What Is Hypoplastic Left Heart Syndrome?

Overview

Hypoplastic left heart syndrome (HLHS) is a congenital (present at birth) heart disease estimated to affect 1 in 5000 newborns.¹ The incomplete development of the left side of the heart is extremely problematic with only 58.6% of patients estimated to survive to age 5 and 32.6% to age 15.²

The main affected structures in the left side of the heart are:

• Left ventricle (LV)
• Aortic valve 
• Ascending aorta
• Aortic arch 
• Mitral valve¹ 

All of these are vital for allowing the effective pumping of oxygen-rich blood around the body. As such, HLHS is classified as a critical congenital heart defect; thus, immediate post-birth intervention is required to give the newborn a chance of survival.³

What is actually happening?

In a healthy heart, blood is pumped from the right ventricle to the lung and then back to the heart, where it is expected to be pumped around the rest of the body by the left ventricle via the aorta. 

In a baby with HLHS, the left side of the heart is underdeveloped thus, unable to pump enough blood to the body. In this case, the right ventricle of the heart takes over to pump blood to both the lung and the rest of the body. This process is happening through ductus arteriosus (a short blood vessel that connects the pulmonary arteries with the aorta). Ductus arteriosus gets smaller until it closes soon after birth. When the ductus arteriosus closes, the babies will die if they don't receive proper treatment.¹

Causes of HLHS

As it is an issue which develops before birth, the direct causes of most cases of HLHS are not well known with most theorising that it is a genetic issue.

However, we do have a theory on the mechanism by which HLHS develops.

This is ‘no flow no grow’. 

The basis of this is that blood (and thus nutrient) flow to both sides of the heart is important for healthy development within the womb but functionally, blood does not need to flow to both sides of the heart pre-birth.⁴ In the case of HLHS, limited blood flow to the left side of the heart can be unproblematic until birth, when both pumping ventricles need to work independently. The same issue can also occur in the other side of the heart, causing hypoplastic right heart syndrome.⁵

Though the root issue is mostly down to ‘faulty genes’, the causes of this lack of blood flow in the developmental phase are theorised to include:

• Aortic stenosis- It happens when the valve between the left ventricle and aorta becomes stiff and narrow, reducing blood flow from the left ventricle to the aorta and then to the rest of the body⁶
• Mitral valve defects - These include defects in the valve between the left atrium (an upper chamber in the heart that initially receives the oxygenated blood) and the left ventricle that result in reducing or blocking blood flow from the left atrium to the left ventricle⁷ 
• Atrial septal defect - A congenital heart defect that develops due to a failure to close the hole between the left and right atria after birth⁸
• Atrial contraction defect - Occurs due to smaller and slower contractions from the left atrium that don’t effectively transfer blood to the left ventricle⁹

Signs and symptoms of HLHS

Visual symptoms of the disease may show upon birth. These symptoms include:¹⁰,¹⁴ 

1. A blue-greyish colour of the gums and lips, known as cyanosis
2. Abnormal rapid breathing
3. Pale appearance
4. Being inactive due to the lack of energy 

Management and treatment for HLHS

After pre-birth screenings, healthcare professionals can begin formulating a plan to treat HLHS. 

There are many options to improve the quality and quantity of life, but few options available to address the cause and reverse/cure the development of HLHS. 

Treatment options that merely relieve the symptoms are known as ‘palliative’. These can be very effective but do not directly address the cause of the disease.

The Norwood, Glenn, and Fontan procedures are the most established palliative surgical techniques and are used in series at different stages of the newborn’s life. Before the surgery, the use of prostaglandin E1 is essential to maintain the small gap between the two main blood vessels.¹¹ 

Norwood procedure

The Norwood procedure is performed to increase the amount of blood flow from the heart around the body. It is usually done in the first two weeks post-birth. The procedure includes connecting the aorta to the right ventricle, followed by inserting a small tube to connect the aorta to the pulmonary arteries.¹² This allows the right ventricle to pump blood around the body; whilst still allowing blood flow to the lungs. 

In some cases, an alternative procedure known as bilateral pulmonary artery banding is performed. The surgeon implants a stent in the ductus arteriosus and bands around the pulmonary arteries to achieve a similar goal.¹⁴

Glenn procedure

It is the second procedure that is performed to relieve stress on the right ventricle and allow more efficient blood flow. It is usually performed between three and six months post-birth. In the Glenn procedure, the vena cava (the veins that returns blood to the right ventricle) that comes from the upper body is disconnected from the heart and re-connected to the pulmonary artery. Additionally, the tube inserted previously during Norwood procedure to hold the gap between the pulmonary artery and the aorta is removed.¹⁵ This reduces the amount of work required by the right side of the heart and alleviates the lack of function on the left side of the heart.

Fontan procedure

It is the third procedure that is performed after the Glenn procedure. It is usually performed between 18 months and four years post-birth. In the Fontan procedure, the vena cava that comes from the lower body is disconnected from the heart and re-connected to the pulmonary artery. A small hole is maintained between the lower vena cava and the right ventricle to allow the lungs some adaptation time.¹⁶

Heart transplant

A heart transplant may seem the best option for improving long-term survival. However, this option has significant drawbacks, like the long-waiting time and limited availability of hearts for transplants. In addition, children who have heart transplants need medications throughout life to avoid rejection of the donor's heart. 

Therefore, it is recommended to undergo classic palliative care procedures before choosing the heart transplant option. Research has shown 70% of HLHS patients receive transplants after a one-year wait, and only 23% survived one year without a transplant.¹⁷

Foetal therapy

In recent years, treatment options designed to address the main cause of HLHS have appeared. One of these options is foetal therapy, including invasive foetal cardiac interventions and non-invasive maternal hyperoxygenation. A foetal cardiac intervention like foetal valvuloplasty is conducted at 21 to 23 weeks of pregnancy. Foetal valvuloplasty involves the insertion of a needle through a fetal (unborn baby) chest wall into the left ventricle. Then, a balloon catheter is inserted and inflated to dilate the aortic valve.¹⁸ This increases blood flow to the left ventricle and allows the left ventricle to grow normally.¹⁹ Although foetal valvuloplasty is a new understudied option, the 88% success rate for this procedure is highly promising.²⁰

On the other hand, non-intensive maternal hyperoxygenation is a non-surgical method that aims to increase blood flow to the left ventricle. This technique provides the mother with additional oxygen for the benefit of the fetus. Though the optimal technique can’t be determined, this treatment has been shown to reverse HLHS development.²¹

Non surgical comfort care

For some patients, the best form of management for this treatment is to alleviate discomfort and prevent further distress by allowing the natural process for this condition to take place.

Since 2017, the National Pediatric Cardiology Quality Improvement Collaborative recommends that all parents are provided with all options when the diagnosis is made.²²

Termination of pregnancy

It is a serious option in the HLHS case, where the quality and quantity of life are likely to be limited. HLHS patients that do survive often suffer from side effects. According to the research, learning difficulties are one of the side effecst for HLHS patients who reach the age of five. Additionally, 40% of heart defect patients have disabilities.^,24

Termination of pregnancy is selected by approximately 21% of the parents.²⁵

Diagnosis of HLHS

Early detection and intervention are important. Without treatment, 95% of newborns die within weeks of birth.²⁶

Fortunately, there are multiple ways to diagnose HLHS.

• Standard pre-birth screening 

Ultrasound is a visualisation technique that utilises high-frequency soundwaves to build an image of soft tissues within the body.²⁷ Using this technique, healthcare professionals can see the underdevelopment of heart structures and thus can detect HLHS as early as 18 weeks after the mother becomes pregnant.²⁸

The specific form of ultrasound to investigate the heart is known as echocardiography.²⁹

• Blood tests

Blood tests can also be used to confirm HLHS. However, these tests cannot take place before birth.⁶

Though post-birth symptoms may be more obvious, the urgency of the situation makes the pre-birth screening highly recommended as a precautionary measure for HLHS.

Risk factors

As the disease appears to be genetic, there seem to be no determined risk factors for HLHS at present. However those with a familial history of heart defects may have a higher potential of HLHS developing in future pregnancies.³⁰

Complications

 Potential complications include:

• The Norwood/Glenn/Fontan procedures are only  palliative
• Infection and rejection of heart transplants are common
• Lifelong immunosuppressant use is required post-heart transplant, which is associated with serious side effects ³¹
• Surgical foetal therapy has been shown to lead to a 10% foetal mortality rate and 42% premature births ²⁰ 
• Hyperoxygenation effectiveness can be limited by multiple underdeveloped blood vessels ³²

FAQs

How can I prevent HLHS?

There is no way to prevent HLHS.  If you have a family history of heart defects or have a child with a congenital heart defect, consult your doctor before getting pregnant.

How common is HLHS?

1 in 5000 pregnancies can be affected by HLHS.

When should I see a doctor?

Standard pre-birth echocardiogram screenings detect HLHS.

Most babies with HLHS are diagnosed either before or after birth. However, you should see a doctor if your baby has any of the HLHS symptoms. 

Summary 

HLHS is a congenital heart defect characterised by an underdeveloped left side of the heart. It affects newborn babies and significantly reduces the quantity and quality of their lives. Many treatment options are available, that range from palliative to heart transplant and foetal therapy. Despite the risks and complications of the treatment, the improvements and survival rates for HLHS patients are significant. As further research is taking place into foetal therapy, and with the possibility of combining multiple types of treatment, the improvement in the quality of life for HLHS patients is promising.