10.3 Urticaria, Angioedema & Cutaneous Mastocytosis

10.3 Urticaria, Angioedema & Cutaneous Mastocytosis

Classification and Pathophysiology of Urticaria

Urticaria is a vascular reaction pattern characterized by the transient appearance of wheals (hives), angioedema, or both. A classic urticarial wheal displays three cardinal features:

1. A central swelling of variable size, almost invariably surrounded by a reflex erythema.
2. Associated itching or sometimes burning sensation.
3. Fleeting nature: Individual wheals typically arise within minutes and resolve completely within 2 to 24 hours without leaving purpura, bruising, or post-inflammatory hyperpigmentation.

High-Yield Exam Trap: Urticarial Vasculitis vs. True Urticaria If individual wheals persist at the exact same anatomical location for longer than 24 to 48 hours, feel painful/tender or burning rather than intensely itchy, and resolve with residual petechiae, ecchymosis, or post-inflammatory hyperpigmentation, suspect urticarial vasculitis. A punch biopsy is mandatory, demonstrating leukocytoclastic vasculitis with post-capillary venule endothelial swelling, fibrinoid necrosis of vessel walls, and neutrophilic karyorrhexis (leukocytoclasia).

                                [CLASSIFICATION OF URTICARIA]
                                              │
                     ┌────────────────────────┴────────────────────────┐
                     ▼                                                 ▼
            [ACUTE URTICARIA]                                 [CHRONIC URTICARIA]
            • Duration < 6 weeks                              • Duration >= 6 weeks
            • Viral infections (children)                     • Subdivided into CSU and CIndU
            • Adverse drug reactions (NSAIDs, β-lactams)      • Severe QoL and sleep impairment
            • IgE food allergy / Hymenoptera stings           
                     │
                     ▼
          ┌────────────────────────────────────────────────────────────────────────┐
          ▼                                                                        ▼
[CHRONIC SPONTANEOUS URTICARIA (CSU)]                   [CHRONIC INDUCIBLE URTICARIA (CIndU)]
• Arises without specific external trigger               • Specific reproducible physical stimulus
• Autoimmune Type I (Auto-allergic IgE against TPO)      • Symptomatic Dermographism (friction/stroking)
• Autoimmune Type IIb (IgG anti-FcεRIα or anti-IgE)      • Cold Urticaria (cold water/wind; TempTest)
• Positive Autologous Serum Skin Test (ASST)             • Delayed Pressure Urticaria (sustained pressure)
• Monitored by Weekly UAS7 Diary                         • Cholinergic Urticaria (core temp elevation; tiny wheals)
                                                         • Solar, Heat, Aquagenic, Vibratory

Acute Urticaria (<6 Weeks)

• Etiology: Highly prevalent (lifetime incidence up to 20%). The vast majority of episodes in children and young adults are triggered by acute viral respiratory or gastrointestinal infections. Non-allergic and allergic drug hypersensitivity reactions (beta-lactam antibiotics, non-steroidal anti-inflammatory drugs [NSAIDs]) and IgE-mediated food allergies are additional common causes. Extensive laboratory workups are unwarranted in uncomplicated acute urticaria.

Chronic Spontaneous Urticaria (CSU, ≥6 Weeks)

In CSU, wheals recur spontaneously for 6 weeks or longer without an identifiable external physical trigger. Approximately 40% to 50% of CSU cases possess an autoimmune etiology, broadly divided into two major pathophysiological endotypes:

• Type I Autoimmunity (Auto-Allergic CSU): Driven by functional IgE autoantibodies directed against endogenous self-antigens, including thyroid peroxidase (TPO), interleukin-24 (IL-24), and double-stranded DNA. These auto-allergic IgE molecules bind FcεRI on mast cells and cross-link upon encountering circulating self-antigens, causing rapid degranulation. Patients with this endotype typically display elevated total serum IgE, positive anti-TPO antibodies, and achieve exceptionally rapid responses to omalizumab.
• Type IIb Autoimmunity (Autoantibody-Mediated CSU): Driven by functional IgG autoantibodies directed against the alpha subunit of the high-affinity IgE receptor (anti-FcεRIα) or directly against IgE (anti-IgE). These IgG autoantibodies cross-link adjacent FcεRI receptors and fix complement (generating C5a, a potent mast cell secretagogue). Clinically characterized by:
  • Positive Autologous Serum Skin Test (ASST) or positive basophil activation test (BAT/CD63 expression).
  • Low or normal total serum IgE, marked basopenia, and eosinopenia.
  • A more refractory clinical course, slower or poor response to omalizumab, but dramatic responsiveness to ciclosporin.

Chronic Inducible Urticaria (CIndU)

In CIndU, wheal and flare reactions occur exclusively in response to specific, reproducible physical triggers:

• Symptomatic Dermographism (Factitious Urticaria): Linear wheals and reflex erythema develop within 1 to 5 minutes after shearing, stroking, or scratching the skin, resolving within 1 to 2 hours. Confirmed using a calibrated dermographometer.
• Cold Urticaria: Wheals triggered within minutes of exposure to cold wind, liquids, or cold surfaces. Diagnosed using an ice cube placed on the volar forearm for 5 minutes or computerized thermal testing (TempTest®). Critical clinical hazard: swimming in cold unmonitored water can provoke massive whole-body mast cell degranulation, precipitating hypovolemic shock, drowning, and death.
• Delayed Pressure Urticaria: Deep, erythematous, painful or burning swellings that arise 4 to 8 hours following sustained perpendicular pressure (tight clothing, tool use, carrying heavy backpack straps). Does not respond well to standard antihistamines.
• Cholinergic Urticaria: Triggered by an increase in core body temperature (active exercise, hot baths, emotional stress, spicy foods). Characterized by tiny, punctate, 1 to 3 mm wheals surrounded by broad, vivid erythematous flares, appearing predominantly on the trunk, neck, and upper limbs.
• Solar Urticaria: Wheals appearing within minutes of exposure to sunlight (UVA, UVB, or visible light). Tested via monochromator phototesting.

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Disease Activity Assessment: The UAS7 Score

The Urticaria Activity Score over 7 days (UAS7) is the validated international standard for quantifying disease activity and monitoring treatment response in CSU:

$\text{Weekly UAS7 Score} = \sum_{i=1}^{7} (\text{Wheals Score}_i + \text{Pruritus Score}_i) \quad [\text{Range: 0 to 42}]$

• UAS7 Disease Activity Stratification:
  • 0: Complete disease control / remission.
  • 1 to 6: Well-controlled disease.
  • 7 to 15: Mild disease activity.
  • 16 to 27: Moderate disease activity.
  • 28 to 42: Severe active disease.

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European Algorithmic Escalation (EAACI / GA²LEN / EuroGuiDerm / APAAACI Guideline)

The international consensus guideline establishes a strict 4-step treatment escalation ladder for chronic urticaria:

                    [EAACI / GA²LEN / EuroGuiDerm / APAAACI STEPWISE ESCALATION]

  STEP 4: Ciclosporin Add-on (3–4 mg/kg/day)
  ═══════════════════════════════════════════════════════════════════════════════
  • Add to 4-fold sgH1-antihistamines in patients uncontrolled by Step 3
  • Monitor blood pressure and serum creatinine bi-weekly; limit course to 3–6 months
                                        ▲
                                        │ Uncontrolled after >= 3–6 months
  STEP 3: Omalizumab Add-on (300 mg SC every 4 weeks)
  ═══════════════════════════════════════════════════════════════════════════════
  • Add to 4-fold sgH1-antihistamines
  • Highly effective, licensed for CSU; flat dose (does not depend on IgE/weight)
                                        ▲
                                        │ Uncontrolled after 2–4 weeks
  STEP 2: Updosing sgH1-Antihistamines (Up to 4-Fold Standard Dose)
  ═══════════════════════════════════════════════════════════════════════════════
  • Bilastine (up to 80 mg), Levocetirizine (up to 20 mg), Fexofenadine (up to 720 mg)
  • Off-label high-level guideline recommendation; excellent safety profile
                                        ▲
                                        │ Uncontrolled after 2–4 weeks
  STEP 1: Standard-Dose Second-Generation H1-Antihistamine (sgH1-AH)
  ═══════════════════════════════════════════════════════════════════════════════
  • Cetirizine 10 mg, Levocetirizine 5 mg, Fexofenadine 180 mg, Bilastine 20 mg, Desloratadine 5 mg
  • Avoid sedating 1st-generation antihistamines (diphenhydramine, hydroxyzine)

Step 1: Second-Generation H1-Antihistamines (Standard Dose)

• Monotherapy with a modern, non-sedating second-generation H1-antihistamine (bilastine 20 mg, cetirizine 10 mg, levocetirizine 5 mg, fexofenadine 180 mg, or desloratadine 5 mg) once daily.
• Crucial Rule: First-generation sedating antihistamines (e.g., hydroxyzine, diphenhydramine, chlorpheniramine) are strongly deprecated by European guidelines due to anticholinergic adverse effects, REM-sleep disruption, impairment of psychomotor performance, and increased vehicular accident rates.

Step 2: Antihistamine Updosing (Up to 4-Fold)

• If symptoms persist or UAS7 remains elevated after 2 to 4 weeks (or sooner in severe cases), the guideline explicitly recommends increasing the dose of the second-generation H1-antihistamine up to 4-fold the licensed daily dose (e.g., bilastine 40–80 mg daily, levocetirizine up to 20 mg daily, fexofenadine up to 360–720 mg daily).
• Updosing a single second-generation agent is strongly preferred over mixing different antihistamines.

Step 3: Omalizumab Add-On

• If inadequate control persists after 2 to 4 weeks on 4-fold antihistamine therapy, omalizumab is added to the high-dose antihistamine regimen.
• Mechanism: Omalizumab is a humanized IgG1k monoclonal antibody that selectively binds to the Cε3 domain of free circulating IgE, preventing IgE from binding to the high-affinity receptor (FcεRI) on mast cells and basophils. This depletes free IgE and induces gradual downregulation of cell-surface FcεRI expression.
• Standard Dosing: 300 mg administered subcutaneously every 4 weeks.
• Exam Fact: In contrast to its dosing in allergic asthma, omalizumab dosing in CSU is fixed (300 mg flat dose) and does NOT depend on the patient's baseline serum total IgE level or body weight.

Step 4: Ciclosporin Add-On

• Reserved for patients who remain refractory after 3 to 6 months of omalizumab at maximum doses (or omalizumab-intolerant patients).
• Dosing & Safety: 3 to 4 mg/kg/day orally in two divided doses. Ciclosporin inhibits calcineurin, suppressing NFAT-mediated cytokine transcription in mast cells and basophils. Blood pressure and serum creatinine must be checked at baseline and every 2 to 4 weeks. Duration is typically restricted to 3 to 6 months to minimize nephrotoxicity.

Exacerbation Management: Systemic Corticosteroids

• Systemic corticosteroids (prednisolone 20 to 50 mg/day) should be used strictly for short rescue courses (maximum 3 to 7 days) to manage severe acute flares. Chronic maintenance corticosteroid therapy in chronic urticaria is categorically unacceptable due to long-term adverse events and severe rebound upon withdrawal.

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Angioedema: Histaminergic vs. Bradykinin-Mediated

Angioedema involves sudden, pronounced, non-pitting edema of the deep dermis, subcutaneous tissue, or submucosa. Distinguishing between mast cell-mediated (histaminergic) and bradykinin-mediated angioedema is a critical, life-or-death diagnostic requirement:

                    ┌────────────────────────────────────────────────────────┐
                    │                ANGIOEDEMA SUBTYPES                     │
                    └───────────────────────────┬────────────────────────────┘
                                                │
                 ┌──────────────────────────────┴──────────────────────────────┐
                 ▼                                                             ▼
    [HISTAMINERGIC ANGIOEDEMA]                                    [BRADYKININ-MEDIATED ANGIOEDEMA]
    • Mediators: Histamine, LTC4, PGD2                            • Mediator: Excessive Bradykinin
    • Wheals present in >80% of cases                             • Wheals ABSENT (NO hives)
    • Marked pruritus; rapid onset (mins/hrs)                     • Painful / tightness; slower onset (hours to days)
    • Responsive to Antihistamines, Steroids,                     • RESISTANT to Antihistamines, Steroids, Adrenaline
      and Intramuscular Adrenaline                                • Abdominal colic (bowel edema mimicking acute abdomen)
    • Subtypes: Acute urticaria, CSU, Anaphylaxis                 • Life-threatening laryngeal edema
                                                                  • Subtypes: HAE (1, 2, nC1-INH), AAE, ACE-Inhibitor

1. Mast Cell / Histaminergic Angioedema

• Accompanied by urticarial wheals in >80% of cases. Rapid onset (minutes to hours).
• Highly responsive to second-generation H1-antihistamines, systemic corticosteroids, and intramuscular adrenaline (epinephrine).

2. Bradykinin-Mediated Angioedema

• Hallmark Features: Absolute absence of urticarial wheals, absence of pruritus, severe visceral involvement (excruciating colicky abdominal pain from transient intestinal wall edema mimicking a surgical acute abdomen), and life-threatening asphyxiation from laryngeal edema.
• Therapeutic Resistance: Completely unresponsive to antihistamines, corticosteroids, and adrenaline!

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Subtypes and Diagnostic Laboratory Tree for Angioedema

High-Yield Diagnostic Algorithm

1. Screening Test: Serum Complement C4 is the single best, cost-effective initial screening test. In untreated HAE Type 1 and Type 2, C4 is persistently depressed even between attacks (sensitivity >95%). A normal C4 level during an acute attack essentially rules out HAE Type 1 and Type 2.
2. Confirmatory Testing: Measure C1-INH antigenic protein and C1-INH functional activity. If protein is low, HAE Type 1 is confirmed. If protein is normal/elevated but function is low, HAE Type 2 is confirmed.
3. Distinguishing HAE from AAE: Measure C1q. C1q is strictly normal in Hereditary Angioedema, but markedly decreased in Acquired Angioedema (AAE) due to immune complex-mediated consumption. Adult-onset angioedema with low C1q mandates immediate screening for an underlying hematological B-cell malignancy (serum protein electrophoresis, immunofixation, bone marrow biopsy).

Management of Bradykinin-Mediated Angioedema

Acute Attack Emergency Therapy

• Airway Security: Priority one. Early flexible nasopharyngoscopy; immediate endotracheal intubation or emergency cricothyroidotomy if laryngeal edema is advancing.
• C1-INH Concentrates:
  • Plasma-derived human C1-INH: Berinert (20 IU/kg body weight IV) or Cinryze (1000 IU IV).
  • Recombinant human C1-INH: Ruconest (conestat alfa, 50 IU/kg IV, produced in transgenic rabbit milk; contraindicated in rabbit dander allergy).
• Bradykinin B2 Receptor Antagonist:
  • Icatibant (Firazyr): Synthetic decapeptide that competitively blocks the bradykinin B2 receptor. Administered as a single 30 mg subcutaneous injection into the abdominal wall. Patients can self-administer at home; symptoms typically begin resolving within 30 to 60 minutes.
• Kallikrein Inhibitor: Ecallantide (Kalbitor) (recombinant plasma kallikrein inhibitor, 30 mg SC; carries risk of anaphylaxis). It is approved in the US only and is not available in the EU.

Long-Term Prophylaxis in HAE

• Lanadelumab (Takhzyro): Fully human IgG1 monoclonal antibody that potently inhibits active plasma kallikrein, preventing bradykinin generation. Dosed 300 mg SC every 2 weeks (can extend to every 4 weeks once well-controlled). Transforms HAE management with dramatic reduction in attack frequencies.
• Berotralstat (Orladeyo): Oral, once-daily small-molecule plasma kallikrein inhibitor (150 mg daily).
• Subcutaneous C1-INH: Haegarda (60 IU/kg SC twice weekly).
• Attenuated Androgens: Danazol or stanozolol (historically used to stimulate hepatic C1-INH synthesis; heavily restricted today due to virilization, hepatotoxicity, liver adenomas, and lipid derangements).

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Cutaneous Mastocytosis

Definition and WHO / ECNM Classification

Mastocytosis is a heterogeneous group of disorders defined by the abnormal clonal proliferation and accumulation of morphologically and immunophenotypically aberrant mast cells within tissues. When restricted exclusively to the skin, it is termed Cutaneous Mastocytosis (CM):

1. Maculopapular Cutaneous Mastocytosis (MPCM / Urticaria Pigmentosa):
   • Most common clinical form (>70% of cases).
   • Multiple symmetrical, round-to-oval, reddish-brown macules, papules, and plaques located predominantly on the trunk and limbs, usually sparing the face, palms, and soles.
   • Polymorphic variant: Lesions of variable sizes, common in infants and children; carries an excellent prognosis with frequent spontaneous involution around puberty.
   • Monomorphic variant: Uniformly small maculopapules, common in adolescents and adults; frequently persists into adulthood and signifies underlying Systemic Mastocytosis (SM) in >95% of adult patients!
2. Solitary Mastocytoma: One to three localized, discrete, reddish-brown or yellow-orange nodules occurring almost exclusively in infants. Spontaneous resolution within several years.
3. Diffuse Cutaneous Mastocytosis (DCM): Rare, severe form presenting in neonates/infants. Diffuse, leathery, thickened infiltration of the entire integument ('peau d'orange' texture), marked dermographism, and extensive blistering/bullae following minor friction.

Pathognomonic Sign: Darier's Sign

• Performance: Mechanical stroking or gentle rubbing of a pigmented cutaneous lesion with the tip of a pen or wooden tongue depressor.
• Positive Reaction: Rapid development of localized erythema, edema, and a prominent urticarial wheal accompanied by intense itching within 2 to 5 minutes strictly over the rubbed lesion.
• Mechanism: Mechanical trauma directly triggers degranulation of hyperplastic dermal mast cells, dumping histamine, leukotrienes, and heparin into the local extracellular space.

Molecular Genetics: The KIT D816V Mutation

• Mast cell growth and differentiation are critically dependent on the interaction between the KIT receptor (CD117) and its ligand, Stem Cell Factor (SCF).
• Over 80% to 90% of adult patients with mastocytosis harbor an acquired somatic activating point mutation in the KIT gene: c-KIT D816V (aspartate replaced by valine at codon 816 in the catalytic kinase domain).
• This mutation induces ligand-independent, constitutive autophosphorylation of the KIT tyrosine kinase, driving continuous mast cell proliferation and survival.
• Crucial Pharmacological Fact: The D816V mutation alters the kinase pocket conformation, conferring absolute resistance to imatinib! (Non-D816V mutations in pediatric mastocytosis, such as exon 8/9/11 mutations, may remain imatinib-sensitive).

Laboratory Workup and Diagnostic Criteria

• Histopathology: Dense mast cell infiltrate in the upper and mid-dermis (perivascular and interstitial). Mast cells are highlighted using Giemsa, toluidine blue (metachromatic granules), and immunohistochemistry for CD117 (KIT), tryptase, and aberrant expression of CD25 and/or CD2.
• Serum Total Tryptase:
  • Normal baseline: <11.4 ng/mL.
  • A persistently elevated baseline serum total tryptase >20 ng/mL is a minor WHO criterion for Systemic Mastocytosis.
  • In any adult patient presenting with urticaria pigmentosa, an elevated serum tryptase (>20 ng/mL), unexplained syncope, or organomegaly mandates a bone marrow aspiration and biopsy (evaluating multifocal mast cell aggregates, KIT D816V mutation analysis, and flow cytometry for CD25/CD2 expression) alongside bone densitometry (DXA for osteoporosis/osteolytic fractures).

Clinical Management of Mastocytosis

• Strict Trigger Avoidance: Patients must be educated to avoid non-specific mast cell degranulating triggers:
  • Physical: Rapid temperature changes, friction, vigorous rubbing, heat, hot baths, emotional stress.
  • Pharmacological: Morphine, codeine, opioids, non-steroidal anti-inflammatory drugs (NSAIDs), neuromuscular blocking agents (rocuronium, atracurium), and iodinated radiocontrast media.
  • Biological: Hymenoptera venom stings (wasp and bee stings provoke fatal anaphylaxis; mandatory venom immunotherapy [VIT] if sensitized).
• Pharmacological Symptom Control: Second-generation H1-antihistamines (up to 4-fold dose) combined with H2-antihistamines (famotidine); oral sodium cromoglicate (mast cell stabilizer, particularly effective for abdominal cramping and diarrhea); leukotriene receptor antagonists (montelukast).
• Emergency Preparedness: All adult patients with mastocytosis, patients with diffuse cutaneous involvement, or any patient with a history of systemic mediator symptoms must be prescribed and carry two adrenaline auto-injectors (0.3 mg) at all times.