Introduction and Background
Hypoplastic Left Heart Syndrome (HLHS) is a condition which happens when the systemic or left ventricle is unsuccessful to develop adequately. It is associated with varying degrees of hypoplasia of the aorta, aortic valve, mitral valve, and left ventricle. It is the second most common critical cardiac defect to present in the first week of life, and is fatal in infancy if palliative surgery is not performed. Genetic syndromes and non-cardiac malformations have been reported in up to three-tenths of neonates with the syndrome. (Bellinger, Wypij, duDuplessis, 1386)
Early survival has now improved, due to prompt diagnosis and stabilization in the neonatal period, as well as advances in surgical techniques and perioperative management. Studies to date, however, have indicated that survivors of both reconstructive surgery and transplantation have a greater likelihood of decreased intelligence, delays in speech and language, attentional and behavioural difficulties, and an increased need for special services in school. Accordingly, greater attention has been directed toward understanding factors that may influence undesirable outcomes.
Hypoplastic Left Heart Syndrome
The Hypoplastic Left Heart Syndrome is basically a heterogeneous group of cardiac malformations consisting of various degrees of underdevelopment of the left heart and the aorta, resulting in significant obstruction to flow of blood into the systemic circulation, and difficulty in the left heart supporting adequately this circulation. (Forbess, Visconti, 98) In the past, precise definitions for this syndrome have been lacking, and the term has been used interchangeably with a variety of others, creating not only varying degrees of confusion, but also lack of consistency in usage.
The variable pathophysiology of patients diagnosed with hypoplastic left heart syndrome becomes important because of the implications as to whether it is possible successfully to achieve biventricular as opposed to functionally uni-ventricular repair. At the severe end of the spectrum of patients classified within the syndrome, the systemic circulation is completely dependent on the right ventricle and flow through the arterial duct, with flow in the aortic arch and the ascending aorta being retrograde. This pathophysiology is seen in the setting of aortic atresia with mitral atresia or stenosis. At the mild end of the spectrum, the systemic circulation may only be partially dependent on the right ventricle and flow through the arterial duct, as seen in some patients with aortic stenosis and mitral stenosis, those with aortic stenosis with left ventricular hypoplasia, or in those diagnosed with the hypoplastic left heart complex. (Dorman, 276) In this setting, the flow to the descending aorta is dependent on the right ventricle and arterial duct, while flow to the ascending aorta and various portions of the aortic arch and branches are supplied by antegrade flow from the left ventricle.
In a very few patients with the complex, the systemic circulation may be entirely dependent on the left ventricle in the presence of lesser degrees of obstruction within the left heart, although such patients are usually in severe congestive cardiac failure, and require surgical correction. In the greater majority of patients with hypoplastic left heart syndrome, the only surgical reconstruction possible is the functionally univentricular approach using the Norwood, or Norwood-like, operation in the neonatal period, transferring the systemic circulation so that it exits directly from the right ventricle, followed eventually by a Fontan-type operation. (Ivanovic, Leiva , Perez, 118) Biventricular repair may be possible, however, for some patients at the milder end of the syndrome. The potential exists for left ventricular growth after birth and after biventricular repair. Accurate understanding of the pathophysiology in each patient is important, as it may help to determine whether biventricular repair should be considered.
Therapeutic options for patients with the hypoplastic left heart syndrome. Three surgical approaches have evolved over the years and are available today:
· Multistage reconstructive surgery based on a single ventricle physiology, popularized by Norwood et al.
· cardiac replacement championed by Bailey and associates
· Biventricular repair, as used for the patients at the mild end of the spectrum of hypoplastic left heart syndrome, as well as for the patients with aortic atresia with ventricular septal defect and a well developed left ventricle, technically a “hypoplastic left heart syndrome-related malformation. (Watemberg, Silver, Harel, Lerman-Sagie, 117)
The Norwood operation to management of the hypoplastic left heart syndrome is a staged surgical palliation. The first stage, which is usually performed in the first week of life, consists of reconstruction of the aortic arch, using the main pulmonary artery as a single outlet from the right ventricle of the heart. (Norwood, Lang, Hansen, 23) The second stage of the Norwood protocol involves surgical anastamosis of the superior vena cava to the pulmonary arteries.
The pioneering work of Bailey and his associates has resulted in the establishment of neonatal and infant cardiac transplantation as a viable alternative to multistage reconstructive procedures. (Bailey, Nehlsen-Cannarella, Doroshow, 949) The main advantage of cardiac transplantation is the replacement, in one operation, of a functionally univentricular heart in which the right ventricle is fulfilling a systemic role with a normal heart possessing four chambers and with normal physiology.
Recently, there has been increased interest in biventricular repair for some patients at the milder end of the syndrome. Thus, Tchervenkov and associates have identified a group of patients who had hypoplasia of the structures of the left ventricle and aorta without intrinsic valvar stenosis or atresia, called by them the hypoplastic left heart complex. (Tchervenkov, Tahta, Jutras, Béland, 1350) Most of these patients have successfully undergone primary biventricular repair, consisting of extensive enlargement of the aortic arch and ascending aorta with a pulmonary homograft and closure of the interventricular and interatrial communications,
Hypoplastic left heart syndrome may be accurately diagnosed during fetal life. Prenatal diagnosis provides the opportunity for parents to make an informed choice about their options, including surgery, non-intervention post-natally or termination of pregnancy. Short to medium term survival continues to improve for a condition that was previously invariably lethal. There continues to be a significant mortality and morbidity associated with hypoplastic left heart syndrome, and the long-term prognosis is unknown. Knowledge of the condition prior to birth means that babies who are to undergo surgery present in optimal condition for such interventions. Parents who have had an affected fetus or child should be offered detailed fetal echocardiography to exclude a recurrence in subsequent pregnancies.
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