal phototherapy and children without neonatal phototherapy among the total population and among children with available measurement of bilirubin. For children with measurement of bilirubin, we estimated mean and standard deviation of the maximum of bilirubin measured in the neonatal period and the age of neonates at time of bilirubin measurement.

Crude and adjusted hazard ratios (HRs) of epilepsy for children with neonatal phototherapy compared to those without neonatal phototherapy were estimated using Cox proportional regression models. Children were followed from day 29 after birth until the onset of epilepsy, death, emigration, or the end of follow-up on December 31, 2016. Age served as the time scale in the Cox proportional regression models.

Adjusted HRs of epilepsy were calculated in two ways, the traditional multivariable model and propensity score matching model, since the propensity score matching model enable us to take more covariates into account than the traditional one. In the analyses using the multivariable model [23], each factor (including sex, gestational age, intrauterine growth [SGA, AGA, LGA], congenital malformation, Apgar score, neonatal and maternal risk factors, birth year, family income, maternal age, and parity) was included as an independent variable. In the analyses using propensity score matching, propensity scores of neonatal phototherapy were computed based on child and maternal risk factors (27 in total, see Table 1). We employed a 1:3 ratio using nearest neighbor matching with a caliper of 0.1 of the standard deviation of the logit of the propensity score and ensured common support of propensity score (overlap in the propensity score distribution between the treatment and control groups) [24]. We did balancing test of neonatal and maternal factors between children with neonatal phototherapy and children without neonatal phototherapy before and after propensity score matching.

Table 1 Characteristics of children according to neonatal phototherapy among the total population (n = 65,365) and among children with available measurement of bilirubin (n = 9,378)
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We conducted similar analyses in the subgroup of children who had at least one bilirubin measurement in the neonatal period enabling us to further adjust for bilirubin related factors and estimate the influence of phototherapy itself on epilepsy risk. In the multivariable model, we further adjusted for gestational age- and age-specific quartiles of bilirubin level (1st, 2nd, 3rd, 4th quartile) and age at the time of bilirubin measurement (1st, 2nd,3rd, 4th, 5–7, 8–14, 15–21, and 22–28 days after birth). For propensity score matching, we included gestational age- and age-specific quartiles of bilirubin level and age at the time of bilirubin measurement in the calculation of the propensity score of neonatal phototherapy (see Table 1). To obtain gestational age- and age-specific quartiles of bilirubin, children were categorized into quartiles within each stratum based on gestational age (35, 36, 37, 38, or ≥ 39 gestational weeks) and age at the time of bilirubin measurement (< 48 h, 48–71 h, 72–95 h, 4–14 days, and 15–28 days).

All analyses were performed with STATA, version 16 (StataCorp LLC, College Station, TX, USA).

Results

In the total study population (n = 65,365), 958 (1.5%) children received neonatal phototherapy treatment in the neonatal period. Children undergoing neonatal phototherapy exhibited a higher prevalence of adverse conditions compared to those without neonatal phototherapy (Table 1). Over a mean follow-up period of 7.2 years (up to 15 years), seven children (incidence rate [IR] 10.8/10,000 person-years) were diagnosed with epilepsy among those receiving neonatal phototherapy, while 354 children (IR 7.7/10,000 person-years) were diagnosed with epilepsy among those without neonatal phototherapy. The adjusted hazard ratio (HR) for epilepsy in children with neonatal phototherapy compared to those without was 0.95 (95% CI: 0.43–2.09) in the multivariable model and 0.94 (95% CI: 0.39–2.28) in the propensity matching model (Table 2). In the analyses using the propensity score matching model, child and maternal factors were balanced between children with and without neonatal phototherapy (Fig. 1A, Supplementary Table 5).

Table 2 Hazard ratio of epilepsy for children with neonatal phototherapy compared with children without neonatal phototherapy (n = 65,365) in the Cox regression analyses with multivariable model and propensity score matching model for confounders adjustment
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Fig. 1
figure 1

Balancing test (standardized % bias) of child and maternal factors between children with neonatal phototherapy and children without neonatal phototherapy before (marked with a solid black dot) and after (marked with a cross) propensity score matching in the total population (A) and in a sub-population of children with a measurement of bilirubin in the neonatal period (B). In the analyses for the sub-population, two more factors were adjusted for including bilirubin level (quartile_bilirubin) and neonatal age at time of bilirubin measurement (age_bilirubin_test). The label of each factor and balancing test of these factors before and after propensity score matching in detail is presented at Supplementary Table 5

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In the subpopulation (n = 9,378) with bilirubin measurement in the neonatal period, 928 (9.9%) children received neonatal phototherapy. Similarly, children with neonatal phototherapy exhibited a higher prevalence of adverse conditions compared to those without neonatal phototherapy, except for birth asphyxia, acidosis, and neonatal convulsion/intracranial hemorrhage, where children with neonatal phototherapy showed lower prevalence rates of these factors (Table 1). Children who underwent neonatal phototherapy had higher bilirubin levels compared to children without neonatal phototherapy (Table 3). Bilirubin measurements for children with neonatal phototherapy were more likely to occur around 4–7 days after birth, while children without neonatal phototherapy had a more varied distribution of bilirubin measurements (Table 1, Table 3). During follow-up, seven children (IR 11.2/10,000 person-years) were diagnosed with epilepsy among those receiving neonatal phototherapy, compared to 86 children (IR 15.9/10,000 person-years) among those without neonatal phototherapy. The adjusted HR for epilepsy in children with neonatal phototherapy compared to children without neonatal phototherapy was 1.26 (95% CI: 0.54–2.97) in the multivariable model and 1.24 (95% CI: 0.47–3.25) in the propensity matching model (Table 4). In the analyses using the propensity score matching model, child and maternal factors were balanced between children with and without neonatal phototherapy (Fig. 1B, Supplementary Table 5).

Table 3 The distribution (mean and standard deviation, SD) of the maximum of bilirubin and the age of neonates when bilirubin was measured according to gestational age and neonatal phototherapy
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Table 4 Hazard ratio of epilepsy among children treated with neonatal phototherapy compared to children without neonatal phototherapy among children with a measurement of bilirubin in the neonatal period (n = 9,378) in the Cox regression analyses with multivariable model and propensity score matching model for confounders adjustment
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Discussion

This study showed that children who underwent neonatal phototherapy did not have an increased risk of epilepsy compared to children without neonatal phototherapy in the general population. We also found no increased risk of epilepsy associated with neonatal phototherapy in children who had bilirubin measurements, in which bilirubin level and age at the time of bilirubin measurement were further taken into consideration besides many other maternal and neonate’s factors. The finding was robust when accounting for potential confounders using different statistical models.

Our study findings diverged from those of previous studies conducted in Denmark [13] and the US [10]. Maimburg et al. (2016) reported a 1.6-fold (95%CI: 1.23–2.24) increased risk of epilepsy among children who underwent neonatal phototherapy, particularly in boys [13], whereas Newman et al. (2018) found a 1.3-fold (95%CI: 1.10–1.61) increased risk of seizures in boys who received neonatal phototherapy compared to those who had bilirubin measurements but did not receive neonatal phototherapy [10]. Methodological differences and variations in healthcare systems may contribute to these discrepancies. The study by Maimburg et al. (2016) collected information on neonatal phototherapy reported by mothers and included children born both ≥ 35 gestational weeks and below 35 gestational weeks [13]. The primary outcome in the US study by Newman et al. (2018) was defined as those with at least one seizure diagnosis and at least one antiseizure medication prescription, providing a slightly different perspective [10]. Notably, the proportion of children receiving neonatal phototherapy in the US study (7.6%) was higher than in ours (1.5% in the study population in which neonatal death were not included and 1.6% in the general population [17]), despite both focusing on children born ≥ 35 gestational weeks. Furthermore, the proportion of children receiving neonatal phototherapy in the US study increased significantly from 2.4% to 15.9% during the study period (1995–2011), [25] whereas the proportion in our study remained stable during the study period (2002–2016) [17]. Fortunately, we could conduct similar analyses by restricting to children with bilirubin measurement, akin to the US study.

The lower epilepsy incidence rate (unadjusted) and a non-significant increased risk after adjusting for potential confounders among children with bilirubin measurements undergoing neonatal phototherapy compared to children with bilirubin measurements without neonatal phototherapy, present an intriguing observation. It emphasizes the importance of adjusting for confounders and considering the complex interplay between phototherapy, bilirubin levels, and other potential risk factors for neurological outcomes. While phototherapy effectively reduces bilirubin levels, thereby mitigating the risk of bilirubin-induced neurotoxicity, it's possible that other mechanisms or confounding variables influence the risk of epilepsy independently [26].

Our study possessed both strengths and limitations. It was a population-based study with nearly complete follow up. Both information on neonatal phototherapy and serum bilirubin levels were derived from registers and recall bias was unlikely. The register provided detailed information on bilirubin measurements, enabling adjustment for bilirubin levels and measurement timing. However, information on the timing, duration, intensity, and frequency of neonatal phototherapy in our study was lacking, which may have impacted our ability to assess its effects comprehensively, for example dose–response relationship between neonatal phototherapy and risk of epilepsy. A Japanese observational cohort study demonstrated that a long duration of neonatal phototherapy was positively associated with the risk of allergic disorders in childhood [27]. Our study benefited from rich data on pregnancy, birth, and the post-natal period for both mothers and children. We employed two methods to adjust for potential confounders, including multivariable regression modeling and propensity score matching, with consistent findings. Our study's small sample size resulted in limited statistical power and our ability to conduct additional analyses, like stratification analyses for boys and girls. Perhaps therefore, this study was not able to assess previous findings of an increased risk of epilepsy among boys treated with neonatal phototherapy. [10, 13] While previous research has shown inconclusive results regarding a link between neonatal phototherapy and childhood neurological development, including epilepsy, our study adds to the growing body of evidence suggesting minimal to no risk of adverse neurocognitive effects. [28] However, larger and more comprehensive data sets, including timing and dose in terms of duration, intensity, and frequency of neonatal phototherapy are needed to elucidate relationship between neonatal phototherapy and children neurological development. Additionally, applying methodologies such as regression discontinuity design [29, 30] could provide rigorous causal inference regarding the safety of neonatal phototherapy on child neurological development [31,32,33,34]. Notably, our study took place in a setting where neonatal phototherapy was maintained at a reasonable low level (1.6% children born at 35 gestational weeks and later were registered with neonatal phototherapy). [17] Avoiding overtreatment of neonatal phototherapy may be recommended both to reduce health care costs and potential adverse effects, without increasing the rates of kernicterus. [28, 35]

Conclusions

Our study, based on a population-based cohort in Denmark, neonatal phototherapy for neonatal hyperbilirubinemia was not associated with an increased risk of epilepsy. The study had limited power, and further research is warranted to clarify the relationship between neonatal phototherapy and epilepsy risk.