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Cerebral fat embolism

Cerebral fat embolism most commonly occurs following long-bone fractures (especially femur), orthopedic surgery, or cardiac surgery, presenting with highly variable neurological symptoms ranging from headache and lethargy to coma and seizures.

Causes and pathophysiology

Most common trigger is orthopedic trauma, particularly long-bone (femoral) and pelvic fractures, and orthopedic procedures/intramedullary instrumentation.

Nontraumatic causes are rare and include sickle cell disease/hemoglobinopathies (bone marrow necrosis during vaso-occlusive crisis), pancreatitis, diabetes mellitus, liposuction, bone marrow harvest/transplantation, and alcoholic liver disease.

Two complementary mechanisms are proposed: a mechanical theory (marrow fat globules enter venous sinusoids after bone injury, then reach the systemic/cerebral circulation, often paradoxically via a patent foramen ovale or by passing through the pulmonary bed) and a biochemical/chemical theory (systemic stress triggers lipolysis, releasing free fatty acids that cause microvascular inflammation and a prothrombotic state).

Key Images

Cerebral Fat Embolism

Cerebral Fat Embolism - SWI

Case 1

Case 1

Case 2

Case 2

Case 2

Look For First
  • Bilaterally symmetric punctate foci of cytotoxic edema (starfield pattern) on DWI in subcortical white matter and deep brain
  • Profuse microhemorrhages in white matter on SWI (walnut kernel pattern)
  • External watershed distribution of lesions involving corpus callosum, internal capsule, and subcortical U-fibers
Key Image Findings
  • DWI shows scattered punctate foci of cytotoxic edema (starfield pattern in ~70% of cases) early at 1-4 days, with confluent areas of cytotoxic edema developing later at 5-14 days in ~50% of cases
  • SWI is abnormal in nearly all (~98%) patients, demonstrating profuse microhemorrhages throughout white matter in a characteristic walnut kernel pattern (~90% of cases)
  • DWI is abnormal in nearly all (~98%) patients, making it the second most sensitive sequence after SWI for detecting cerebral fat microembolism
  • Distribution pattern is bilaterally symmetric and predominantly involves subcortical white matter, deep white matter, corpus callosum, internal capsule, and subcortical U-fibers with variable external watershed distribution
  • T2/FLAIR may show small areas of high signal intensity indicating vasogenic edema in ~30% of cases
  • T1 post-contrast imaging may show enhancement in some areas of vasogenic edema, with corresponding low T1 signal in focal regions
  • ADC changes may be much less commonly seen compared to DWI high signal intensity alone
  • In macroembolism, CT may show hypodense vessel sign and features of acute ischemic stroke, while MRI changes mirror those of large vessel occlusion from other causes
Differential Diagnosis
  • Disseminated intravascular coagulation from systemic infection or sepsis - distinguished by clinical context and absence of the characteristic starfield and walnut kernel patterns
  • Cardiogenic cerebral emboli or septic emboli - typically fewer, larger lesions without the profuse microembolic pattern
  • Diffuse axonal injury - associated with head trauma history and different distribution pattern on MRI
  • Cerebral vasculitis - may mimic on imaging but typically shows different clinical presentation and CSF findings
  • Watershed infarction - typically in boundary zones between major arterial territories rather than bilateral symmetric subcortical distribution
  • Minute hemorrhagic cerebral metastases - typically fewer lesions with history of malignancy and different size/distribution pattern
Discussion

Fat emboli reach the brain through right-to-left cardiac shunt or intact pulmonary circulation, although patent foramen ovale prevalence is not higher in fat embolism patients than the general population.

Microhemorrhages - result primarily from fat-induced toxic injury to the cerebral microvascular endothelium rather than from simple mechanical vessel rupture

Microembolism presents with highly variable and non-specific symptoms (headache, lethargy, delirium, stupor, convulsions, coma) and can be aided by concurrent pulmonary or cutaneous features of fat embolism syndrome

Macroembolism is rare and typically presents with acute ischemic stroke symptoms from large vessel occlusion, often without systemic fat embolism syndrome

MRI patterns are classified into three types based on chronicity: acute (scattered cytotoxic edema), subacute (confluent edema, vasogenic edema, or petechial hemorrhages), and chronic (atrophy and gliosis)

Prognosis is often better than imaging severity suggests, with ~90% achieving minimal-to-mild disability in patients with mild mental status changes or seizures, though mortality is ~10%

Males are disproportionately affected (~80% of cases) and tend to be younger (~30 years) compared to affected females (~50 years)

Reporting Pearls

When reporting cerebral fat microembolism, describe the bilateral symmetric distribution of numerous tiny foci on both DWI (starfield pattern) and SWI (walnut kernel pattern) predominantly in the subcortical and deep white matter; this profuse and characteristic microembolic pattern is the key differentiator from other causes of multifocal brain lesions. Differential and reporting In trauma, diffuse axonal injury (DAI) is the most important imaging mimic. DAI hemorrhages favor characteristic shear-injury sites, especially the gray–white junction and corpus callosum; CFE more often produces a very large burden of tiny, widespread lesions with associated bilateral diffusion abnormalities. The two can coexist. Other considerations depend on the pattern and context: shower embolic infarcts for punctate DWI lesions, and hypoxic, hypoglycemic, or toxic leukoencephalopathy for confluent white-matter restriction. Neither “starfield” diffusion nor microbleeds alone establishes the diagnosis. A useful impression, when the history fits, is: “Numerous bilateral punctate foci of restricted diffusion with widespread petechial susceptibility foci. In the setting of recent long-bone fracture/orthopedic intervention and acute encephalopathy, findings strongly support cerebral fat embolism.” If there is a focal territorial infarct or a visible hypodense artery, describe it separately rather than folding it into the typical microembolic pattern

Pitfalls
  • CT brain is normal in most cases of microembolism, so normal CT does not exclude fat embolism — always obtain MRI if clinical suspicion is high
  • DWI and SWI sensitivity is ~98%, so absence of findings on both sequences makes fat microembolism unlikely, but if only one sequence is performed the diagnosis may be missed
  • The starfield pattern on DWI and walnut kernel pattern on SWI are the most specific findings; focusing on only T2/FLAIR or T1 post-contrast sequences may underestimate or miss the diagnosis entirely
  • Confusing fat macroembolism with fat microembolism — macroembolism presents as large vessel occlusion with acute stroke pattern, while microembolism shows profuse tiny bilateral lesions
Source: Reference source

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