Visualization of Ventricular Septal Defects on High-resolution Fetal Ultrasonography: Real or Artifact?

In recent years, prenatal diagnosis has been influenced by a prosperous economy, easy access to the Internet, and an increased level of knowledge among patients. For example, in Taiwan, where the use of ultrasound has become prevalent in prenatal diagnosis, increasingly people are choosing to undergo high-resolution fetal ultrasound scans at their own expense. Increasing use of the Internet and economic prosperity have contributed to the perception that high-resolution fetal ultrasonography is a necessity for prenatal testing. Consequently, prenatal ultrasound is more prevalent in Taiwan than anywhere else in the world. Highresolution fetal ultrasound is generally performed in such a way that fetal anomalies can be detected in a timely fashion, thus facilitating informative counseling of patients, and promoting optimal prenatal and postnatal care [1]. Although timely diagnosis of fetal anomalies is beneficial, ultrasound may be a double-edged sword. The overuse of ultrasound has raised concern because ambiguous “suspicious diagnoses” without the benefit of expert counseling may lead to unfortunate misunderstanding by the expectant mother and her family. This may, at a minimum, cause emotional stress for the pregnant woman, and may also unduly influence a decision to terminate the pregnancy. Although ultrasound is considered to be a safe, noninvasive diagnostic tool that “lacks the evidence of causing any fetal harm,” we remain cautious about the unanticipated sequelae of this procedure.

Of the common fetal anomalies diagnosed by ultrasound, a fetal ventricular septal defect (VSD) is among the most disturbing ones [2]. A VSD is a congenital cardiac anomaly most commonly detected prenatally, suggesting that VSDs are encountered fairly often by physicians. Nevertheless, due to the complex structure of the heart, shadowing caused by the movement of a valve during ultrasound can sometimes give rise to overdiagnosis. Furthermore, no reliable data are internationally available to calculate the rate of misdiagnosis of VSDs. Rather, it is generally believed that a VSD can be diagnosed definitively only when it is visible in at least two different planes. Perplexed by the aforementioned circumstance, the author describes two cases, thus seeking commentary from other clinicians.

The four-chamber view should be the standard plane for the diagnosis of a VSD [3], nonetheless, according to the author’s own experience, only large VSDs, which include muscular-type VSDs or even atrioventricular defects, can be detected via the four-chamber view [4]. In addition, a postnatal review of midterm ultrasound for many cases of perimembranous-type (Type II) VSDs, especially when the probe was held perpendicular to the septum, demonstrated a nearly intact septum with no observed abnormalities (Figure 1A). In contrast, an area of discontinuity in the septum can be visualized in the four-chamber view. This observation may result from the probe being held parallel to the septum, or from the shadow formed under the vertical angle between the probe and the septum. In such a case, the infant may eventually be observed to have normal cardiac development.

Figure 1

In a newborn diagnosed with a VSD by early postnatal echocardiography, the midterm ultrasound showed (A) a continuous septum in the four-chamber view; however, (B) a larger hole with an intact and blunt edge appears in the LVOT view, and (C) blood flow through the hole was visualized under color Doppler ultrasound. (D) The infant was diagnosed with Type II VSD after birth. LVOT =left ventricular outflow tract; VSD = ventricular septal defect.

In order to be diagnosed at an early stage, the aforementioned perimembranous-type (Type II) VSDs are more likely to be visualized in the left ventricular outflow tract view (see below). By contrast, in the left ventricular outflow tract view, continuous movement of the aortic valve during cardiac contraction can cause a shadow, which may easily be misinterpreted as a hole in the septum (Figure 2B). Hence, researchers suggest that, in order to avoid misleading shadows originating from a single angle, VSDs should be diagnosed definitively only when one or more holes are noted in at least two views (Figures 1B, 1C, 3A, and 3B). When the edge of the hole appears distinct rather than ill defined, it is less likely to have been caused by a shadow (Figures 1B and 3B).

Figure 2

In a newborn with a normal early postnatal echocardiogram, the midterm ultrasound showed (A) a discontinuity in the septum in the four-chamber view, and (B) a smaller hole with a sharp edge, which may be caused by a shadow, in the LVOT view. LVOT= left ventricular outflow tract.

Figure 3

In a newborn diagnosed with a VSD by early postnatal echocardiography, the midterm ultrasound showed (A) a discontinuous septum in the four-chamber view, and (B) a smaller hole with an intact, blunt edge in two planes in the LVOT view. (C) The newborn was diagnosed with Type II VSD. (D) Blood flow through the hole was also seen on color Doppler ultrasound. LVOT= left ventricular outflow tract; VSD= ventricular septal defect.

I hope to achieve more accurate diagnosis of VSDs in order to avoid unnecessary anxiety for pregnant women. I invite discussion on my observations.