Heterotaxy

A patient with abnormal organ situs (left-right patterning defect). The first distinction is between situs inversus totalis (a complete mirror image of normal anatomy: dextrocardia + right-sided stomach + left-sided liver) and heterotaxy (situs ambiguus, with chaotic asymmetric placement of thoracic and abdominal organs, almost always with complex congenital heart disease). Situs inversus totalis is often clinically silent; heterotaxy is high-stakes from birth because of the cardiac, immune, and biliary comorbidities that ride along with it.

Three questions structure the work-up:

1. Situs inversus totalis vs heterotaxy? Mirror-image anatomy with concordant chamber relationships behaves benignly in most cases. Heterotaxy is the dangerous bucket and divides further into two flavors.
2. Right vs left isomerism? Each has a recognizable visceral and cardiac signature and a different downstream management protocol.
3. Cilia dysfunction signs? Chronic sinopulmonary disease, otitis, male infertility. Combined with situs inversus this is Kartagener / primary ciliary dyskinesia; cilia drive embryonic left-right determination, so PCD has situs inversus in ~50% of patients.

Right isomerism (asplenia syndrome, Ivemark)

Bilateral right-sided structures throughout the body.

• Spleen: asplenia (often complete absence).
• Lungs: bilateral trilobed lungs with bilateral eparterial bronchi.
• Atria: bilateral right atria (no morphologic left atrium).
• Liver: midline.
• Cardiac: complex CHD is the rule (AVSD, double-outlet right ventricle, transposition, pulmonary stenosis or atresia, totally anomalous pulmonary venous return). Dextrocardia common.
• Immune: functional asplenia → high risk of pneumococcal sepsis. Lifelong penicillin prophylaxis + full asplenia immunization schedule.

Left isomerism (polysplenia syndrome)

Bilateral left-sided structures throughout the body.

• Spleen: polysplenia (multiple small spleens).
• Lungs: bilateral bilobed lungs with bilateral hyparterial bronchi.
• Atria: bilateral morphologic left atria.
• IVC: interrupted IVC with azygos continuation to the SVC (almost pathognomonic on imaging).
• Biliary: biliary atresia risk (the polysplenia variant of biliary atresia, ~10% of all BA cases).
• Cardiac: generally less severe than right isomerism but still complex AVSD and conduction disease (heart block from abnormal AV node development).

Genes (high-yield)

• ZIC3 (X-linked heterotaxy): the most common Mendelian cause; males severely affected with complex CHD + situs anomalies, carrier females usually unaffected but can have mild features.
• NODAL (AD): heterotaxy with variable expressivity.
• LEFTY2 (AD): heterotaxy.
• CFC1 (Cryptic, AR or AD): heterotaxy, often with conotruncal CHD.
• GDF1 (AR): heterotaxy.
• MMP21 (AR): heterotaxy.
• Primary ciliary dyskinesia (AR; DNAH5, DNAH11, DNAI1, many others): situs inversus in 50% (Kartagener triad: situs inversus + chronic sinusitis + bronchiectasis); also heterotaxy in a smaller subset because nodal cilia set up the body axis.

1. Abdominal ultrasound for spleen number, liver position, IVC anatomy.
2. Echocardiogram to characterize cardiac anatomy fully; look for AV node disease.
3. Chest x-ray for bronchial branching pattern (eparterial vs hyparterial), lung lobation, and cardiac position.
4. Howell-Jolly bodies on peripheral smear to confirm functional asplenia in right isomerism.
5. Pneumococcal vaccination schedule + daily penicillin prophylaxis for any patient with asplenia or functional asplenia.
6. Hepatobiliary work-up (direct bilirubin, HIDA, liver ultrasound) in any infant with polysplenia + cholestasis. Time-critical for Kasai.
7. Chromosomal microarray as first-tier genetic test, then heterotaxy panel or exome. Trio testing is high yield given the de novo and X-linked options.
8. Nasal nitric oxide screening + PCD gene panel if any cilia-disease signs are present.