Syndrome, Barlow: Barlow syndrome is mitral valve prolapse (also known as "click murmur syndrome"), the most common heart valve abnormality, affecting 5%-10% of the world population. Most patients have no symptoms and require no treatment. However, the condition can be associated with fatigue and/or palpitations. The mitral valve prolapse can often be detected by a doctor during examination of the heart and can be confirmed with an echocardiogram. .