Clinical meaning
Acute chest syndrome (ACS) is the leading cause of death and the second most common reason for hospitalization in patients with sickle cell disease (SCD). It is defined as a new pulmonary infiltrate on chest radiograph involving at least one complete lung segment, accompanied by one or more of the following: fever (temperature above 38.5 degrees Celsius), respiratory symptoms (cough, dyspnea, tachypnea, chest pain), or hypoxemia. Understanding the complex pathophysiology of ACS requires a thorough knowledge of sickle cell hemoglobin polymerization, vaso-occlusion, and the interplay between infection, fat embolism, and pulmonary vascular dysfunction.
Sickle cell disease results from a point mutation in the beta-globin gene on chromosome 11, where valine replaces glutamic acid at position 6, producing hemoglobin S (HbS). Under conditions of deoxygenation, acidosis, dehydration, hypothermia, or infection, HbS molecules polymerize into rigid rod-like fibers that distort the normally flexible biconcave erythrocyte into a rigid sickle (crescent) shape. These sickled erythrocytes have several pathological properties: they are rigid and unable to deform through narrow capillaries, they are sticky and adhere to the vascular endothelium via adhesion molecules (VCAM-1, P-selectin, and thrombospondin), they activate the coagulation cascade promoting microthrombus formation, and they have a shortened lifespan (10-20 days versus 120 days for normal erythrocytes), causing chronic hemolytic anemia.
The pathogenesis of ACS involves multiple overlapping mechanisms. Pulmonary infection is the most common trigger in children, with Chlamydia pneumoniae, Mycoplasma pneumoniae, and respiratory viruses (respiratory syncytial virus, influenza, parvovirus B19) being the most frequent pathogens. In adults, fat embolism from bone marrow necrosis during vaso-occlusive crisis (VOC) is the most common cause. During a VOC, sickling within the bone marrow microvasculature causes infarction of bone marrow tissue, releasing fat droplets and necrotic cellular debris into the venous circulation. These fat emboli travel to the pulmonary vasculature, where phospholipase A2 converts the fat into free fatty acids that cause direct injury to the pulmonary endothelium, triggering an intense inflammatory response.