Fig. 4.16 Unusualtypesofemboli.(A)Bonemarrowembolus.Theembolusiscomposedofhematopoieticmarrowandmarrowfatcells(clear spaces) attachedtoathrombus.(B)Amnioticfluidemboli.Twosmallpulmonaryarteriolesarepackedwithlaminatedswirlsoffetalsquamouscells.Thesurroundinglungisedematousandcongested.(Courtesy of Dr. Beth Schwartz, Baltimore, MD.) symptomatic fat embolism syndrome characterized by pulmonary insufficiency, neurologic symptoms, anemia, thrombocytopenia, and a diffuse petechial rash that is fatal in 10% of cases. Clinical signs and symptoms appear 1 to 3 days after injury as the sudden onset of tachypnea, dyspnea, tachycardia, irritability, and restlessness, which can progress rapidly to delirium or coma. Thrombocytopenia is attributed to platelet adhesion to fat globules and subsequent aggregation or splenic sequestration; anemia can result from similar red cell aggregation and/or hemolysis. A diffuse petechial rash (seen in 20%–50% of cases) is related to rapid onset of thrombocytopenia and can be a useful diagnostic feature.
The pathogenesis of fat emboli syndrome involves both mechanical obstruction and biochemical injury. Fat microemboli occlude pulmonary and cerebral microvasculature, both directly and by triggering platelet aggregation. This deleterious effect is exacerbated by fatty acid release from lipid globules, which causes local toxic endothelial injury. Platelet activation and granulocyte recruitment (with free radical, protease, and eicosanoid release) (Chapter 3) complete the vascular assault. Because lipids are dissolved by the solvents used during tissue processing, microscopic demonstration of fat microglobules (i.e., in the absence of accompanying marrow elements) requires specialized techniques (frozen sections and fat stains).
Amniotic fluid embolism is an uncommon, grave complication of labor and the immediate postpartum period occurring in 1 in 40,000 deliveries. The mortality rate approaches 80%, making it the most common cause of maternal death in the developed world and the fifth most common cause of maternal death in the United States, accounting for 10% of maternal deaths in this country; 85% of survivors suffer some form of permanent neurologic deficit. Onset is characterized by sudden severe dyspnea, cyanosis, and hypotensive shock, followed by seizures and coma. If the patient survives the initial crisis, pulmonary edema typically develops, along with (in about half the patients) disseminated intravascular coagulation secondary to release of thrombogenic substances from amniotic fluid. Indeed it is thought that morbidity and mortality in such cases results not from mechanical obstruction of pulmonary vessels but from biochemical activation of the coagulation system and the innate immune system caused by substances in the amniotic fluid.
The underlying cause is the entry of amniotic fluid (and its contents) into the maternal circulation via tears in the placental membranes and/or uterine vein rupture. Histologic analysis shows squamous cells shed from fetal skin, lanugo hair, fat from vernix caseosa, and mucin derived from the fetal respiratory or gastrointestinal tracts in the maternal pulmonary microcirculation (
Fig. 4.16B ). Other findings include marked pulmonary edema, diffuse alveolar damage (Chapter 13), and systemic fibrin thrombi generated by disseminated intravascular coagulation.
Gas bubbles within the circulation can coalesce and obstruct vascular flow and cause distal ischemic injury.