When you look at somebody from the outside they appear symmetric. In other words, one side of the body looks exactly the same as the other. On the inside, however, this is not the case. Each side of the body is unique. For example, the right side of the body contains certain organs (for example, the liver) that the left side of the body does not contain. Likewise, the left side contains organs (stomach, spleen, etc.) that the right-sided does not contain. Each side forms in a unique fashion. A good analogy to use when thinking about the symmetry or asymmetry of the body is that of a city. From an airplane at 20,000 feet, the city of Dallas looks symmetric. Up close, however, it is very clear that Dallas is completely unique; one side of the city is not simply a mirror image of the other.
Heterotaxy syndrome refers to a collection of disorders where either one side of the body is duplicated or the sides do not form in the correct manner. The most common organ systems involved include the cardiovascular system (the heart), the GI tract (stomach and intestines), and the spleen and liver.
Incidence
The incidence of heterotaxy syndrome is about 1 in 10,000 births with a male to female ratio of 2:1. Heterotaxy accounts for approximately 3% of cases of congenital heart disease.
Heart Anomalies
The heart is almost always involved in cases of heterotaxy syndrome. What type of defects are found depends on a number of things, including the type of heterotaxy syndrome (eg bilateral right sidedness or left-sidedness). Listed below are the most common heart anomalies associated with heterotaxy:
- Dextrocardia (the heart is located in the right side of the chest instead of the left)
- Single atrium (single top chamber of the heart)
- Single ventricle (single bottom chamber of the heart)
- Transposition of the great arteries (the aorta and the pulmonary artery have switched positions)
- Total or partial anomalous pulmonary venous return (the pulmonary veins enter into the heart in the incorrect location)
- Atrioventricular canal defect (large hole in the center of the heart with a single common valve)
- Coarctation of the aorta (narrowing of the aorta)
- Pulmonary valve atresia (absence of the pulmonary valve)
- Pulmonary valve stenosis (narrowing of the pulmonary valve)
- Double outlet right ventricle (where both main arteries of heart arise from the right ventricle)
- Complete atrioventricular block (no electrical signal from the top chambers of the heart reaches the bottom chambers of the heart resulting in a slow heart rate)
- Multiple sinus nodes or absent sinus node (the sinus node is the impulse-generating tissue of the heart)
Physiology
Heterotaxy syndrome is a complex syndrome that occurs when the axes of the body fail to rotate correctly when developing in the womb. This can result in many different organ systems being affected, and also means that each individual with heterotaxy is unique. Paired organs, such as the lungs or kidneys, are often mirror images of one another instead of having the unique characteristics of right and left that are normally present. Those individuals having a duplicated right side are referred to as having "right atrial isomerism", while those with a duplicated left side are termed "left atrial isomerism". Many individuals have a variety of severe congenital heart disease, while a smaller percentage have minor heart problems or none at all. Some have lung complications, while for others the major concern is the gastro-intestinal or immune system.
The physiology is typically determined by the type of heart defects present. Those defects associated with decreased blood flow to the lungs often present with cyanosis, a blue discoloration of the skin caused by decreased oxygen levels in the blood. Less commonly children who have increased blood flow to lungs may present with symptoms of excessive pulmonary blood flow, for example tachypnea (rapid breathing) or poor growth.
Associated Defects
The organs such as the stomach, intestines, liver and lungs may be in abnormal places in the chest and abdomen. The intestines may have malrotation, in which the loops of bowel are lined up incorrectly and may be inclined to develop blockages. Many children require a relatively straightforward abdominal surgery to correct this condition. There may also be irregularities with the skeleton, central nervous system and urinary tract. The spleen may not work correctly or may be missing entirely. When the spleen is missing or doesn’t work correctly, patients may be more susceptible to infections. In some cases, there may be a functioning spleen, but it may be divided into several smaller spleens (polysplenia). Most of these patients require a daily antibiotic to minimize the risk of infections.
Treatment
Nearly all patients with right atrial isomerism, and some patient with left atrial isomerism, will require a series of major heart surgeries. Surgeons will reconfigure the heart and circulatory system so that the heart functions with one ventricle instead of two. This is called a Fontan circulation. Patients with a Fontan circulation are referred to as single ventricle patients.
Prognosis
Without corrective surgery, most children with heterotaxy syndrome and significant heart problems will not survive beyond the first year of life. Prognosis is difficult to determine for the group as a whole because of the varying degree of heart defects. Fortunately, with advances in medical technology over the past 20 years, survival rates have increased significantly. The most serious risk factor for poor outcome is the presence of complete heart block, especially if diagnosed prenatally. Patients who survive the first few months of life often end up doing reasonably well long-term with a good quality of life and only minimal limitations.