Terminology

  Heterotaxy: "heteros" means different, "taxis" means arrangement (from the Greek).

      The internal thoracic organs and the abdominal organs exhibit abnormal left-right relations

      in heterotaxy syndromes.

  Isomerism: "isos" means equal, "meros" means part (from the Greek).

      Isomerism refers to the similarity of bilateral structures that are normally dissimilar.


  Right isomerism: Bilateral structures with morphologic right characteristics, such as

            bilateral morphologic right bronchi,

            bilateral trilobed lungs,

            bilateral right atria,

            atrial appendages exhibiting right morphologic features.

  Left isomerism: Bilateral structures with morphologic left characteristics, such as

            bilateral morphologic left bronchi,

            bilateral bilobed lungs,

            bilateral left atria,

            atrial appendages exhibiting left morphologic features,

            bilateral superior vena cavae attached to bilateral morphologic left atria,

            an absent or atretic sinoatrial node,

            common atrium,

            common atrioventricular valve, atrioventricular septal defect,

            partial anomalous pulmonary venous connection.


  Atrioventricular discordance: A morphologic right atrium connects to a morphologic left ventricle,

      and a morphologic left atrium connects to a morphologic right ventricle.

  Ventriculoarterial discordance: A morphologic right ventricle gives rise to the aorta,

      and a morphologic left ventricle gives rise to the pulmonary trunk.

  Double discordance: Atrioventricular discordance together with ventriculoarterial discordance.

      The result is physiologically correct circulatory flow.




Heterotaxy syndromes,

  Heterotaxy syndrome affects up to 1 in 10,000 live births and is associated with high mortality rates.

  There may be levocardia, dextrocardia, or mesocardia (midline or indeterminate apical orientation).

  Left isomerism is usually associated with polysplenia.

  Right isomerism is usually associated with asplenia (
Ivermark syndrome).

  A chest radiograph allows appreciation of tracheobronchial anatomy, the position of the cardiac apex,

      the gastric bubble, liver, and spleen.

  The tracheobronchial branching pattern usually reflects atrial situs,

      a symmetrical branching pattern being suggests atrial isomerism.

  Discordance of the cardiac apex and abdominal viscera is often suggestive of heterotaxy.

  The sinus node is usually present bilaterally in right isomerism and absent bilaterally in left isomerism,




Right Atrial Isomerism

  There is no left atrium to receive pulmonary venous drainage,

      and total anomalous pulmonary venous drainage into a systemic vein is seen in > 50% of cases.

  Right atrial isomerism is usually associated with severe cyanotic heart disease in infancy.

  Accompanying heart defects may include transposition of the great arteries, common atrioventricular

      (AV) valve, ventricular hypoplasia or single ventricle physiology, pulmonary atresia and pulmonary vein

      obstruction.

  The syndrome is associated with a poor prognosis and high mortality rate (80%-90%) in the first year of life.




ECG in Right Atrial Isomerism

  The sinus node is usually present bilaterally in right isomerism.

  The sinus nodes are paired because bilateral superior vena cavae are attached to

      bilateral morphologic right atria.

  The atrioventricular conduction system is equipped with 2 nodes often but not always connected

      by a sling of tissue.

 
Alternating QRS patterns can be seen due to antegrade conduction through either one of the two AV

      nodes
.

  The P-wave axis is generally normal because the right sinus node is usually the dominant atrial pacemaker.

 
Supraventricular tachycardia is attributed to reentry between paired atrioventricular nodes.



Left Atrial Isomerism

  In left isomerism there is often polysplenia.

  The most common associated cardiovascular anomalies in polysplenia are noncyanotic congenital heart

      defects, including atrial septal defect, endocardial cushion defects, partial anomalous pulmonary venous

      return, and up to 50% of cases are associated with dextrocardia.

  The most distinctive and therefore the most diagnostically useful clinical feature is inferior vena caval

      interruption with azygous continuation, in which the suprarenal segment of the inferior cava is absent,

      and the infrarenal segment continues as the azygos or hemiazygous vein.

  The sinus node is
usually absent bilaterally in left isomerism.

 
Fetal complete heart block is presumptive evidence of in utero left isomerism.

      Among fetuses with complete heart block and accompanying structural heart defects,

      left atrial isomerism is the most common cause.




ECG in Left Atrial Isomerism

  The normal sinus node is a right-sided structure located at the junction of a right superior vena cava

      and a morphologic right atrium.

  In left isomerism, vena caval connections are necessarily to a morphologic left atrium,

      so
the sinus node is absent or hypoplastic.

  The atrial pacemaker is therefore ectopic, and the P-wave axis is abnormal.

      The ectopic atrial pacemaker can shift from one site to another (
wandering atrial pacemaker) or

      it may fire slowly (ectopic atrial bradycardia).

  More than half of the patients have a
left atrial rhythm.

  Complete atrioventricular block occurs in approximately 1 in 5 cases and

      has a significant impact on morbidity and mortality in the fetus and neonate,

  Complete atrioventricular (AV) block is an uncommon congenital lesion, occurring in about

      1 of 20,000 newborns. Left atrial isomerism is the most commonly associated structural cardiac defect.

  Atrial fibrillation and atrial flutter may be seen occasionally.

  In some patients, a "
dome and dart" P wave in lead V1 or V2 may be seen,

      a configuration associated with a left atrial ectopic focus irrespective of atrial situs.




References

  Am Heart J 2002;144:159-164.

  Am J Cardiol 2011;108:1352-1361.

  J Am Coll Cardiol 1998;32:773-779.

  Am J Cardiol 1987;59:1156 -1158.

  J Am Coll Cardiol 1998;31:1120-1126.

  J Am Coll Cardiol 1992;20:904-910.

  J Am Coll Cardiol 2000;36:908-916.

  Journal of Cardiovascular Computed Tomography 2012;6:127-136.

  Cardiovascular Pathology 2012;21:421-427.

  Best Practice & Research Clinical Obstetrics and Gynaecology 2008;22:31-48,





ECG 1a. The ECG above belongs to a 15 years-old boy with left atrial isomerism. The echocardiogram showed left atrial
isomerism, complete AVCD (AtrioVentricular Canal Defect), left ventricular hypoplasia, DORV (Double Outlet Right Ventricle)
and severe pulmonary stenosis.

Pediatric cardiologist Dr. Mahmut Gokdemir has donated the above ECG to our website.

Click here for a more detailed ECG




ECG 1b. The above ECG tracing is from the Holter recording of the same patient.
P waves with multiple different shapes show wandering atrial pacemaker.

Pediatric cardiologist Dr. Mahmut Gokdemir has donated the above ECG to our website.

Click here for a more detailed ECG




ECG 1c. The above ECG tracing is from the Holter recording of the same patient.
P waves with multiple different shapes show wandering atrial pacemaker.

Pediatric cardiologist Dr. Mahmut Gokdemir has donated the above ECG to our website.

Click here for a more detailed ECG




ECG 1d. The above ECG tracing is from the Holter recording of the same patient.
P waves with multiple different shapes show wandering atrial pacemaker.

Pediatric cardiologist Dr. Mahmut Gokdemir has donated the above ECG to our website.

Click here for a more detailed ECG




ECG 1e. The ECG above belongs to the same patient. It was recorded 1 year after ECG 1a.
The P waves have different shapes in this ECG.
The ectopic atrial pacemaker can shift from one site to another in atrial isomerism.

Pediatric cardiologist Dr. Mahmut Gokdemir has donated the above ECG to our website.

Click here for a more detailed ECG





ECG 2. The above ECG is from an 8 months-old infant with left atrial isomerism and complete AV canal defect.
P wave axis in lead II is variable.

Pediatric cardiologist Dr Ahmad Usaid Qureshi has donated the above ECG to our website.

Click here for a more detailed ECG