Partial Atrioventricular Septal Defect

A rare congenital cardiac malformation that is a variant of an atrioventricular septal defect (AVSD) with an interatrial communication (ostium primum defect) just above the common atrioventricular (AV) valve, no interventricular communication just below the atrioventricular valve, a common atrioventricular junction but separate right and left atrioventricular valvar orifices, and a three-leaflet, left-sided component of the common atrioventricular valve (''cleft''). Shunting is restricted to the atrial level because of fusion of the leaflets of the common AV valve with the crest of the ventricular septum.

Epidemiology

Partial atrioventricular septum defect (PAVSD) accounts for 1-2% of all congenital heart malformations and its prevalence is estimated to be 1/5,000-1/2,500.

Clinical description

Most patients with PAVSD are asymptomatic until late in life. The clinical presentation depends on the degree of mitral regurgitation and on the associated cardiac defects. The two most common clinical manifestations are impaired exercise capacity and exertional dyspnea. Rarely, cardiac failure may occur in infancy. Additional features in adults include palpitations, presyncope or syncope, and sustained atrial arrhythmias. The ventricles may be equal or nearly equal in size (balanced) or one of the ventricles may be significantly larger than the other (unbalanced). Unbalanced ventricles are associated with hypoplasia of the arterial valve above and aortic coarctation if the left ventricle is hypoplastic.

Etiology

CRELD1 (3p25.3), GATA4 (8p23.1-p22), GATA6 (18q11-q12) and NR2F2 (15q26) have been associated with a small fraction of PAVC cases. These genes encode developmental transcription factors that are critical for normal cardiac morphogenesis.

Diagnostic methods

Diagnosis of PAVC is established by means of 2D-echocardiography. Transesophageal echocardiography or cardiac catheterization can be useful in adults (to assess pulmonary vascular resistance and status of the left AV valve and, in patients > 40 years, to exclude coexisting coronary arterial disease). An elevated pulmonary arterial pressure and moderate to severe left atrioventricular valve regurgitation may be observed. First-degree AV block, right bundle branch block, and a superior QRS axis may be noted on electrocardiogram.

Differential diagnosis

Differential diagnosis includes the complete and intermediate forms of atrioventricular septal defects. Intermediate (transitional) atrioventricular septal defect is a variant of complete atrioventricular septal defect with a restrictive ventricular component due to multiple attachments of the bridging leaflets on the crest of the ventricular septum. PAVSD may be associated with syndromes including the RASopathies (particularly Noonan syndrome caused by mutations in PTPN11 and RAF1), Ellis Van Creveld and Down, and CHARGE syndrome.

Antenatal diagnosis

PAVSD may be detectable prenatally by 4-chamber view screening during obstetric ultrasonography.

Management and treatment

The early diagnosis of PAVSD requires cardiology follow-up and elective complete repair between 3 and 5 years of age, which involves closure of a primum atrial septal defect with an appropriately shaped patch through right atriotomy, and partial suture of the ''cleft'' of the left component of the common atrioventricular valve. A pacemaker insertion may be required for complete AV block, which may spontaneously develop after repair. A continuous lifelong follow-up is recommended to avoid late complications. Symptom free patients may be referred for PAVSD repair because of a substantial left-to right atrial shunt (pulmonary to systemic flow ratio 1.8:1) and echocardiographic evidence of right heart volume overload.

Prognosis

Long-term survival after repair of partial AVSD is the rule. The elective age for repair is 3-5 years, but some patients will not present until later in life. The later the repair is made, the greater the loss of ventricular function that occurs. High morbidity and need for reoperation, often related to residual problems of the systemic AV valve or LAVV regurgitation, may be observed. Furthermore, the prognosis can be severely impaired if the left ventricle and left component of the common atrioventricular valve are hypoplastic or severely malformed. Norwood-type intervention in infancy can be required, as well as prosthetic replacement of the left AV valve.