Teaching Case

A 32-year-old man with multiple fractures was admitted to the emergency department after a motorbike accident. Initial CT of the head was unremarkable, 7 days later a brain MRI was performed due to altered level of consciousness.

X-ray showed shaft fracture of the right femur and displaced shaft fractures of the left tibia and fibula (Fig. 1).
Brain MRI revealed bilateral T2 confluent hyperintensity lesions with restricted diffusion located in subcortical white matter, middle cerebellar peduncles and splenium of corpus callosum (Fig. 2, 3). Multiple supra and infratentorial hypointense lesions on susceptibility-weighted imaging (SWI) images were also noted, due to haemorrhagic lesions involving sub-cortical and periventricular white matter, cerebellar hemispheres, cerebellar peduncles, corpus callosum and brain stem (Fig. 4).

Fat embolism syndrome (FES) has been known to be associated with displaced long bone fracture of the lower extremities and it is characterised by respiratory disability, petechial skin rash and neurological symptoms, typically seen between 12 and 72 hours after the injury. Neurological findings vary from drowsiness to acute confusional state and seizures, decorticate posturing and focal deficits [1].
End organ pathology is thought to be mediated by two main processes: mechanical obstruction and biochemical injury. Fat cells in the bone marrow gain access to the vein sinusoids after trauma and travel through the venous system lodging in the pulmonary circulation. Fat cells enter the arterial circulation via a patent foramen ovale or directly through the pulmonary capillary bed. Fat emboli trapped in the small capillaries release free fatty acids and glycerol, which are toxic and initiate an inflammatory cascade resulting in endothelial injury, permeability oedema and haemorrhage, leading to end-organ dysfunction [2].

Brain MRI has been reported to be the most sensitive means of diagnosing CFE [3]. MRI findings are varied and 5 distinct image patterns have been described [4]. Type 1 is the most well-known pattern, characterised by scattered spot lesions with restricted diffusion on DWI, representing cytotoxic oedema, mainly in the acute stage. The lesions are distributed bilaterally in watershed zones and deep grey matter.
In the subacute stage, type 2A pattern is often present, where confluent symmetric T2 hyperintense lesions with restricted diffusion on DWI in periventricular and subcortical white matter can be found bilaterally. The cerebellar peduncles, corpus callosum and posterior internal capsule may be involved. Type 2B is also frequent in the subacute stage. The difference is that these lesions usually have a small patch shape and the signal on the DWI image shows increased diffusion instead. Furthermore, these lesions may enhance. Type 2C is a pathognomonic pattern of CFE, characterised by punctuate hypointense foci on SWI distributed bilaterally in the cerebral and cerebellar white matter and the corpus callosum on SWI.
Type 3 pattern is found in the late stage, with chronic sequelae including brain atrophy or demyelination.

Another traumatic brain injury accompanied by multiple microhaemorrhages is diffuse axonal injury (DAI). However, DAI mainly occurs in the grey-white matter interface of the frontotemporal lobes and corpus callosum [4].

In the setting of neurologic deterioration after long bone fracture, the scattered SWI hypointensities are indicative of CFE. Early recognition may determine appropriate management and improve the outcome.