Embolic versus thrombotic occlusion

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Acute limb ischaemia describes the threatened perfusion state, while embolism and thrombosis describe mechanisms. An embolus forms elsewhere and travels to an artery; in-situ thrombosis forms at the occlusion site, often over diseased native artery, an aneurysm, stent or bypass graft. The distinction changes the anatomy expected, the likely revascularisation technique and long-term prevention, but not the need to grade viability immediately.

Classic embolism is dramatic: sudden severe symptoms, a previously asymptomatic limb, a potential cardiac source and relatively normal pulses in the opposite leg. Classic thrombosis is superimposed on chronic PAD: preceding claudication or rest pain, reduced pulses in both legs, vascular scars or known aneurysm, and more extensive calcified disease. Real patients overlap. Atrial fibrillation may coexist with PAD, and an embolus can lodge in an already diseased vessel.

Clinical clues generate a working probability rather than a final verdict. CTA, duplex and angiography define lesion length, underlying plaque, collateralisation, graft patency and distal runoff. The experienced vascular team integrates these findings with symptom duration, Rutherford category, comorbidity and bleeding risk. When motor deficit is present, the imaging pathway must serve treatment rather than become a prerequisite that delays it.

Key points

Sudden maximal symptoms, a normal opposite limb, absent prior claudication and atrial fibrillation favour embolism; previous claudication, diffuse pulse loss and bilateral PAD favour thrombosis in situ.
Do not let cause-finding delay limb rescue: map sensory loss and weakness, contact vascular surgery immediately, and give intravenous unfractionated heparin unless active bleeding or current or previous HIT contraindicates it.
Neurological deficit requires emergency revascularisation and imaging only when it will not delay treatment; profound anaesthesia, paralysis and absent arterial plus venous signals suggest Rutherford III.
Emboli often lodge at arterial bifurcations and may leave a sharply demarcated angiographic cutoff in otherwise less diseased vessels.
Thrombosis commonly complicates atherosclerotic stenosis, aneurysm, graft or stent disease and may span longer calcified segments.
Established collaterals can soften the onset of thrombosis but do not guarantee limb viability.
Atrial fibrillation is an important embolic clue, yet finding it does not prove the leg event arose from the heart.
After immediate treatment, investigate the source because secondary prevention differs for cardiac embolism, aneurysm, graft failure and atherosclerosis.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Distant embolism

Thrombus may travel from the left atrium in atrial fibrillation, a damaged ventricle, a diseased valve or the aorta and lodge at a limb artery bifurcation.

In-situ thrombosis

Plaque rupture or critical stenosis can thrombose within native PAD, while low flow or structural failure can occlude an aneurysm, stent or bypass graft.

Mixed mechanisms

A cardiac or aortic embolus may lodge on chronic atherosclerosis, and thrombosed aneurysms can shower secondary distal emboli, producing overlapping clinical and imaging patterns.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Abrupt inflow loss
A newly occluded artery reduces oxygen delivery below tissue demand; nerves and skeletal muscle lose function as ischaemia duration and severity increase.
2
Collateral reserve
Previously normal arteries have little compensatory collateral flow, whereas chronic stenosis may have developed channels that transiently soften symptoms after superimposed thrombosis.
3
Distal propagation
Stagnant blood around the original obstruction can extend thrombus proximally or distally and occlude runoff, progressively reducing the chance of durable reperfusion.
4
Reperfusion burden
Restoring flow to injured muscle releases potassium, acid and myoglobin, creating arrhythmia, kidney injury and compartment swelling in proportion to ischaemic damage.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Tempo of onsetRed flag

Instantaneous, maximal-at-onset pain and dysfunction favour embolism, whereas thrombosis may worsen over hours on a background of chronic symptoms, although either mechanism can present abruptly.

Previous limb symptoms

No preceding claudication supports an embolic event; established claudication, rest pain, tissue loss or prior revascularisation makes thrombosis on chronic arterial disease more likely.

Contralateral examination

Normal pulses and warmth on the opposite side support an embolic pattern, while bilateral pulse deficits, trophic change and arterial bruits suggest systemic PAD.

Source history

Ask about atrial fibrillation, recent myocardial infarction, valve disease, endocarditis, aortic aneurysm and recent catheterisation, while recognising that each association requires confirmation.

Threat overrides mechanismRed flag

Sensory loss beyond the toes or any motor weakness identifies an immediately threatened limb requiring emergency revascularisation regardless of aetiological confidence.

Graft or stent failure

Prior bypass or endovascular treatment with loss of a previously palpable graft pulse strongly suggests thrombosis within a reconstructed arterial segment.

Aneurysm clues

A popliteal mass, bilateral popliteal aneurysms or a known abdominal aortic aneurysm can indicate thrombosis with distal embolisation and multilevel occlusion.

Red flags requiring action

Any new motor weakness or sensory loss beyond the toes means immediate limb threat irrespective of whether embolism or thrombosis is more likely.
Profound anaesthesia, paralysis or rigor with absent arterial and venous Doppler signals suggests irreversible tissue injury and major reperfusion hazard.
Chest or back pain, pulse asymmetry or haemodynamic instability raises aortic dissection or proximal aortic occlusion rather than an isolated leg lesion.
Massive swollen cyanotic limb with preserved arterial inflow can indicate phlegmasia and also requires emergency vascular assessment.
Active major bleeding or known immune HIT changes initial anticoagulation choice, but the vascular referral remains immediate.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
CT angiographyFirst step
Why
Define occlusion morphology, background arterial disease, inflow and distal runoff.
Interpretation and limitations
A focal bifurcation cutoff in relatively smooth vessels supports embolism; long irregular calcified occlusion, collaterals, aneurysm or graft disease supports thrombosis. Morphology informs but does not replace the clinical timeline.
02
Duplex ultrasound
Why
Assess flow, thrombus extent, aneurysm and previous graft or stent patency.
Interpretation and limitations
Duplex may reveal underlying stenosis, chronic collateral flow or aneurysm. It is operator dependent and can miss iliac or deep segments, so nondiagnostic results must not delay limb-saving treatment.
03
ECG and monitored rhythm
Why
Identify atrial fibrillation or another rhythm associated with systemic embolisation.
Interpretation and limitations
Atrial fibrillation raises embolic probability and later anticoagulation needs, but does not prove causation because PAD and AF commonly coexist in older adults.
04
Echocardiography
Why
Search for structural or intracardiac embolic sources after immediate limb management.
Interpretation and limitations
Findings such as ventricular thrombus or important valve disease can redirect prevention. The timing and transthoracic or transoesophageal approach depend on stability and suspected pathology.
05
Aortic imaging
Why
Assess aneurysm, mural thrombus or dissection when history or anatomy suggests a proximal source.
Interpretation and limitations
Aortic pathology may explain embolisation to one or both limbs and can change the operative plan. Chest or back pain with instability requires a dedicated emergency aortic pathway.
06
Blood count, coagulation, renal profile and CK
Why
Prepare for anticoagulation, contrast and reperfusion while estimating systemic injury.
Interpretation and limitations
Anaemia, thrombocytopenia and coagulopathy alter bleeding management; renal function affects contrast planning; high CK supports muscle injury but a normal early CK cannot classify the limb as viable.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Aortic dissection

Chest or back pain, asymmetric arm pressures, pulse deficits and systemic instability can accompany dissection that compromises iliac inflow or causes embolisation.

Phlegmasia cerulea dolens

Massive venous thrombosis produces a swollen cyanotic painful limb and can secondarily reduce arterial inflow, contrasting with the typically cold, less swollen arterial presentation.

Acute compartment syndrome

Severe pain, pain on passive stretch, tense compartments and evolving nerve dysfunction may occur despite palpable pulses and require a different decompressive strategy.

Neurological mimic

Radiculopathy or spinal cord disease can cause pain and weakness, but usually does not produce a cold pale limb with lost arterial Doppler signals.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01Initial mechanism assessmentSeparate probability from emergency actionFirst stepA patient presents with acute painful cold limb and the cause is uncertain.

1Grade Rutherford viability from symptom duration, sensory loss, motor power and Doppler signals while contacting the vascular service immediately.
2Give analgesia and intravenous unfractionated heparin unless contraindicated; arrange emergency treatment without waiting to prove embolism or thrombosis.
3Compare both limbs and obtain the history of claudication, revascularisation, aneurysm, atrial fibrillation, myocardial infarction and recent instrumentation.
4Use rapid CTA, duplex or angiography as chosen by the vascular team when it informs intervention without delaying a threatened limb.
5Record the working mechanism and competing evidence, then revise it after operative, angiographic and cardiac findings.

02Embolic patternTreat the limb and find the sourceAbrupt maximal symptoms occur in a previously well limb with a plausible distant embolic source.

1Plan urgent embolectomy, endovascular extraction or another reperfusion method according to anatomy and Rutherford category.
2Inspect the treated artery for residual thrombus and underlying stenosis because an embolus can expose occult PAD.
3After stabilisation, investigate cardiac and aortic sources and choose long-term antithrombotic treatment for the confirmed cause.

03Thrombotic patternAddress thrombus and diseased arteryAcute deterioration occurs on chronic PAD, aneurysm, bypass graft or stent disease.

1Define inflow, lesion length, conduit status and distal targets, accepting that collaterals may mask the severity of the new occlusion.
2Select thrombectomy, thrombolysis, endovascular correction, bypass or a combined strategy from viability, anatomy and bleeding risk.
3Treat the underlying stenosis, aneurysm or graft problem and establish secondary prevention after flow is restored.

04Unsalvageable limbAvoid harmful reperfusionProfound anaesthesia and paralysis accompany fixed skin change and absent arterial and venous signals.

1Obtain immediate experienced vascular confirmation of Rutherford III because the consequence of misclassification is major.
2Do not pursue revascularisation when irreversible injury is established, as necrotic muscle reperfusion can cause severe systemic toxicity.
3Consider primary amputation or comfort-focused care with senior anaesthetic and multidisciplinary input.

Key medicines and prescribing safety1 treatment · regimens, roles and cautions

Prevents propagation and recurrent embolisation while anatomy and definitive reperfusion are addressed.

Unfractionated heparin

The licensed UK regimen for acute peripheral arterial occlusion is 5,000 IU intravenously followed by 1,000–2,000 IU/hour continuous intravenous infusion, with APTT sampled 4–6 hours after initiation and dose adjusted.

Withhold in active major haemorrhage and avoid with current or previous immune heparin-induced thrombocytopenia; urgently balance other bleeding sites and recent brain, spinal or eye surgery. Check baseline platelets and coagulation, then use the hospital UFH nomogram for preparation and assay-specific targets.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Irreversible limb loss

Persistent severe ischaemia causes muscle necrosis, permanent nerve injury and tissue loss, eventually requiring major amputation when a salvageable window is missed.

Reperfusion syndrome

Potassium, acid and myoglobin released after flow restoration can provoke arrhythmia, shock, acute kidney injury and a systemic inflammatory response.

Compartment syndrome

Post-ischaemic capillary leak within fixed fascial compartments raises tissue pressure, compromises renewed perfusion and causes additional muscle and nerve necrosis.

Recurrent occlusion

An untreated cardiac source, residual thrombus, diseased runoff or uncorrected stenosis can cause early re-embolisation or thrombosis after apparently successful intervention.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Repeat sensory distribution, toe and ankle power and named Doppler signals because worsening viability changes the permissible delay.
Monitor cardiac rhythm continuously when embolism is possible and preserve ECG evidence of intermittent atrial fibrillation.
Adjust UFH using the local APTT or anti-Xa nomogram and assess cannula sites, urine, haemoglobin and platelets for bleeding or HIT.
After reperfusion, document haemodynamic success and observe for recurrent occlusion, distal embolisation and compartment syndrome.
Trend potassium, acid-base status, renal function, CK and urine output after prolonged ischaemia or extensive thrombus removal.
Before discharge, record the final aetiology, source investigation, antithrombotic indication and ownership of vascular follow-up.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Atrial fibrillation is not proof

It is a strong source clue, but a person with AF can still thrombose a severely atherosclerotic artery or an occluded graft.

Collateral vessels tell a history

Prominent collaterals suggest pre-existing obstruction and favour thrombosis, while their absence can explain dramatic tissue threat after an embolus.

Embolus can uncover plaque

A discrete embolus may lodge at a diseased bifurcation, so successful extraction should be followed by assessment for a residual causative stenosis.

Popliteal aneurysm behaves differently

Thrombosis can obstruct the aneurysm and repeatedly embolise tibial vessels, leaving poor distal runoff that complicates straightforward thrombectomy.

Procedure history matters

Acute ischaemia shortly after catheterisation, arterial closure or cardiac intervention may be iatrogenic and requires access-site and proximal imaging.

The final diagnosis may be mixed

Thromboembolism onto chronic PAD is common; document the evidence rather than forcing anatomy into a purely embolic or purely thrombotic label.

11Common pitfallsFrequent interpretation and management errors.

01
Assuming every patient with atrial fibrillation has embolic occlusion without examining the opposite limb or background PAD.
02
Calling a gradual presentation safe when sensory or motor loss already indicates a threatened limb.
03
Delaying heparin and vascular contact while arranging echocardiography to prove a cardiac source.
04
Treating the extracted clot but leaving an underlying stenosis, aneurysm or graft lesion unrecognised.
05
Using a normal early CK result to downgrade clinical neurological threat or justify slower treatment.
06
Applying thrombolysis without considering current bleeding risk, recent surgery and the speed required for Rutherford IIb reperfusion.

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