Abdominal pain is a common complaint in patients of all ages. Thirty-eight percent of 8- to 15-year-old children report abdominal pain weekly (1). Consultations for abdominal pain account for 2% to 4% of all pediatric office visits (2,3). A review of health-related calls to a US telephone medical triage center found that abdominal pain was the most common complaint among adults (8%) (4). Within the pediatric gastrointestinal (GI) community, the presence of a pattern of higher frequency of abdominal pain consultations in winter than in summer months has long been perceived, and is widely accepted. This perceived pattern has led investigators to conduct multiple community- and clinic-based investigations that consistently confirmed that both abdominal pain consultations and abdominal complaints at the school level followed a distinct seasonal pattern (1,5). In contrast, the presence of a seasonal effect in abdominal pain is not commonly accepted by the adult GI community, and it is not considered as a factor in adult epidemiological studies on functional gastrointestinal disorders (FGIDs). The absence of extensive adult epidemiological data and the lack of comparative studies between adult and pediatric populations do not allow a conclusion of whether perceived differences between adult and pediatric specialists constitute a real epidemiological difference or a mere difference in perception.
The pathogenesis of functional abdominal pain is complex and incompletely understood. The most accepted framework, the biopsychosocial model, proposes that functional abdominal pain results from the interaction of biological, psychosocial, and environmental factors in predisposed individuals (6). The presence of a seasonal periodicity in the prevalence of FGIDs should be viewed within the context of a multifactorial model. Seasonal periodicity in the prevalence of FGIDs likely reflects the complex interplay between the different subsystems of this biopsychosocial model. Various factors including disturbances of the circadian time structure of the GI tract, seasonal variations in stress, and climatologic or other environmental factors could explain the seasonality in presentation of abdominal pain. It is plausible that some of these factors may affect children and adults differently. A clearer characterization of the seasonal patterns of abdominal pain in children and adults may provide further insight into the pathophysiological framework associated with FGIDs and support previous clinic-based studies (1,5).
We have conducted a large epidemiological study to investigate consultation patterns of abdominal pain in adults and children. The present investigation compares abdominal pain consultation patterns across a large database of nationally insured adults and children and assesses possible seasonal, regional, and sex differences. We hypothesized that there was a seasonal effect on the frequency of abdominal pain consultations in children but not in adults.
This was a retrospective claims database analysis using eligibility and medical claims data from a large US health plan affiliated with i3 Innovus (Medford, MA). The individuals covered by this health plan are geographically diverse across the United States. The database combines health care records from >30 different contributing managed-care plans for approximately 42 million covered lives. The health plans are primarily from the Northeast, Mid-West, South Atlantic, and West regions.
Medical claims or encounter data are collected from all available health care sites (inpatient hospital, outpatient hospital, emergency room, physician's office, surgery center, and so on) for virtually all types of provided services, including specialty, preventive, and office-based treatments. Each individual provider service record contains information on up to 4 diagnoses, recorded with International Classification of Diseases-9 (ICD-9)-CM diagnosis codes.
Sample Selection and Study Design
Patients with a diagnosis of abdominal pain of nonspecified origin (ICD-9 code 789.0) from May 2000 to December 2008 were identified from the database; all cases with the ICD-9 code of 789.0 were included, regardless of the number of diagnoses and claim-filing position. Study subjects with ≥1 claim with a diagnosis of abdominal pain of nonspecified origin were termed as abdominal consultations. Additional ICD-9 codes for pain associated with other conditions (eg, IBD, surgical procedures, and so on) were analyzed to assess the number of patients associated with these additional codes; however, as they proved to represent <1% of the study group, they were not included in any further analyses. Patients were categorized as children (ages 5–17 years) and adults (age 18 years or older). Children younger than 5 years were excluded from the analysis. The primary outcome measure was the rate of abdominal pain consultations (total number of abdominal pain consultations/total number of distinct patients by month × 1000) by season in children and adults. To analyze the variation in consultation frequency, we selected 4 time periods. The time periods were defined as winter (December–February), spring (March–May), summer (June–August), and fall (September–November). Sex and regional differences were explored by season to support the primary analysis.
Descriptive statistics (ie, frequency counts) were used to examine the distribution of patients across seasons for sex, age group, and region. Additionally, cross-tabulation by sex and age group was conducted by season. Linearity was qualitatively evaluated to assess the degree of linearity among adults and children. Percentage point difference for children by season was calculated as (Summer Consultations − Alternate Season Consultations)/(Summer Consultations). Among children, subanalyses by age 5 to 11 years and 12 to 17 years and sex were conducted to assess whether the seasonal patterns were consistent within these subgroups.
A total of 172.4 million distinct patients (17.3% children, 82.7% adults) were identified in the administrative claims database between May 2000 and December 2008 (Fig. 1). The adult population was 63.5% females and the average age was 42 years (range 18–88); the child population was 59.2% females and the average age was 11 years (range 5–17). During the same time period, a total of 15.6 million patient consultations with abdominal pain (ICD-9 789.0) were identified (10.1% children, 89.9% adults) (Fig. 1).
Abdominal Pain Consultations by Month and Season
Children demonstrated a nonlinear seasonal trend in abdominal pain consultations, with consultation rates 20% to 25% lower during the summer months (June–August) compared with other seasons of the year (Fig. 2). Seasonal patterns best fit a quadratic regression curve (y = 5.571x3−39.315x2 + 79.987x + 8.357). In contrast, seasonal trends among adults appeared linear, with abdominal pain consultations ranging between 2% and 7% higher during the summer months compared with other seasons (Fig. 2).
Abdominal Pain Consultations in Children Stratified by Age and Sex
Seasonal patterns in abdominal pain consultations among children stratified by age and sex remained consistent with the overall child population, with abdominal pain consultations lowest during the summer months for each subgroup (Fig. 3A and B). Abdominal pain consultations were consistently higher among females, as compared with males. In children ages 5 to 11 years (Fig. 3A), the ratio of abdominal pain consultations (females:males) remained remarkably constant throughout the different seasons at approximately 1.4:1.0; whereas among children ages 12 to 17 years (Fig. 3B), the proportional ratio of females to males again remained constant throughout the seasons but at a higher rate of 2.1:1.0.
Depression and Anxiety Consultations by Month
Depression and anxiety consultation patterns for children followed similar seasonal trends to those observed for childhood abdominal pain (Figs. 4 and 5); consultation patterns for both depression and anxiety followed a quadratic regression curve, with consultation rates 5% to 25% lower during the summer months (June–August) compared with other seasons of the year. For adults, seasonal depression and anxiety consultation patterns paralleled those for abdominal pain, and followed a linear curve (Figs. 4 and 5).
This is the first study comparing abdominal pain consultations in children and adults. Our study has shown that both the pediatric and adult gastroenterologists’ perceptions of presence and absence of seasonal patterns of abdominal pain consultations, respectively, were true. Our large retrospective study of an administrative claims database supports previous work demonstrating seasonal variation in the presentation of abdominal pain complaints and consultations for chronic or recurrent abdominal pain in children (1,5). The present study found consultation rates for abdominal pain in children best fit a quadratic regression curve with lowest rates found during the summer months. When stratified by age and sex, seasonal patterns in abdominal consultations remained consistent with the overall child population, which proves the reliability of the observed overall trends. In contrast, seasonal consultation patterns for abdominal pain in adults were not evident.
The reasons behind the seasonal variation in abdominal pain in children remain unknown. Parental influences should be considered. Parents intervene in the decision of consulting in children and not in adults. Abdominal pain during winter months, at times that children have to attend school, may be more worrisome to the parents than in summer months. Still, we cannot explain the seasonality in consultation merely by parents being keener to consult during winter months because previous studies have also shown seasonality of symptoms in children who did not consult (1,7) and in children consulting the school nurse on their own (8). We can only speculate at the mechanisms by which consultation patterns for abdominal pain differ in children and adults.
The biopsychosocial model, the most accepted framework explaining the pathogenesis of FGIDs, is complex and not completely understood. This model proposes that FGIDs result from the interplay of multiple biological, psychological, and social factors in predisposed individuals. The type and relative importance of the different biopsychosocial factors involved in the pathogenesis of FGIDs are likely to differ in children and adults. Potential areas worthy of further exploration include stress/anxiety, sleep, melatonin metabolism, infections, vitamin D, dietary variations, and other school-related factors.
Studies in children have described a positive association among stress (9), higher anxiety and depression scores, and increasing abdominal pain (1,10,11). The authors propose that stress and anxiety may trigger GI symptoms in patients with autonomic liability (9). A recent study described decreased pain tolerance in response to stress in children with recurrent abdominal pain (12). Given the similar seasonal patterns in consultation rates for abdominal pain, depression, and anxiety found in our study, one may speculate that school-related stress and anxiety may explain our findings; however, the relation between these factors may not be linear or easy to explain. A study in children who completed daily assessment of abdominal pain during winter and summer months showed no variation in abdominal pain during weekdays or weekends during school time or vacations, results that suggest that factors other than school stress may contribute to explain the seasonal variation (13). In agreement with our present study, the study showed higher intensity of pain during winter than summer months. One or more factors may explain the described differences in abdominal pain patterns between adults and children. Altered sleep is highly prevalent in children with abdominal pain–associated FGIDs (14). Changes in sleeping patterns between winter and summer are more likely to occur in children who attend school than in adults who are more likely to have a similar working schedule during the entire year. Relationships between circadian-phase delays/advances as well as other sleep disturbances have been associated with GI disturbances and FGIDs (15–23). Serotonin and melatonin are both essential neurotransmitters with distinct roles in regulating mood and GI motility. Serotonin is transformed through acetylation and methylation into melatonin and is an active compound with effects in multiple organs and systems including the central nervous system and GI tract (24). Serotonin-modulating drugs are frequently used in the treatment of FGIDs. Melatonin is known to be a major regulator of the “internal clock” and sleep-wake cycles, and interestingly, has been shown to improve irritable bowel syndrome (IBS) symptoms including abdominal pain (25–28). Recent studies investigating the role of melatonin in the pathogenesis of IBS have reported lower levels of melatonin in patients experiencing IBS compared with control subjects (29,30). The seasonal periodicity in abdominal pain consultations found in the present study may reflect disturbances in melatonin metabolism and secretion resulting from seasonal changes in environmental stimuli. Whether school-related stress influences or a combination of other factors including changes in melatonin or other environmental modifiers are responsible for the seasonal variation of abdominal pain consultations found in the present study cannot be answered through our design.
Environmental factors may help explain differences between adults and children. During school times, children spend several hours in class in close contact with other children increasing the chances of acquiring viral infections. Viral acute gastroenteritis has been associated with abdominal pain lasting for few months (31). It is plausible that mild forms of viral infections may manifest with only few abdominal symptoms. Transient GI neurogenic inflammation could explain the presence of abdominal pain in a subgroup of children. Similarly to the case of abdominal pain, headaches, another disorder explained by the biopsychosocial model, are more common in children during winter than in summer months (32). The seasonal variation of headaches has been linked to the seasonal variations of vitamin D levels. Several studies have shown an association between low vitamin D levels and higher incidence of chronic pain (33,34) and inflammatory conditions (35). A recent study has shown a higher incidence of Crohn disease and ulcerative colitis in higher latitudes than in lower latitudes. Vitamin D status has been proposed as one of the possible mechanisms explaining this geographical association (36). A population study has shown that present American Academy of Pediatrics daily vitamin D recommendations are inadequate for maintaining vitamin D sufficiency in children (37). Dietary insufficiency and inadequate sunshine exposure both may contribute to lower vitamin D levels in children. Dietary variations between summer and winter are more likely to occur in children who eat at school than in adults who may have more consistent dietary habits throughout the year. Food allergies and intolerances have been associated with FGIDs (38). Parents frequently report that abdominal pain symptoms occur after their children consume specific types of foods. Children eating at school have a lower ability to select their food what may contribute to the presence of abdominal pain in a subset of children. Other school-related factors may also play a role in the seasonal variation of abdominal pain in children. Children frequently refuse to use school bathrooms. Stool withholding and constipation may also result in abdominal pain.
The limitations of our study are those inherent to the retrospective design and the use of an established database. In the absence of an ICD-9 code for functional abdominal pain, we were obligated to use a surrogate for this diagnosis. A review of the ICD-9 code revealed that code 789.0 (unspecific abdominal pain) most closely correlated with functional abdominal pain. Because of the lack of an ICD-9 code specific to functional abdominal pain, it is possible that other subjects who may qualify as functional abdominal pain and were included in the database with other code may have not been included in our study. The ICD-9 diagnosis for each patient was provided by a physician at the time the patient consulted, and we relied on the accuracy of this diagnosis for our study analysis. The research team did not confirm the ICD-9 diagnosis. Patients in the database were unidentified and it is possible that some patients could have consulted on >1 occasion; however, the extremely large number of patients included in our study most likely accounts for the event of repeated visits in some of the subjects. Other limitations of our study include our inability to definitively explain our findings and longitudinally track each of the children to establish individual or familial patterns of consultation throughout the years. Future studies should prospectively investigate these and other factors through validated questionnaires and biochemical and environmental investigations administered throughout the whole year.
The large number of consultations analyzed across several years and the widespread distribution of our population constitute significant strengths of our study. Our study may open a new line of investigation that may lead to a better understanding of the seasonal variation found in GI and extra-GI disorders such as acute myocardial infarction, leukemia, eating disorders, anxiety, and mood disorders (39–48). In addition, our study has shown that similar phenotypes and motives of consultation may likely result from the interplay of different factors in children and adults or that similar factors influence adults and children differently.
Previous studies considering the sex-specific prevalence of recurrent abdominal pain in children have provided a range of results, possibly reflecting changing prevalence rates on moving from childhood through adolescence (49–54); however, the overall evidence to date suggests that in childhood (younger than 8 years), there are no significant differences between males and females in the prevalence abdominal pain symptoms, whereas in older children (8 years or older), there is a female predominance in symptoms (2,52). The present investigation found female predominance in abdominal pain consultations in both age groups studied (5–11 years and 12–17 years); however, differences in consultation rates increased dramatically in the older age group, with female pain consultations rates more than double that found in males. The observed female predominance in pain consultations in the older age group may reflect hormonal changes associated with the onset of puberty.
In conclusion, the present investigation reports seasonal variation in abdominal pain consultations among children, with higher rate of pain consultations observed during the winter months. We speculate that interplay between the psychological and environmental (circadian-phase disturbances) components of the biopsychosocial model proposed for the pathogenesis of FGIDs may be responsible for these trends.
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