Right atrial isomerism is characterized by bilateral triangular, morphologically right atrial, appendages, both joining the atrial chamber along a broad front with internal terminal crest.

Heterotaxy ('heter' meaning 'other' and 'taxy' meaning 'arrangement'), or situs ambiguus, is a developmental condition characterized by randomization of the placement of visceral organs, including the heart, lungs, liver, spleen, and stomach. The organs are oriented randomly with respect to the left-right axis and with respect to one another (Srivastava, 1997). Heterotaxy is a clinically and genetically heterogeneous disorder. For a discussion of genetic heterogeneity of visceral heterotaxy, see HTX1 (306955).

Primary ciliary dyskinesia-18 (CILD18) is an autosomal recessive disorder characterized by early infantile onset of recurrent sinopulmonary infections due to ciliary dysfunction and impaired airway clearance. Males are infertile and about half of patients have situs inversus. Electron microscopy of cilia shows a defect of the outer and inner dynein arms and impaired ciliary function (summary by Horani et al., 2012).

Primary ciliary dyskinesia-35 (CILD35) is an autosomal recessive disorder characterized by recurrent upper and lower respiratory infections due to defective ciliary function. Examination of respiratory cilia shows lack of outer dynein arms (ODAs) and immotile cilia. Some patients may have laterality defects (summary by Wallmeier et al., 2016). For a phenotypic description and a discussion of genetic heterogeneity of primary ciliary dyskinesia, see CILD1 (244400).

Visceral heterotaxy-12 (HTX12) is an embryonic developmental disorder characterized by defects in the asymmetric positioning of visceral organs across the left-right axis, known as laterality defects. The phenotype is highly variable, ranging from complete organ reversal (situs inversus totalis) to selective misarrangement of organs (situs ambiguus) such as the liver, spleen, and pancreas. The disorder is often associated with dextrocardia or variable complex congenital heart defects. Early death may occur in the most severe cases (summary by Szenker-Ravi et al., 2022). For a discussion of the genetic heterogeneity of visceral heterotaxy, see HTX1 (306955).

Primary ciliary dyskinesia-52 (CILD52) is an autosomal recessive disorder characterized by laterality defects and mild respiratory symptoms due to subtle ciliary beating defects (summary by Leslie et al., 2022).