Prevalence of Congenital Cardiac Disease

Introduction

Examination of the frequency of congenital cardiac disease, either as a rate or as a proportion, has important implications for the study of congenital cardiac malformations, as well as their clinical management. However, there is much confusion and misuse regarding terminology and methodology, with important implications for the accuracy, validity, and comparability of findings reported in the published literature. Knowledge of how critically to appraise these reports is important in defining their value when applied to issues of diagnostic likelihood, surveillance and trends, etiologic associations, burden of disease, and requirements for resources. These issues have more recently been impacted by fetal diagnosis and termination, with individual decisions influenced by contemporary estimates of prognosis related to the natural and modified natural history. As more patients survive into adulthood, estimates of the burden of disease, and the requirements for resources, have also achieved greater importance. Therefore the question of “how much congenital cardiac disease?” continues to evolve. Providing the correct answer has important relevance for both the providers of health care and the health care system itself.

Definitions

In considering and discussing the prevalence of congenital heart disease, key terms to be defined include congenital cardiac disease itself, frequency, ratio, proportion, rate, incidence, and prevalence.

Congenital Cardiac Disease

In examining reports of congenitally malformed hearts, it is important to know how the lesions were defined and what conditions were included or excluded. However, the definitions in Table 13.1 do not address all controversy. There is no current consensus as to whether several groups of lesions should be considered to represent congenital cardiac malformations because they do not manifest clinically until later in life. These conditions include genetic conditions, arrhythmic conditions, primary cardiomyopathies, and structural defects that do not have functional importance in many, but not all, circumstances. Considerations regarding the inclusion or exclusion of these lesions are important because these lesions are common and their inclusion might inflate a prevalence estimate. Comparability of previous reports has somewhat suffered from a lack of common nomenclature regarding the description and classification of congenital heart defects. To some extent, there has been important work in consolidating nomenclature toward an internationally accepted standard, which may facilitate future work.

Table 13.1

Clinical Definitions Used in Identifying the Frequency of Congenital Cardiac Disease

Term	Definition and Examples
Congenital cardiac disease	A gross structural abnormality of the heart or intrathoracic great vessels that is actually or potentially of functional significance. Excludes normal variants without functional consequence (e.g., persistent patency of the left superior caval vein, or abnormal patterns of branching of the systemic arteries).
Genetic conditions	Present from conception and may not have manifest cardiovascular consequences until much later in life (e.g., Marfan syndrome, Williams syndrome, and hypertrophic cardiomyopathy).
Arrhythmic conditions	Abnormalities at the physiologic or ultrastructural level that result in arrhythmias (e.g., long QT syndrome and ventricular preexcitation).
Primary cardiomyopathies	Myocardial structural abnormalities with a genetic or metabolic etiology (e.g., hypertrophic cardiomyopathy and ventricular noncompaction).
Structural defects with functional abnormalities only in some cases	Defects that do not have functional significance in many, but not all, circumstances or resolve without becoming clinically manifest (e.g., aortic valve with two leaflets, the prolapsing mitral valve, silent persistent patency of the arterial duct or small septal defects, included patency of the oval foramen).

Ratios, Rates, and Proportions

Confusion and misuse regarding the terms incidence and prevalence are rooted in misunderstanding regarding what constitutes a ratio, a rate, and a proportion. These definitions are summarized in Table 13.2 .

Table 13.2

Statistical and Methodologic Definitions, Some Considerations, and Examples Used in Identifying the Frequency of Congenital Cardiac Disease

Term	Definition, Some Considerations, and Examples
Ratio	A relationship between two quantities where the numerator and a denominator are mutually exclusive (e.g., the ratio of males to females with transposed arterial trunks).
Proportion	The frequency of a condition or category within the population specified in the denominator independent of time (e.g., the proportion of muscular septal defects among all ventricular septal defects).
Rate	The frequency of occurrence of a condition that takes into account the time period over which the numerator was accumulated (e.g., the rate of the occurrence of a thrombotic episode in patients during their first year subsequent to the Fontan operation).
Kaplan-Meier estimates	The cumulative frequency of occurrence of a condition that reflects instantaneous changes in rates and risk over time and, unlike rate, does not assume that the condition occurs uniformly over the specified period of time (e.g., freedom from of a thrombotic episode in patients subsequent to the Fontan operation over 10-year period, showing maximal occurrence in first postoperative year).
Incidence	The rate of new occurrences of a condition over a specified period of time in a population at risk (e.g., new cases per year of Kawasaki disease per 100,000 children <5 years).
Prevalence	The proportion of preexisting and new occurrences of a condition identified in a population at risk, either at a single point in time or over a specified period (e.g., prevalence of congenital heart disease at birth amongst all live born individuals).
Denominator	Represents the population of interest or at risk over a defined time period and should be the first consideration when estimating the prevalence.
Numerator	Represents the number of cases of congenital heart disease that were identified from a population at risk over a defined period of time.
Ascertainment of cases	The completeness of identification of all the cases from the population at risk. This is a key aspect of appraisal of the numerator of prevalence estimates (e.g., active surveillance is better than nonmandatory reporting).
Verification of cases	The accuracy of definition or classification of all cases. The method of verification can affect the prevalence (e.g., clinical identification versus autopsy, angiography, and echocardiography).
Definition of cases	The specification and use of clear and standard definitions of congenital heart lesions described with accepted nomenclature. It is also important to consider which lesions were included and excluded.

Incidence and Prevalence

Using the definitions in Table 13.2 , what many mistakenly refer to as the incidence of congenital cardiac disease is in reality the prevalence, consisting of the number of newborns who are subsequently confirmed to have congenitally malformed hearts as observed within a defined population of live born individuals over a specified time. However, incidence and rates are very relevant in the study of prognosis and the natural and modified natural history.

The prevalence at or for a variable period following live birth is important in defining the maximal burden of congenital cardiac disease in the population. The natural and modified natural history specific to certain defects and strategies for their management influence the changing prevalence in the population over time, with the numerator and denominator both decreasing due to deaths, and the numerator perhaps decreasing due to spontaneous resolution of some lesions. The absolute number and characteristics of patients at any given time point are important for defining the burden posed by disease, which is a key determinant in defining the requirements for resources. With the increasing survival of patients into adulthood, and their transition into the system of health care providing for adults, this number has taken on increasing importance, but its accurate estimation is fraught with numerous methodologic challenges.

Denominator

The definition of the denominator is a key piece of information because it characterizes the population to which the estimate of incidence or prevalence may be applied. Knowledge of the population characteristics is also necessary to evaluate similar populations to which the estimate might be extrapolated. The most valid denominators are ones that are enumerated during the time of ascertainment of the cases, which is best achieved by using a prospective cohort study design. However, such an approach is intensive in terms of both time and resources. Most estimates of prevalence use as their denominator the total number of live births occurring over a specified period of time derived from a geographically defined population.

A convenient and readily available denominator that is used frequently is the number of births reported to a governmental or administrative system of registration or vital statistics. If used, information regarding the completeness of ascertainment/reporting of births, usually through audits of the data, should be sought. Some registries will include stillbirths and fetal deaths, and this information should be clearly indicated. Registries of births usually collect additional information, specifically demographics of the birth parents, but may also include data regarding clinical diagnoses and characteristics present or evident at birth. Some estimates of prevalence will additionally rely on this information for ascertainment of cases. If so, then the validity and reliability of that data need to be carefully scrutinized.

Numerator

To evaluate the numerator of a prevalence estimate, one must know how the cases were identified and verified from the data source, what types of lesions were included and excluded, and what nomenclature and classification scheme was used.

Ascertainment of Cases

A comprehensive and active prospective surveillance of all sources of cases is likely to yield the most complete ascertainment because case ascertainment is a specific and planned endeavor. Most studies of this nature rely on clinical presentation or evaluation of a living subject as the initial entry point. The duration of follow-up for case ascertainment must be sufficiently long that all cases are identified, especially because some important congenital heart disease may not manifest very early in life.

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