26 Isomerism of the Atrial Appendages Rohit S Loomba, Diane E Spicer, Lauren E Raczka, Robert H Anderson Abstract Isomerism, or so-called heterotaxy, is a multiorgan entity that has anatomic and functional consequences In the thorax, there is mirror imagery of the usually lateralized organ systems In the heart, the mirror imagery is seen in the atrial appendages, which can be of either left or right morphology Clinicians need to recognize isomerism and to be aware of its associated findings because they impact both morbidity and mortality As many children with isomerism will require cardiac surgery, this recognition is very important for cardiologists Patients with isomerism may have anomalies of the nervous, sinopulmonary, pulmonary, cardiac, gastrointestinal, genitourinary, hematologic, immune, and reproductive systems The genetic underpinnings of isomerism are still being clarified, with multiple genes having been implicated Many of these genes also play a role in ciliary function This is of particular interest, as many of those with isomerism also have ciliary dysfunction Thus the genetics and embryology of isomerism may be related to those of ciliary dysfunction Children with isomerism who have functionally univentricular congenital heart disease have decreased survival compared to those with the same congenital heart disease but without isomerism Decreasing this difference depends on thorough knowledge of the anatomic and functional consequences of, the inheritance of, and symptomatic care of, isomerism Key Words Isomerism; heterotaxy; asplenia; polysplenia; functionally univentricular; Kawashima; Fontan; Glenn; ciliary dyskinesia