2. Catastrophic Antiphospholipid Syndrome (CAPS)

Quick Recap

Autoimmune & Rheumatology System, Protocol 2/5. Includes Scleroderma Renal Crisis and a rheumatologic laboratory interpretation reference as closely related content covered in the same source chapter.

1. Definition

Catastrophic antiphospholipid syndrome (CAPS) = a rapidly progressive, life-threatening disease causing MULTIORGAN THROMBOSES and dysfunction in the presence of antiphospholipid antibodies — the most severe, fulminant expression of the broader antiphospholipid antibody syndrome (APS) spectrum.

2. When to Suspect

Rapid multiorgan thromboses, especially in the setting of SLE, infections, or known APS. An elevated PTT that does NOT correct with a 50:50 mixing study (patient plasma + normal plasma) in the right clinical setting — particularly with an active clot — is suggestive of an inhibitor and APS, a genuinely counterintuitive lab pattern (prolonged clotting time in a patient who is actually THROMBOSING, not bleeding) that can confuse clinicians unfamiliar with lupus anticoagulant physiology. Certain anticoagulant medications will alter this testing — interpret with awareness of concurrent anticoagulation.

3. Clinical Features by Organ System

Genuinely difficult to distinguish from other multiorgan thrombotic processes — HUS, TTP, and DIC all overlap clinically; see the dedicated HUS/TTP/DIC protocols (Hematology System) for their distinguishing lab patterns, and apply the CAPS-specific diagnostic criteria below to help differentiate.

4. Diagnostic Criteria

1. Evidence of involvement of THREE OR MORE organs, systems, or tissues
2. Manifestation development SIMULTANEOUSLY or within <1 WEEK
3. Histopathologic confirmation of small-vessel occlusion in at least ONE organ/tissue
4. Laboratory confirmation of antiphospholipid antibodies (lupus anticoagulant, anticardiolipin antibodies, and/or anti-beta2-glycoprotein I antibodies)

Definite CAPS: ALL FOUR criteria met.

Probable CAPS: absence of criterion 2 OR 3, OR only two organs/systems involved — this "probable" category is clinically important since treatment should not wait for "definite" status to be established given the rapidly progressive, life-threatening natural history.

5. Immediate Stabilization / Treatment — The "Triple Therapy" plus Escalation

High suspicion, early diagnosis, and AGGRESSIVE therapy are essential — this is a genuine treat-first, confirm-second emergency, paralleling the Pulmonary-Renal Syndrome protocol's empiric-treatment principle.

Core management (apply ALL simultaneously, not sequentially):

1. Treat any possible underlying infection — infection is a recognized CAPS trigger, and untreated infection will undermine any immunosuppressive/anticoagulant therapy
2. Anticoagulation with heparin — the foundational antithrombotic therapy given the multiorgan thrombotic pathophysiology
3. Plasma exchange for FIVE CONSECUTIVE DAYS, with or without IVIG (400 mg/kg/day for 5 days) given AFTER plasma exchange has been completed — note the specific sequencing (IVIG after, not during/before, plasma exchange, since plasma exchange would remove the administered IVIG)
4. Pulse-dose systemic glucocorticoids: methylprednisolone 1g IV daily x3 days, followed by high-dose oral/parenteral therapy (prednisone 1 mg/kg daily, usually not exceeding 60-80mg)

This combination is sometimes referred to as "triple therapy" (anticoagulation + plasma exchange/IVIG + glucocorticoids) — all three components together, not any single element alone, constitute standard CAPS management.

Refractory cases: rituximab and eculizumab have also been used IN ADDITION TO standard cyclophosphamide therapy — escalation options for cases not responding to the core triple therapy.

Checklist:

6. Investigations

Lupus anticoagulant testing, IgG and IgM anticardiolipin antibodies (aCL), IgG and IgM anti-beta2-glycoprotein I antibodies, 50:50 plasma mixing study (rapid screening test, does NOT substitute for formal antiphospholipid antibody testing), coagulation panel, organ-specific workup per the affected systems (echocardiogram for valvular involvement, neuroimaging for CVA, imaging for visceral ischemia), infectious workup to identify/exclude a triggering infection, histopathology if biopsy of an affected organ is feasible and would support the diagnosis.

7. Scleroderma Renal Crisis — Related Rheumatologic Emergency

Risk factors: scleroderma, particularly with DIFFUSE skin involvement (up to 20% risk); palpable tendon friction rubs; antibodies to RNA polymerase III; use of systemic glucocorticoids (a recognized PRECIPITANT, not just a treatment — an important, counterintuitive point given glucocorticoids' central role in most other rheumatologic emergencies in this system).

Clinical manifestations: abrupt onset of SEVERE hypertension; acute onset, rapidly progressive renal failure; microangiopathic hemolytic anemia and thrombocytopenia (another thrombotic-microangiopathy-mimicking presentation); urinalysis ranges from normal to mild proteinuria to abundant cells/casts — but glomerulonephritis is NOT a feature, distinguishing this from the RPGN/active urinary sediment pattern of the ANCA vasculitides in the Pulmonary-Renal Syndrome protocol.

Management: antihypertensives, with the CRITICAL, counterintuitive instruction to ALWAYS INCLUDE AN ACE INHIBITOR even in the setting of acute kidney injury and a rising creatinine — ACE inhibitors are the disease-specific, outcome-improving therapy for scleroderma renal crisis specifically, and withholding them due to the AKI (as might be reflexive in other AKI contexts) would be a significant management error in this specific disease. Supportive measures otherwise.

8. Rheumatologic Laboratory Interpretation Reference

Vasculitis serology: high-titer anti-PR3 (c-ANCA) is EXTREMELY specific (>95%) for GPA; low complement usually present in mixed cryoglobulinemia and SLE; high rheumatoid factor titers seen in RA/Sjögren/cryoglobulinemia-associated vasculitis. Additional workup for unclear vasculitis presentations: cryoglobulins, hepatitis B/C, SPEP/immunofixation, anti-GBM antibody; strongly consider angiographic study or biopsy if diagnosis remains unclear with ongoing clinical suspicion.

Systemic sclerosis serology: anti-Scl-70 (topoisomerase) -> diffuse disease, can progress rapidly, severe ILD risk; anticentromere antibody -> limited disease (CREST syndrome), associated with pulmonary hypertension and digital ischemia; anti-RNA-polymerase III -> diffuse disease, SCLERODERMA RENAL CRISIS, and GAVE (gastric antral vascular ectasia); ANA present in 95% of SSc (classically nucleolar or centromere pattern) — suspicion for SSc should depend on the presence of BOTH ANA AND Raynaud phenomenon, since Raynaud's is nearly universal in this disease.

Rheumatoid arthritis serology: RF is more sensitive (75%)/less specific (75%); anti-CCP antibody is more specific (95%)/less sensitive (65%) — the two tests have complementary performance characteristics.

Myositis workup: CK, aldolase, AST, ALT, LDH (isolated aldolase elevation may suggest perimysial disease specifically); anti-Jo-1 antibody (most common, fastest-turnaround) and full myositis-specific antibody panel; EMG/NCS; muscle biopsy (most useful if a site is first identified via MRI/ultrasound). Inflammatory arthritis, skin involvement, and ILD are also commonly present in the inflammatory myopathies, not just muscle findings.

Synovial fluid interpretation (arthrocentesis):

9. Organ Support

Anticoagulation (heparin) as foundational antithrombotic therapy; plasma exchange (5-day course); IVIG post-exchange; pulse-dose then oral glucocorticoids; standard organ-specific ICU supportive care for the affected systems (respiratory support for ARDS/DAH, RRT for renal involvement, neurologic supportive care for CVA); rituximab/eculizumab/cyclophosphamide for refractory cases.

10. Consultation Matrix

11. Monitoring Framework

Serial coagulation panel, organ-specific monitoring per affected systems, antiphospholipid antibody titers, blood pressure (especially scleroderma renal crisis, where abrupt severe hypertension is the presenting sign), renal function trend.

12. Complications

Multiorgan failure/death (the dominant CAPS risk), ongoing thrombosis despite anticoagulation, treatment-related complications (plasma exchange access complications, immunosuppression-related infection risk), scleroderma renal crisis progressing to ESRD if ACE inhibitors withheld inappropriately. Prevention: prompt triple-therapy initiation without waiting for "definite" CAPS criteria, correct 50:50 mixing study interpretation, ACE inhibitor use in scleroderma renal crisis despite AKI. Rescue: rituximab/eculizumab/cyclophosphamide escalation, standard organ-specific rescue therapies (ECMO for refractory ARDS/DAH, RRT for renal failure).

13. Escalation & De-escalation

Escalate: refractory CAPS despite triple therapy -> add rituximab, eculizumab, or cyclophosphamide.

De-escalate: thrombotic activity controlled, organ function stabilizing/improving -> transition from plasma exchange to maintenance anticoagulation, continue steroid taper, ongoing rheumatology follow-up for long-term APS management.

14. ICU Discharge Criteria

Thrombotic activity controlled, no new organ involvement, anticoagulation established for long-term APS management, plasma exchange course completed, steroid taper underway, organ-specific complications stabilized or on an appropriate recovery trajectory.

15. Documentation & Medicolegal Checklist

16. Key Guidelines

Asherson RA, Cervera R, de Groot PG, et al. Catastrophic antiphospholipid syndrome: international consensus statement on classification criteria and treatment guidelines. Lupus. 2003;12(7):530-534.

17. Controversies

The precise role and sequencing of rituximab vs eculizumab vs cyclophosphamide in refractory CAPS lacks head-to-head comparative trial data, given the disease's rarity, and selection is largely case-series/expert-opinion-guided. The overlap between CAPS, TTP, HUS, and DIC in presentation means diagnostic classification can remain genuinely ambiguous in individual patients even after applying the formal criteria, and some patients may warrant empiric multi-etiology coverage pending clarification.

18. References

1. Autoimmune Emergencies (CAPS, Scleroderma Renal Crisis, and laboratory interpretation sections). Washington Manual of Critical Care, 4th ed, 2025 (Ch. 69).
2. Asherson RA, Cervera R, de Groot PG, et al. Catastrophic antiphospholipid syndrome: international consensus statement on classification criteria and treatment guidelines. Lupus. 2003;12(7):530-534.
3. Cervera R, Bucciarelli S, Plasín MA, et al. Catastrophic antiphospholipid syndrome (CAPS): descriptive analysis of a series of 280 patients from the "CAPS Registry". J Autoimmun. 2009;32(3):240-245.
4. Cartin-Ceba R, Peikert T, Ashrani A, et al. Primary antiphospholipid syndrome-associated diffuse alveolar hemorrhage. Arthritis Care Res (Hoboken). 2014;66(2):301-310.

See also: DIC, TTP, HUS (Hematology System) for the essential differential diagnosis; Pulmonary-Renal Syndrome (Autoimmune & Rheumatology System) for the related empiric-treatment-first management philosophy; Diffuse Alveolar Hemorrhage (Respiratory System) for CAPS-associated DAH management.