Unravelling a Multifaceted Case of Cyanotic Congenital Heart Disease Complicated by Secondary Polycythemia and Induced Hyperviscosity

Double inlet left ventricle (DILV) is a rare and complex cyanotic congenital heart disease (CCHD), characterized by chronic hypoxia resulting from the admixture of oxygenated and deoxygenated blood. In response, patients often develop compensatory secondary polycythemia, which increases blood viscosity. While polycythemia is known to elevate the risk of thromboembolic events such as stroke, overt hyperviscosity syndrome is uncommon. Classically, hyperviscosity syndrome has been associated with monoclonal gammopathies, less frequently with primary polycythemia, and only rarely with secondary polycythemia. We report the case of a 19-year-old male with DILV who was relatively asymptomatic till few months back, presented with worsening cyanosis, progressive dyspnea, edema, and symptoms of hyperviscosity, including holocranial headaches and visual disturbances. Investigations revealed an exceptionally elevated hematocrit level of 84.3%, which has not been reported yet. Despite such extreme erythrocytosis, the patient did not demonstrate any focal neurological deficits or cerebrovascular sequelae, highlighting a compensated erythrocytosis state. He was managed with serial phlebotomies, intravenous fluids, and anticoagulation, leading to symptomatic relief. Reporting such cases is critical, as they not only emphasize the rare natural survival of uncorrected complex congenital heart diseases into adulthood but also underscore the clinical challenge of balancing oxygen delivery and viscosity-related complications. Early recognition and tailored management are essential to prevent life-threatening consequences. Documenting such presentations contributes to the growing body of evidence needed to optimize treatment protocols and long-term outcomes in patients with uncorrected or palliated CCHD.