What is Truncus Arteriosus

Truncus arteriosus (TA) is a very rare, congenital disorder of the heart. ‘Congenital’ means that the patient was born with this defect. In TA, there is one, singular vessel leaving the heart, instead of two vessels.

This is quite a serious condition which requires surgical intervention to avoid dire consequences for the child. TA can be scary for patients and their parents, and thus, this article aims to provide all the necessary details and information about truncus arteriosus.

Anatomy and pathophysiology

In a normal heart, there are four chambers; the two smaller chambers found in the upper part of the heart are called atria and the larger, lower chambers are called ventricles. Deoxygenated blood is brought to the right atrium of the heart from all over the body, by the superior and inferior vena cava. This blood then flows through a valve located between the atrium and the ventricle and then into the right ventricle. From here, the blood is pumped through the pulmonary arteries, which leave the heart and carry blood to the lungs to be oxygenated. After this, the newly oxygenated blood is sent back to the left atrium of the heart via the pulmonary veins. After passing from the left atrium to the left ventricle via a valve, the oxygenated blood is pumped throughout the body via a large artery leaving the heart called the aorta.¹  Usually, the pulmonary artery and aorta each have a valve to prevent the backflow of blood. 

The heart begins to develop in the embryo from the third week of gestation and is complete by the 9th week.² The truncus arteriosus is a normal embryonic heart structure which later separates to form a part of the aorta and the pulmonary artery. The condition ‘TA’ refers to the failure of this vessel to separate into two distinct vessels resulting in one common vessel leaving the heart. This singular vessel only has one large valve. Additionally, there is often another heart defect present along with TA, which is known as a ‘ventricular septal defect’ (VSD). A VSD means there is a hole between the two ventricles which can cause oxygenated and non-oxygenated blood to mix. 

The formation of a VSD and a single vessel leads to the mixing of the oxygenated and deoxygenated blood, which does not usually occur. This reduces the oxygen saturation of the blood, therefore, less oxygen is transported around the body. There would also be an excessive amount of blood being delivered to the heart and lungs resulting in overloading of these organs and eventually causing a buildup of fluid around them. The blood vessels may also be damaged causing the heart to have to exert itself further in order to pump the excess blood through the damaged vessels. This may subsequently result in pulmonary hypertension which is a higher than normal blood pressure in the lungs.

Causes and risk factors

The cause of TA is largely unknown. Some cases are caused by genetic mutations (an abnormal change in gene structure) and some are associated with a family history of congenital heart disease. TA is also linked with a disorder known as DiGeorge syndrome. 

It is thought that some cases might be associated with maternal factors such as smoking, drinking and taking certain medications during pregnancy. Contracting rubella during pregnancy may also increase the risk of truncus arteriosus.

Clinical presentation

The main symptoms³ of TA are driven by the poor supply of oxygenated blood to the body due to the mixing of oxygenated and deoxygenated blood. As a result, there is also an oversupply of blood to the heart which can cause congestive heart failure and pulmonary hypertension. 

The following symptoms are common in TA:

• Cyanosis (blue discolouration of the skin)
• Lethargy and fatigue
• Poor growth and feeding
• Rapid breathing (tachypnoea)
• Shortness of breath (dyspnoea)
• Signs of congestive heart failure
  • Noisy and fast breathing 
  • Wheezing 
  • Difficulty breathing (exhibited by nostril flaring and pulling under the ribs) 
  • The liver might also be enlarged (hepatomegaly) 

Diagnosis

At times, TA can be diagnosed antenatally⁴ (before the baby is born). This requires a foetal echocardiogram which visualises the defects present in the heart, such as a single vessel leaving the heart with one valve and a VSD. 

After the baby is delivered, TA may be suspected based on the presence of the symptoms mentioned above. An electrocardiogram (ECG) can be done which will show non-specific, abnormal changes whereas an echocardiogram would be able to show the actual defective heart anatomy and thus, confirm the diagnosis. 

A chest X-ray can also be done which may reveal an enlarged heart and the signs of lung congestion. 

The anatomy of the heart can be visualised in more detail using a cardiac MRI, catheterisation and angiography. This may be done after surgery to assess the efficacy of treatment.

Genetic testing should be offered to any family who exhibits a family history of congenital cardiac defects. 

Treatment 

The first step in the management of TA is to stabilise the patient. This involves ensuring that the patient is not in any immediate danger and correcting any abnormalities which do not require surgery. 

Medication may be administered to achieve proper fluid balance and to reduce the overloading effect on the heart. Any electrolyte imbalance is corrected as well as any anaemia (decrease in red blood cells), or hypoglycaemia (low blood sugar). If the patient displays any signs of respiratory distress, this must be urgently addressed, although supplemental oxygen is not generally administered as it can worsen the condition. In some severe cases, the patient may need to be intubated to help them breathe properly.  

Surgery is the most definitive treatment option for TA and should be done within the first month of life.⁴ A corrective procedure is performed to correct the single vessel abnormality. Usually, this single vessel is retained as the new aorta whereas the new pulmonary artery is formed using a man-made tube. The VSD is also repaired during this surgery. 

After this surgery, the patient will be monitored very closely in the cardiac intensive care unit. As the heart function gradually becomes more stable over time, the respiratory support, intravenous medication and monitoring lines can be slowly withdrawn. When this is eventually achieved, oral medication can be given and more attention can be given to proper feeding. Overall, the expected hospital stay can be up to three weeks.³ 

Prognosis 

It is imperative to have corrective surgery for TA as early as possible to ensure a good prognosis. Over 90% of patients survive this surgery and go on to lead relatively normal lives. However, life-long follow-up appointments will be needed, as well as ECGs and echocardiograms to monitor heart function. 

Follow-up surgeries will also be required as the artificial tube used to replace the pulmonary artery will not grow with the child and will need to be reconstructed after some time. These follow-up surgeries are not considered major procedures and will only require hospital stays of about a week or so. 

Parents will need to keep a close eye on their children who have undergone surgery. They should watch for the emergence of any new symptoms such as dyspnea, palpitations, fainting, changes in exercise tolerance or oedema.

Further advances are being made in the management of TA. 3D-printed models of hearts are being used to better visualise the heart and practice repair of the defects.⁵ There has also been progress in antenatal diagnosis and surgical techniques which has improved the mortality rate as well as the long-term survival rate.⁶

Summary

In conclusion, TA is very rare and often its cause can be unknown. It might be associated with a genetic mutation or several maternal factors. Whereas there is normally an aorta and pulmonary artery leaving the heart in TA these are combined into only one large vessel. This defect is also associated with a VSD. These defects result in the mixing of deoxygenated and oxygenated blood as well as overloading of the heart which can produce the symptoms listed in the section above.  

Nowadays, TA can be detected before the child is born and thus, preparations for management of the condition can be put in place. After birth, TA may be detected by the emergence of classical signs and symptoms. The diagnosis must be confirmed with further tests to visualise the heart defects. 

Surgery is the mainstay of treatment for TA and needs to be done soon after birth. If the surgery is successful, the child can have a good prognosis and lead a relatively normal life. However, follow-up appointments, tests and surgeries will be needed and any new symptoms will need to be further evaluated.