Q3 LONG ANSWER 10M ★ High-yield

Define embolism. Classify emboli and describe pulmonary, fat, air and amniotic fluid embolism.

Pathology · 2nd Professional — Para-Clinical

Definition

An embolus is a detached intravascular solid, liquid or gaseous mass that is carried by the blood to a site distant from its point of origin.

Over 99% of emboli arise from a dislodged thrombus — hence the term thromboembolism
The consequence is almost always ischaemic necrosis (infarction) of the tissue supplied

Classification

By composition	By source and destination
Thromboembolism — over 99%	Venous emboli → lodge in the lung
Fat and marrow	Arterial (systemic) emboli → lower limbs, brain, spleen, kidney, intestine
Air or gas	Paradoxical embolism — a venous embolus reaching the systemic circulation through a patent foramen ovale
Amniotic fluid	—
Tumour, septic, atheromatous, foreign body, parasitic	—

Pulmonary Thromboembolism

Source — deep veins of the leg above the knee in over 95%; also pelvic and prostatic veins
Risk factors — immobility, surgery (especially hip and knee), malignancy, pregnancy and the puerperium, oral contraceptives, obesity, previous thromboembolism, thrombophilia

Size of embolus	Consequence
Massive — saddle embolus at the bifurcation	Sudden death from acute right ventricular failure or electromechanical dissociation; often instantaneous
Medium	Pulmonary infarction if the bronchial circulation is also compromised; pleuritic pain, haemoptysis, a wedge-shaped haemorrhagic infarct
Small, recurrent	Pulmonary hypertension and right ventricular hypertrophy over time
Silent	60–80% cause no symptoms and resolve

CLINICAL PEARL

The lung has a dual blood supply, which is why most pulmonary emboli do not infarct. The bronchial arteries continue to perfuse the tissue when a pulmonary artery branch is blocked. Infarction follows only when that second supply is also inadequate — typically in a patient who already has cardiac failure. The infarct is then red (haemorrhagic), because blood seeps in from the bronchial circulation.

Fat Embolism

Source — fracture of a long bone (femur, tibia), orthopaedic surgery, severe burns, soft tissue trauma, and rarely acute pancreatitis or liposuction
Fat globules and marrow enter torn venous sinusoids in the medullary cavity
Two mechanisms — mechanical obstruction of pulmonary and cerebral vessels, and biochemical injury, as lipase releases free fatty acids that damage endothelium
Classical triad — respiratory distress, neurological disturbance, and a petechial rash on the chest, axilla and conjunctiva
Appears 24–72 hours after the injury — a latent period that distinguishes it from other causes of post-traumatic collapse
Fat globules in urine and sputum; treatment is supportive, with early fixation of the fracture as the best prevention

Air and Amniotic Fluid Embolism

Type	Causes	Features
Air embolism	Neck and thoracic surgery, obstetric procedures, chest trauma, central venous catheter insertion or removal, laparoscopic surgery	About 100 mL is needed for a clinical effect; air is churned into a froth in the right ventricle that will not pump. Treated by the left lateral head-down (Durant) position
Decompression sickness	Rapid ascent from depth; nitrogen comes out of solution	"The bends" — joint and muscle pain; "the chokes" — respiratory distress; caisson disease — multifocal bone necrosis of the femoral and humeral heads. Treated in a hyperbaric chamber
Amniotic fluid embolism	Tear in the placental membranes and rupture of uterine veins during labour	Sudden dyspnoea, cyanosis, shock, seizures, then DIC. Squames, lanugo, mucin and fat from the fetus are found in the pulmonary microcirculation. Mortality over 60%

Other Types of Embolism

Type	Source	Consequence
Systemic (arterial) thromboembolism	80% from intracardiac mural thrombi — after myocardial infarction or in atrial fibrillation; also aortic aneurysm and ulcerated plaques	Infarction of the lower limbs (75%) and brain (10%); also intestine, kidney and spleen
Septic embolism	Infected vegetations in infective endocarditis	Abscesses and mycotic aneurysms at the site of lodgement
Tumour embolism	Invasion of vessels by carcinoma	The mechanism of haematogenous metastasis
Atheromatous (cholesterol) embolism	Ulcerated plaque, or dislodged during angiography or surgery	Blue toe syndrome, livedo reticularis, renal impairment; cholesterol clefts in vessels on biopsy
Foreign body	Broken catheters, bullets, talc in drug users	Variable
Parasitic	Hydatid, schistosome ova	Pulmonary hypertension

The destination is decided by the source — venous emboli go to the lung, arterial emboli to the systemic circulation, and portal emboli to the liver. Stating this rule first answers most questions about where an embolus will lodge

Applied Aspects

Prevention of venous thromboembolism is one of the most effective interventions in hospital practice — risk assessment on admission, early mobilisation, and prophylactic heparin
Massive pulmonary embolism is treated with thrombolysis or embolectomy; submassive and small emboli with anticoagulation
An inferior vena cava filter is used when anticoagulation is contraindicated or has failed
Paradoxical embolism should be suspected when a systemic embolus occurs in a patient with a deep vein thrombosis and no cardiac source — look for a patent foramen ovale, present in about a quarter of people
Septic emboli from infective endocarditis cause abscesses and mycotic aneurysms wherever they lodge
Amniotic fluid embolism cannot be predicted or prevented; management is entirely supportive, with aggressive treatment of the DIC
Air embolism during central line insertion is prevented by placing the patient head-down, so venous pressure at the puncture site stays positive
Divers must ascend slowly, and the treatment of decompression sickness is recompression in a hyperbaric chamber followed by slow decompression
Caisson disease is the chronic form, with avascular necrosis of the femoral and humeral heads — an occupational disease of tunnel and bridge workers
Bone marrow embolism is found incidentally at autopsy after cardiopulmonary resuscitation and fractures, and is usually of no clinical consequence
Nitrogen is far more soluble in fat than in water, which is why obese divers are at greater risk and why the nervous system and marrow are affected
Trophoblastic emboli reach the lung in normal pregnancy without causing harm, and only massive amniotic fluid entry produces the syndrome
Every embolus should prompt a search for its source — treating the embolus without finding where it came from invites a second one
Talc granulomas in the lung of an intravenous drug user are a characteristic form of foreign body embolism, birefringent under polarised light

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