Hypoplastic Left Heart Syndrome

Hypoplastic left heart syndrome (HLHS) is one of the rarest, most severe and complicated types of congenital heart defects that forces parents to make life-changing decisions for their child. The lack of information and research available regarding this disorder has made it even more life-threatening.

What is Hypoplastic Left Heart Syndrome?

Hypoplastic left heart syndrome is a specific disease under the broad umbrella of congenital heart defects. Congenital refers to defects that do not form later in life but instead are present from birth. Hypoplastic left heart syndrome is a specific category of defect which occurs when the left side of the heart does not develop normally. These defects can include various parts of the central and left side of the heart, including the left ventricle, valves, and the ascending aorta. Normally, the left side of the heart pumps oxygenated blood to the rest of the body, but due to these deformations, blood cannot circulate effectively.¹

So, why does the left side of the heart develop abnormally? The direct cause or mutation is still unknown. While there are many theories surrounding the cause of this syndrome, most conclude that there is a multitude of genetic and environmental factors at play. Genetically, most cases of this disease are not inherited. Instead, interactions between unidentified genes and the environment are thought to cause this abnormal development. These environmental factors are generalized risk factors that contribute to multiple types of birth defects and include a decrease in folic acid, smoking and drinking alcohol. These interactions have not been specifically defined yet and are an area requiring intensive research. ¹

Despite the ambiguity of the cause of this disease, there are some rare cases that are genetically inherited. There is an autosomal recessive inheritance of this disease where the individual needs two copies of the mutated GJA1 gene. Another way of inheriting the disease is through an autosomal dominant inheritance where an individual needs 1 copy of the mutated NKX2-5 gene. ¹ Additionally, those with genetic disorders such as Turner syndrome, Jacobsen syndrome, trisomy 12 and 18 are also more likely to have hypoplastic left heart syndrome.¹ Individuals with hypoplastic left heart syndrome are also more likely to have genetic defects resulting in atrial septal defects as well. Atrial septal defects are another type of congenital heart defect which results in a hole causing an abnormal connection between the left and right side of the heart. Additionally, those with genetic disorders such as Turner syndrome, Jacobsen syndrome, trisomy 12 and 18 are also more likely to have hypoplastic left heart syndrome.¹ Due to various genetic and environmental factors, this disease can present with a wide range of abnormalities resulting in it presenting differently from person to person.

Diagnosis:

Diagnosis of hypoplastic left heart syndrome can occur either during pregnancy or shortly after birth. During pregnancy, an image of the baby’s heart is taken through a noninvasive, specialized type of ultrasound called a fetal echocardiogram, which identifies any malformations or defects. After birth, diagnosis relies heavily upon signs and symptoms which tend to develop within the first few hours or days. This may include cyanosis, which is a blue discoloration of the skin, lethargy, a weak pulse and problems breathing, due to the insufficient circulation of deoxygenated blood. To investigate these symptoms, an echocardiogram and/or a newborn pulse oximetry test are conducted to visualize the heart and to determine levels of oxygen within the blood respectively.¹

Prognosis:

Hypoplastic left heart syndrome is considered a critical heart defect because if left untreated, it can be fatal. Current survival rates are improving due to novel surgeries. Those who undergo a Norwood surgery, the first of three surgeries to help correct the defect, have a 3-5 year survival rate of 70% while those who survive to 1 year of age have long-term survival rates of around 90%.² Although scientists are working to improve these survival rates, there has been very little data collected regarding the quality of life for those with this disease. Some studies have suggested that those with hypoplastic left heart syndrome have an increased risk of other diseases such as thrombotic complications including stroke, a decrease in the ability to exercise, and an increased risk for neurodevelopmental impairment.²

Treatments:

When a baby is born with hypoplastic left heart syndrome, they are immediately admitted to neonatal intensive care. The initial goals are to keep the baby breathing and to keep the ductus arteriosus open. The ductus arteriosus is an opening between the aorta and pulmonary trunk that allows blood to bypass the underdeveloped lungs of the fetus and be sent to the rest of the body. This opening is normal in utero but usually closes at birth. A medication called prostaglandin E1 is administered to keep the ductus arteriosus open so that more blood can enter the aorta, thus aiding the delivery of blood to the organs.⁴

Once the baby is stable, many parents decide on surgery to help restore heart function. It is important to note that this series of surgeries do not cure this defect but can help restore normal function. In this treatment, 3 surgeries need to take place at various time points after birth: the Norwood procedure at 2 weeks, the Hemi-Fontan procedure at 4-6 months and the Fenestrated Fontan procedure at 18-30 months. The goal of these surgeries is to create a new path of flow for blood so that oxygenated blood can get to the body. Figure 1 shows the differences in the blood flow path between a normally developed heart and one with hypoplastic left heart syndrome that has been surgically corrected. The surgically corrected path has the deoxygenated blood moves directly into the pulmonary arteries, instead of moving into the right atrium and ventricle. This allows the blood to get to the lungs to get oxygenation. Then the blood returns to the heart and enters the left atrium and then into the right atrium. Finally, the blood returns to the right ventricle where it will go into the aorta which will pump the oxygenated blood around the body.⁴

Other treatments include heart transplants although they tend to be rare because very few donor hearts are available4. Additionally, new treatments for hypoplastic left heart surgery are being researched. For example, stem cell therapy after surgery is being investigated to help the right ventricle deal with the increased stress of pumping the blood into the aorta instead of the lungs.³ Although surgery is the recommended form of treatment, not all babies will survive or they may require additional surgeries. Thus, palliative care measures can be offered at the parents’ request.⁴

Figure 1: Top image shows normal heart blood flow. The bottom image shows hypoplastic left heart syndrome which has been corrected with surgery to alter blood flow.¹

Support Groups and Policy:

Hypoplastic left heart syndrome is a rare disease and thus it may be difficult to locate and access resources for afflicted individuals and their families. Sisters by Heart is a support group for those specifically affected by hypoplastic left heart syndrome and single ventricle defects. Additionally, Hope for Hearts is an organization started by McMaster student, Homayra Ahmed, which seeks to increase awareness through sharing the stories of those affected by congenital heart defects.

Daksha Marfatia

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Works Cited:

1. Congenital Heart Defects - Facts about Hypoplastic Left Heart Syndrome | CDC. Accessed February 27, 2021. https://www.cdc.gov/ncbddd/heartdefects/hlhs.html
   
   
2. Hypoplastic left heart syndrome | Genetic and Rare Diseases Information Center (GARD) – an NCATS Program. Accessed February 27, 2021. https://rarediseases.info.nih.gov/diseases/6739/hypoplastic-left-heart-syndrome
   
3. J. BG, David M, B. DK, et al. Stem Cell Therapy for Hypoplastic Left Heart Syndrome. Circ Res. 2018;123(2):288-300. doi:10.1161/CIRCRESAHA.117.311206
   
   
4. Fruitman DS. Hypoplastic left heart syndrome: Prognosis and management options. Paediatr Child Health (Oxford). 2000;5(4):219-225. doi:10.1093/pch/5.4.219
   
   

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Cite This Article:

Marfatia D. Hypoplastic Left Heart Syndrome. Illustrated by Z. Hasan. Rare Disease Review. January 2022. DOI: 10.13140/RG.2.2.13604.55688