Laboratory Studies
An otherwise unexplained increase in pulmonary shunt fraction alveolar-to-arterial oxygen tension difference, especially if it occurs within 24-48 hours of a sentinel event associated with fat embolism syndrome (FES), is strongly suggestive of the syndrome. Thrombocytopenia, anemia, and hypofibrinogenemia are indicative of FES; however, they are nonspecific.
Urinary fat stains are not considered to be sensitive or specific enough for diagnosing FES or for determining the risk of it. Fat globules in the urine are common after trauma. [11]
Preliminary investigations of the cytology of pulmonary capillary blood obtained from a wedged pulmonary artery catheter revealed fat globules in patients with FES and showed that this method may be beneficial in early detection of patients at risk.
In the future, genotyping for polymorphisms associated with increased susceptibility to inflammatory stimuli may help identify those at risk for FES. Specific antibody therapy targeting inflammatory molecules has not been useful.
Radiography and Computed Tomography
Serial chest radiographs reveal increasing diffuse bilateral pulmonary infiltrates within 24-48 hours of the onset of clinical findings.
Findings from noncontrast computed tomography (CT) of the head performed because of alterations in mental status may be normal or may reveal diffuse white-matter petechial hemorrhages consistent with microvascular injury.
Because the embolic particles are lodged in the capillary beds, helical CT findings may be normal. Parenchymal changes consistent with lung contusion, acute lung injury, or acute respiratory distress syndrome (ARDS) may be evident, depending on the burden of secondary lung injury.
Nodular or ground-glass opacities in the setting of trauma suggest fat embolism. [22] In one retrospective review, ground-glass opacities in the setting of trauma were the most common findingings on helical chest CT; 67% of all FES subjects enrolled had ground-glass opacities. [23] The presence of ground-glass opacities involving more than 75% of lung parenchyma with the concomitant presence of consolidation correlates with disease severity. [23, 21]
Ultrasonography
In a small case study, five patients with trauma were monitored with intracranial Doppler ultrasonography, two during intraoperative nailing of long-bone fractures. [24] Cerebral microembolic signals were detected as long as 4 days after injury.
Transesophageal echocardiography (TEE) may be of use in evaluating the intraoperative release of marrow contents into the bloodstream during intramedullary reaming and nailing. The density of the echogenic material passing through the right side of the heart correlates with the degree of reduction in arterial oxygen saturation.
Repeated showers of emboli on TEE have been noted to increase right heart and pulmonary artery pressures. Embolization of marrow contents through a patent foramen ovale also has been noted. However, evidence of embolization obtained by means of TEE is not correlated with the actual development of FES.
Other Imaging Studies
Scant data exist regarding magnetic resonance imaging (MRI) findings in patients with FES; however, in one small patient group, multiple nonconfluent, hyperintense lesions were seen on proton-density– and T2-weighted images. [25]
Nuclear ventilation-perfusion imaging of the lungs may be performed when pulmonary embolism is suspected. The findings from this scan may be normal or may demonstrate subsegmental perfusion defects.
Bronchoalveolar Lavage With Staining for Fat
Bronchoalveolar lavage (BAL) specimens have been evaluated in trauma patients and sickle-cell patients with acute chest syndrome, and the results have been mixed. [26, 27]
Lipid inclusions commonly appear in patients with traumatic and nontraumatic respiratory failure; the standard cutoff in the BAL studies—5% fat-containing macrophages—results in a low specificity for the test. To improve specificity, some authors suggest raising the cutoff to 30%. At present, using BAL to aid in the diagnosis of FES or to predict its likelihood is controversial.
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Hematoxylin-eosin stain of section of lungs showing blood vessel with fibrinoid material and optical empty space indicative of presence of lipid dissolved during staining process. This 55-year-old woman died of massive fat embolism after developing pancreatitis due to endoscopic retrograde cholangiopancreatography. Image courtesy of Wikimedia Commons. Originally published in Kanen BL, Loffeld RJLF. Pancreatitis with an unusual fatal complication following endoscopic retrograde cholangiopancreaticography: a case report. Journal of Medical Case Reports. 2008;2:215.
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Frozen section of lung stained with oil red O showing multiple orange red fat globules of varying sizes in septal vasculature. Image courtesy of Dr AVC Rao, Senior Lecturer in Pathology, The University of the West Indies at St Augustine, Trinidad and Tobago. Originally published in Journal of Orthopaedics (http://www.jortho.org/2008/5/4/e8/).
