Behçet Disease

• HLA-B51 Association and Genetic Susceptibility: The strongest genetic association is with HLA-B51 (found in >90% of Turkish patients, 50-70% in other Mediterranean populations, 5-10% in Northern Europeans), though HLA-B51 alone is neither necessary nor sufficient for disease development. This suggests additional genetic modifiers (MICA, ICAM-1, FAM167A-BLK locus, IL-10, IL-23R) contribute to disease susceptibility. HLA-B51 likely presents pathogenic peptides (potentially derived from streptococcal antigens or oral pathogens) that activate CD8+ T cells, which then cross-react with endothelial antigens. The genetic contribution accounts for only ~30% of disease heritability, underscoring the critical importance of environmental triggers.

• Aberrant Innate Immunity and Inflammasome Activation: Patients with BD demonstrate constitutive hyperactivation of innate immunity with elevated baseline levels of IL-1β, IL-6, IL-8, TNF-α, and IL-17 compared to healthy controls. This hyperinflammatory state is driven by enhanced NLRP3 inflammasome activation in monocytes and neutrophils, leading to excessive production of pro-inflammatory cytokines, particularly IL-1β. Pathogenic triggers (infectious agents, trauma, microbial-associated molecular patterns) activate pattern recognition receptors (TLRs, NOD-like receptors) on innate immune cells, amplifying inflammasome signaling. The result is inappropriate innate immune amplification in response to normal stimuli or commensals, explaining the pathergy phenomenon (exaggerated response to minor trauma). This aberrant innate immunity is independent of adaptive immunity dysfunction and represents a key therapeutic target.

• Adaptive Immune Dysfunction with Th1/Th17 Skewing: Detailed immunophenotyping reveals abnormalities in T cell regulation characterized by elevated Th1 (IFN-γ-producing) and Th17 (IL-17-producing) CD4+ T cells, with reciprocal reduction in regulatory T cells (Tregs). This Th17 skewing is particularly prominent and correlates with disease activity, especially in patients with ocular manifestations. IL-17 acts on endothelial cells to increase adhesion molecule expression (ICAM-1, VCAM-1), promote vascular permeability, and amplify recruitment of neutrophils and Th17 cells. Th1 responses are similarly enhanced with elevated IFN-γ production driving macrophage activation and granuloma formation (seen in pathergy lesions). Additionally, B cells demonstrate hyperactivity with elevated immunoglobulin production, though serum autoantibodies (ANA, ANCA) are typically absent. The defective Treg compartment—with reduced numbers and impaired suppressive function—fails to restrain this pro-inflammatory Th17/Th1 response.

• Molecular Mimicry and Pathogen-Driven Activation: Emerging evidence suggests environmental pathogens, particularly Streptococcus sanguis and other oral streptococci, trigger disease through molecular mimicry and cross-reactive T cell responses. Specific streptococcal epitopes share homology with endothelial antigens (MICA, vascular adhesion molecules); T cell responses primed against streptococcal antigens (particularly in the context of poor oral hygiene or periodontal disease) cross-react with endothelial and neutrophil antigens. This molecular mimicry extends to HSV-1, which reactivates frequently in BD patients and may amplify local mucocutaneous inflammation. The repeated exposure to oral pathogens combined with genetic predisposition explains the oral-ulcer predominance and the role of poor oral hygiene as a risk factor.

• Endothelial Dysfunction and Vasculitis Pathogenesis: At the tissue level, activated T cells and neutrophils infiltrate blood vessel walls (endothelium and media of small and medium vessels), causing vasculitis. Endothelial cells in BD are primed for activation: they constitutively overexpress adhesion molecules and produce elevated levels of IL-6 and IL-8. In response to pro-inflammatory cytokines, endothelial cells further increase vascular permeability through disruption of tight junctions (VE-cadherin and claudin disruption), allowing transendothelial migration of leukocytes. The resulting vasculitis manifests as leukocytoclasia (neutrophil infiltration with fragmentation) in skin lesions and frank vasculitis with fibrinoid necrosis in affected vessels. Endothelial damage leads to increased tissue factor expression and a prothrombotic state, explaining both thrombotic and hemorrhagic manifestations. The pathergy phenomenon—a sterile pustule developing at sites of minor skin trauma within 24-48 hours—reflects this heightened endothelial reactivity and neutrophil recruitment.

• Oxidative Stress and Complement Activation: BD patients demonstrate elevated markers of oxidative stress (increased reactive oxygen species production by neutrophils and macrophages, elevated malondialdehyde levels) coupled with reduced antioxidant defenses (decreased catalase activity). Oxidative stress perpetuates inflammatory signaling through NF-κB activation and further inflammasome priming. Complementary evidence shows increased C3a generation and elevated markers of complement activation, though complete complement deficiency is not a feature. The combination of oxidative stress and complement activation amplifies tissue damage during vasculitic flares.

• HLA-B51 Positivity: The most significant genetic risk factor, present in 90% of Turkish patients, 50-70% of Mediterranean/Middle Eastern patients, and conferring approximately 20-40-fold increased risk in positive individuals. However, only 3-5% of HLA-B51+ individuals develop BD, emphasizing that genetic predisposition alone is insufficient. Population-specific HLA associations also exist: HLA-B52 in Japanese patients, HLA-A26 in Latinos. These associations suggest population-specific epitopes presented by different HLA alleles.

• Infectious Triggers: Multiple lines of evidence implicate oral pathogens, particularly Streptococcus sanguis (found with increased frequency in oral flora of BD patients and showing elevated serum antibodies). HSV-1 reactivation correlates temporally with mucocutaneous flares and has been detected by PCR in ulcer lesions. Cytomegalovirus (CMV) has been identified in ocular lesions of some patients. The precise mechanism remains unclear—whether infectious agents directly persist in tissue or trigger cross-reactive immune responses through molecular mimicry—but the strong epidemiologic association is established.

• Environmental and Behavioral Factors: Poor oral hygiene and periodontal disease increase disease activity and flare frequency, supporting the role of oral pathogens. Geographic clustering along the Silk Road correlates with specific HLA alleles and suggests environmental exposures (dietary factors, specific pathogens, UV exposure) vary geographically. Smoking paradoxically appears associated with reduced ocular disease severity but increased mucocutaneous disease. Trauma (pathergy) triggers localized inflammation within 24-48 hours, reflecting the hyperreactive endothelium.

• Socioeconomic and Demographic Factors: Lower socioeconomic status (associated with poor oral hygiene and increased infection rates) correlates with disease incidence. Age of onset typically 20-40 years. Male gender predicts more severe disease, particularly ocular manifestations (up to 3:1 ratio in some populations). These demographic patterns suggest both genetic and environmental contributions.

• Secondary Associations: Rarely, BD-like presentations occur with other diagnoses (chronic infections, inflammatory bowel disease with vasculitis, systemic lupus erythematosus overlap), but true secondary BD is not well-established. Some patients with HLA-B27+ ankylosing spondylitis or inflammatory bowel disease may develop BD-like mucocutaneous lesions, representing overlap syndromes rather than true secondary disease.

• Recurrent Oral Aphthous Ulcers (Pathognomonic Mucocutaneous Feature): Present in >95% of BD patients and often the first manifestation, preceding other features by months to years. Ulcers appear as painful, round to oval lesions with erythematous borders and yellowish fibrin-covered base, typically 2-10 mm in diameter (minor aphthae being most common; major and herpetiform varieties less common). They occur on buccal mucosa, gingiva, palate, and tongue—anywhere in the oral cavity. Clusters of 5-10 ulcers commonly appear simultaneously. Individual ulcers heal within 1-3 weeks without scarring, but new ones recur constantly, creating near-continuous presence of oral ulceration. The pathophysiologic basis involves infiltration of activated CD8+ T cells and Th17 cells into the oral epithelium, followed by apoptosis of keratinocytes and ulceration. Minor trauma (cheek biting, aggressive brushing) precipitates ulcers, reflecting pathergy. Patients report significant pain with eating and speaking.

• Genital Ulcers (Highly Specific for BD): Present in 50-75% of patients, appearing as painful ulcers similar in appearance to oral ulcers but often solitary and deeper. In males, ulcers typically occur on scrotum, penis, or perineum; in females, on vulva, vagina, or perineal area. Individual genital ulcers heal slowly (1-6 weeks) often with superficial scarring. Genital ulceration reflects the same vasculitic pathology as oral ulcers but on different mucosal surfaces. The presence of both oral and genital ulcers together is highly specific for BD (present in 44-65% of patients with BD) and significantly increases pre-test probability for the diagnosis.

• Ocular Manifestations (Most Vision-Threatening Features): Occur in 50-70% of patients overall but present in up to 90% of young males in high-prevalence areas. Anterior uveitis is the most common presentation, featuring photophobia, pain, conjunctival injection, anterior chamber inflammation (with hypopyon—pus level in anterior chamber—in 5-10%, pathognomonic for BD when present), and posterior synechiae formation. Patients develop floaters, blurred vision, and photophobia acutely. Posterior uveitis (vitritis, retinal vasculitis, optic disc involvement) often accompanies anterior disease and manifests as floaters, photopsia, and vision loss. Retinal vasculitis—both occlusive (branch retinal artery occlusion, BRAO) and hemorrhagic (retinal hemorrhages, vitreous hemorrhage)—can develop acutely. Over time, repeated inflammation drives cataracts, glaucoma (either from inflammation or from peripheral anterior synechiae), and optic atrophy. Macular edema from chronic inflammation causes the most permanent vision loss. Ocular disease tends to be bilateral (70% of cases) and follows a chronic relapsing course with exacerbations every 3-12 months in untreated patients. Male gender, young age at disease onset, and presence of HLA-B51 predict more severe ocular disease.

• Skin Manifestations (Variable and Diverse): Present in 75-80% of patients with varied morphologies reflecting the underlying vasculitis. Erythema nodosum (painful, tender nodules typically on anterior shins) represents septal panniculitis and occurs in 30-50% of patients, particularly women. Superficial thrombophlebitis appears as cord-like, tender nodules along superficial veins. Pustules and papules (non-follicular, sterile pustules on extremities and trunk) appear spontaneously or after minor trauma (positive pathergy test). Acneiform lesions, urticaria, and livedoid vasculitis (lacy purpuric pattern on extremities) also occur. The pathergy test—intradermal injection of 0.05 mL sterile saline with examination at 24-48 hours—represents the heightened reactivity of endothelium and skin; a pustular or papular reaction is positive and supports BD diagnosis (sensitivity 50-70%, highly specific). These skin manifestations reflect leukocytoclastic vasculitis on histology, with neutrophil infiltration and fibrinoid necrosis.

• Vascular Manifestations (Major Source of Morbidity and Mortality): Occur in 5-40% of patients (higher in cohorts with longer disease duration and in males). Both venous and arterial involvement occurs. Venous thrombosis accounts for 80-90% of vascular events: superficial thrombophlebitis (low risk), deep vein thrombosis of lower extremities (intermediate risk, may be recurrent), superior vena cava syndrome, portal vein thrombosis, cerebral vein/sinus thrombosis (high risk). Arterial involvement (aneurysms, pseudoaneurysms, arterial occlusion) is less common (5-20% of vascular manifestations) but carries higher mortality (10-15% with untreated arterial disease). Pulmonary artery aneurysm represents the most common fatal arterial complication, presenting with massive hemoptysis. The pathophysiology involves vasculitis of the vessel wall with fibrinoid necrosis; in veins, thrombosis develops secondary to endothelial damage and tissue factor expression; in arteries, aneurysmal dilatation develops from medial necrosis. Thrombosis in BD is paradoxically associated with a prothrombotic state despite often normal or only mildly elevated thrombophilia markers, reflecting the local endothelial damage and tissue factor expression rather than systemic thrombophilia.

• Neurological Manifestations (Paraneuro-Behçet): Occur in 5-10% overall but up to 25% in some cohorts. Neuro-BD typically presents with either parenchymal disease (brainstem syndrome with diplopia, dysarthria, ataxia; meningoencephalitis; optic neuritis; transverse myelitis; movement disorders) or vasculitic disease (cerebral venous sinus thrombosis, stroke from arterial involvement). Brainstem syndrome (affecting CN III, IV, V, VI nuclei) is the most common parenchymal manifestation. MRI shows T2/FLAIR-hyperintense lesions in brainstem, basal ganglia, or thalamus. Pathologically, there is vasculitis of small vessels with perivascular inflammation and demyelination.

• Gastrointestinal Manifestations: Present in 5-60% depending on geographic region (higher prevalence in Middle Eastern and Asian populations). Recurrent oral ulcers (technically GI) occur universally, but esophageal, gastric, and colonic involvement also occurs. Patients present with dysphagia (esophageal ulcers), epigastric pain (gastric ulcers), diarrhea, or abdominal pain (colonic ulcers). Ileocecal region is most commonly affected. Histologically, ulcerations show vasculitis and leukocytoclastic infiltration similar to skin lesions. Rarely, intestinal perforation occurs. GI bleeding from large ulcerations has caused fatalities.

• Arthralgias and Arthritis: Present in 40-60% of patients, typically affecting knees, ankles, and wrists symmetrically. Arthritis is usually non-erosive and self-limited, though rarely progresses to chronic destructive disease. The pathophysiology is unclear but likely reflects vasculitis of synovial vessels and inflammatory synovitis.

Immediate stabilisation

• Sight-threatening posterior uveitis/retinal vasculitis, massive hemoptysis from pulmonary artery aneurysm, brainstem neuro-Behçet, or GI perforation: admit, give high-dose systemic glucocorticoids (methylprednisolone pulse), and start a rapidly acting immunosuppressive the same day. Same-day ophthalmology consultation is the single best next step for any acute red eye or vision change.

First-line by manifestation

• Mucocutaneous: topical glucocorticoids for individual ulcers; colchicine is first-line systemic therapy, most effective for erythema nodosum and genital ulcers. Apremilast, a PDE4 inhibitor, is FDA-approved specifically for the oral ulcers of Behçet disease.
• Uveitis: systemic glucocorticoids plus a steroid-sparing agent — azathioprine for milder disease; infliximab or adalimumab (anti-TNF) or interferon-alfa for severe/sight-threatening or relapsing posterior involvement. Cyclosporine is an alternative but not in neuro-Behçet.
• Acute venous thrombosis: glucocorticoids plus an immunosuppressive (azathioprine, cyclosporine, or cyclophosphamide). The clot is inflammatory and adherent to a vasculitic wall — treat the vessel, not just the blood.
• Arterial aneurysm and parenchymal neuro-Behçet: glucocorticoids plus cyclophosphamide, with anti-TNF for refractory disease; long-term azathioprine for maintenance.

Definitive/procedural

• Pulmonary artery aneurysm: medical immunosuppression first; endovascular embolisation is preferred over open repair. Bowel perforation or uncontrolled hemorrhage requires surgery, though anastomotic recurrence is high; peri-operative immunosuppression reduces it.

Contraindicated/avoid

• Anticoagulation as sole therapy for Behçet thrombosis, and anticoagulation of any kind before pulmonary artery aneurysm is excluded — fatal hemoptysis risk.
• Cyclosporine in neuro-Behçet (neurotoxicity, associated with parenchymal CNS events).
• Thalidomide and cyclophosphamide in pregnancy (teratogenic).

Disease-related — emergencies

• Pulmonary artery aneurysm rupture: medial necrosis from arterial vasculitis produces pseudoaneurysm; signalled by massive hemoptysis in a young man who often also has DVT. The classic trap is treating this as pulmonary embolism and anticoagulating — a lethal error.
• Cerebral venous sinus thrombosis: presents with headache, papilledema, and elevated opening pressure on LP with otherwise bland CSF; confirm with MR venography.
• Irreversible vision loss: recurrent posterior uveitis and occlusive retinal vasculitis cause macular edema, optic atrophy, and secondary glaucoma from posterior/peripheral anterior synechiae. Painless progressive visual decline between flares signals cumulative damage.
• Intestinal perforation or hemorrhage: deep, punched-out ileocecal ulcers; peritonitis or hematochezia with hemodynamic instability.
• Parenchymal brainstem neuro-Behçet: cranial neuropathies, ataxia, and pseudobulbar affect; the leading cause of long-term disability and death in neuro-Behçet.

Disease-related — non-emergent

• Budd-Chiari syndrome and SVC syndrome: hepatic vein or caval thrombosis; ascites, hepatomegaly, or facial/upper-limb plethora with collateral veins.
• AA (secondary) amyloidosis: chronic serum amyloid A elevation from persistent inflammation; heralded by new proteinuria or nephrotic syndrome.

Treatment-related

• Glucocorticoids: hyperglycemia, osteoporosis, adrenal suppression, and infection risk with prolonged tapers.
• Cyclophosphamide: acrolein-mediated hemorrhagic cystitis (prevented with mesna and hydration), bladder malignancy, and gonadal failure.
• Azathioprine: myelosuppression, exaggerated in TPMT or NUDT15 deficiency — check activity/genotype before starting; never combine with allopurinol at full dose.
• TNF inhibitors: reactivation of latent tuberculosis and hepatitis B; screen with an IGRA/tuberculin test, chest radiograph, and HBV serologies before the first dose, per CDC and ACR biologic-screening guidance.
• Cyclosporine: nephrotoxicity, hypertension, gingival hyperplasia, and neurotoxicity.
• Colchicine: diarrhea; myopathy and marrow suppression in renal impairment or with CYP3A4/P-gp inhibitors.
• Apremilast: diarrhea, weight loss, and depressed mood.

• The classic triad stem: recurrent painful oral aphthae plus painful genital ulcers plus uveitis in a young adult of Turkish, Middle Eastern, or East Asian descent. Hypopyon — a layered sterile pus level in the anterior chamber — is the single most recognizable buzzword.
• Pathergy: a sterile pustule at the site of a needle stick 24–48 hours later. Highly specific, only modestly sensitive, and far more often positive in Mediterranean than in Northern European or US patients.
• The one association examiners test: HLA-B51. Know that it is a risk marker, not a diagnostic test — it is neither necessary nor sufficient, and ordering it is never the best next step.
• Best next step for vision change: urgent slit-lamp examination and dilated fundoscopy, then systemic glucocorticoids with anti-TNF or interferon-alfa for posterior involvement, per EULAR. Do not treat sight-threatening uveitis with topical steroids alone.
• Best next step for hemoptysis: CT angiography of the chest to look for pulmonary artery aneurysm before any anticoagulation. Behçet thrombus is inflammatory and adherent, so embolization is rare and anticoagulation adds bleeding risk without benefit.
• Vascular management principle: immunosuppression, not anticoagulation, is the treatment of Behçet-associated DVT.
• Distractors to avoid: reactive arthritis and SLE cause painless oral ulcers (and circinate balanitis is painless); herpes gives grouped vesicles on an erythematous base with positive PCR; Crohn disease can mimic ileocecal Behçet but shows granulomas and skip lesions. Behçet is ANA-negative and ANCA-negative, and there is no diagnostic serology — diagnosis is clinical.
• Prognosis pearl: young male sex and early onset predict severe ocular and vascular disease; mucocutaneous activity tends to wane with age even as vascular risk persists.