HLHS is a severe congenital heart defect where the left side of the heart is critically underdeveloped, including the left ventricle, mitral valve, aortic valve, and aorta. This underdevelopment prevents the left heart from effectively pumping oxygenated blood to the body, making the condition life-threatening. Despite advances, misdiagnosis or an initial uncertain diagnosis remains a reality. Understanding the standard diagnostic process and factors leading to diagnostic error is crucial for seeking appropriate care.
The Standard Diagnostic Process
HLHS detection occurs in two phases: before birth and following delivery. Prenatal diagnosis relies on the fetal echocardiogram, a specialized ultrasound providing detailed images of the developing heart. This test is performed if a routine obstetric ultrasound shows a suspected heart abnormality or if the pregnancy is high-risk.
A definitive prenatal diagnosis relies on visualizing a small, underdeveloped left ventricle, along with atresia or severe stenosis of the mitral or aortic valves. Identifying these structural abnormalities allows medical teams to prepare for specialized care and delivery at a pediatric cardiac surgical unit. Prenatal diagnosis, often occurring between 18 and 22 weeks, is associated with improved neonatal outcomes.
If not diagnosed prenatally, it presents clinically in the newborn period, usually within the first 48 hours of life. Symptoms appear as the temporary fetal connections—the foramen ovale and the patent ductus arteriosus—begin to close. The newborn exhibits signs of poor systemic circulation, including cyanosis, weak pulses, rapid breathing, and lethargy.
Postnatal confirmation is quickly achieved with a transthoracic echocardiogram, which maps the heart’s structure and function. This imaging confirms the small left-sided structures and assesses the function of the right ventricle, which must take over the work of the entire heart. Rapid diagnosis is paramount, as the baby requires immediate medication to keep the ductus arteriosus open and maintain blood flow to the body.
Factors Leading to Diagnostic Inaccuracy
Despite sophisticated technology, several factors can contribute to an inaccurate or missed HLHS diagnosis. One factor is the timing of the initial fetal echocardiogram. While early scans are possible, certain features of HLHS, such as the progressive restriction of the foramen ovale, may not be fully apparent until later in the second trimester.
The technical quality of the image also presents a significant challenge. Factors such as fetal position, amniotic fluid amount, and maternal body habitus can obscure the small, complex structures of the developing heart. Poor image quality can prevent precise measurement of the left ventricle or clear visualization of the aortic and mitral valves, leading to an uncertain assessment.
Interpreting fetal cardiac anatomy requires specialized skill and experience from the sonographer and the interpreting physician. Fetal echocardiography is an operator-dependent technique, and features can be missed by less experienced imagers. Studies show that the highest rate of diagnostic discrepancies occurs when the initial scan is performed by a non-specialist.
The defect itself can also evolve over time, complicating the diagnostic process. A heart that initially appears to have a “borderline” left ventricle due to critical aortic stenosis can progress in utero to develop the full underdevelopment characteristic of HLHS. This progression necessitates serial follow-up scans to monitor the heart’s development.
Conditions That Mimic Hypoplastic Left Heart Syndrome
The challenge of diagnosing HLHS is compounded by other congenital heart defects that share a similar visual presentation. These conditions may cause the left side of the heart to appear small on initial imaging, but they are not true HLHS. Distinguishing them is crucial because their treatment pathways are often very different.
Critical Aortic Stenosis and Mitral Atresia
Critical aortic stenosis involves a severely narrowed aortic valve, restricting blood flow out of the left ventricle. This obstruction causes the ventricle to struggle and fail to grow, visually resembling HLHS. The underlying issue is the valve, not a primary developmental failure. Similarly, severe mitral atresia, where the mitral valve is closed, prevents blood from entering the left ventricle, leading to its underdevelopment.
Aortic Arch Anomalies
Certain arch anomalies, such as a severe coarctation of the aorta or an interrupted aortic arch, can also cause the left ventricle to appear small. These conditions impede the outflow of blood to the body, secondarily impacting ventricular growth. Although they result in a hypoplastic left-sided circulation, the presence of a structurally intact left ventricle may allow for ventricle-sparing surgical options not available for classic HLHS.
Pulmonary Atresia with Intact Ventricular Septum (PA/IVS)
PA/IVS is sometimes confused with HLHS because both result in ductal-dependent systemic circulation and can cause cyanosis. However, in PA/IVS, the obstruction is on the right side of the heart, preventing blood flow to the lungs. The left ventricle is typically normal in size, which is a key distinguishing factor from HLHS.
Seeking a Second Opinion and Confirmation
Given the complexity of the diagnosis, seeking a second opinion is a prudent step for any family receiving an initial HLHS diagnosis. A second review allows an independent specialist to confirm the diagnosis or identify a subtle difference pointing to a less severe or more treatable condition.
The most effective second opinion is obtained from a specialized Fetal Heart Center or a pediatric cardiology institution. These centers employ advanced imaging technology and multidisciplinary teams, including specialized fetal cardiologists and cardiac surgeons. The consultation should involve a thorough review of all existing images and may include a follow-up fetal echocardiogram or advanced postnatal imaging.
The goal of a second opinion is to confirm the diagnosis and ensure the most accurate anatomical classification. Confirmation allows the family to move forward with treatment planning. A revised diagnosis can fundamentally change the child’s prognosis and surgical pathway.