Cerebral edema with tentorial and tonsillar herniation secondary to fat emboli
Fat embolism is a descriptive term for the presence of fat droplets in the lung or peripheral circulation with or without clinical sequelae. First described by Zenker in 1861 it is potentially catastrophic and is characterised clinically by the triad of petechiae, neurological dysfunction and pulmonary involvement (1). It typically occurs after long bone fractures, but has been described after orthopaedic procedures. The number of orthopaedic procedures has increased sharply in recent years and is likely to continue to increase as the population's age increase. Likewise numbers of hip and pathologic fractures are likely to increase. The complication rate is lower for elective procedures but is greater than 5% for emergency procedures (1). The incidence of fat embolism and FES in this population is unknown but may be higher than reported in retrospective studies.
The pathogenesis of FES is poorly understood but two hypothesis have been put forward - the mechanical and the biochemical (1, 2). The former proposes that an increase in intramedullary pressure either iatrogenic or after injury forces fat into open venous sinuses. Several mechanisms have been suggested to explain how fat globules bypass the pulmonary circulation and reach other organs. Smaller droplets 7-10um may pass through the capillaries. Additionally a patent foramen ovale is found in 20-35% of the population. The biochemical hypothesis proposes physiochemical alterations when fat globules are acted on by lipoprotein lipase releasing free fatty acids, thus precipitating a local and systemic inflammatory response. It is probable that a combination of both factors contribute to the clinical syndrome (2). It is interesting to note that there was no microscopic evidence of an inflammatory reaction in this case.
Clinical diagnosis of FES is usually made using the Gurd criteria but this is of limited value in patients presenting with neurological problems (3). The full clinical triad is not always present but neurological complications are estimated to occur in as many as 80% (2). Typically acute in onset they vary from focal neurological signs to coma as in this case. Outcome is poor in patients presenting with coma. Neurological manifestations of FES have been attributed to cerebral oedema which in turn is thought to be due to hypoxemic, ischemic or vascular injury due to the toxic effects of free fatty acids or other inflammatory molecules (1). In this case the cause of death is cerebral oedema which in turn is due to hypoxia due to occlusion of small capillaries by fat emboli
Computerised Tomography and Magnetic Resonance Imaging may be helpful in the diagnosis or confirmation of FES (2, 4). CT is frequently normal or demonstrates non specific findings such as generalised swelling (2, 4). It was not particularly helpful in this case. The typical 'starfield' pattern was not seen on MR in our case. This feature is considered highly specific for cerebral FES. Diffusion weighted imaging was more helpful here.
The true incidence of FES is difficult to establish. Mild forms make a full clinical recovery but presentation with coma, as in this case is associated with a poor prognosis.
Contributed by Seamus Looby, Derval Royston and Francesca Brett