Case of the Week

Background:

• Cerebral fat embolism (CFE) usually occurs in the setting of long bone and pelvic fractures and less commonly with marrow necrosis in sickle cell crises. Fat microemboli travel to the brain and become trapped in capillary beds resulting in many tiny infarcts, which in severe cases are innumerable and produce a pathognomonic appearance on MR (“starfield pattern”). In severe cases, there is diffuse involvement of the cortex and subcortical white matter in an external watershed distribution with involvement of deeper brain structures and the cerebellum/brainstem also being possible. Microemboli can also disseminate throughout the body and result in infarctions within the spinal cord and organs.

Clinical Presentation:

• Early clinical diagnosis of CFE is challenging as the signs and symptoms are variable and nonspecific. CFE should be considered in non-head trauma patients or in those with severe sickle crises who present with or develop altered mental status. In one study the classic clinical triad of fat embolism syndrome consisted of altered mental status (60%), respiratory distress (95%), and widespread petechial rash (33%). No cutaneous findings were demonstrated in our case. 

Key Diagnostic Features:

• CT findings of CFE are either normal or show nonspecific changes such as ill-defined areas of hypoattenuation and/or cerebral edema. MRI is key for diagnosis, with DWI and SWI being the most sensitive and specific. Acute CFE produces a very characteristic appearance on DWI referred to as the “starfield pattern,” where there are extensive small foci of diffusion restriction consistent with innumerable microembolic infarcts. These infarcts can have associated petechial hemorrhages (microbleeds), and if extensive can produce another characteristic imaging sign of CFE referred to as the “walnut kernel” sign on SWI (not seen in our case). T2/FLAIR images in acute CFE will show a heterogeneous appearance of the brain secondary to increased signal from a combination of vasogenic and cytotoxic edema affiliated with the extensive infarct burden.

Differential Diagnosis:

• The imaging appearance described above is suggestive of CFE, especially in the setting of recent non-head trauma, as there are a limited number of disease processes that could result in this appearance, which would include: 
  • Diffuse axonal injury (DAI): DAI appears as microhemorrhages at the locations of axonal shearing in the setting of trauma. These are best seen on susceptibility-weighted images as punctate foci of magnetic susceptibility classically at the gray-white interface. The extent of cerebral involvement is related to the severity of trauma. The key for differentiation of DAI from CFE is the lack of significant DWI and FLAIR signal changes, which are expected with CFE. Additionally, involvement of deep white matter and deep gray nuclei is seen less commonly with DAI.
  • Disseminated intravascular coagulation (DIC): Intracranial manifestations of DIC are nonspecific and not always present, but typically would include entities such as extra-axial or parenchymal hemorrhages (not microhemorrhages) and/or infarctions. These are related to coagulation factor consumption and intravascular coagulation/thrombus formation. Findings of intracranial DIC may be less extensive than that of CFE and usually occur in the absence of trauma. Hematologic and coagulation abnormalities would also be demonstrated on laboratory testing.
  • Septic/cardiogenic cerebral emboli: Bland thromboemboli will result in acute infarcts affecting multiple vascular territories, while ongoing thromboembolism formation will produce multiple infarcts of varying ages. Septic emboli can also result in acute infarcts and are often associated with hemorrhage and abscess formation. The middle cerebral artery vascular territory is most often involved with the posterior circulation only rarely being involved. 
  • Cerebral vasculitis: Generally, cerebral vasculitis will appear as multiple infarcts in varying stages of evolution affecting multiple vascular territories. Associated hemorrhage may be present. Angiographic imaging will show multifocal areas of vessel irregularity with intracranial stenoses and/or occlusion.
  • Disseminated, miliary pattern of hemorrhagic metastasis: These will appear as numerous small enhancing lesions with blood products with or without vasogenic edema throughout the brain. DWI signal changes are not expected with punctate cerebral metastases. These lesions would progressively enlarge over time and are unlikely to result in sudden changes in mentation.

Treatment:

• Supportive care is the mainstay of treatment and can lead to complete recovery. Mortality is estimated to be 5–15%.