Antiphospholipid Syndrome

Antiphospholipid antibody generation and persistence

• Persistent aPL (present ≥12 weeks apart) develop through mechanisms involving molecular mimicry, defective regulatory T cells, and aberrant B cell responses
• Three major classes are clinically significant: lupus anticoagulant (LA), anticardiolipin antibodies (aCL), and anti-β2 glycoprotein-I (anti-β2GPI) antibodies
• Anti-β2GPI antibodies are the most pathogenic; they target domain I of β2-glycoprotein-I, a natural anticoagulant protein that binds to phospholipid membranes
• LA is detected through abnormalities in phospholipid-dependent coagulation tests (prolonged APTT that does not correct with mixing study due to inhibitory antibodies)
• The presence of all three antibody types confers "triple-positive" status with highest thrombotic risk (>20% annual recurrence)

Thrombotic mechanism: multiple convergent pathways

• Direct endothelial activation: aPL antibodies bind to endothelial cells and trophoblastic cells expressing phosphatidylserine and β2GPI, triggering inflammatory cascades
• Tissue factor induction: Antibody-induced endothelial activation upregulates tissue factor (TF), initiating extrinsic coagulation cascade
• Complement activation: β2GPI-aPL immune complexes fix complement via the classical pathway, particularly activating C3 and C5, leading to further cellular activation and inflammatory cytokine release (TNF-α, IL-6)
• Platelet activation and aggregation: aPL activate platelets through toll-like receptors and complement receptors, promoting P-selectin expression and microparticle release carrying phosphatidylserine
• Impaired natural anticoagulation: aPL block the anticoagulant function of protein C, protein S, and thrombomdulin; anti-β2GPI antibodies prevent β2GPI's native anticoagulant role
• Reduced fibrinolysis: aPL inhibit tissue plasminogen activator (tPA) activity and elevate plasminogen activator inhibitor-1 (PAI-1)
• Microthrombi formation: The cumulative effect of these mechanisms leads to platelet-rich microthrombi and widespread microvascular occlusion

Pregnancy-specific pathophysiology

• Trophoblastic cells express high levels of phosphatidylserine and β2GPI on their apical surface
• aPL antibodies and resulting complement activation (especially C3 and C5a) directly injure trophoblasts, causing syncytiotrophoblastic necrosis
• Inflammasome activation releases IL-1β, which impairs trophoblast differentiation and placental development
• Reduced placental perfusion from complement-mediated inflammation and microthrombosis leads to placental insufficiency, explaining recurrent fetal loss (especially second and third trimester), intrauterine growth restriction (IUGR), and preeclampsia
• Pregnancy increases thrombotic tendency independently, and aPL magnifies this risk substantially

Primary antiphospholipid syndrome

• Occurs in isolation without underlying autoimmune disease; represents approximately 50% of APS cases
• Pathogenic trigger remains incompletely understood but likely involves environmental factors (infections, medications) in genetically predisposed individuals
• Some cases associated with HLA-DR7, β2GPI gene polymorphisms, and STAT4 variants

Secondary antiphospholipid syndrome

• Systemic lupus erythematosus (SLE): Present in 40-60% of SLE patients; aPL detected in up to 70% of SLE patients but thrombosis occurs in only 10-15%, indicating that antibody presence does not predict clinical events
• Other autoimmune diseases: Sjögren's syndrome, rheumatoid arthritis, systemic sclerosis, polymyositis, inflammatory bowel disease
• Malignancy (particularly lymphoproliferative disorders) can produce aPL through paraproteinemia; these antibodies are often transient and rarely cause thrombosis

Environmental and infectious triggers

• Infections: HIV, hepatitis C, syphilis, and other chronic infections have been associated with transient or persistent aPL development
• Medications: Minocycline, hydralazine, procainamide, and other drugs can induce aPL, typically without thrombotic manifestations
• Oral contraceptives and hormone replacement therapy: Increase thrombotic risk in aPL-positive women; progestin-only formulations may be safer alternatives

Genetic and acquired prothrombotic conditions

• Co-existence of other thrombophilias (Factor V Leiden, prothrombin G20210A mutation) increases thrombotic risk beyond aPL alone
• Elevated Lipoprotein(a) [Lp(a)] synergizes with aPL to increase arterial thrombosis risk
• Hypertension, dyslipidemia, and smoking act as independent risk factors that compound APS-related thrombotic risk

Venous thromboembolism (most common manifestation)

• Deep vein thrombosis (DVT): Most frequent thrombotic manifestation (approximately 40% of thrombotic APS cases); typically presents with unilateral leg swelling, pain, and erythema; often lower extremity but can occur in unusual sites (hepatic, mesenteric, renal veins)
• Pulmonary embolism (PE): Occurs in 10-15% of thrombotic APS cases; presents with dyspnea, pleuritic chest pain, hemoptysis; may be recurrent despite anticoagulation if APS not recognized
• Cerebral venous sinus thrombosis (CVST): Presents with headache, focal neurological deficits, seizures; high mortality if untreated; APS accounts for 5-10% of hypercoagulable CVST cases

Arterial thrombosis

• Ischemic stroke: Most common arterial manifestation; occurs in young patients (typically <50 years) without traditional cardiovascular risk factors; may be recurrent; pathophysiology involves both large-vessel thrombosis and small-vessel disease
• Myocardial infarction: Premature coronary artery thrombosis; can occur in young patients without atherosclerotic disease; may result from epicardial coronary artery thrombosis or coronary vasculitis
• Peripheral arterial occlusion: Less common but important; requires urgent recognition to prevent limb loss
• Splenic and mesenteric infarction: Can present with severe abdominal pain; often requires imaging for diagnosis

Pregnancy-related manifestations (obstetric APS)

• Recurrent early pregnancy loss: Three or more consecutive spontaneous abortions before 10 weeks gestation (or one or more unexplained fetal death after 10 weeks) represent cardinal features; mechanism involves placental insufficiency and complement-mediated inflammation
• Intrauterine growth restriction (IUGR): Occurs in 15-20% of pregnancies in aPL-positive women; results from placental insufficiency
• Preeclampsia and eclampsia: Occurs more frequently in APS pregnancies, often with severe features and earlier onset (<34 weeks); complement inhibition may contribute pathophysiology
• Placental insufficiency: Manifested by abnormal umbilical artery Doppler studies and decreased amniotic fluid
• Maternal thrombotic events: 5-10% of pregnant APS patients experience thrombosis during pregnancy or postpartum period; hypercoagulable state of pregnancy compounds APS-related risk

Hematologic manifestations

• Thrombocytopenia: Mild to moderate (50,000-150,000/μL in most cases); <20,000/μL is uncommon and should prompt consideration of other diagnoses; mechanism involves antibody-mediated platelet consumption and splenic sequestration; paradoxically increases thrombotic risk
• Microangiopathic hemolytic anemia: Less common; indicates more severe disease with microvascular involvement
• False-positive serologic tests for syphilis: Detected in approximately 20% of aPL-positive patients; results from aPL reactivity with lipoidal antigens in RPR/VDRL tests; fluorescent treponemal antibody absorption (FTA-ABS) and treponemal-specific tests are negative

Cutaneous and other manifestations

• Livedoid vasculopathy: Characteristic reticular (net-like) purple or red discoloration with pale or atrophic centers, typically on legs and trunk; pathophysiology involves occlusive microvasculopathy; occasionally results in skin ulceration or gangrene
• Superficial thrombophlebitis: May affect unusual sites; recurrent episodes suggest APS
• Cardiac manifestations: Valvular disease (valve thickening, regurgitation, stenosis) in 5-10% of chronic APS; myocardial infarction; intracardiac thrombi; dilated cardiomyopathy from recurrent coronary events
• Renal involvement: Thrombotic microangiopathy causing APS-associated renal disease; can present with hypertension, proteinuria, elevated creatinine; distinguished from lupus nephritis by absence of immune complex deposition on immunofluorescence
• Neurologic manifestations beyond stroke: Migraine, seizures, cognitive dysfunction, transverse myelitis, multiple sclerosis-like demyelinating disease

Catastrophic antiphospholipid syndrome (CAPS)

• Rare but life-threatening variant (occurs in <1% of APS patients) characterized by simultaneous or sequential thrombotic events in multiple organ systems over days to weeks
• Clinical presentation: Multiorgan thrombosis (typically affecting kidneys, lungs, brain, heart, skin); microangiopathic hemolytic anemia; thrombocytopenia; high fever; acute respiratory distress syndrome
• Mortality rate: 50% despite aggressive treatment; requires immediate recognition and intervention
• Precipitating factors: Infection, malignancy, surgery, anticoagulation discontinuation, pregnancy/postpartum period, SLE flare
• Pathophysiology: Massive complement activation and endothelial injury with disseminated intravascular coagulation-like picture but usually with normal fibrinogen (distinguishes from true DIC)

Clinical suspicion and historical features

• Key history questions: Unexplained thrombosis (especially at young age or in unusual sites), recurrent pregnancy loss, family history of thrombosis or SLE, prior SLE diagnosis, recurrent superficial thrombophlebitis
• Red flags: Arterial stroke in patient <50 years old without atherosclerotic disease or atrial fibrillation; spontaneous abortion at >10 weeks gestation; thrombosis despite anticoagulation; thrombocytopenia without clinical bleeding; livedoid vasculopathy pattern

Laboratory diagnosis (requires careful interpretation)

• Lupus anticoagulant (LA): Detected through phospholipid-dependent coagulation assays
• Initial test: Prolonged activated partial thromboplastin time (APTT) on routine coagulation panel
• Confirmatory step: Mixing study (adding normal plasma)—in LA, APTT remains prolonged (mixing does NOT correct it, distinguishing LA from Factor deficiency)
• Platelet neutralization procedure: Addition of excess platelets (which provide phospholipids) corrects APTT in LA but not in Factor VIII inhibitors
• Dilute APPT: More specific than standard APPT for detecting LA
• Sensitivity/specificity: 80-90% sensitivity for thrombotic APS; highly specific (>99%)

• Anticardiolipin antibodies (aCL): Detected by ELISA
• IgG aCL: More predictive of thrombosis than IgM; titer ≥40 GPL units or >99th percentile considered moderate-to-high positive
• IgM aCL: Less specific; associated with infections and transient positivity
• IgA aCL: Emerging as potentially important but not required for diagnosis

• Anti-β2 glycoprotein-I antibodies (anti-β2GPI): Detected by ELISA
• IgG anti-β2GPI: Strongly associated with thrombosis; isolated anti-β2GPI positivity increases thrombotic risk significantly
• IgM and IgA anti-β2GPI: Less predictive of clinical events
• Domain-I specific anti-β2GPI: Represents the pathogenic epitope; emerging biomarker for thrombotic risk

• Persistence requirement: Antibodies must be present on ≥2 occasions separated by ≥12 weeks for diagnosis; transient positivity in acute infections or medications should not lead to lifelong anticoagulation without careful consideration
• Triple-positive status (presence of LA, aCL, and anti-β2GPI): Carries highest thrombotic risk (~20% annual recurrence); associated with more severe clinical disease
• Seronegative APS: Approximately 2-5% of clinically typical APS cases are negative for all three standard antibody tests; may have anti-β2GPI domain-I antibodies or other emerging aPL epitopes
• Antithrombin III and protein C deficiency: May be observed in aPL-positive patients but are likely secondary to antibody effects rather than inherited deficiencies; should not be tested during acute thrombotic event or anticoagulation

Coagulation and platelet studies

• Complete blood count: Assess for thrombocytopenia (mild-moderate most common); moderate thrombocytopenia (50,000-100,000/μL) in 10-15% of APS patients is nonspecific but supports diagnosis when combined with clinical features
• Prothrombin time (PT): Usually normal; may be prolonged in some aPL-positive patients
• Thrombin time (TT): May be prolonged; indicates potential aPL interference with thrombin-fibrinogen interaction
• Fibrinogen level: Typically normal; helps distinguish from DIC in CAPS presentations

Imaging studies

• For thrombotic events: Determine location and extent
• DVT: Compression ultrasound of lower extremities; CT venography if upper extremity or central veins suspected
• PE: CT pulmonary angiography (CTPA); ventilation-perfusion scan if contrast contraindicated
• Stroke: Brain MRI with diffusion-weighted imaging (DWI) to detect acute ischemia; MRA to assess for large-vessel thrombosis; head CT to exclude hemorrhage
• Coronary ischemia: Coronary angiography may reveal thrombosis with minimal atherosclerotic disease

• For chronic manifestations:
• Cardiac echocardiography: Assess for valvular disease (valve thickening, regurgitation); screen for intracardiac thrombus
• Renal ultrasound or kidney biopsy: Evaluate for thrombotic microangiopathy; kidney biopsy shows intracapillary thrombi without immune complex deposition (negative immunofluorescence distinguishes from lupus nephritis)

Diagnostic criteria (Sydney criteria, 2006)

1. 1.Vascular thrombosis: One or more episodes of arterial, venous, or small-vessel thrombosis, confirmed by imaging or Doppler studies (not clinical suspicion alone)
2. 2.Pregnancy complications: Three or more consecutive unexplained spontaneous abortions (<10 weeks gestation with normal maternal and paternal karyotypes) OR one or more unexplained fetal death

Acute thrombosis (stabilise first)

• Parenteral anticoagulation: low-molecular-weight heparin (enoxaparin) or IV unfractionated heparin started immediately once imaging confirms thrombus, then overlapped with a vitamin K antagonist. Standard stroke/VTE care (thrombolysis, thrombectomy) is not withheld because of APS.
• Monitoring caveat: lupus anticoagulant prolongs the baseline aPTT, so titrate UFH by anti-Xa activity rather than aPTT; if LA interferes with the INR reagent, follow chromogenic factor X.

First-line long-term therapy (EULAR 2019 recommendations for APS in adults)

• Vitamin K antagonist: warfarin to INR 2.0-3.0 indefinitely after a first venous event in definite APS — duration is lifelong, not 3-6 months, because recurrence risk persists.
• Arterial thrombosis: VKA is preferred over antiplatelet therapy alone; EULAR endorses either standard-intensity warfarin plus low-dose aspirin or higher-intensity anticoagulation, individualised to bleeding risk.
• Hydroxychloroquine: added in SLE-associated APS for its immunomodulatory and mild antithrombotic effect.
• Asymptomatic aPL carriers with a high-risk profile (triple positivity, persistent high titres): low-dose aspirin for primary prophylaxis, plus aggressive control of hypertension, lipids, and smoking.

Obstetric APS (ACOG and EULAR)

• Low-dose aspirin plus prophylactic LMWH through pregnancy, with LMWH continued into the postpartum period; therapeutic-dose LMWH if there is a prior thrombotic event.
• Aspirin alone may be used for aPL positivity without prior loss or thrombosis.

Catastrophic APS — escalation

• Triple therapy: full-dose heparin plus high-dose glucocorticoids plus plasma exchange and/or IVIG, with treatment of any precipitating infection.
• Refractory disease: rituximab or complement inhibition with eculizumab.

Contraindicated / avoid

• DOACs: EULAR advises against rivaroxaban in triple-positive APS and generally against DOACs in arterial APS after trials showed excess thrombotic events.
• Warfarin in pregnancy: teratogenic (warfarin embryopathy) — use LMWH.
• Estrogen-containing contraceptives and smoking.

Disease-related — emergencies first

• Catastrophic APS (CAPS): explosive multiorgan microthrombosis from massive complement and endothelial activation; signalled by fever, rapidly rising creatinine, ARDS, encephalopathy, thrombocytopenia and schistocytes with a normal or near-normal fibrinogen (unlike DIC). Medical emergency — start triple therapy without waiting for confirmatory antibody repeat testing.
• Adrenal infarction/haemorrhage: adrenal vein thrombosis causes venous congestion and gland necrosis; suspect in the APS patient with abdominal or flank pain, hypotension, hyponatraemia and hyperkalaemia — adrenal crisis is an emergency requiring stress-dose hydrocortisone.
• Recurrent thrombosis on therapy: breakthrough events, especially in triple-positive patients or those switched to a DOAC; signalled by new ischaemia despite therapeutic INR.
• Cerebral venous sinus thrombosis and ischaemic stroke: young patient, headache with focal deficit or seizure.

Chronic organ complications

• Libman-Sacks (non-bacterial thrombotic) endocarditis: sterile fibrin-platelet vegetations on valve surfaces, most often mitral; signalled by a new regurgitant murmur or embolic stroke with negative blood cultures.
• APS nephropathy: intrarenal thrombotic microangiopathy causing hypertension, proteinuria and rising creatinine; biopsy shows thrombi with negative immunofluorescence, separating it from lupus nephritis.
• Chronic thromboembolic pulmonary hypertension: unresolved organised PE; exertional dyspnoea with a loud P2 and right heart strain on echocardiography.
• Skin ulceration and digital gangrene from occlusive microvasculopathy.

Treatment-related

• Major and intracranial haemorrhage: highest with higher-intensity warfarin or warfarin plus aspirin.
• Warfarin-induced skin necrosis: transient protein C depletion at initiation without heparin bridging.
• Warfarin embryopathy: nasal hypoplasia and stippled epiphyses with first-trimester exposure.
• Heparin-induced thrombocytopenia: platelet fall days 5-10 with new thrombosis — stop all heparin.
• Heparin-associated osteoporosis with prolonged pregnancy dosing; glucocorticoid and rituximab infection risk in CAPS.

• The paradox is the buzzword: a patient who clots but has a prolonged aPTT that fails to correct on mixing study. In vitro the antibody inhibits phospholipid-dependent assays; in vivo it is prothrombotic. Adding excess phospholipid or platelets corrects it — that is the confirmatory step.
• Single best next step after a first positive test: repeat the antibody panel in ≥12 weeks. A one-off positive during acute thrombosis or infection does not establish APS and does not commit the patient to lifelong anticoagulation.
• Classic association examiners test: SLE — a young woman with malar rash, arthralgias and recurrent fetal loss. Also expect the false-positive RPR/VDRL with a negative FTA-ABS.
• Obstetric stem: recurrent loss after 10 weeks, or three or more losses before 10 weeks → answer is low-dose aspirin plus prophylactic LMWH, per ACOG. Warfarin is the wrong answer in pregnancy (embryopathy), and aspirin alone is wrong if there is prior thrombosis.
• Anticoagulation choice: warfarin to INR 2-3, indefinitely. A DOAC is the classic distractor — EULAR advises against DOACs in triple-positive and arterial APS after excess breakthrough events in trials.
• Young stroke without atherosclerosis or atrial fibrillation → check aPL. Look for livedo reticularis and mild thrombocytopenia as supporting clues.
• Thrombocytopenia here does not mean bleeding — do not withhold anticoagulation for platelets in the 50,000-150,000/µL range; the patient is still hypercoagulable.
• Fever, multiorgan failure, schistocytes, but normal fibrinogen = CAPS, not DIC. Treat with heparin, steroids, and plasma exchange and/or IVIG.
• Do not anticoagulate the asymptomatic incidental aPL carrier — aspirin and risk-factor control only.