Introduction
Congenital abnormalities of the left outflow tract can be categorized into sub-valvar, valvar and supra-valvar components. Hypoplasia of the whole left outflow tract is usually part of the spectrum of hypoplastic left heart syndrome (see Chapter 20) and should be managed accordingly, but this chapter considers left ventricular outflow tract (LVOT) problems in the setting of an otherwise adequate-sized left ventricle.
Sub-Aortic Stenosis.
The commonest form of sub-aortic stenosis is a circumferential fibrous membrane developing immediately beneath the aortic valve leaflets (Figure 14.1). It is usually well defined but can extend onto the underside of the leaflets and along the ventricular septum. The aetiology may be due to the initial presence of a small, restrictive sub-aortic ventricular septal defect (VSD) that is closed by fibrous tissue generated by the turbulent flow. The build-up of fibrous tissue in the outflow tract then generates more turbulence, and there is further accumulation, creating a circumferential ridge of tissue. The lesion most commonly occurs in isolation but can be seen in association with atrio-ventricular septal defect (AVSD). The dimensions and structure of the aortic valve are usually normal, but if the membrane is extending onto the valve leaflets, they can become tethered with progressive aortic regurgitation. There may be a muscular component to the obstruction, partly due to the compensatory hypertrophy of the left ventricle (LV) in response to the stenosis. Purely muscular sub-aortic stenosis is usually an acquired lesion related to asymmetrical septal hypertrophy as part of hypertrophic obstructive cardiomyopathy (HOCM). Longer, tubular stenosis of the LVOT is rare and most likely seen in the setting of repaired AVSD (with its ‘goose-necked’ LVOT, which predisposes to stenosis; see Chapter 10) or repaired double-outlet right ventricle (DORV) or transposition of the great arteries (TGA)/VSD, where the VSD has been closed with a patch that commits the LV through to the displaced aorta.
Figure 14.1 Echo image of (A) discrete sub-aortic membrane and (B) operative view through the aortic valve.
Investigation and Management.
Trans-thoracic echo will provide all the necessary information in the majority of cases, but occasionally trans-oesophageal echo (TOE) or even MRI is helpful in more complex tunnel-like stenoses. It is important to establish how close any membrane is to the valve and whether or not the leaflets are tethered. A peak gradient of more than 50 mmHg is generally regarded as an indication for surgery. Stenosis is not amenable to ballooning or stenting as it is a fixed fibro-muscular obstruction. The only situation where non-surgical treatment is an option is in the rare situation of asymmetrical septal hypertrophy in HOCM where radio-frequency muscle ablation has been attempted, as well as selective alcohol injection into the first septal perforator branch of the left anterior descending (LAD) coronary artery. However, these techniques require highly specialized interventional cardiologists and are usually reserved for adults or older adolescents.
Surgery for isolated sub-aortic stenosis is performed through a standard aortotomy, working through the aortic valve (Figure 14.1B). The fibrous shelf is incised where it meets the septal muscle underneath the commissure of the left and right coronary cusps. The membrane can then be peeled away from the muscle. Care should be taken to peel away any extensions of the membrane onto the underside of the valve cusps. If possible, a wedge resection of septal muscle is also performed (again, working beneath the commissure between the left and right coronary cusps so as to be distant from the bundle) to maximize the area beneath the aortic valve and so reduce the risk of recurrence. The leaflets must be carefully checked at the end of the procedure to ensure that they have not been injured.
More complex tunnel obstruction may have a fibrous component that can be resected in a similar way together with any hypertrophied muscle bulging into the outflow tract – especially if there has been previous surgery with subsequent fibrous reaction. Long-length tubular obstruction in the setting of an adequate-sized aortic valve is exceedingly rare. A Konno procedure can be considered in this situation: a transverse incision is made in the infundibulum of the right ventricle, and the aorta is opened through a standard incision (Figure 14.2). An instrument is then passed through the aortic valve and pushed through the thickened septal muscle to appear on the right ventricular side. The intervening muscle is then cored out to create a large VSD under the aortic valve, and the defect repaired with a patch placed on the right ventricular side of the septum. This is an unusual and difficult procedure, carrying significant risk of heart block and ventricular dysfunction. More commonly, the aortic valve is also small, and the septum can be opened in combination with excising the aortic valve, splitting the aortic annulus and then combining this with a Ross procedure (see below and Figure 14.3), such as the ‘Ross-Konno’ procedure or with patch enlargement and placement of a prosthetic aortic valve (Konno-Rastan).
Figure 14.2 Konno procedure for LVOT stenosis.
Figure 14.3 Ross-Konno procedure: note that the pulmonary autograft is harvested with a ‘skirt’ of RV muscle attached which can be used to augment the defect created in the left outflow tract in performing the Konno incision.
In the setting of functionally univentricular circulations, complex sub-aortic stenosis can sometimes be managed by creating a ‘double outlet’ to the heart and joining the pulmonary and aortic roots together in what is called the ‘Damus-Kaye-Stansel (DKS) procedure’ (Figure 14.4). This is only possible where there is a normally developed pulmonary valve and root and is typically seen in the setting of double-inlet left ventricle (DILV), where the aorta arises from a vestigial right ventricle (see Chapter 4).
Supra-aortic Stenosis.
This is rare and usually occurs in isolation and in the setting of a normal tri-leaflet aortic valve of normal dimension. The lesion has a characteristic ‘hourglass’ deformity immediately above (sometime involving) the sino-tubular junction and can be very pronounced (Figure 14.5). It is most commonly associated with Williams syndrome, a deletion of part of the long arm of chromosome 7 which is characterized by ‘elfin-like’ facial features, mild to moderate developmental delay and an overly sociable personality. Supra-aortic stenosis is the commonest cardiac lesion, but there may also be multiple stenoses in the central and branch pulmonary arteries, the latter of which tend to regress with age. Williams accounts for about 60 to 70 per cent of all cases of supra-aortic stenosis. The aortic wall is very thickened, contributing to the stenosis, and there can be associated origin stenosis of head and neck vessels and tubular narrowing of the transverse arch.
