HLHS is short for hypoplastic left heart syndrome, a severe form of birth defect of the heart. The left sided chambers of the heart are underdeveloped (hypoplastic) in HLHS. That means the left atrium and ventricle are small. Along with the valves between these chambers (mitral valve) is also narrow (mitral stenosis) or with no opening at all (mitral atresia). In addition the valve at the outflow of the left ventricle may also be narrow (aortic stenosis) or without an opening (aortic atresia). The largest blood vessel (aorta) may also be hypoplastic. HLHS is a complex congenital heart disease, which may require a series of corrective surgeries.