Fat embolism syndrome follows long bone fractures. Its classic presentation consists of an asymptomatic interval followed by pulmonary and neurologic manifestations combined with petechial hemorrhages. The syndrome follows a biphasic clinical course. The initial symptoms are probably caused by mechanical occlusion of multiple blood vessels with fat globules that are too large to pass through the capillaries. Unlike other embolic events, the vascular occlusion in fat embolism is often temporary or incomplete since fat globules do not completely obstruct capillary blood flow because of their fluidity and deformability. The late presentation is thought to be a result of hydrolysis of the fat to more irritating free fatty acids which then migrate to other organs via the systemic circulation.
Many aspects of the fat embolism syndrome remain poorly understood, and disagreement about its etiology, pathophysiology, diagnosis and treatment persists. It is therefore difficult to determine the incidence of this complication. It ranges from less than 2% to 22% in different studies. Fat embolism has been associated with many nontraumatic disorders. It is most common after skeletal injury, and is most likely to occur in patients with multiple long bone and pelvic fractures. Patients with fractures involving the middle and proximal parts of the femoral shaft are more likely to experience fat embolism. Age also seems to be a factor in the development of FES: young men with fractures are at increased risk.
Fat embolism and FES are also more likely to occur after closed, rather than open, fractures. Two events promote entrance of marrow contents into the circulation following a fracture: movement of unstable bone fragments and reaming of the medullary cavity during placement of an internal fixation device. Both of these cause distortion of and increased pressure within the medullary cavity, permitting entry of marrow fat into torn venous channels that remain open even in shock because they are attached to the surrounding bone.
Multiple fractures release a greater amount of fat into the marrow vessels than do single fractures, increasing the liklihood of FES.
There are two theories which have gained acceptance:
- The mechanical theory: FES results from physical obstruction of the pulmonary and systemic vasculature with embolized fat. Increased intramedullary pressure after injury forces marrow into injured venous sinusoids, from which the fat travels to the lung and occludes pulmonary capillaries. Fat emboli can cause cor pulmonale if adequate compensatory pulmonary vasodilation does not occur.
- The biochemical theory: Circulating free fatty acids are directly toxic to pneumocytes and capillary endothelium in the lung, causing interstitial hemorrhage, edema and chemical pneumonitis.
It is also possible that coexisting shock, hypovolemia and sepsis, all of which reduce liver flow, facilitate the development of FES by exacerbating the toxic effects of free fatty acids.
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.
Clinical fat embolism syndrome presents with tachycardia, tachypnea, elevated temperature, hypoxemia, hypocapnia, thrombocytopenia, and occasionally mild neurological symptoms.
A petechial rash that appears on the upper anterior portion of the body, including the chest, neck, upper arm, axilla, shoulder, oral mucous membranes 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.
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, or convulsions. Cerebral edema contributes to the neurologic deterioration. Hypoxemia is present in nearly all patients with FES, often to a Pa02 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.
The chest X-ray may show evenly distributed, fleck-like pulmonary shadows (Snow Storm appearance), increased pulmonary markings and dilatation of the right side of the heart.
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 and thrombocytopenia.
In summary, the diagnosis of FES may be difficult because, except for the petechiae, there are are no pathognomonic signs.
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.
Mechanical ventilation and PEEP may be required to maintain arterial oxygenation.
High dose corticosteroids have been effective in preventing development of FES in several trials, but controversy on this issue still persists.
A high index of suspicion is needed to make the diagnosis of the often fatal fat embolism syndrome.