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Fat embolism syndrome (FES) is distinct from the presence of fat emboli. Symptoms usually occur 1–3 days after a traumatic injury and are predominantly pulmonary (shortness of breath, hypoxemia), neurological (agitation, delirium, or coma), dermatological (petechial rash), and haematological (anaemia, low platelets). The syndrome manifests more frequently in closed fractures of the pelvis or long bones. The petechial rash, which usually resolves in 5–7 days, is said to be pathognomonic for the syndrome, but only occurs in 20–50% of cases.
Fat emboli occur in almost 90% of all patients with severe injuries to bones, although only 10% of these are symptomatic. The risk of fat embolism syndrome is thought to be reduced by early immobilization of fractures and especially by early operative correction. There is also some evidence that steroid prophylaxis of high-risk patients reduces the incidence. The mortality rate of fat-embolism syndrome is approximately 10–20%.
The pathogenesis occurs due to both mechanical obstruction and biochemical injury. It is aggravated by local platelet and erythrocyte aggregation. The release of fatty acids from the fat globules also causes local toxic injury to endothelium. The vascular damage is aggravated by platelet activation and recruitment of granulocytes.
Several mechanisms have been proposed to explain the pathogenesis of fat embolism. They may be acting together or singly.
- Mechanical. Mobilisation of fluid fat following trauma to bone and soft tissue.
- Emulsion instability. Explains the pathogenesis of fat embolism in non-traumatic cases. Fat embolus formed by aggregation of plasma lipids (chylomicrons and fatty acids) due to disturbances in emulsification of fat. Symptoms include fatty liver (hepatic steatosis).
- Intravascular coagulation. May result from disseminated intravascular coagulation (DIC).
- Toxic injury. Blood vessels injured by high plasma levels of free fatty acid, results in increased vascular permeability and consequently pulmonary edema.
Complications from a fat embolism tend to be serious:
- Pulmonary fat embolism. Widespread obstruction causes sudden death.
- Systemic fat embolism. These may get lodged in capillaries of organs like brain, kidney, skin etc., causing minute hemorrhage and microinfarcts.
Signs and symptoms
A thorough knowledge of the signs and symptoms of the syndrome and a high index of suspicion are needed if the diagnosis is to be made. An asymptomatic latent period of about 12–48 hours precedes the clinical manifestations. The fulminant form presents as acute cor pulmonale, respiratory failure, and/or embolic phenomena leading to death within a few hours of injury.
A petechial rash that appears on the upper anterior portion of the body, including the chest, neck, upper arm, axilla, shoulder, oral mucosa and conjunctivae is considered to be a pathognomonic sign of FES, however, it appears late and often disappears within hours. It results from occlusion of dermal capillaries by fat, and increased capillary fragility.
Central nervous system (CNS) signs, including a change in level of consciousness, are not uncommon. They are usually nonspecific and have the features of diffuse encephalopathy: acute confusion, stupor, coma, rigidity (neurology), or convulsions. Cerebral edema contributes to the neurologic deterioration. Hypoxemia is present in nearly all patients with FES, often to a PaO2 of well below 60 mmHg. Arterial hypoxemia in these patients has been attributed to ventilation-perfusion inequality and intrapulmonary shunting. Acute cor pulmonale is manifested by respiratory distress, hypoxemia, hypotension and elevated central venous pressure.
Diagnosis of FES may be difficult because, except for the petechiae, there are no pathognomonic signs. Laboratory tests are mostly nonspecific:
- Serum lipase level increases in bone trauma – often misleading.
- Cytologic examination of urine, blood and sputum with Sudan or Oil Red O staining may detect fat globules that are either free or in macrophages. This test is not sensitive, however, and does not rule out fat embolism.
- Blood lipid level is not helpful for diagnosis because circulating fat levels do not correlate with the severity of the syndrome.
- Decreased hematocrit occurs within 24–48 hours and is attributed to intra-alveolar hemorrhage.
- Alteration in coagulation (thrombogenesis) and thrombocytopenia.
The most effective prophylactic measure is to reduce long bone fractures as soon as possible after the injury.
Maintenance of intravascular volume is important because shock can exacerbate the lung injury caused by FES. Albumin has been recommended for volume resuscitation in addition to balanced electrolyte solution, because it not only restores blood volume but also binds fatty acids, and may decrease the extent of lung injury.
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- Harsh Mohan, textbook of pathology, pages 124–125
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- Duke Orthopedics fat_embolism_syndrome
- Fat Embolism MRI Brain Injury from Fat Embolism