Acute chest syndrome

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Overview
Acute Chest Syndrome is a vaso-occlusive crisis of the pulmonary vasculature commonly seen in patients with Sickle Cell Anemia. Acute Chest Syndrome is often initiated by a lung infection and the resulting inflammation and loss of oxygen tension leads to sickling of red cells and further vaso-occlusion. Acute Chest Syndrome is one of the most common causes of death for Sickle Cell patients.

Historical Perspective
The term acute chest syndrome (ACS) was first proposed in 1979, by Charache et.al. ACS is the second most common cause of hospitalization in patients with sickle cell disease (SCD), and is responsible for up to 25% of deaths. Recurrent episodes have been associated with an increased risk of both chronic lung disease and early mortality. The Cooperative Study of Sickle Cell disease is a prospective study that followed 3,751 patients with SCD from 1979 through 1988. Vichinsky et.al., from that group, reviewed 1,722 cases of ACS in 939 patients. They defined ACS as a new pulmonary infiltrate on chest x-ray (CXR), or a defect on a perfusion scan, however, 99% of patients, were diagnosed with ACS on the basis of their CXRs.

Epidemiology and Demographics

 * The incidence of ACS peaks in children 2-5 years old, and declines as patients age.

Risk Factors

 * Factors associated with an increased incidence of ACS are:
 * The SS genotype
 * A higher steady-state white blood cell (WBC) count.
 * It seems that smoking status does not influence the incidence of ACS.
 * A higher level of Hb F may protect children younger than 2 years old.

Screening
See sickle cell anemia

Causes
Genetics: The SS genotype is associated with an increased incidence of ACS. See sickle cell anemia for more information regarding the genetics of the disease.

Differential Diagnosis

 * The primary acute pulmonary diseases which affect patients with SCD include pneumonia, infarction due to in-situ clot, fat / bone marrow emboli and ACS. Patients with SCD are predisposed to pneumonia due to loss of antibody protection (secondary to auto-splenectomy), and defective opsonization and phagocytosis resulting from a defect in the alternate pathway of complement activation. As blood and sputum cultures are often negative in any patient with pneumonia it is difficult to distinguish infection from ACS. However, unlike children, who have a higher incidence of infectious causes of ACS, adults are thought to have a higher incidence of vascular occlusion. One study that examined the causes of pulmonary infiltrates in patients with ACS with bronchoscopy, found a 21% incidence of bacterial infection. Interestingly, patients with SCD do not have a higher rate of large vessel fibrin thromboembolism compared with the general population. They do, however, have a higher rate of in-situ clot, and fat embolism. In-situ clot results from sickle cells being less deformable than normal cells, an increased adherence of sickle cells to the endothelial cells, and local vasoconstriction secondary to hypoxemia. Patients with SC disease have a higher rate of thrombosis as compared to those with SS disease, due to a higher blood viscosity and hematocrit. Fat embolism probably results from bone marrow infarction and is often associated with mental status changes, thrombocytopenia, a falling hematocrit (HCT), disseminated intravascular coagulation (DIC), hypocalcemia, hyperuricemia, and severe hypoxemia. Unfortunately, there is no test that will confirm the diagnosis of fat emboli. Lipemia retinalis and petechiae on the conjunctiva and upper thorax are suggestive of fat emboli syndrome. Bronchoalveolar lavage findings of greater than 5% lipid laden macrophages, and fat droplets obtained from a pulmonary capillary wedge aspirate are also highly suggestive, however their sensitivity and specificity remain to be define. Rib and sternal fractures or infarction can also precipitate ACS by causing splinting, atelectasis and hypoxemia.

Natural History

 * Overall mortality in patients with ACS is 1.8%, however in adults the rate is 4.3%.
 * There is no difference between patients with Hb SS and Hb SC.
 * Unfortunately, there have been no presenting signs, symptoms or laboratory data that is predictive of who will die.
 * A trend to increased mortality has been observed in patients with a lower presenting hemoglobin, multilobar involvement on CXR, and bacteremia.
 * Patients who die, tend to do so very rapidly and unexpectedly.

Prognosis

 * Overall mortality in patients with ACS is 1.8%, however in adults the rate is 4.3%.
 * There is no difference between patients with Hb SS and Hb SC.
 * Unfortunately, there have been no presenting signs, symptoms or laboratory data that is predictive of who will die.
 * A trend to increased mortality has been observed in patients with a lower presenting hemoglobin, multilobar involvement on CXR, and bacteremia.
 * Patients who die, tend to do so very rapidly and unexpectedly.

History and Symptoms

 * The most common presenting symptoms for ACS are:
 * Fever
 * Cough
 * Chest pain
 * There is an age dependant frequency of symptoms:
 * Younger patients
 * More fever and cough
 * Older patients
 * More chest pain
 * Shortness of breath
 * Chills
 * Productive cough
 * Hemoptysis
 * Symptomatology generally does not vary between patients with Hb SS and Hb SC disease, except that severe chest pain is seen more frequently in patients with Hb SS. There tends to be a seasonal variation, with more cases occurring in the winter months.

Physical Examination

 * Patients commonly have fevers up to 104 degrees Fahrenheit. The patient will likely have tachypnea.

Eyes

 * Lipemia retinais can be seen in the patient with fat embolism

Heart

 * Tachycardia

Lung

 * Rales and dullness to percussion are the most common findings on physical exam.
 * 35% of patients have a normal auscultatory exam.
 * Patients can also have sternal tenderness.
 * There is little difference in physical findings between patients with Hb SS and Hb SC disease.

Neurologic

 * Mental confusion may suggest hyoxia or fat embolization.

Electrolyte and Biomarker Studies

 * Patients also commonly have a mild anemia and leukocytosis.
 * Severe hypoxemia with a PaO2 <60mmHg can be seen in 20%.
 * Bacteremia has been documented in up to 3.5% of patients.
 * The incidence decreases with age, and only 1.8% of patients older than 10, have positive blood cultures.
 * In the older group, S. pneumo has been isolated in roughly 25%, with other common organisms including H. influenza, S. aureus, Salmonella, Enterobacter, and Clostridia. Bacteremia in adults tends to be more common in patients with Hb SS disease.

Electrocardiogram
Can be used to rule out an acute coronary syndrome as a cause of chest pain and tachypnea.

Chest X Ray

 * Although many patients present with normal CXRs, 36% of adults develop multilobar infiltrates, more commonly in the lower lobes, within several hours to several days of admission. *:* Additionally, pleural effusions are seen in 21-35%.
 * These tend to be exudative with a predominance of polys.

Acute Pharmacotherapies

 * The primary goal is to prevent the development of ACS. It is therefore essential to vaccinate patients with the pneumococcal and haemophilus influenza B (HIB) vaccines and prevent hypoxemia.
 * The acute treatment of ACS is primarily supportive, and depends on the likelihood of the above diagnoses.
 * Antibiotics should be initiated, with the goal of covering the typical and atypical organisms that cause community acquired pneumonia.
 * Supplemental oxygen to maintain a PaO2 >70.
 * Hydration to prevent intravascular sickling are essential.
 * Pain control and incentive spirometry to avoid splinting is also important.
 * There is some data, however, that suggests that vigorous hydration in combination with narcotic analgesics can lead to pulmonary edema. Therefore, euvolemia should be the goal.
 * Empiric anticoagulant therapy is currently not recommended because of the risk of intracranial and renal bleeding.
 * Exchange transfusion is recommended in the setting of progressive infiltrates and hypoxemia, with a goal of reducing the Hb S level to below 30%, while keeping HCT ~30%.
 * Hydroxyurea has been associated with a 50% reduction in the frequency of developing both pain crises and ACS, and studies are underway examining its use in the acute treatment of patients with ACS.

Chronic Pharmacotherapies
The primary goal is to prevent the development of ACS. It is therefore essential to vaccinate patients with the pneumococcal and HIB vaccines and prevent hypoxemia.

Primary Prevention
The primary goal is to prevent the development of ACS. It is therefore essential to vaccinate patients with the pneumococcal and HIB vaccines and prevent hypoxemia.

Secondary Prevention
The primary goal is to prevent the development of ACS. It is therefore essential to vaccinate patients with the pneumococcal and HIB vaccines and prevent hypoxemia.