Urtikaria

01Pathophysiology

Urticaria is fundamentally a mast cell-driven disease. The central event is the activation and degranulation of cutaneous mast cells, which release preformed mediators (primarily histamine, but also tryptase, prostaglandins, and leukotrienes) into the surrounding dermis. This mediator release causes three hallmark tissue responses: vasodilation (erythema), increased vascular permeability (edema, producing the wheal), and sensory nerve stimulation (pruritus). This triad explains why the patient presents with raised, red, intensely itchy plaques.

In acute urticaria, the trigger is usually exogenous and identifiable. IgE-mediated type I hypersensitivity reactions (foods, drugs, insect stings) cause crosslinking of IgE on the mast cell surface via the high-affinity IgE receptor (FcεRI), leading to immediate degranulation. Non-IgE-mediated triggers include direct mast cell degranulation by certain drugs (opioids, radiocontrast media, vancomycin), complement activation (C3a, C5a anaphylatoxins), and viral infections in children.

In chronic spontaneous urticaria (CSU), the mechanism is predominantly autoimmune. Two distinct autoimmune endotypes are now recognized. In type IIb autoimmune CSU, IgG autoantibodies target either the α-subunit of FcεRI or IgE itself on the mast cell surface. These autoantibodies directly crosslink receptors and activate the cell, independent of any allergen. This subtype is associated with low total IgE levels, basopenia, positive autologous serum skin test (ASST), and a tendency toward poorer response to omalizumab but better response to cyclosporine. In type I autoimmune CSU (autoallergic CSU), patients produce IgE autoantibodies against self-antigens (including thyroid peroxidase (TPO), IL-24, and double-stranded DNA). This form tends to show higher total IgE levels and a faster, more robust response to omalizumab.

The reason individual wheals are transient (lasting less than 24 hours) is that the mast cell mediator release is self-limited and the increased vascular permeability reverses as histamine is metabolized. If a wheal persists beyond 24 hours or leaves residual bruising or pigmentation, the pathology is no longer simple urticaria but likely involves leukocytoclastic vasculitis of dermal vessels, and this should prompt a skin biopsy.

Angioedema occurs when the same process extends to the deeper dermis and subcutaneous tissue. It is present in roughly 40% of CSU patients alongside wheals, and in about 10% of patients, it occurs in isolation without surface wheals. Histamine-mediated angioedema (as seen in urticaria) is distinguished from bradykinin-mediated angioedema (as seen in hereditary angioedema or ACE inhibitor-induced angioedema) by its association with pruritus and response to antihistamines. Bradykinin-mediated angioedema is not pruritic and does not respond to antihistamines.

02Classification and Clinical Manifestation

The international guideline (consensus conference December 2024, published February 2026) maintains the following classification:

Subtypes of Chronic Inducible Urticaria

03Diagnostic Workup

Acute urticaria does not require routine diagnostic testing. The 2024 international guideline explicitly recommends against routine laboratory workup in acute spontaneous urticaria. The diagnosis is clinical. History alone is usually sufficient to identify the trigger (a new medication, a food, an infection). Testing should only be done if the history strongly suggests a particular etiology, for instance, allergen-directed IgE testing if there is a clear history of food or venom exposure.

Chronic urticaria demands a stepwise workup, but the scope should be guided by clinical suspicion, not a blanket panel. The best initial diagnostic step is a thorough history and physical examination: duration and morphology of individual lesions (< 24 hours versus > 24 hours), associated symptoms, medication use (particularly NSAIDs and ACE inhibitors), and response to antihistamines.

For the baseline laboratory workup of CSU, most guidelines converge on a minimal panel: CBC with differential, ESR or CRP, and TSH. These serve to screen for underlying inflammation, eosinophilic disorders, and concurrent autoimmune thyroid disease. The addition of total IgE and anti-TPO IgG has become increasingly important since the approval of omalizumab, as these biomarkers help predict treatment response and endotype the disease.

There is no single confirmatory "gold standard" test for CSU itself. The diagnosis is clinical. However, if urticarial vasculitis is suspected (individual lesion persistence > 24 hours, pain > pruritus, purpura or residual pigmentation, associated systemic symptoms), a punch biopsy is the most accurate test to confirm it, showing leukocytoclastic vasculitis with perivascular neutrophilic infiltrate and fibrinoid necrosis of vessel walls.

For chronic inducible urticaria, provocation testing confirms the diagnosis and identifies the threshold for the triggering stimulus. An ice cube placed on the forearm for 5 minutes confirms cold urticaria (wheal formation upon rewarming). Dermatographism is confirmed by standardized skin stroking. Exercise challenge with core temperature monitoring confirms cholinergic urticaria.

04Management and Treatment

Acute urticaria management is straightforward. The mainstay is a second-generation (non-sedating) H1-antihistamine. First-generation antihistamines (diphenhydramine, hydroxyzine) are no longer recommended as first-line due to sedation, anticholinergic effects, and cognitive impairment. For acute flares, a short course of systemic corticosteroids (e.g., prednisolone 0.5 to 1 mg/kg/day for 3 to 5 days, maximum 10 days) may be used but should not become a chronic treatment strategy. If there is associated anaphylaxis, intramuscular epinephrine is the priority.

Chronic spontaneous urticaria management follows the revised three-step algorithm.

Step 1: Begin with a second-generation H1-antihistamine at standard dose. All licensed second-generation antihistamines are considered equivalent in efficacy. The choice is driven by availability and patient tolerance. Continue this for at least 2 to 4 weeks before escalating.

Step 2: If symptoms persist, increase the dose up to fourfold (4x the standard dose). This is the single most important concept tested on exams: the next best step when a patient fails standard-dose antihistamine is to updose, not to switch classes, add a first-generation antihistamine at bedtime, or jump directly to omalizumab or steroids. About 60 to 70% of patients will achieve control at this step. During step-down, patients should maintain complete symptom control for at least 3 consecutive months before reducing the dose, and the reduction should be gradual (no more than one tablet per month).

Step 3: For patients unresponsive to fourfold antihistamine dosing, add omalizumab 300 mg SC every 4 weeks as add-on therapy. Omalizumab is a recombinant humanized monoclonal anti-IgE antibody that reduces free IgE and downregulates FcεRI expression on mast cells and basophils. The starting dose is 300 mg every 4 weeks; dosing in CSU is not based on body weight or total IgE (unlike in asthma). Up to 6 months should be allowed to assess response. If the response is inadequate, dose escalation up to 600 mg every 2 weeks may be considered. The antihistamine is continued alongside omalizumab; it is an add-on, not a replacement.

Step 4 (refractory): Cyclosporine (3 to 5 mg/kg/day, divided BID) is reserved for patients who fail omalizumab or in whom omalizumab is unavailable. It is a calcineurin inhibitor that suppresses T-cell activation and has direct effects on mast cell mediator release. Monitoring for nephrotoxicity and hypertension is mandatory. Regular measurement of serum creatinine, blood pressure, and cyclosporine trough levels is required.

Important contraindications and exam-relevant points:

• First-generation antihistamines are contraindicated or strongly discouraged in the elderly (fall risk, cognitive impairment, anticholinergic burden), in patients operating machinery, and in pregnancy at certain doses.

• Leukotriene receptor antagonists (montelukast) are no longer recommended in the current stepwise algorithm and have been downgraded.

• Chronic oral corticosteroids are not recommended for CSU. They cause well-known adverse effects (osteoporosis, adrenal suppression, metabolic syndrome) and provide only temporary relief with rebound upon discontinuation.

• Cyclosporine is contraindicated in uncontrolled hypertension and significant renal impairment.

• Omalizumab is generally well-tolerated but carries a small risk of anaphylaxis (0.1 to 0.2%), so the first doses should be administered in a monitored clinical setting.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common way students get urticaria questions wrong is by selecting the wrong next step when standard-dose antihistamine fails. The reflex answer for many test-takers is to add a corticosteroid, switch to a different antihistamine class, or prescribe omalizumab. The correct next step is to increase the dose of the same second-generation H1-antihistamine to up to four times the standard dose. This updosing strategy is the cornerstone of the revised algorithm and is one of the most frequently tested management concepts.

The second major trap involves confusing histamine-mediated angioedema with bradykinin-mediated angioedema. When a vignette describes isolated angioedema without wheals, without pruritus, and unresponsive to antihistamines or epinephrine, that is bradykinin-mediated (HAE or ACE inhibitor-induced), not urticaria. Ordering a C4 level and C1-inhibitor assay, not giving more antihistamine, is the correct move.

The third common pitfall is diagnosing urticarial vasculitis as ordinary urticaria. The discriminator is the timeline and aftermath of the individual wheal. Exam vignettes will subtly describe lesions that "persist for more than a day" or "leave a brownish discoloration after resolving." These details signal vasculitis and the need for a skin biopsy, not empiric antihistamine escalation.

Finally, students frequently miss that acute urticaria does not require diagnostic workup. A vignette of a child with a viral URI who develops transient, pruritic wheals for a few days is asking you to recognize the self-limited nature of the condition and manage symptomatically, not order a panel of labs.

The core competency being tested across all urticaria questions is the ability to classify the disease correctly (acute versus chronic, spontaneous versus inducible), select the appropriate level of workup (minimal for acute, stepwise for chronic), and follow the correct treatment algorithm (updose before escalating to biologics).