Laterality defects are also known as defects of embryonic left-right axis patterning or malposition complexes. They include a spectrum of disorders that range in increasing severity from isolated dextrocardia or situs inversus (SI) abdominis to situs inversus totalis (SIT) or heterotaxy. The more severe laterality defects may include situs ambiguous, complex congenital heart defects (CHD), and spleen anomalies (asplenia, polysplenia) [reviewed by Van Praagh, 2006; Chin, 2012]. The cause of most cases of laterality defects remains unknown, although chromosome abnormalities [reviewed by Iida et al., 2006], other genetic causes [Zhu et al., 2006; Fakhro et al., 2011; Cohen, 2012], and maternal environmental influences [Martínez-Frías, 2001] are being increasingly identified as potential causes. Most cases of laterality defects do not have a monogenic etiology [Ferencz et al., 1997; Lin et al., 2000; Aylsworth, 2001], with a few notable exceptions, including X-linked heterotaxy [Gebbia et al., 1997; reviewed in Zhu et al., 2006], autosomal dominant heterotaxy [Alonso et al., 1995], and Kartagener syndrome (a form of autosomal recessive primary ciliary dyskinesia [PCD]) [Kennedy et al., 2007; Brueckner, 2007]. Regardless of the specific cause, laterality defects can be viewed as the result of a broad range of developmental disorders that impact right-left axis patterning [Brueckner, 2012].

In contrast to the abundance of genetic and developmental research, there have been few epidemiologic studies of laterality defects. Previous prevalence estimates drawn from hospital-based [Lin et al., 2000] and population-based studies [Ferencz et al., 1997; Reller et al., 2008; Khoshood et al., 2012] range from 0.9 to 1.7 per 10,000 births. The variability of these estimates is likely due to small sample size or differences in methodology, including case classification and ascertainment. The National Birth Defects Prevention Study (NBDPS) offers a unique opportunity to conduct descriptive epidemiologic analyses in a nationally representative population-based cohort of a relatively large sample of well-classified non-syndromic laterality defect cases.

The NBDPS is an ongoing, multi-site population-based case-control study of genetic and environmental risk factors for more than 30 categories of major structural defects [Yoon et al., 2001]. Cases were ascertained from birth defects surveillance systems at 10 study sites: Arkansas, California, Georgia, Iowa, Massachusetts, New Jersey (through 2002), New York, North Carolina (beginning 2003), Texas, and Utah (beginning 2003). Study cases are livebirths (all sites), fetal deaths ≥20 weeks (Arkansas, California, Iowa, Massachusetts, North Carolina, Texas, and Georgia), and elective pregnancy terminations (Arkansas, California, Iowa, Texas, and Georgia). Controls are singleton livebirths without major birth defects selected randomly from the same source population as cases through review of birth certificates or hospital records, matched for geographic area and birth year [Cogswell et al., 2010]. Cases are identified from the existing state surveillance systems and abstracted data are reviewed for study inclusion by the NBDPS clinical geneticist at each site [Rasmussen et al., 2003]. The NBDPS excludes cases with (1) the mildest end of the clinical spectrum of laterality defects, i.e., isolated dextrocardia or SI abdominis; or (2) a recognized or strongly suspected single-gene condition or chromosome abnormality. This means excluding cases with PCD, an etiologically heterogeneous Mendelian syndrome which overlaps with heterotaxy. In addition to the abstracted medical record data, mothers of both cases and controls are asked to consent to a computer-assisted telephone interview on potential risk factors for birth defects. The NBDPS has been approved by the CDC Institutional Review Board and the institutional review board for each participating site.

The study ascertained 517 cases with nonsyndromic laterality defects (heterotaxy 378, 73.1%; SIT 139, 26.9%) for an estimated birth prevalence of 1.1 per 10,000 (95% CI:1.0–1.2). The prevalence of heterotaxy (0.81, 95% CI: 0.73–0.89) was more than twice that of SIT (0.30; 95% CI: 0.25–0.35) (Table II). Prevalence varied across sites, but not significantly (for heterotaxy, ranging from 0.6 per 10,000 in New Jersey and New York, to 1.9 per 10,000 in California, and for SIT, ranging from 0.3 in New Jersey to 1.4 in California). Prevalence did not vary significantly over time (see Supplemental Table SI–II), or by the inclusion of pregnancy termination (data not shown). Laterality defects as a group, as well as the subtypes of heterotaxy and SIT, were significantly more common among preterm cases (less than 37 weeks gestational age) compared to term cases, and in mothers who were non-white (Hispanic, black, other) compared to those who were white non-Hispanic. All laterality defects combined were also more common among mothers younger than 20 years compared to those age 25–29.

The analysis for SGA included 8029 controls and 469 cases (344 heterotaxy, 125 SIT). Compared to controls, heterotaxy cases were nearly three times more likely to be born SGA (16.9% were SGA vs. 6.5% in controls; OR 2.92, 95% CI 2.17–3.92). This was not the case with SIT (8.0% vs. 6.5%, OR 1.25, 95% CI 0.65–2.40).

The majority of laterality defect cases (350/517, 67.7%) had complex CHD, whereas 9.3% (48/517) had simple CHD, and 23.0% (119/517) had no CHD (Table III). However, the presence and type of CHD varied significantly among laterality defect subtypes. Most cases of SIT had no CHD (82/139, 59.0%); conversely, complex CHD was present in 82.8% (313/378) of heterotaxy cases. For example, the following CHDs were more common among cases with heterotaxy compared to SIT (Table IV): double outlet right ventricle, complete atrioventricular canal defects, pulmonary stenosis, non-tetralogy of Fallot pulmonary atresia with ventricular septal defect, totally and partially anomalous pulmonary venous return, all ventricular and atrial septal defects, vena caval anomalies, and persistent left superior vena cava. A comparison of two familiar forms of heterotaxy showed patterns of CHD frequency (Supplemental Table SI–III), which are roughly similar to previous postmortem and clinical studies [Van Praagh, 2006; Foerster et al., 2008]. Among 66 (48.5%) patients with situs ambiguous, bilateral right-sidedness/right atrial isomerism (“asplenia”), there were significantly (P <0.001) more cases of totally anomalous pulmonary venous return, complete atrioventricular canal defect, and pulmonary atresia with ventricular septal defect (an NBDPS code which differs from tetralogy of Fallot). Also common, but less significantly, were all forms of single ventricle (P <0.01), d-loop transposition of the great arteries (P <0.05), and total cases of valvar pulmonary stenosis and pulmonary atresia/ventricular septal defect (not tetralogy of Fallot) (P <0.5). In contrast, among 70 (51.5%) cases with situs ambiguous, bilateral left-sidedness/left atrial isomerism (“polysplenia”), there were more cases of interrupted inferior vena cava (P <0.001) and primum-type atrial septal defect (P <0.01). As a group, extracardiac defects were more common in cases with heterotaxy than those with SIT, though this finding was statistically unstable due to small numbers in each defect category. Orofacial clefts, esophageal atresia, bowel atresia, and omphalocele were more frequent among heterotaxy cases than among SIT cases, while Dandy-Walker malformation, anotia/ microtia, and limb deficiencies were more frequent among SIT cases than heterotaxy cases (Table IV). In this comparison of extracardiac defects between heterotaxy and SIT, we excluded the abdominal defects that define the complex (i.e., situs ambiguous, spleen anomalies, malrotation, liver anomalies) and occur in many cases with heterotaxy.

Few phenomena are as fundamental in human development as the determination of the position of the body’s organs (sidedness) [Cohen, 2012]. Sidedness is the subject of numerous embryological and anatomic descriptive studies, with extensive research into its molecular genetic basis and developmental biology [Vandenberg and Levin, 2013]. Although laterality defects have been recognized for over a century, there is a lack of agreement about the best way to classify them for clinical management, genetic research, and pathologic studies, which has hampered epidemiologic analyses [Houyel et al., 2011]. This study provides a population-based descriptive analysis of laterality defects and its main phenotypic subgroups from a large multi-site study of birth defects using a systematic and common approach to case review and classification, based on detailed description of cardiac and extracardiac defects.

Over the past 30 years, birth prevalence estimates of laterality defects have been generated by hospital-based studies [Lin et al., 2000], regional cardiology programs [Fyler et al., 1980], and population-based [Ferencz et al., 1997; Reller et al., 2008; Bedard et al., 2012; Khoshood et al., 2012] studies (Table V). Estimates from these studies generally hover at slightly more than 1 per 10,000, ranging between 0.88 and 1.7 per 10,000, with variations by inclusion criteria and anatomic definition. Similar to the Baltimore-Washington Infant Study (BWIS), another population-based case-control study of CHDs, the NBDPS excluded syndromes. Whereas the BWIS used a less specific schema of combining all laterality and cardiac looping defects, the current study separated SIT from heterotaxy subtypes of laterality defects [Ferencz et al., 1997]. Excluded from comparison is the recent large population-based registry data (EUROCAT) [Dolk et al., 2011] which described only “Ivemark atrial isomerism” in their Severity Group II, and provided no data on this group. Despite differences from previous studies in the definition of the phenotypic spectrum (i.e., the inclusion of syndromes or ectopia cordis), the estimated prevalence in our study (1.1 per 10,000) is remarkably similar to that reported by recent population-based studies in Western Canada (1.3 per 10,000) [Bedard et al., 2012] and metropolitan Paris (1.2 per 10,000 [Khoshood et al., 2012], and by a hospital-based study of heterotaxy in Boston (0.99 per 10,000 for heterotaxy) [Lin et al., 2000]. All three studies included some form of familial cases, syndromes (chromosome and Mendelian gene) and/or “other anomalies”, which would generally be predicted to increase the prevalence of laterality defects. However, methodologic differences make detailed comparisons difficult. In the current study, prevalence differed insignificantly across sites with a tendency to lower (New Jersey) and higher (California) rates. It is difficult to know whether the variation is related to the surveillance program’s abstractors’ ability to pursue the type of diagnostic tests and consults which define the phenotypes, or whether there are regional differences among medical specialists such as pediatric cardiologists, geneticists, surgeons, and pathologists who may evaluate these patients differently.

The similarity of the sociodemographic characteristics data between SIT and heterotaxy cases in our study supports the notion that these phenotypes are part of a continuum of laterality maldevelopment. We acknowledge that the studied characteristics, which are relatively imprecise, may be insufficient to distinguish etiologic differences among the phenotypes. The higher rate of SGA among cases of heterotaxy could be related to the higher rate of complex CHDs or could be an indication of a more profound developmental abnormality in this group. Of note, there appears to be no significant impact on prevalence by excluding terminations of pregnancy from the analyses. However, only five sites had available data; terminations were not routinely collected by other surveillance systems.

The detailed analysis of sociodemographic factors in this study differs from other cohorts, notably, the BWIS which did not analyze maternal age and noted only that racial distribution was “slightly different” from controls [Ferencz et al., 1997]. That study compared the sex of births, noting more white females (66.1%) than males (59.1%) and nearly equal frequency of black males (33.1%) and females (31.0%), In the United Kingdom, there was no difference in the male:female sex prevalence ratios of “situs inversus” and “asplenia”. Using data from the Metropolitan Atlanta Congenital Defects Program, Miller et al. [2011] focused on CHDs and major structural noncardiac anomalies in which laterality defects (n =161) were classified as a unique malformation complex rather than a CHD. Table III in their paper showed similar trends with a significant difference in race distribution (black or African American more common than White and Other) and maternal age (< 35 years more common than >35 years). The authors noted the lack of homogeneity in their sample across all study years since Hispanic ethnicity was not recorded during entire time period. Sokal et al. [2014] studied major congenital anomalies in the United Kingdom as part of a national population-based study and international meta-analysis. The laterality defects were limited to “situs inversus” and “asplenia” for which they found no significant difference in the male:female prevalence ratios.

Laterality defects pose a challenge to epidemiologic analysis because of their tremendous anatomic complexity and phenotypic diversity which this study tried to address through careful case classification and rigorous review. This study’s prevalence estimates from a racial-ethnic and geographically diverse cohort provides helpful information in addressing the impact of these rare but complex conditions. In addition, the epidemiologic similarities in socio-demographic patterns between heterotaxy and SIT suggest that they are part of a spectrum of abnormalities in the determination of sidedness, and seeking commonalities in etiology and pathogenesis between these two classes of conditions could prove fruitful. This information may help guide clinical care, future research, and prevention strategies.