What is fat embolism syndrome?
Fat embolism means fat globules are present in the microcirculation. Fat embolism syndrome (FES) is the clinical illness that follows when those globules, and the inflammation they trigger, disturb the lungs, brain, skin, retina and kidneys. Fat embolism is far commoner than the syndrome: StatPearls notes that in most cases fat embolism is only found at autopsy, and that overt FES occurs in about 0.9% of patients with long-bone fractures.
Zenker described fat in lung capillaries after a crush injury in the 1860s, and von Bergmann made the first clinical diagnosis in 1873. Because there is no single confirmatory test, diagnosis rests on clinical criteria, and the Gurd and Wilson criteria remain the most commonly used.
What causes fat embolism syndrome?
Traumatic causes are more common than non-traumatic ones. The highest-risk injuries are fractures of the long bones (femur, tibia) and the pelvis. Procedures that pressurise the marrow cavity, such as intramedullary nailing and reaming and hip or knee arthroplasty, can also release fat. Nailing techniques that increase the risk include faster reaming, over-filling the medullary canal and a wide gap between nail and cortex.
| Group | Examples |
|---|---|
| Traumatic (commoner) | Long-bone fractures (femur, tibia), pelvic fractures, crush injury |
| Orthopaedic procedures | Intramedullary nailing and reaming, knee or pelvic arthroplasty |
| Non-traumatic (rare) | Severe burns, prolonged CPR, decompression sickness, intraosseous lines, caesarean delivery |
| Medical / metabolic | Acute haemorrhagic pancreatitis, sickle cell disease and other haemoglobinopathies, parenteral lipid infusion |
| Cosmetic / marrow | Liposuction, fat grafting, bone-marrow transplant or harvest |
Incidence figures depend on how hard one looks. Autopsy series found pulmonary fat emboli in up to 80% of trauma deaths and 90% after cardiopulmonary resuscitation, and fat globules in the blood of about two-thirds of orthopaedic trauma patients. Clinical FES using Gurd's criteria was reported in 19% in the original series, but recent studies give about 1–11%, falling as early fixation of fractures became routine. Incidence is roughly half as high in children, possibly because they have less marrow fat.
How does fat embolism syndrome develop (mechanical vs biochemical theory)?
Two theories explain how fat in the circulation produces a systemic illness, and they are not mutually exclusive.
| Feature | Mechanical (Gauss) | Biochemical (Baker) |
|---|---|---|
| Idea | Fracture tears marrow veins; fat globules are sucked into the venous system and lodge in pulmonary capillaries | Free fat is emulsified into droplets; lipases release free fatty acids and chylomicron breakdown products that are directly toxic |
| Explains | Typical post-fracture FES; spread to the brain through a patent foramen ovale or pulmonary shunts | FES in non-traumatic settings; the delay of 1–3 days; ARDS |
| Key mediators | Capillary obstruction, raised pulmonary artery pressure | Pro-inflammatory cytokines, thrombogenic fat, consumptive coagulopathy / DIC |
Fat droplets are deformable, which is why they can squeeze through the pulmonary capillary bed into the systemic circulation. In the lung, obstruction raises pulmonary artery pressure and produces ventilation–perfusion mismatch, so the arterial oxygen falls while ventilation is still normal. The released fatty acids and inflammatory mediators damage the capillary membrane, which can progress to acute respiratory distress syndrome (ARDS). In the brain, fat in the microcirculation provokes inflammation and ischaemia; the neurological signs are thought to arise from cerebral oedema rather than from large-vessel ischaemia.
What are the clinical features and when do they appear?
FES typically appears 24 to 72 hours after the insult, with an average onset of about 48 hours. This latent period is a classic examination point: the patient is often stable after resuscitation and fixation, then deteriorates on day 2. An acute fulminant course is also described, with sudden respiratory failure, right heart strain and collapse soon after the injury.
| System | Findings |
|---|---|
| Respiratory (first, commonest) | Tachypnoea, dyspnoea, hypoxaemia, increased work of breathing; may progress to ARDS |
| Cerebral | Restlessness, confusion, drowsiness, seizures, coma; usually non-focal and transient; due to oedema |
| Skin | Petechial rash — upper anterior chest, axillae, shoulders, conjunctivae and oral mucosa |
| Eye | Retinal haemorrhages and exudates on fundoscopy |
| Systemic | Fever, tachycardia; anaemia and thrombocytopenia; renal involvement |
| Cardiovascular | Pulmonary hypertension and right ventricular failure; hypotension and collapse in severe disease |
The petechiae are the most characteristic sign but the least constant. They are usually self-limiting, disappearing within about a week, and are easily missed in dark-skinned patients unless the upper chest, axillae and conjunctivae are examined deliberately. One explanation for the distribution is that fat droplets in the aortic arch vessels are carried to non-dependent areas, where over-distended capillaries leak red cells.
What are the Gurd criteria for diagnosing FES?
No universally accepted criteria exist. Gurd (1970) and Wilson (1974) proposed the criteria below. StatPearls states the requirement as two major criteria, or one major plus four minor criteria; other reviews quote one major plus four minor — so learn both: ≥2 major, or 1 major + ≥4 minor.
| Major criteria | Minor criteria |
|---|---|
| Petechial rash | Fever above 38.5 °C |
| Respiratory insufficiency (hypoxaemia) | Tachycardia above 110 beats/min |
| Cerebral involvement in a patient without head injury | Retinal involvement |
| Jaundice | |
| Renal signs | |
| Anaemia / fall in haematocrit | |
| Thrombocytopenia | |
| Raised ESR | |
| Fat macroglobulinaemia (fat globules in blood, sputum or urine) |
| System | How it works |
|---|---|
| Schonfeld score | Quantitative: petechiae 5, diffuse infiltrates on chest X-ray 4, hypoxaemia 3, and 1 each for fever, tachycardia and confusion (the full scale has seven items). A cumulative score above 5 supports FES |
| Lindeque criteria | Respiratory features alone: sustained PaO2 below 8 kPa (about 60 mmHg), PaCO2 above 7.3 kPa (about 55 mmHg), or respiratory rate above 35 despite sedation, with increased work of breathing |
What investigations help in fat embolism syndrome?
The tests are supportive rather than diagnostic. Arterial blood gas typically shows hypoxaemia with a raised alveolar–arterial gradient, the hallmark of ventilation–perfusion mismatch; an increased shunt fraction within 24–48 hours of a causative event, without ARDS, is strongly suggestive. Anaemia and thrombocytopenia are very common. Fat globules in sputum, urine or bronchoalveolar lavage (lipid-laden macrophages) support the diagnosis but are non-specific, as are raised lipase, free fatty acids and phospholipase A2.
| Modality | Typical finding | Comment |
|---|---|---|
| Chest X-ray | Diffuse bilateral interstitial markings, flake-like infiltrates — the 'snowstorm' appearance | Appears about 24–48 hours after injury; may be normal early |
| CT chest | Vascular congestion, pulmonary oedema, ground-glass change | Helps exclude contusion and pulmonary embolism |
| CT brain | Usually normal; may show a hypodense artery sign | Insensitive; mainly rules out haemorrhage |
| MRI brain (most sensitive) | T2/DWI 'starfield' pattern of punctate hyperintense lesions in the white matter, thalami and basal ganglia; microbleeds ('walnut kernel') | Lesions resolve with clinical recovery |
| Transoesophageal echo | Fat or marrow emboli passing through the right heart during nailing | Used intra-operatively to monitor |
At autopsy, fat emboli are demonstrated with fat stains such as Oil Red O, Sudan III, Sudan IV and Sudan black; routine paraffin processing dissolves the fat. This is why FES is a favourite forensic medicine topic: a trauma victim who survives a day or two and then dies of respiratory failure may have no obvious cause of death until the lungs are stained for fat.
How is fat embolism syndrome managed?
No specific treatment exists. Management is supportive and aims to keep the end organs oxygenated until the syndrome resolves, which it does in most patients who receive adequate care.
- Oxygenation and ventilation — supplemental oxygen; intubation and mechanical ventilation for fulminant ARDS, a GCS below 8, or severe respiratory distress not improving on non-invasive support. Lung-protective ventilation follows the ARDS principles (see mechanical ventilation modes).
- Haemodynamics — resuscitate with fluids; albumin is recommended because it restores intravascular volume and binds free fatty acids. Transfuse red cells if needed for oxygen delivery. Use dobutamine or milrinone if pulmonary hypertension causes right ventricular failure.
- Cerebral oedema — mannitol or hypertonic saline, with intracranial pressure monitoring in severe cases.
- Supportive extras — deep-vein thrombosis prophylaxis with sequential compression devices, nutrition and hydration.
- Early fracture fixation — early open reduction and internal fixation of long-bone fractures is highly recommended; FES is commoner in unfixed long-bone fractures.
| Agent | Status |
|---|---|
| Corticosteroids | A meta-analysis of seven randomised trials of prophylactic steroids showed about a 77% reduction in FES risk in long-bone fracture, but no difference in mortality, infection or avascular necrosis; remains controversial, with no agreed dose |
| Heparin | Helped in animal models but not used — no clinical benefit and bleeding risk |
| Dextrose, ethanol | Tried to reduce free fatty acid mobilisation; no proven clinical benefit |
| IVC filter | Proposed to stop fat dissemination; not adequately studied |
How is fat embolism prevented and what is the prognosis?
Prevention is mainly orthopaedic. Early stabilisation of long-bone fractures lowers the incidence. During fixation, the surgeon should limit intramedullary pressure because higher pressure drives more marrow fat into the circulation. Techniques tried to reduce embolisation include marrow lavage before fixation, venting the femur and drilling small cortical holes, though none has been shown to reduce FES. Prophylactic corticosteroids are discussed above.
Most patients with adequate supportive care recover from the neurological, respiratory and retinal changes. Recent studies report a mortality of about 7–10% (older reviews quote 5–15%). The main causes of death and morbidity are respiratory failure, cerebral oedema and haemodynamic collapse.